Cyclic Monomers: Epoxides, Lactide, Lactones, Lactams, Cyclic Silicon-Containing Monomers, Cyclic Carbonates, and Others

Carlotti, Stéphane; Peruch, Frédéric

doi:10.1007/978-4-431-54186-8_5

Stéphane Carlotti^3,4 &
Frédéric Peruch^3,4

3824 Accesses
9 Citations

Abstract

Cyclic monomers constitute a broad family of monomers which are able to be polymerized by anionic ring-opening polymerization or related nucleophilic ring-opening mechanism. This chapter presents successively the polymerization of cyclic ethers, cyclic esters, cyclic amides, cyclosiloxanes and other cyclic silicon-based compounds, cyclic carbonates, and other cyclic monomers, i.e., cycloalkanes, cyclic sulfides, cyclic amines, cyclic ureas, depsipeptides, and cyclic phosphorus monomers. The main synthetic strategies are reviewed in terms of monomer reactivity, side reactions, and control of macromolecular architectures. Ring-opening polymerization of cyclic monomers utilizing alkali metal derivatives or other initiating systems in conjunction or not with activating systems is described. Emphasis is also put on the use of organic initiators or catalysts to trigger the metal-free ring-opening polymerization.

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Cyclic Polymers Synthesized by Spontaneous Selective Cyclization Approaches

A rational entry to cyclic polymers via spontaneous and selective cyclization reactions

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Article 21 September 2022

Keywords

1 Introduction

For many years, efforts in polymer science were directed toward the control of polymerization methods, a precise control of the structure, topology, and functionality of polymeric chains enabling the design of macromolecular scaffolds that may find applications in high added value domains. For polymers bearing heteroatoms in their backbone, two pathways are generally possible: step-growth polymerization and ring-opening polymerization (ROP). The main advantage of step-growth polymerization is the easy accessibility of a wide range of monomers of various structures. Nevertheless, it suffers from limitations. Indeed, high conversions are needed to get high molar mass polymers, often not controlled, and high polymerization temperatures are generally required. These drawbacks are overcome by the implementation of ROP, which has become a powerful tool for the synthesis of various polymers, mainly polyethers, polyesters, polyamides, polysiloxanes, and polycarbonates (Scheme 1).

The ring strain, coming from distortion of the ring angles and stretching bonds, is generally responsible for conversion of monomer into polymer units. The ROP can be performed according to several mechanisms, namely, anionic, cationic, and coordination-insertion. In this chapter, we will focus on polymerization for which the propagating species is an anion. Few exceptions, where the propagating species are not fully charged, will also be presented. This chapter will be divided into several sub-chapters, each one focusing on one type of cyclic monomer. It will be presented successively the anionic ring-opening polymerization of cyclic ethers, cyclic esters, cyclic amides, cyclic silicon-containing monomers, cyclic carbonates, and other cyclic monomers, i.e., cycloalkanes, cyclic sulfides, cyclic amines, cyclic ureas, depsipeptides and cyclic phosphorus monomers. When well established, the elementary steps involved in the polymerization are given. Recent developments concerning the synthesis of controlled macromolecular architectures are also presented.

2 Cyclic Ethers

2.1 Introduction

The anionic ring-opening polymerization (AROP) of cyclic ethers enables the synthesis of polyethers like poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO), often referred to as poly(ethylene glycol) and poly(propylene glycol), respectively. The worldwide production of these polymers attains several million tons per year for commodity (precursors for polyurethanes, surfactants, and lubricants) or high-performance (biomedical or cosmetic domains) applications. Nucleophiles can initiate the polymerization leading to alkoxides able to attack a new monomer enabling the propagation step. Cyclic ethers reactivity and polymerization kinetics are predominantly influenced by their polymerization enthalpy and ring strain but also by electronic and steric factors associated with the nature of the ring substituent as well as reaction conditions like temperature and solvent. An anionic-related mechanism can be considered after preliminary complexation of the monomer by specific additives (an electrophile), which strongly facilitate nucleophilic attack and ring-opening. Based on this chemistry, as well as organic synthetic tools, many substituted epoxide monomers are able to be polymerized, opening pathways for well-defined polyether structures and functionalities and allowing the preparation of materials with various properties.

2.2 Conventional Anionic Polymerization of Epoxides

Propagation may proceed without side reaction in anionic polymerization of ethylene oxide [1]. Alkali metal derivatives like hydrides, alkyls, aryls, and amides and mainly alkoxides of sodium, potassium, and cesium represent the most common initiators used for the AROP of epoxides [2, 3]. Lithium alkoxides do not lead to the polymerization of such monomers due to strong aggregation between lithium species after insertion of the first epoxide unit. Polymerizations initiated by alkali metal alkoxides are generally carried out in aprotic and apolar media or in coordinative solvents like dimethylsulfoxide (DMSO) or dimethylformamide (DMF) in order to dissociate active species. The driving force for the ring-opening reaction is the relief of the strain energy of the epoxide ring. High temperatures are usually needed for the AROP of long-chain alkylene oxides.

The different steps of conventional AROP of epoxides are shown in Scheme 2. The initiation step consists of a nucleophilic substitution – S_N2 type – of the alkoxide species leading to the formation of new alkoxide species able to further attack monomer molecules resulting in polyether chains with an atactic structure. The termination step is achieved by addition of an acidic compound with labile hydrogen. Alcohols and water are the most commonly used termination agents in order to obtain hydroxyl end groups. The transfer to monomer is observed with most of the initiating systems when substituted epoxides are polymerized.

2.2.1 Ethylene Oxide

Alkali metal salts of carbanions [4–6] and nitranions [7] are efficient initiators for the polymerization of ethylene oxide (EO). Alkali metal alkoxides were also investigated in detail due to the fact that the structure of these derivatives is similar to the one of propagating species. In aprotic solvents with low to medium polarity, i.e., ethers, alkali metal alkoxides exhibit a strong tendency to aggregate leading to complex reaction kinetics. This is particularly significant for small-sized metals such as lithium or sodium. If the reaction follows a monomer order of 1, the order in alkoxide propagating species varies according to counterion and solvent. This can be related to the presence of aggregates, ion pairs, and free ions of different intrinsic reactivity (Scheme 3). EO polymerization with alkoxide aggregates is extremely slow and does not even proceed in most cases.

The combination of alkoxide with its parent hydroxy compound was used to limit aggregation and preserve solubility [8–10], allowing a better control of the initiation and propagation, to the detriment of reaction kinetics [11, 12]. This would be referred today as a “degenerative transfer,” already reported by Flory in the 1940s.

The use of potassium tert-butoxide as initiator was reported to yield living PEO in DMSO with molar masses controlled by the ratio [monomer]/[initiator] [13–15]. With K⁺ and Cs⁺ salts, EO polymerization in DMSO proceeds almost exclusively by free ions in agreement with a higher dissociation constant in this solvent in line with a high DMSO permittivity (ε = 48 at 20 °C). With Na⁺ as counterion, both ion pairs in equilibrium with a small proportion of free ions contribute to the propagation. In tetrahydrofuran (THF) (ε = 7.6 at 25 °C) and in the presence of sodium, potassium, and cesium naphthalene as initiators, a living polymerization takes place, the rate of propagation increasing with the size of the counterion [16, 17]. However, kinetics are complicated due to the strong association of alkoxide end groups.

2.2.2 Monosubstituted Epoxides

Although alkali metal derivatives are efficient initiators for AROP of ethylene oxide, they are much less efficient for monosubstituted epoxides, e.g., propylene oxide (PO), glycidyl ethers, etc. Indeed, as alkoxide species are relatively strong bases, the abstraction of monomer substituent proton can take place. This side reaction leads to the formation of polyether chains, initiated by an allyloxy group, limiting the molar masses of polyethers (Scheme 2) [14, 18].

Similarly to ethylene oxide, the size of the counterion and the temperature were shown to influence the polymerization rate of monosubstituted epoxides. Increasing the size of the counterion leads to more dissociated active species and thus to higher polymerization rate, faster polymerizations being observed with cesium counterion. Higher temperatures increase the polymerization rate [18, 19] with the time frame being generally in the magnitude of several hours or days to reach high yields. In the presence of potassium as counterion (t-BuOK as initiator), the reactivity of racemic propylene oxide in hexamethylphosphoramide (HMPA) at 40 °C is about four times lower than that of EO on the basis of overall polymerization rates [20]. The reactivity of other monosubstituted epoxides depends on both electronic and steric factors induced by the substituent attached to the epoxide ring [21]. For instance, the reactivity of 2,2-dimethyloxirane (DMO) is ten times lower than that of PO, whereas glycidyl ethers such as tert-butyl glycidyl ether (t-BuGE) are more readily polymerized than PO [20].

Conventional anionic ring-opening polymerization suffers in general from slow kinetics and, more particularly for substituted epoxides, of transfer reactions. Other systems were therefore developed.

2.3 Systems for Activated Epoxide Polymerization

2.3.1 Alkali Metal Derivatives Associated to Crown Ether

Addition of complexing agents to alkali metal cations, such as crown ethers or cryptands (Scheme 4), was shown to drastically increase ethylene oxide propagation rate in ethereal solvents [8, 22, 23], reducing the aggregation of alkoxide polymer ends and increasing the proportion of free ions. For example, the dissociation constant of PEO⁻ K⁺ at 20 °C in THF is 1,700 times higher when K⁺ is complexed by cryptand 222. In this system, the reactivity of free ions is about 60 times higher than that of cryptated ion pairs.

For monosubstituted epoxides, 18-crown-6 was also shown to increase the reactivity, the propagation rate constant being up to 14 times at 25 °C [10, 24–26]. In line with the acidic character of hydrogen on the α-carbon of the monomer, the nature of epoxide substituent plays an important role in the chain transfer process. For instance, the anionic polymerization of long-chain alkylene oxides initiated by potassium and cesium alkoxides is much less subjected to chain transfer processes than alkoxides deriving from PO. However, relatively high temperatures were needed to reach reasonable polymerization times, which caused residual side reactions limiting molar masses. The breakthrough of using the additive 18C6 was associated with a decrease of polymerization temperatures, minimizing transfer reaction to monomer. This is reflected by the production of poly(2-butyloxirane) of higher molar masses at 20 °C (Table 1) [19]. The polymerization temperature of 2-butyloxirane and of higher 2-alkyloxiranes like 2-hexyloxirane and 2-octyloxirane could even be reduced below 0 °C, which almost eliminate completely all side reactions. However, very long reaction times were required, i.e., 4–8 days, and conversion did not go to completion [19]. Alkali metal hydrides were also associated to 18C6 for the AROP of glycidyl butyl ether [21, 27]. Polyethers with molar masses lower than 5,000 g/mol and low dispersity were obtained. For this range of molar masses, polymerization time was considerably reduced, from several days to a few hours, as well as transfer reactions.

Table 1 Polymerization conditions and characteristics of poly(2-butyloxirane) synthesized in presence of K or Cs tert-butoxide in toluene with or without crown ether 18C6 (molar ratio 18C6/metal = 3)

Full size table

2.3.2 Aluminum Systems: From Bulky to Simpler Compounds

In the 1980s, Inoue and coll. used metalloporphyrin as catalyst, in particular aluminum-based porphyrin, for the polymerization of methacrylates [28, 29], lactones [30], and epoxides [31] and, in some extent, oxetane [32] which is usually polymerized by a cationic route. The equimolar combination between diethylaluminum chloride and α, β, γ, δ-tetraphenylporphyrin (TPPAlCl) led to a high catalytic activity in the polymerization of propylene oxide. The covalent nature of the Al-Cl bond suggests a polymerization via a coordination mechanism. Synthesized polyethers, by the so-called “immortal” polymerization, reached molar masses up to 70,000 g/mol with a narrow distribution [33]. A monomer molecule is first inserted between the Al-Cl bond of the initiator (Scheme 5). Aluminum porphyrin, due to its nucleophilic character, was used as initiator enabling coordination with epoxide. Under such conditions, transfer reactions were considerably decreased, allowing the synthesis of a series of block copolymers, including poly(PO-b-EO), poly(1,2-butene oxide-b-PO), poly(PO-b-epichlorohydrin), poly(lactones-b-PO), poly(lactones-b-EO), etc. [34, 35]. Although EO polymerization could be achieved rapidly (half-time reaction is about 30 min at room temperature for [EO]/[TPPAlCl] = 400 in dichloromethane), other epoxides exhibited a lower reactivity. In similar conditions, several hours were required for the polymerization of PO and 1,2-epoxybutane [31] in order to reach complete conversion, whereas for styrene oxide or 1,2-epoxy-2-methylpropane conversions did not exceed 15 % after 8 days of reaction [33].

In order to increase polymerization rate of propylene oxide, aluminum porphyrin was used in association with a bulky Lewis acid [36–38]. For instance, methylaluminum bis(2,4,6-tri-tert-butylphenolate) (MAlBP) was used to coordinate the epoxide and to activate the monomer substrate toward a nucleophilic attack. As compared to the previous system, the catalytic species and the initiator are independent, i.e., employed as a bi-component initiating/activating system. There is no interaction between these two aluminum derivatives due to their bulkiness. Polymerization rates of propylene oxide and 1,2-butene oxide were strongly enhanced due to the presence of the bulky Lewis acid [36]. Only 0.25 % of MAlBP, with respect to propylene oxide concentration, was added to increase the polymerization rate by a factor of 460. With a [MAlBP]/[TPPAlCl] ratio equal to 0.5, 3 min was enough for propylene oxide to be polymerized with 86 % conversion, leading to a PPO with a molar mass up to 12,000 g/mol. Without MAlBP, in 7 h, the conversion is around 20 % and molar mass reached 3,300 g/mol. The Lewis acid alone did not initiate the polymerization under similar conditions. The “living” nature of the polymerization could be demonstrated by the successful formation of block copolymers based on propylene oxide and 1,2-butene oxide, though complete conversions could not be obtained.

Using relatively similar systems based on quaternary ammonium and quaternary phosphonium halides associated to sterically hindered methyl(diphenoxy)aluminum, poorly reactive four-membered ring oxetane, e.g., 1,3-propylene oxide, was polymerized according to a coordinated-anionic mechanism, as a result of strong monomer activation by complexation with the aluminum derivative [39].

Braune and Okuda used porphyrin-free aluminate complexes for the polymerization of propylene oxide activated by their neutral Lewis acid precursors [40]. The nucleophilic species were easily obtained by reaction of a bulky Lewis acid based on diphenoxyaluminum compounds, with a cesium alkoxide or an ammonium salt. The ring-opening polymerization proceeds under the synergic interaction of a phenolate-aluminum-oxirane complex forming an activated monomer with the corresponding “ate” complex which initiates the reaction (Scheme 6). Ring-opening takes place by transfer of an alkoxy group from the “ate” complex, regenerating an aluminate able to activate a new monomer. The synthesis of poly(propylene oxide)s with molar masses up to 4,000 g/mol was reported following an anionic (or coordinative) mechanism because of exclusive head-to-tail linkages.

Tsvetanov reported in 1985 the polymerization of ethylene oxide initiated by sodium tetrabutylaluminate [41]. PEO were prepared in toluene in the range 15–70 °C with a high kinetic order with respect to the initiator. The polymer chain growth was explained by the presence of aggregates of NaAlBu₄, predominantly trimers of the “ate” complex. Interaction between the oxygen atom of EO and the alkali metal, as well as EO and aluminum to a lesser degree, was shown, in line with some activation of the epoxide.

Using epoxide monomers and combinations of a Lewis acid – typically a trialkylaluminum – and alkali metal alkoxides or onium salts [42–47], Carlotti and Deffieux developed efficient synthesis of various polyethers. An excess of Lewis acid with respect to the initiator was required. The formation of an “ate” complex, which was able to ring-open the activated epoxide by the excess of activator, was observed (Scheme 7).

Propylene oxide polymerization, based on a monomer-activated anionic polymerization, occurs at room or lower temperature. Control over the polymerization strongly depends on the nature of the counterion. Sodium and potassium enable the polymerization, whereas lithium does not. In few hours, controlled poly(propylene oxide) chains, up to 20,000 g/mol, were obtained with nevertheless the presence of some residual transfer reactions, i.e., transfer to monomer leading to allyloxy groups in the α-position and transfer to triisobutylaluminum (i-Bu₃Al), which generates initiation by a hydride coming from an isobutyl group. Ammonium salts were more successful as much higher molar masses could be achieved in a controlled manner, especially at −30 °C, yielding polyethers of low dispersity (e.g., Đ = 1.34 for $ \overline{M_n} $ = 170,000 g/mol) [45]. The strong decrease of transfer reactions was explained by the decrease of basicity of the active bi-component complex. The exclusive preparation of regioregular polymers (head-to-tail) was indicative of an anionic/coordination type mechanism. Polymerization proceeds at [Al]/[Initiator] ratio higher than unity, indicating that only complexed PO molecules are susceptible to ring-open, thanks to significant electron-withdrawing effect that makes the ring carbon atoms much more electrophilic. Increasing trialkylaluminum concentration, at constant monomer and initiator concentrations, was shown to yield a drastic increase in polymerization rate, whereas the number of PPO chains remained unchanged.

These initiating systems were also applied to the polymerization of a broad variety of epoxides including several alkylene oxides and glycidyl ethers, epichlorohydrin (ECH), etc. Compared to conventional alkali metal initiators, the tetraoctylammonium bromide/i-Bu₃Al-initiating system strongly enhanced the rate of ethylene oxide polymerization while retaining the living character of the reaction [48]. At a ratio [i-Bu₃Al]/[NOct₄Br] = 1.5, the synthesis of PEO of 20,000 g/mol was completed within 2 h at room temperature in dichloromethane. By changing the ammonium salt by an alkyllithium, a PEO of 10,000 g/mol was synthesized at low temperature and in nonpolar media, e.g., toluene [49]. The presence of trialkylaluminum used in excess with respect to the lithium initiator permits disaggregation of lithium alkoxide species by forming lithium/aluminate complexes which are able to ring-open the AlR₃-complexed EO molecules. However, ligand exchanges in the lithium/aluminate complex lead to slow deactivation of the propagating species during the polymerization which limits the access to high molar mass PEO.

Monomer-activated anionic polymerization of epichlorohydrin utilizing similar conditions was also described. In contrast to conventional anionic polymerization, aluminate species that ensures propagation in AlR₃/onium systems selectively react with activated ECH ring, keeping the chloromethyl function [50]. Syntheses of poly(glycidyl methyl ether) (PGME) [51], linear poly(2-ethoxyethyl glycidyl ether) (PEEGE), and poly(tert-butyl glycidyl ether) (PtBuGE) [52] with narrow chain distribution and controlled molar masses were also reported. The amount of Lewis acid required to trigger the reaction and achieve quantitative monomer conversions was shown to increase with the number of oxygen atoms in the monomer.

This anionic living/controlled polymerization, employing onium salt/triisobutylaluminum systems and involving a monomer-activated mechanism, was applied to the synthesis of a series of random and block copolymers. For instance, EO/PO random copolymers with a gradient structure, molar masses up to 70,000 g/mol, and narrow dispersity were prepared [48]. PPO-co-PECH [50] and amphiphilic poly(alkylene oxide-co-glycidol) were also synthesized via the synthesis of PPOx-co-PEEGE and PBO-co-PtBuGE copolymers [52], followed by the deprotection of hydroxyl groups under acidic conditions. Despite the determining role of the monomer complexation in this polymerization process, the copolymerization ratios remain close to those reported for conventional anionic copolymerization. Different diblock and triblock copolymers of various compositions and lengths were also prepared by sequential monomer addition. Synthesis of PEO-b-PPO-b-PEO triblock copolymers with NOct₄Br/i-Bu₃Al initiating system was first achieved [48]. PPO-b-PECH block copolymers with molar masses ranging from 6,000 to 30,000 g/mol and with various PPO and PECH block lengths were also prepared by sequential addition of the two monomers [50]. The re-initiation efficiency was shown to be quantitative, no matter the order of addition of the two monomers. Finally, block copolymerization of EO initiated with lithium derivatives was also lately described [49]. Although it is known that EO polymerization could not proceed properly when lithium alkoxide species are involved [53, 54], it was shown that living polystyryllithium and polyisoprenyllithium chains can play the role of a macroinitiator for EO polymerization in presence of trialkylaluminum. Block copolymers polystyrene(or polyisoprene)-b-poly(ethylene oxide)s were obtained in hydrocarbon within a few hours with a PEO block molar mass up to 10,000 g/mol and a re-initiation efficiency of about 80 %.

2.3.3 Calcium-Based Systems

The association of two metals was also proposed in 1980 to polymerize ethylene oxide [55]. A calcium amide/alkali metal alkoxide initiating system was used at high temperature in order to produce poly(ethylene oxide)s with high molar masses. EO polymerization involves monomer coordination at the catalyst active site via σ bond formation between the monomer heteroatom and the catalyst metal atom. It is followed by a nucleophilic attack of the alkoxide active group [56]. Tsvetanov and coworkers investigated this combination for the synthesis of poly(propylene oxide) and miscellaneous copolyethers [57–61]. The authors explained that calcium derivatives are much weaker than other metals, like aluminum or zinc, in the formation of oxiranes complexes. As a result, Ca-EO complexes are more readily formed in comparison with substituted epoxides where low-rate polymerizations are observed. In addition, the polymerization takes place under heterogeneous condition as the calcium-based initiator is not soluble in the solvents used. As an example, the synthesis of an amphiphilic poly(ethylene oxide)-b-poly(alkylglycidyl ether) copolymer with a molar mass up to 10⁶ g/mol and a high dispersity (5–8) was achieved in relatively short times (several hours, but not days) at 97 °C [59].

In summary, various systems such as metals associated to crown ethers or metallic activators as well as combinations or organic initiators associated to organometallic compounds were used to polymerize epoxides affording control and fast kinetics. Nowadays, complementary challenges concerns metal-free systems which are able to fulfill all criteria so far discussed.

2.4 Toward Organic Initiating Systems

2.4.1 Tertiary Amines

Amines were essentially used for the anionic polymerization of di- and polyepoxides, mainly diglycidyl ether of bisphenol A and its derivatives as well as glycidyl phenyl ether [62]. Reaction with amines leads to slow polymerization rates, to long induction periods, and to the formation of short chains due to transfer reactions. Benzyldimethylamine, pyridine, triethylamine, 4-dimethylaminopyridine (DMAP), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) are the most common amines used as initiators. Ammonium salts and imidazoles are also employed to open epoxides by anionic polymerization. The main drawback of all these compounds is the limitation in terms of molar masses. Although the initiation mechanism is not well established, two pathways are generally considered (Scheme 8) [63, 64]. The addition of alcohol decreases induction periods and increases polymerization rates but limits molar masses [62]. The alkoxide formed by the second way (Scheme 8b) is more reactive than species resulting from usual initiation step (Scheme 8a). Williams and coll. succeeded to decrease the time of initiating step by using DMAP thanks to its conjugated structure with a negative charge on the nitrogen atom of the cycle and a positive charge on the tertiary amine [64]. The molar masses were about 1,000 g/mol at 80 °C.

2.4.2 N-Heterocyclic Carbenes

N-Heterocylic carbenes (NHCs) can be used as ligands of transition metallic complexes [65] but also as organic catalysts for various metal-free reactions [66, 67]. Ring-opening polymerization of some epoxides was triggered by NHCs [68–70]. Taton and coll. showed that 1,3-bis-(diisopropyl)imidazole-2-ylidene is able to initiate ethylene oxide polymerization according to a zwitterionic mechanism (Scheme 9) [68]. NHC plays in this case the role of initiator controlling PEO molar masses and forming zwitterionic imidazolium. It can be released by attack of functionalizing terminators added to the medium. Polymerizations were performed at 50 °C in DMSO and required long times, i.e., several days. Molar masses up to 13,000 g/mol were obtained with narrow distribution and good agreement between theoretical and experimental molar masses, in line with a good control of the polymerization. This approach was less effective for the polymerization of propylene oxide due probably to transfer reactions to monomer. Poly(propylene oxide) oligomers with a relative low dispersity were nevertheless prepared [70].

Controlled EO polymerization was found to proceed also with a catalytic amount of NHC, in DMSO at 50 °C, and in presence of a variety of chain regulators, e.g., benzyl alcohol, propargyl alcohol, or trimethylsilyl azide. They possess a nucleophilic part and an electrophilic one (Nu-E). In this case PEO molar masses match the [EO]/[Nu-E] ratio, typically in molar proportions [NHC]/[Nu-E]/[EO] = 0.1/1/100 [69]. Reversible exchanges with Nu-E molecules, involving the formation of active and dormant chains, yield α-Nu-ω-E-poly(ethylene oxide) opening a way for chain-end functionalization.

2.4.3 Ammonium, Phosphonium, and Phosphazene Bases

Ammonium salts were first used by Hemery, Ganachaud and coll. for the AROP of glycidyl phenyl ether (GPE) in miniemulsion [71]. Didodecyldimethylammonium hydroxide was used as an inisurf (initiator-surfactant) exhibiting both surface-active properties and the ability to initiate polymerization. Average molar masses increased with conversion and were dependent on the initiator concentration. A critical polymerization degree of 8 was reached. Using a similar initiator, i.e., tetrabutylammonium fluoride, in solution polymerization, endo achieved the synthesis of poly(glycidyl phenyl ether) oligomers with controlled molar masses up to 4,000 g/mol (Scheme 10) [72] and the synthesis of poly(ethylene oxide-b-glycidyl phenyl ether) when the polymerization of glycidyl phenyl ether was performed in the presence of poly(ethylene glycol) monomethyl ether [73].

As already mentioned, the size of the counterion plays a preponderant role in epoxide polymerization, in particular on the kinetics. As a result of their bulky size, some phosphonium and phosphazenium derivatives arising from strong Brönsted phosphazene bases, developed by Schwesinger, revealed potential interesting counterions in order to limit aggregation phenomena [74, 75]. Several of these commercially available bases were used as organic deprotonating agents of –OH (−CH, −SH, −NH, or –COOH) containing initiators, to polymerize epoxides like ethylene oxide [76–84], propylene oxide [85–87], butene oxide [82, 88, 89], styrene oxide [90], as well as ethoxy ethyl glycidyl ether [88, 91, 92] and other protected form of glycidol [93], affording homopolymers, copolymers [94–96], grafted or block copolymers [81, 84, 93, 97–100], and star-shaped structures [89].

Phosphonium and phosphazene bases can be used with an alcohol to form a protonated counterion. The use of t-BuP₄H⁺ enables an increase of the polymerization rate of propylene oxide, but transfer reactions still occur. Polymerization rates increase when the positive charge is delocalized into the molecule, which is due to better ion separation [80, 81]. Phosphazene bases were also utilized in order to complex lithium alkoxides allowing the (co)polymerization of ethylene oxide and ethoxyethyl glycidyl ether with lithium derivatives (Scheme 11) [76, 79, 88, 91, 92, 101], which is generally not possible because of strong aggregation between lithium alkoxides except when a trialkylaluminum is added [49]. The base behaves as a cryptand for Li⁺ ions via polar amino and imino groups located inside the globular molecule, and the outer shell is formed by alkyl substituents. The equilibrium between complexed lithium alkoxide ion pairs and reactive free anions is thus shifted toward the latter species allowing polymerization. Copolymers based on polystyrene and polyethers were prepared without the need of cation exchange. However, the presence of residual transfer reactions and an induction period resulting from the slow disaggregation of the lithium alkoxide ends still complicated the epoxide polymerization [79].

A broad range of polyethers can be synthesized leading to an important class of materials used in many applications. Controlled structures and dimensions as well as easy, inexpensive, and rapid polymerizations remain the major challenges to be addressed. Preparation of functional polyethers used as reactive precursors, by an anionic route, is also of major interest.

2.5 Functionalization of Polyethers Prepared by Anionic Ring-Opening Polymerization

Functionalization of polyether chains can occur either to chain ends or along the polymer backbone. In the first case, a functional initiator and/or a termination agent is employed. Further polymerization or other reactions can be conducted from the as-introduced functional groups. The functionalization into the chains is carried out by the use of functional monomers. Hydroxy, amine, epoxide, carbonate, thiol, azide, as well as double and triple bonds are the most commonly used functions in order to obtain a versatile range of functional polymers for many applications. Carlotti and coll. [102], Frey and coll. [103, 104], and Riffle and coll. [105] have reviewed main syntheses, properties, and applications of various functional polyethers. This part will focus on the functions introduced into polyethers prepared by AROP.

2.5.1 Polyethers with Hydroxyl Functions

Low molar masses polyether polyols are mainly used as precursors of polyurethanes [106]. The majority of hydroxy telechelic polyethers are synthesized by AROP initiated by potassium or cesium hydroxide [18]. Initiation by a hydroxy group followed by termination reaction using water or an alcohol yields α-, ω-dihydroxy polyethers. As shown previously, this approach is efficient for ethylene oxide but much less with monosubstituted epoxides. Branched and star polyethers were synthesized from tri-, tetra-, penta-, and octa-alkoxides of potassium with molar masses close to 5,000 g/mol and a relatively broad dispersity [107–114]. With a similar approach and using hyperbranched polyglycerol initiators, Lutz, Frey and coll. prepared functional multiarm star PEO [9].

Bulkier counterion like phosphazene bases was used as deprotonating agents with dipropylene glycol as initiator showing some decrease of usual transfer reactions [86]. Using the monomer activation methodology [45], di-, tri-, and penta-hydroxy telechelic poly(propylene oxide)s were obtained in very short times (hours) at room temperature in hydrocarbon solvents with molar masses up to 80,000 g/mol with a relatively low dispersity (Scheme 12) [115].

Using an organo-catalysis approach described previously (Scheme 9), Taton and coll. reported that N-heterocyclic carbenes (NHC) could lead to α-,ω-dihydroxy poly(ethylene oxide) using water as terminating agent. Hydroxide (OH⁻) was shown to behave as a nucleophile that displaces the α-imidazolium moiety from PEO chains, thus releasing the NHC, whereas the terminal alkoxide was transformed into a ω-hydroxy function [69].

Hydroxyl functions can also be introduced after post-modification of other reactive functions introduced via initiation. An amino function can lead to an OH group after reaction with a molecule of ethylene oxide [116]. The use of tetrabutylammonium acetate as initiator proved efficient to introduce acetate-end groups to poly(glycidyl phenyl ether) [117]. Hydroxyl functions were obtained after hydrolysis with an acidic treatment. A narrow molar mass distribution and molar masses up to 4,000 g/mol were thus obtained.

The introduction of pendant hydroxyl functions is predominantly obtained from anionic polymerization of glycidol and its derivatives. These polymers are very attractive for biological and medical applications [118]. Direct reaction of glycidol with bases such as triethylamine, pyridine, alkali metal hydroxide, and sodium methoxide leads to two distinct initiation steps at room temperature (Scheme 13) [119–123]. The first way corresponds to a basic attack toward the hydrogen atom of the hydroxyl group, and thus an oxyanion bearing an epoxy group is formed. The second way is the attack of the base over the methylene of the epoxide leading to ring-opening. Hyperbranched polyethers with a high functionality in hydroxyl groups were achieved [103, 124–126].

In order to access to well-defined linear polyols derived from glycidol, hydroxyl functions of that monomer had to be protected. Dworak, Tsvetanov and coll. [114, 127–131], and Möller and coll. [132–134] investigated the AROP of ethoxyethyl glycidyl ether using metal alkali alkoxides such as KOH, CsOH, and t-BuOK. These systems enabled the synthesis of linear polyols up to 30,000 g/mol. Quantitative conversion was obtained between 17 and 48 h, depending on targeted molar masses, and required high temperatures (up to 120 °C). Due to the initiating systems used, residual transfer reactions were observed, limiting somehow the control over the structures [91]. Linear polyglycidols with molar masses up to 85,000 g/mol, starting from ethoxyethyl glycidyl ether (EEGE), were obtained using NOct₄Br/triisobutylaluminum as initiating system at room temperature in a few hours [52]. The formation of an initiating and propagating “ate” complex of weak basicity was proposed to explain the decrease of side reactions. Allyl glycidyl ether or isopropylidene glyceryl glycidyl ether was also used as a protected form of glycidol to obtain polyethers polyols [114, 133, 135, 136].

Taking advantages of the anionic route, various well-defined functional block copolymers were prepared [128, 133, 137–141].

2.5.2 Polyethers with Amine Functions

Jeffamines® represent the most industrially produced polyetheramines. Such polyamino telechelic polyethers cannot be directly obtained by initiation and termination steps. They are synthesized from hydroxytelechelic random copolymers of ethylene oxide and propylene oxide via reaction between hydroxyl functions and ammoniac gas at 300 °C. Considering the nature of the polymerization, only polyetheramines with low molar masses can be prepared. They are thermoresponsive polymers possessing a lower critical solubility temperature around 30 °C in water. Amino end groups are mainly used as curing agents in epoxy resins [142, 143] and as chain extenders for polyurethane applications [144, 145].

The introduction of amino groups at the head of polyether chains is generally carried out using an initiator bearing a primary amine function and another more reactive function, i.e., an alcohol, which enables the initiation. Aminoalcohols can be thus used to polymerize ethylene oxide by an anionic mechanism [116]. In basic media, both metal alkoxide and amide are formed, and the equilibrium is driven toward the formation of alkoxide by increasing the size of the counterion. Molar masses of α-amino-ω-hydroxy functional polyethers reached 1,000 g/mol. Transfer and termination reactions occur due to the hydrogen of amino groups. Protection of the primary amino group by a tertiary one was considered to overcome those limitations [96, 100, 116, 135]. Molar masses increased up to 6,000 g/mol and required long times, i.e., 150 h [146]. Schlaad used α-methylbenzyl cyanide as a CH-acidic compound to obtain α-cyano, ω-hydroxy poly(ethylene oxide) with controlled molar masses up to 2,500 g/mol with narrow distributions [78]. The cyanide function was then reduced in –NH₂ group by LiAlH₄.

Polyethers bearing primary amino groups are also generated by using epoxide monomers with protected amines such as N,N-dibenzyl amino glycidol or N,N-diallylglycidylamine prepared from epichlorohydrin and N,N-dibenzylamine or N,N-diallylamine, respectively [147, 148]. The synthesis of copolymers with EO was next performed using cesium alkoxide as initiator giving controlled copolyethers with molar masses up to 10,000 g/mol. Pendant amine functions can also be introduced from chlorine atoms. Statistical or block copolyethers, made by NOct₄Br/i-Bu₃Al systems and having both hydroxy and amine pendant groups, were brought, respectively, by ethoxyethyl glycidyl ether (protected glycidol) and epichlorohydrin monomers (Scheme 14) [149]. Chlorine atoms were subsequently transformed into azido groups using sodium azide. Consequent reaction with triphenylphosphine enables quantitative formation of amino groups. Such type of copolymers was proposed to enable selective electrophilic reactions due to the difference of reactivity between –OH and –NH₂ functions.

2.5.3 Polyethers with Alcene Functions

Double bonds are known to be stable toward anionic ring-opening polymerization conditions but can be further easily modified into various functions which make them attractive. They can be introduced via initiation with an alcohol bearing such a function [96, 150–152]. Allyl alcohol and 10-undecen-1-ol can be deprotonated, for instance, by naphthalene potassium or by sodium hydride, respectively. However, during polymerization, double bonds can be isomerized into propenyl group to form CH₃CH = CHO⁻ as initiating species. To avoid this side reaction, polymerization has to occur between 15 and 20 °C, rather than at high temperatures [151]. As one example, thiol-ene reactions were applied to PEO oligomers initiated by 10-undecen-1-ol [152]. The synthesis of α,ω-diallyl PEO was achieved by reaction between α,ω-dihydroxy PEO, deprotonated with diphenylmethyl potassium, and allyl bromide. Hydrogels were obtained by their post-reaction with octafunctional silsesquioxanes via hydrosilylation [153, 154].

Monomers bearing allyl functions, like allyl glycidyl ether (AGE), can be directly used to synthesize polyethers with pendant double bonds through AROP with alkali metal alkoxides as initiators [21, 133, 155]. Generally, molar masses lower than 10,000 g/mol and long polymerization times are required. Ammonium salts/trialkylaluminum systems [156] allowed controlled and high molar mass structures in a few hours. With potassium alkoxide/naphthalenide initiators, Lynd and Hawker could also obtain molar masses up to 100,000 g/mol with low dispersity within 20–144 h [157]. Similarly, ethoxy vinyl glycidyl ether (EVGE) was shown to be selectively polymerized and copolymerized with ethylene oxide to give functional polyethers able to be post-modified [158].

2.5.4 Polyethers with Azide Functions

Most of the PEO azidation routes reported in the literature involve the chemical modification of previously formed hydroxyl-terminated PEO [159–162]. Kataoka achieved the synthesis of azido-terminated heterobifunctional poly(ethylene oxide)s in a multistep process, involving ring-opening polymerization of ethylene oxide initiated by allyl alcohol and subsequent transformation of R-allyloxy and ω-hydroxy PEO end groups by a series of chemical reactions [163]. The azide group was introduced by mesylation of the hydroxyl terminus, followed by its subsequent substitution with sodium azide [164, 165].

The monomer-activated approach allowed the direct synthesis of a broad series of heterofunctional polyethers bearing an azido head group, and a hydroxy-terminated chain end, starting from tetrabutylammonium azide in the presence of triisobutylaluminum [166]. Poly(alkylene oxide)s, protected polyglycidol, and polyepichlorohydrin were obtained in this way with a high functionalization efficiency. Reduction reactions can lead to amine functionalization, or Huisgen’s coupling reaction with alkyne moieties can be applied [167].

2.5.5 Polyethers with Other Functions

Carboxylic acid groups were, for instance, introduced from the initiation step. The use of dipotassium-3-mercaptopropionate synthesized from 3-mercaptopropionic acid and two equivalents of potassium naphthalide allowed the direct synthesis of α-carboxy, ω-hydroxy poly(ethylene oxide) up to 25,000 g/mol with narrow distribution [168]. Only the thiolate was shown to react; the carboxylate did not participate in the polymerization (Scheme 15), but the usual way to introduce such a function is post-modification of chain ends [152, 169–171].

Aldehyde functions in α-position can also be introduced by using an initiator bearing a protecting group, i.e., 4-(diethoxymethyl)benzyl alcohol [172].

As a last example, polyethers with pendant methacrylate functions were achieved by selective ring-opening polymerization of glycidyl methacrylate using a monomer-activated anionic approach affording cross-linkable low Tg polymers [173].

The method based on chemical modification of previously and anionically formed polyethers is the most common way to get various functionalized and reactive structures. In many cases a simple chemistry can be used which makes this way very attractive. But, in the other cases, the interest is much more limited particularly for an industrial application. The recent advances in the control of the anionic polymerization of epoxide offer nowadays direct routes to prepare reactive and new polymers. The research and development of novel technologies will certainly contribute to an increasing use of such recent methods.

3 Cyclic Esters

3.1 Introduction

The biodegradability and biocompatibility of aliphatic polyesters render them very attractive for a wide range of applications as environmental friendly thermoplastics and biomaterials [174]. Moreover, many of them could be obtained from renewable resources, which is one of the great challenges for polymer chemists. Three different polymerization mechanisms can be implemented to synthesize aliphatic polyesters: the step-growth polymerization through esterification reaction of hydroxyl acids or diacids/diols, the ring-opening polymerization (ROP) of cyclic ketene acetals, and the ROP of cyclic esters. The step-growth polymerization is highly used as its main advantage is the easy availability of a very wide range of acid and alcohol precursors. Nevertheless, this polymerization suffers from severe limitations: extremely high conversion has to be reached to get high molar masses, high temperatures are generally needed, control of the molar masses is very difficult, and dispersity is quite large. The ROP of cyclic ketene acetals could proceed through cationic and radical processes [175]. Even if this polymerization is known for long, its development remains limited due to many drawbacks: there is competition between direct vinyl polymerization and indirect ring-opening of the cycle depending on the ring size, substituents, and temperature; monomers are not readily accessible; and branching reactions could occur. All these limitations can be overcome by implementing the ROP of cyclic esters either by ionic (cationic or anionic) or coordination-insertion mechanisms. Indeed, this technique allows living and/or controlled polymerization with fast initiation and high molar masses with low dispersity. Availability of the monomers occupies an intermediate position between step-growth polymerization and ROP of ketene acetals. The question of polymerizability of cyclic esters arises since the polymerization rate is highly dependent on the ring size and the substituents. Moreover, in the case of anionic mechanism, the active species varies with the ring size. Many recent reviews could give other details than those presented in this subchapter [175–183].

3.2 Thermodynamics of Cyclic Esters Ring-Opening Polymerization

The ability of a cyclic ester to be polymerized by the ring-opening mechanism has to be allowed both thermodynamically and kinetically. Indeed, the monomer-polymer equilibrium has to be shifted to the polymer formation, and the polymerization time has to be reasonable. ROP of cyclic esters could be sometimes limited by the presence of a relatively high concentration of the unreacted monomer when at the equilibrium. This is typically the case for γ-butyrolactone which is hardly polymerizable. The driving force for the polymerization of the majority of cyclic esters is their ring strain. As a consequence, large-ring lactones are more difficult to polymerize than small ones.

3.3 Lactide

The AROP of lactide was not as extensively studied as its coordination-insertion polymerization, this latter being most investigating because it could enable stereoselective polymerization [179, 182, 184–186]. Nevertheless, several types of initiators were able to perform polymerization of lactide.

3.3.1 Initiators with Alkali Metals

Studies concerning the AROP of lactide with alkali metals initiators were essentially conducted in the 1990s [187–202]. It was shown that strong bases were needed as carboxylates and phenolates were not able to initiate the polymerization [192]. On the contrary, potassium tert-butoxide and butyllithium allowed the polymerization of lactide with yields below 80 % and with the presence of racemization, transesterification reactions, and macrocyclics formation whatever the polymerization conditions [192]. Moreover, it was demonstrated that the initiation did not proceed through a nucleophilic attack of the strong base but through a proton abstraction from lactide to give an enolate which was the actual initiator of the polymerization (Scheme 16). After the nucleophilic attack of the enolate onto a lactide molecule yielding acyl cleavage, the active species responsible of the propagation was an alkoxide. Better results were obtained with primary and secondary lithium and potassium alkoxides as in this case the initiation proceeded mainly through the nucleophilic attack, but still uncontrolled molar masses were achieved (Scheme 16) [194, 201]. With potassium methoxide as the initiator, polymerizations were completed in less than 2 h in THF at room temperature allowing a good control of the molar masses with dispersities around 1.3 and low extent of racemization [188–190].

The addition of a crown ether onto potassium tert-butoxide or naphthalenide potassium revealed beneficial for the dispersity that dropped below 1.2 and for the reduction of racemization reactions but detrimental for the polymerization rate that slowed down dramatically [191, 196, 197]. Finally, it was demonstrated that the AROP of rac-lactide initiated by lithium tert-butoxide could yield to the synthesis of disyndiotactic polylactide (PLA), provided that yield remained quite low (below 35 %) [199, 200].

In spite of some side reactions, many studies described the synthesis of block copolymers with at least one PLA block via AROP of lactide. For example, poly(ethylene oxide)-b-polylactide synthesis was highly investigated as these copolymers could be used in the biomedical field. They were either synthesized through the sequential AROP of ethylene oxide and lactide [203–212] or by the AROP of lactide using commercial poly(ethylene oxide)s [213–217]. Starting from a difunctional (macro)initiator, tri- [218, 219] and penta-blocks [220] copolymers were also produced. The sequential polymerization allowed also the synthesis of poly(ethylene oxide)-b-polyglycidol-b-polylactide copolymer [221]. Finally, the synthesis of polysaccharides-g-polylactide was described [222, 223].

3.3.2 Organocatalyzed Polymerization

Since 2001 and the description of the first nucleophilic organocatalyzed ROP of lactide [224], this field was highly investigated [225–228], with the use of imidazole [229], amines [224, 230], amidines [231], phosphines [232], phosphazene [233, 234], or N-heterocyclic carbenes (NHC) [235–250].

Tertiary amines and phosphines are among the simplest metal-free catalysts. The controlled ROP of lactide was thus performed in the presence of 4-dimethylaminopyridine (DMAP) or 4-pyrrolidinopyridine (PPY) in dichloromethane at 35 °C or in the melt at 135 and 185 °C yielding PLA with controlled molar masses and low dispersities [224]. The polymerization was proposed to proceed through a monomer-activated mechanism with a nucleophilic attack of the amine onto the lactide monomer resulting in a zwitterionic species that was attacked by the initiating or propagating alcohol chain end (Path A, Scheme 17). Nevertheless, computational calculation suggested that the propagation occurred preferentially through an alcohol-activated mechanism (Path B, Scheme 17).

Phosphazenes were described to also induce such an alcohol-activated mechanism to produce polylactides with predictable molar masses, low dispersities, and high chain-end fidelity [233, 234]. By analogy with cyclic ethers, one can ask about the mechanism implying the deprotonation of an alcohol by a phosphazene base and propagation through a phosphazenium alkoxide. With phosphines, polymerizations had to be conducted in bulk at high temperature to proceed with at least one equivalent of phosphine compared to initiator in order to control the polymerization [232]. A monomer-activated ROP mechanism comparable to that of tertiary amine was proposed for these catalysts. In the absence of any alcohol, it was shown that imidazoles [229] and amidines [231] were able to polymerize lactide in bulk at high temperature or in solution at room temperature, respectively. In both cases, it was shown that the catalyst was capable of nucleophilic attack onto lactide and that cyclic polylactides were almost exclusively obtained.

NHCs were also investigated as organocatalysts for lactide ROP. They proved to be active, with complete monomer conversion in a few hours at room temperature to afford PLAs with high and controlled molar masses and low dispersities [237, 242]. The mechanism is supposed to be an activated monomer mechanism (Scheme 18). NHCs can be obtained from ionic liquids, with the advantage that the use of a biphasic system (THF/ionic liquid) allowed an easy polymer and catalyst recovery [240, 242]. NHCs can also be produced in situ from thermally activated NHC adducts, but in this case, lactide racemization occurred [247]. Alcohol adducts of NHCs proved to act as single-component catalyst/initiators with various alcohols as initiators. Some of them are stable solids and readily release the alcohol and the carbene in solution at room temperature [248], whereas for some others, polymerization could only take place at 90 °C [236, 238]. In the absence of an alcohol, NHCs promoted the polymerization of lactide, and the propagating species was demonstrated to be a zwitterion [241, 245]. Under these conditions, exclusively cyclic PLAs with rather low dispersities and a good degree of control were obtained at room temperature. The presence of both even and odd numbers of lactate units deduced from Maldi-Tof analyses indicated the presence of transesterification reactions. Very recently, carbene carboxylates were able to perform the polymerization of lactide [235]. Finally, the use of bulky NHCs allowed the synthesis of highly isotactic and heterotactic polylactides from rac-lactide and meso-lactide, respectively, at low temperatures [244]. In aprotic conditions (absence of exogenous alcohol), it was shown that sparteine was able to perform the zwitterionic ROP of l-lactide from both nitrogen atom to end up with macrocyclic polylactide [251].

Finally, the ROP of O-carboxyanhydride catalyzed by NHCs yielded to the formation of polylactide with controlled molar masses and chain ends when the polymerization was performed at room temperature in THF [252]. Star-shaped structures were also successfully obtained.

3.4 β-Lactones

As polymer chemists would like to be able to synthesize polyesters that would resemble polyhydroxyalkanoates (PHA) that are natural polyesters produced by many bacteria [253], AROP of β-lactones was by far the most studied ROP of cyclic esters [188, 189, 193, 254–261]. Besides, β-lactones behave differently than other larger lactones due to their high polarity and high internal strain. Their polymerization can occur through the nucleophilic attack onto the carbonyl carbon or onto the carbon adjacent to the endocyclic oxygen atom (Scheme 19).

Another important difference from other lactones is the easy α-proton abstraction as a side reaction that would produce acrylate or crotonate ions that are also able to initiate the polymerization (Scheme 20). As a consequence, the control over the molar masses and chain-end fidelity could be problematic.

3.4.1 β-Propio- or β-Butyrolactone

3.4.1.1 Alkali Metal-Based Initiators

From the 1960s, the AROP of β-propiolactone (PL) was shown to be easily initiated by weak bases like alkali metal carboxylates but also by stronger bases such as alkali metal alkoxides. On the contrary, β-butyrolactone (BL) was shown to be polymerized by such bases only if they are activated by the addition of macrocyclic ligands (crown ethers or cryptands) [262]. When carboxylates salts were employed as initiating species, the polymerization of PL was not living as transfer reactions to the monomer occurred leading to side initiation by acrylate ions (Scheme 20). It was shown independently in 1976 by Penczek [263] and Boileau [264] that the introduction of crown ethers or cryptand could enable the living polymerization of PL (Scheme 21). Since then, many carboxylate salts were utilized as initiators [262, 265–275]. Polymerizations were generally performed in THF at room temperature with long reaction times (generally more than 100 h for BL).

When the polymerization of β-lactones was initiated by strong bases such as alkali metal alkoxides, active species involved in the propagation were highly debated in the literature [259, 274, 276–283]. Penczek and coll. suggested that, when polymerization of PL was initiated by potassium methoxide in DMF at room temperature, both acyl-oxygen and alkyl-oxygen bond could occur yielding alkoxide or carboxylate propagating species, as indicated in Scheme 19 [274, 276]. Nevertheless, as alkyl-oxygen bond cleavage is preferred, after few monomers addition, the only propagating species are carboxylates. Later, as double bonds were observed as chain ends, some other authors showed that initiation occurred through a nucleophilic attack at the carbonyl carbon atom of a monomer by the alkoxide anion of the initiator, cleaving the acyl-oxygen bond to yield the corresponding potassium alkoxide of the β-hydroxycarboxylic acid esters, followed by the formation of an unsaturated ester due to KOH elimination (Scheme 22) [259, 278–281]. Finally, KOH acts as the actual initiator of the polymerization, and block copolymers were obtained with this type of initiator [284].

In few examples, naphthalenide potassium was used as the initiator for the polymerization of β-lactones (BL, PL) in THF at 20 °C [262, 285, 286]. The initiation only occurred in the presence of a crown ether (18C6) or a cryptand ([222]), but even in this case, polymerization rate was very slow as more than 100 h or 10 h were needed to rich a conversion higher than 90 % for BL and PL, respectively, yielding polyesters with molar masses up to 10,000 g/mol with dispersities around 1.3. Concerning the mechanism involved with this type of initiator, as indicated on Scheme 23, it was demonstrated first the α-deprotonation of β-lactones followed by the ring-opening of the monomer yielding potassium crotonate for BL (or acrylate for PL) which is the actual initiator. Again, the active species are carboxylates. It is thus possible to produce macromonomer of BL or PL with a good control as the molar masses are in good agreement with the theoretical ones, and each chain bears a double bond at one chain end coming from the initiation step. In the same vein, very similar results were obtained with potassium hydride as the initiator in the same conditions [287].

Finally, potassium solutions (obtained from 18C6 THF solution in the presence of a potassium mirror) revealed also powerful initiators for the polymerization of β-lactones [285, 288–293]. When polymerizations were performed in THF at 20 °C, the polymerization rate was quite high (at least higher than with other anionic initiators) as PPL with 12,000 g/mol was obtained in 3 h. Very high molar mass PPL could also be obtained. For BL again, polymerization rate was much slower yielding atactic PBL with molar masses up to 6,000 g/mol in more than 100 h. As the polymerization is living, block copolymers of BL and PL were also achieved [291]. Again, concerning the mechanism and more specifically the initiation step, controversy can be found in the literature. The last suggested mechanism is depicted in Scheme 24. The initiation proceeds through a 2e ⁻ transfer in two steps. After the first e ⁻ transfer, an anion radical is formed, and after the second e ⁻ transfer, the resulting dianion lactone is decomposed with the heterolytic cleavage of the acyl-oxygen bond. This compound deprotonates the monomer giving potassium enolate and potassium β-alkoxide aldehyde, this latter being unstable, it decomposes into crotonaldehyde and potassium hydroxide. Both potassium enolate and potassium hydroxide are able to generate initiators as indicated on Scheme 24. As a side reaction, the β-lactone anion radical can undergo a homolytic alkyl-oxygen bond cleavage yielding finally potassium butyrate that can also initiate the polymerization.

Few studies investigated the tacticity of PBL through anionic polymerization. Starting from racemic butyrolactone, mainly atactic PBL were obtained, whatever the initiating system [262, 270, 281, 287, 293, 294]. Nevertheless, it was also demonstrated that reducing polymerization temperature or adding tartrate esters could induce the synthesis of partially syndiotactic PBL (up to 60–65 %) starting from racemic butyrolactone [295, 296]. Finally, isotactic PBL were also synthesized efficiently using R-butyrolactone [295] (around 80–85 %) or S-butyrolactone [270, 273] (up to 95 %).

3.4.1.2 Organic Initiators

Organic bases, like phosphines, pyridines, tertiary amines, and betains, were among the first to be used for the initiation of β-lactones [259, 297–302]. It was first suggested that the initiation step involved the formation of betain species that are the actual initiator with active species being carboxylates. The propagation would then proceed via alkyl-oxygen bond cleavage yielding macrozwitterions [297, 298, 300]. Nevertheless, this was inconsistent with the respective nucleophilicity/basicity ratio of the engaged initiators, at least for amines. It was thus demonstrated that after the betain formation, the protonated base was released yielding acrylate or crotonate ions that were the actual initiators (Scheme 25) [299]. Eventually, PPL or PBL can be produced with unsaturated chain ends. In the case of phosphine, both phosphonium and unsaturated chain end were detected indicating the concomitance of both types of initiation [299]. These side reactions proved to be a limiting factor in the control over molar mass and molar mass distribution.

More recently, other organic (co-)initiators, like carbenes [235, 236, 303], guanidine [304], amidine [304], and phosphazenes [304, 305], revealed powerful for the polymerization of β-lactones. Polymerizations initiated by a carboxylic acid/phosphazene base led to atactic PBL with good control over the molar masses, the polymerization rate being dependent on the basicity of the phosphazene used. With triazole carbenes, it was demonstrated that the polymerization of BL in toluene at 80 °C with methanol as the initiator was controlled for DP up to 200 with good chain-end fidelity (few crotonate chain end detected) in the presence of tert-butanol as the co-solvent [236, 303]. Concerning the initiation mechanism, it was shown that the initiator was deprotonated by the carbene, and it was thus an alkoxide that was the actual initiator (Scheme 26). Moreover, the propagation proceeded both via alkyl-oxygen or acyl-oxygen bond cleavage, yielding concomitantly alkoxides and carboxylates as active species. Nevertheless, the acyl bond cleavage being less favored, after a couple of monomer additions, carboxylates were the only active species.

Besides, with some other carbenes, the polymerization mechanism revealed different, yielding only cyclic polymers with a good control of the molar masses [306]. As spirocycles were formed all along the polymerization, it was proposed that 1,3-dimesitylimidazol-2-ylidene was able to perform a nucleophilic attack onto the carbon of the carbonyl group of BL yielding a zwitterion that ring-closed after each monomer addition (Scheme 27).

3.4.2 Other β-Lactones

3.4.2.1 α,α-Disubstituted-β-Propiolactones

The AROP of α,α-disubstituted-β-propiolactones (Scheme 28) was highly investigated, pivalolactone (α,α-dimethyl-β-propiolactone, PVL) being the most studied monomer [307–316]. Due to the absence of proton in α position compared to other β-lactones, the polymerization of α,α-disubstituted-β-propiolactones could be easily controlled, especially the chain ends, as the side reactions yielding crotonate or acrylate groups (Scheme 20) could not occur with these monomers.

In most of the studies, potassium or ammonium carboxylates were used as the initiating species, the propagating species being carboxylates in this case. When tertiary amines were the initiating species, again carboxylates were the propagating species [316]. On the contrary, when the initiator was a metal alkoxide, the propagation proceeded through an alkoxide, and the formation of macrocyclic structures was noticed [311, 312].

3.4.2.2 β-Substituted-β-Lactones

The AROP of β-substituted-β-lactones was also highly studied (Scheme 29) [317–345]. More specifically, researchers were interested in the monomers that could be precursors for the synthesis of poly(β-malic acid), which is a water-soluble, biodegradable, and biocompatible polymer that exhibits biological properties (proteinase inhibitor, for instance). Whereas this polymer is available from natural and/or bacterial resources, many studies deal with its chemical synthesis, especially since the first description of the synthesis of β-malolactonate in 1979 [346]. The racemic or the optically active versions of benzyl-β-malolactonate were the most studied monomers. Optically active polymers could be obtained (no racemization) with an inversion of the configuration [320].

Several types of initiators were able to polymerize β-substituted-β-lactones. With triethylamine, only low molar masses were obtained with poor control [337]. Great enhancement was achieved when tetraalkylammonium benzoate was employed as the initiator. Polymerizations were generally performed in bulk at temperature ranging from 30 to 70 °C, but like for BL and PL, transfer reactions occurred through proton abstraction, preventing a good control over the molar masses and the chain-end fidelity [317, 318, 330, 340]. Transfer reactions were shown to be highly reduced when polymerizations were performed in THF at 0 °C, at low reactant concentration [330], or when monomers were highly purified [327]. Another possibility to suppress transfer reactions is to start from an α,1;α-disubstituted-β-substituted-β-lactones instead of β-substituted-β-lactones; as for pivalolactone, proton abstraction is no more possible in this case [319, 336, 343]. More recently, carbenes [236], phosphazenes [336, 344, 345], amidines [344, 345], and guanidine [344, 345] were described as efficient initiators for the polymerization of β-substituted-β-lactones. Nevertheless, the transfer reactions were still present. Despite the presence of transfer reactions, the synthesis of macromolecular architecture was possible since it was described the synthesis of block copolymers with the first block constituted of β-substituted-β-lactones and the second block constituted of another β-substituted-β-lactones [328], butyrolactone [317], lactide [334, 335], or caprolactone (CL) [330, 331, 334], these two latter being polymerized through organometallic catalyzed ROP. The synthesis of random copolymers was also performed [317, 322, 323, 326, 329, 332, 343], as well as the synthesis of graft copolymers [329, 332].

3.5 Larger-Ring Lactones

3.5.1 ε-Caprolactone and δ-Valerolactone

3.5.1.1 Polymerization Initiated with Alkali Metal Compounds

The AROP of other lactones was by far less investigated than that of β-lactones. The polymerization of CL could be initiated by metal alkoxides [264, 274, 279, 347–356], cyclopentadienyl sodium [357], phenyllithium [358], carbazole potassium [359], lithium diisopropyl amide [360], or sodium hydride [361]. Depending on the initiator, the initiation proceeded via monomer deprotonation (Path A, Scheme 30) or via nucleophilic attack, the monomer being opened at the acyl-oxygen bond and the growing species being an alkoxide (Path B, Scheme 30). For example, using cyclopentadienyl sodium as the initiator, no cyclopentadienyl groups were present on the polymeric chain ends, and the polymerization is said to proceed through deprotonation of the monomer [357].

The main drawback of this method is the occurrence of significant intramolecular transesterification reactions, also called “back-biting,” which were very dependent on the polymerization conditions and the initiator and resulted in the formation of generally low molar mass polymers and in cyclic polymers. For instance, it was demonstrated that with tert-butoxide potassium as the initiator, high dilution favored the formation of cyclics (less than 6–7 CL units) to the detriment of polymer formation [351, 352]. With cyclopentadienyl sodium as the initiator, in bulk and in nonpolar solvents, molar masses up to 130,000 g/mol were obtained, whereas in polar solvents, only oligomers were produced [357]. When the polymerization of CL was initiated with lithium diisopropylamide, medium molar mass polymers were obtained [360]. The polymerization of CL initiated by phenyl lithium in bulk at 170 °C led to high molar mass polymers (50–70,000 g/mol) [358]. Polymerizations performed in supercritical carbon dioxide exhibited low yields probably because of the occurrence of side reactions between the anionic species and carbon dioxide [350]. It was also demonstrated that alkali graphitides allowed the synthesis of very high molar mass polylactones with nevertheless the presence of a low molar mass fraction [362–368].

In spite of many possible side reactions and like for the other cyclic esters, the possible synthesis of poly(δ-valerolactone)-b-polylactide [188, 189, 369], poly(ethylene oxide)-b-poly(ε-caprolactone) [370], and polyglycidol-b-poly(ε-caprolactone) [371] was described in the literature.

3.5.1.2 Organocatalyzed Polymerization

Few organocatalysts were shown to be able to perform the polymerization of lactones compared to lactide. Indeed, most of the studies employed carbenes [237, 242, 372–379], with few examples using phosphazenes [234, 380] or TBD [381]. With tBuP₁ or BEMP, in the presence of alcohols, only δ-valerolactone (δ-VL) was polymerized with high conversion in 2–4 days, whereas for ε-caprolactone only 15 % conversion was obtained after 10 days. On the contrary, with tBuP₂, CL was polymerized in few hours to yield PCL with controlled molar masses. NHCs were shown to polymerize both δ-valerolactone and ε-caprolactone giving access not only to linear but also to cyclic aliphatic polyesters. While the NHCs were generally highly active in the polymerization of lactide, less efficiency was observed toward the ROP of CL. The mechanism is supposed to be an activated monomer mechanism, and several NHCs, either in their “bare” or masked form, were shown to perform the polymerization of lactones (Scheme 31).

The ROP of CL was generally performed at room temperature in THF solution (0.5–2.0 M), in the presence of monofunctional initiators or multihydroxylated initiators such as ethylene glycol, 1,1,1-tris(hydroxymethyl)propane, pentaerythritol, or a six-arm poly(propylene glycol) (Scheme 31), yielding well-defined linear or star polycaprolactone (PCL). Catalytic activity was sensitive to steric and electronic properties of the carbene, as more electron-rich and less bulky substituted carbenes were more active for the synthesis of well-defined PCL [242, 374]. A so-called “abnormal” NHC, in which the carbene center is no longer located between the two nitrogen atoms but between a nitrogen and a carbon atom, was reported to exhibit a high catalytic activity in the ROP of CL [376]. Again, like for lactide ROP, in the absence of alcohol, the polymerization was shown to be zwitterionic, and it was thus synthesized cyclic polycaprolactones at room temperature with relatively high dispersity (between 1.4 and 2.1) for a wide range of molar masses (41,000–114,000 g/mol) [372, 377, 378]. Copolymerization of CL and δ-VL led to the formation of cyclic copolyesters with a gradient microstructure due to the difference in the reactivity ratios between the two monomers, which is usually not observed with metal-based alkoxides [375].

3.5.2 Other Lactones

Some other lactones were shown to be polymerizable through AROP. For instance, bicyclic oxalactone (Scheme 32) could be polymerized with butyllithium to afford polymers of moderate molar mass and high dispersity [382]. The polymerization of large-ring lactones (undecanolide, λ-lauryllactone, and pentadecanolide) was also performed in bulk at high temperature or in solution at moderate temperature in the presence of metal alkoxides [383, 384]. The presence of back-biting reactions leading to the formation of macrocycles was still detected. α-Methylenemacrolides were successfully polymerized with butyllithium at low temperature [385].PEG-containing macrolactones were also successfully polymerized using thiols and tBuP₄ [386]. More recently, 8-membered lactones obtained from 1,3-benzoxazine were polymerized in bulk at high temperature in the presence of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) yielding low molar mass polymers with high dispersity due to intensive back-biting reactions [387].

5-Membered γ-butyrolactones do not generally afford polymers because the rate of ring closure is faster than that of ring-opening, and therefore only insufficient formation of the corresponding polyesters was achieved. On the contrary, Endo et coll. showed that copolymerizations of bis(γ-lactones) with epoxides led to the synthesis of alternated copolymers (Scheme 33) [388–398]. This was possible due to the presence of an isomerizable structure onto the monomer that prevents backward ring closure. Concerning the mechanism, the bislactone reacts with alkoxide-type propagating chain end exclusively, to undergo double ring-opening reaction. The formed acyclic carboxylate is thermodynamically stable and thus does not undergo backward ring closure. At the same time, the nucleophilicity of the carboxylate is not high enough to react with the bislactone but is reactive with epoxide to regenerate an alkoxide. Moreover, as the formation of the copolyester synthesis was accompanied by low volume shrinkage during polymerization, such polymers are useful for network formation when bisepoxides were used. It was also shown that phosphines could replace alkali metal alkoxide to perform a zwitterionic polymerization.

The AROP of 3,4-dihydrocoumarin (DHCM, an aromatic lactone, Scheme 34) was also studied with an imidazole as the initiator in bulk at high temperatures (100–120 °C) [399–405]. Whereas DHCM is a 6-membered ring like δ-valerolactone, its homopolymerization was not possible. On the contrary, it was shown that DHCM was easily copolymerized with an epoxide to yield alternated copolymers. When the bislactone was copolymerized with epoxides, only one of the two lactones participated in the copolymerization yielding only linear polymers. Networks were only obtained by post-reaction of the remaining lactone with a diamine, for example.

4 Cyclic Amides (Lactams)

4.1 Introduction

Polyamides are well-known polymers that are present in markets such as fibers, engineering plastics, and specialties, due to specific and various properties depending on their structures. Ring-opening polymerization of lactams (cyclic amides) initiated by water, referred to hydrolytic polymerization (i.e., reactions between the amine chain-end group and the lactam and/or carboxylic group of its hydrolyzed derivative), is carried out for industrial polymerization of ε-caprolactam (ε-CL) to form polyamide 6 (PA6, Nylon 6), though nylon 6–6, 4–6, and 6–10 are synthesized by stepwise reactions of a diacid monomer with a diamine monomer. Cationic initiation is also possible, but not useful because of the low conversion and molar mass of the resulting polyamides [406]. Anionic initiation following an activated monomer mechanism is mainly used for polymerization in molds in order to prepare polyamides (PA6, 10, 12) directly from the corresponding lactams [407]. Polymerization mechanism of lactams, their polymerizability, and the properties of the resulting polymers were largely investigated. Reviews published by Reimschuessel [408, 409], Šebenda [410], Sekiguchi [406], Hashimoto [411], Roda [412], and Russo and Casazza [413] can precise this presented overview.

The anionic route is the fastest method for producing polyamides due to the low activation energy needed. This fast kinetic makes nowadays this route of high interest for industrial processes producing lightweight composite materials for automotive and wind energy. The anionic polymerization of lactams may be accomplished in solution or in the bulk either below or above the melting point of the polymer for the latter.

4.2 Mechanism of the Anionic Polymerization of Lactams

The mechanism differs from the anionic polymerization of most of the unsaturated and heterocyclic monomers because the growth center is not an anionically activated end group but is represented by an N-acyllated neutral chain. The anionic polymerization of lactams is initiated, under anhydrous conditions, by formation of the lactamate anion. Strong bases are able to deprotonate lactams and produce N anion of lactam effective for initiating the polymerization.

4.2.1 Initiating and Activating Systems

The anionically activated species is the monomer in the form of lactamate anion which is a very strong nucleophile (Scheme 35). The negative charge is delocalized on the amide group due to resonance stabilization by conjugation with the carbonyl group.

The lactamate anion is acylated by a lactam monomer, although the acylating ability of the latter is poor, with the amide group being stabilized by resonance. The lactamate anion reacts with the monomer by a ring-opening transamidation reaction forming N-acyl lactam structures carrying primary amine anions. Assuming a free ion mechanism [414], the imide anion is formed, in the first slow step, by nucleophilic attack of the lactamate on the carbonyl of the lactam molecule (Scheme 36). As it is not stabilized by resonance, rapid proton exchange undergoes with lactam monomer, yielding imide dimer (N-acyl lactam) and regenerating the lactamate. Result of these two combined reactions is the disproportionation between two amide groups (present in lactam monomer and in lactamate anion) to give amine and acyl lactam moieties (in the N-acyl lactam species) and the reaction rate dependent on factors like the nature of counterion and reaction medium, lactam ring size, substituents, and structure of the resulting linear monomeric unit. N-substituted lactams are observed to react with lactamate anion with a rate significantly higher than that of the initial reaction, depending on the size and the electrophilicity of the substituent, and are generally used as activators in the activated anionic polymerization. The use of high reaction temperatures (>250 °C) is required in the absence of activator, and only the more reactive lactams, such as ε-caprolactam, undergo polymerization in the presence of a strong base in a non-activated method.

The initiators, which are the monomers carrying the anionic charge able to attack the chain end, are prepared by reaction of a lactam with strong bases such as mainly metal alkoxide, metal halide, alkali metal, and Grignard reagent [406, 414, 415] but also pentamethylene guanidine [406], quaternary ammonium salts [416], phosphazene [417], bicyclic “superbase” protophosphatranes [418, 419], or carbenes [420, 421]. The association of a strong base (NaH, LiH, BuLi) with a reducing agent such metal dialkyl/dialkoxy aluminum hydrides [422–424] or metal dialkyl boron hydride [423] can also be used as precursors of lactamates. In this case, the nucleophilic species obtained, i.e., the activated monomer, is a metal salt of 2-(dialkyoxyaluminoxy)-1-azacycloheptan.

As usual in anionic polymerization, the nature and concentration of the initiator play a crucial role. The rate is directly related to the concentration of active species and in particular to the dissociation constant yielding free ions. It is known that the concentration of free lactam anion increases with temperature, starting to be predominant above 150 °C, alkali metal lactamates being considered completely dissociated at higher temperatures [425, 426]. The rate of polymerization becomes here independent of the nature of the cation. In general, the activity of alkali metals follows the order of electropositivity, except with Li and despite its highest ionization energy: Na⁺ < Li⁺ < K⁺ < Cs⁺ = R₄N⁺. Lactamates of transition metals (e.g., Cr³⁺) and other metals (e.g., Al³⁺), exhibiting high electronegativity values and having very low dissociation constants, hardly dissociate even at high temperatures. In the molten monomer medium, without solvating or complexing species, the lactamate dissociation depends on both the lactam properties (i.e., acidity, dielectric permittivity, donor-acceptor capability, substituents) and the electropositivity of the metal. For example, higher lactam permittivity, such as in ω-laurolactam as compared to ε-caprolactam, makes easier the salt dissociation [427].

An induction period and slow kinetics are observed with non-activated anionic polymerization of lactams, whereas opposite behaviors are obtained when an activator is added. The induction period is absent, and the AROP can be performed at much lower temperatures (130–180 °C for ε-CL) [410]. Poorly reactive lactams, such as 2-pyrrolidone and 2-piperidone, can be polymerized by initial reaction of monomers with an acylating agent. In this activated mechanism, the slow self-initiation step is strongly minimized to the detriment of a fast acylation reaction and propagation step. The interest to work with milder conditions for shorter times allows to strongly reducing side reactions, yielding more regular macromolecular chains. These observations are nevertheless dependent on structure and concentration of the activators. Many substances can be used such as N-substituted lactams (N-acyllactam (1) [428, 429] and carbamoyllactam (2) [430–434] with electronegative substituent (R) increasing the acylating ability of the cyclic acyl group), compounds capable of producing N-substituted lactams, under the conditions of the anionic polymerization (e.g., isocyanate (4), acid halides or esters (5), carbon dioxide (6)) [428, 435–437] and derivatives from side reactions (C-, N-, O-acylation) in low ring strain lactam monomers such as oxoamides type (8), N-acylamidine (3), and others (7) (Scheme 37).

4.2.2 Propagation Reaction

The slow formation of an N-acyllactam by reaction between monomer and lactamate ion (k _i = 10⁻⁷ L·mol⁻¹·s⁻¹ for sodium ε-caprolactamate at 160–190°C) [438] is followed by an extremely fast neutralization reaction, i.e., monomer deprotonation. For pyrrolidone at 35 °C and laurolactam at 160 °C, the rate constant of proton exchange (k _H) is 10⁵ L·mol⁻¹.s⁻¹ [439] and 10² L·mol⁻¹·s⁻¹ [440], respectively, which prevent the process of ring-opening of lactam via an active chain-end (ACE) mechanism (i.e., k _p(ACE) ~ k _i ~ 10⁻⁷ L·mol⁻¹·s⁻¹) (Scheme 38). The nucleophilic attack of the lactamate anion on the carbonyl group of the monomer is much slower than that of the carbonyl group in an N-acyllactam-type chain end (10⁻¹ < k _p(AM) < 10³ L·mol⁻¹·s⁻¹, k _p(AM) = 68 L·mol⁻¹·s⁻¹ in the case of ε-caprolactam) [441], which refers to the process of ring-opening polymerization of lactam via activated monomer (AM) mechanism (Scheme 38).

The propagation step is therefore composed of a nucleophilic attack to the acyllactam-type growing chain end (k _p < 10³ L·mol⁻¹·s⁻¹) and a subsequent very fast proton transfer from the monomer to the amidate (k _H ~ 10²–10⁵ L·mol⁻¹·s⁻¹) (Scheme 39). The neutral N-acyl lactam acts as the growth center at the chain end as the exocyclic carbonyl group in the N-acyl lactam increases the electron deficiency of the amide group and, thus, the acylating ability. The polymerization rate is first order with respect to lactamate (L⁻) and N-acyllactam (Act) concentrations and zero order with respect to lactam monomer (L) concentration [438] and can be written as follows .

$$ -\frac{\mathrm{d}\left[L\right]}{\mathrm{d}t}={k}_{\mathrm{p}}\left[\mathrm{Act}\right]\kern0.05em \left[{\mathrm{L}}^{-}\right] $$

In the activated polymerization, the number average molar mass is determined by the concentration of the activator as compared to monomer concentration. Experimental values are generally higher than the theoretical ones. This is mainly due to the lowering of the number of growth centers due to the side reactions and to the cross-linking between polymer chains, for example, by Claisen-type condensation reactions, which are more and more relevant as the medium basicity and the polymerization temperature increase [442]. Such a polymerization is thus not living because of these side reactions. When the polymerization is run at temperatures below the melting point of the polyamide, side reactions are largely reduced, and, even at equimolar concentrations of initiator and activator, the polymerization proceeds essentially by the reaction of lactam anions with a constant number of growth centers, resulting in a narrower molar mass distribution (D < 2) [443] and the formation of high molar masses [434, 444]. In the non-activated polymerization where the growth centers are both formed at the very beginning and in the final stage, the molar mass distribution is expected to be broader than the ones observed with the use of an activator.

The structure of the activators, in particular the nature of the exocyclic acyl group in N-acyl lactam, was shown to play a crucial role on the polymerization rates [445]. “Very fast” activators, like of N-carbamoyl lactams in appropriate concentration, allowed to drastically reducing the polymerization time (less than a minute) [434, 446] to get high polymer yield, with the advantage to enable a decrease of polymerization temperature down to 140 °C, minimizing side reactions.

Concerning the mechanism, lactamolytic mechanism proposed by Sekiguchi [447–449] (Scheme 40a) assumes transfer of the alkali metal cation from the activated monomer species to the imide group at the end of the growing chain and its coordination to the carbonyls of the imide. A conductivity increase was attributed to a higher concentration of free ions. The reaction proceeds via formation of an alkoxide-type anion by nucleophilic attack of the lactam anion on the endocyclic carbonyl, proton exchange with monomer, and rearrangement with ring-opening. Alternatively, Frunze et al. [450, 451] proposed the participation of ion pairs of lactam salts in the propagation step and suggested an ion-coordination mechanism. According to this mechanism, complex between the lactamate and the two carbonyl groups of the growing center is formed (Scheme 40b). As already mentioned, side reactions are observed whatever the assumed mechanisms. In any case, free ions play a decisive role at high temperatures and in media of high permittivity, while at low temperatures and in low polar media, the involvement of ion pairs is more expected.

4.2.3 Side Reactions

Species involved in anionic polymerization are generally highly reactive, leading to a series of side reactions, in particular at high temperatures for long polymerization times. Reversible and irreversible side reactions can occur, consuming both the growth centers and the monomer anions. The strongly basic conditions in AROP of lactams promote mainly polymer branching and β-keto compounds, yielding to side products and chain irregularities. UV spectrometry was shown to be a powerful tool for monitoring the occurrence of such side reactions [414, 442, 452].

4.2.3.1 Formation of Acyllactams, Amines, and Imides

The polymer amide groups may be involved in disproportionation reactions, forming acyl lactams and amine end groups (Scheme 41). The presence of amide N anions along the polymeric chain, derived from equilibrium reactions with lactam anions in strongly basic medium, may also produce imide groups and polymer branching (Scheme 41). Transacylation reactions between polymer amide anions and acyl lactams (N-acylations) (Scheme 42) may cause depolymerization or incorporation of a lactam unit when the exocyclic (Scheme 42a) or the endocyclic carbonyl groups (Scheme 42b) are involved. The nature of the counterion affects not only the degree of dissociation of the corresponding lactamates but also the whole polymerization rate [453].

4.2.3.2 Formation of β-Ketoimides and β-Ketoamides

The acidity of the hydrogen atoms in α position of the carbonyl of the imide group in the N-acyl lactam chain end is comparable to that of hydrogen in an amide group. As a consequence, in the presence of lactamate, deprotonation may occur, leading to the formation of two distinct carbanions (Scheme 43). The Claisen-type condensation reactions on exo- and endocarbonyls then happen, giving four different β-ketoimide structures. The concentration of these carbanions is generally low meaning that C-acylations and, only in some specific cases, O-acylations are competitive reactions with regard to the propagation step [454]. One can also consider the formation of carbanion in the α-position of the carbonyl of a branched structure. Reactivities are related to the lactam size and their substituents, the nature of the activator, the initial ratio of initiator and activator concentrations, the permittivity of the reaction medium, and the reaction temperature as well as the nature of the counterion [454–456].

Neutral β-ketoimides are strong acylating agents and may be involved in reactions, acting as growth centers and leading to either linear or branched chains. β-Ketoimides may be converted to β-ketoamides (2-oxoamides) by nucleophilic attack of the N anion on the carbonyl of the imide group (Scheme 44). These keto derivatives decrease the concentration of active species and thus influence kinetics. They are also very reactive under basic conditions or at high temperatures and responsible for complex secondary reactions, i.e., formation of water, carbon dioxide, amines, and heterocyclic structures which are able to act as branching and cross-linking points [454]. The thermal or base-catalyzed decomposition of β-ketoamides can afford ketones and isocyanates. The latter reactive functions can also reform N-acyl lactams capable to react in the expected polymerization way. Formation of water can contribute to the deactivation of lactamate and to the hydrolysis of N-acyl lactams, β-keto compounds, and imide branching points leading to carboxylates, amine groups, ketones, carbon dioxide, or carbonates [410].

4.2.3.3 Formation of Cyclic Oligomers

The formation of cyclic oligomers was particularly investigated by Russo and coll. for ε-caprolactam [452, 457], their amount depending on the polymerization temperature (e.g., 3.5 % at 280 °C). The main reaction leading to cyclic oligomers is a back-biting reaction which is an intramolecular reaction of the neutral end groups with amidic groups inside the chain (Scheme 45). The counterion involved in the polymerization also directly influences the occurrence of such a side reaction [458]. With magnesium salts of ε-CL, cyclization reactions are strongly reduced both below and above the melting temperature of the polymer as compared to sodium systems, due probably to coordination between magnesium-based compounds and polyamides end groups. Cyclic structures can have a negative influence on processing or applications as they are able to modify the crystalline structure in the solid phase [459, 460].

4.3 Anionic (Co)polymerization of ε-Caprolactam and ω-Laurolactam

4.3.1 Homopolymerization of ε-Caprolactam and ω-Laurolactam

The polymerization of ε-caprolactam, a 7-membered ring, is usually conducted in bulk conditions, above the melting temperature of the monomer (80 °C) in the presence of initiator and activator. Initially liquid, the mixture turns turbid and then solidifies in the course of the polymerization which can be as fast as few tens of seconds for “very fast” systems. The beginning of solidification is considered as the moment at which the growing chains attain a critical length that enables their crystallization, forming spherulites insoluble in the monomer.

As discussed previously, playing with the structure and concentration of both activator and initiator allows tuning the polymerization rate of ε-CL. The initial polymerization temperature and isothermal, nonisothermal, or adiabatic conditions are also considered as tools to modify the time of reaction. “Very fast”, “fast,” or “slow” processes affect the structure and properties of the resultant anionic polyamide 6. For a very fast bulk polymerization of ε-CL at 155 °C, conditions close to adiabatic ones are obtained due to high rate and poor heat exchange with the surroundings. A temperature increase of 50 °C is observed with a resultant polymer with high molar mass, low residual monomer content, and low cyclic oligomers [408, 434]. To decrease even more side reactions, quasi-isothermal conditions near 150–160 °C were proposed [433].

Similar polymerization systems and conditions were employed for the synthesis of polyamide 12 obtained by ring-opening of ω-laurolactam (ω-LL), a 13-membered ring. The polymer gained attention for its low level of absorbed moisture, easily removable during heating and melting of the monomer at 150 °C. Moreover, it possesses an excellent ductility, good electrical properties, and significant chemical resistance. However, due to the long methylene sequences between the amide linkages, it has a lower melting point (172 °C) compared to PA6 (210 °C). To get a low content of residual monomer due to favorable monomer-polymer equilibrium, temperatures above 150 °C are generally required [444, 461]. Some specific polymerization systems, based on alicyclic carbodiimide as activator and sodium caprolactamate as initiator, were also developed to allow, for instance, a long-term storage of the initiating species, an efficient control of the polymerization rate, and an accurate tailoring of polyamide molar masses [462]. As for ε-CL, initiation and activation influence also polymerization kinetics and thermodynamics but also the degree of crystallinity [463].

As discussed in the previous paragraph, the AROP of lactams suffers from numerous (ir)reversible side reactions, depending on the experimental conditions. The usual kinetic law depending on activator and initiator concentration (R _p = −d[M]/dt = k _p·f [Activator] [Initiator]₀) appears not efficient to get right values but might be sufficient to compare polymerization systems. The autocatalytic model of Malkin, based on a phenomenological approach, seems the most successful to follow activated polymerization in bulk [464]. It describes the nonisothermal kinetics of both ε-caprolactam and ω-laurolactam, monitoring the temperature rise inside the reactor:

$$ -\frac{\mathrm{d}\left[\uplambda \right]}{\mathrm{d}t}=k\frac{{\left[A\right]}^2}{{\left[M\right]}_0}\kern0.15em \left(1-\uplambda \right)\;\left(1+\frac{\mathrm{b}\uplambda}{\left[A\right]}\right)\kern0.15em \exp \kern0.15em \left(-\frac{E\mathrm{a}}{RT}\right) $$

with λ the conversion, [A] the activator concentration, [M]₀ the initial monomer concentration, k the reaction rate constant, Ea the activation energy, and b the autocatalytic term characterizing the intensity of the self-acceleration effect during chain growth. Both k and b depend on the chosen activator. The rate constant can be evaluated for low conversions where polymerization and crystallization are not overlapped.

4.3.2 Copolymerization of ε-Caprolactam and ω-Laurolactam

Anionic polymerization is well known and used for the synthesis of copolymers, in particular blocks, due to its living character. Despite the nonliving character of lactam polymerization, copolymers based on polyamide can be synthesized and offer interesting and specific properties. ω-Laurolactam is in general used as co-monomer of ε-caprolactam in order to extend the range of PA6 properties [429], in particular by increasing the notched Izod impact strength at low temperature, and the decrease of water absorption [465]. Roda and coworkers showed the influence of the initiator toward the copolymer structures and therefore properties [461, 466]. As compared to ε-caprolactam magnesium bromide, sodium caprolactamate exhibits higher polymerization activity especially in copolymerization with high content of ω-LL at high polymerization temperatures. The copolymers have only one melting endotherm in the whole range of monomer feed and one single crystalline form, when two melting endotherms (140 °C and 210 °C) and two types (α and β) of crystalline forms are observed from 30 to 70 mol% of ε-CL with the magnesium-based initiator. This is explained by a copolymer microstructure composed of PA6 blocks linked to sequences of ε-CL/ω-LL random copolymer. PA6 is preferentially formed at the beginning of the polymerization, due to the much higher reactivity of ε-CL as compared to ω-laurolactam. Random copolymers are then formed from the remaining ε-CL and slowly reacting with ω-LL. In the case of sodium caprolactamate which is a strong base as compared to the magnesium derivative, transamidation reactions cause full randomization of the sequences.

4.3.3 Copolymerization of Lactams and Lactones

Interesting degradable polyamides could be obtained through the synthesis of polyesteramides by copolymerizing lactams and lactones even if different anionic ring-opening mechanisms are involved in their homopolymers formation [412]. Some lactones were shown to act both as activator of lactam polymerization and as co-monomer for the synthesis of a polyester block [436, 467–469]. The initiation step corresponds to the acylation reaction between a lactamate and the reactive lactone (Scheme 46). The oxyanion formed is then able to initiate the rapid ring-opening polymerization of some cyclic esters such as ε-caprolactone or δ-valerolactone by usual chain growth mechanism (active chain end).

Playing with ε-caprolactam and ε-caprolactone ratios as well as experimental conditions, various random or multiblock copolymers were prepared. Fast transacylation reactions between ester and amide groups in the copolymer chains were proposed to be responsible for the observed copolymer randomness. Using reactive processing like a twin-screw extruder, diblock or triblock copolymers were prepared from various lactams and lactones using suitable sequential monomer feeding and specific temperature profiles [470–472]. The block lengths can be adjusted by controlling the feed rate. The use of poly(ε-caprolactone) (PCL) was also proposed as additive to the polymerization of ε-CL with ε-caprolactam magnesium bromide as initiator, with or without activator. PCL was shown to act as an activator, and random copolymers were prepared [473].

4.3.4 Polyamide-Based Copolymers with Non-polyamide Blocks

The use of macroactivators obtained from appropriately terminated prepolymers is the main route leading to lactam-based block copolymers. In general, hydroxy telechelic polymers are reacted with diisocyanates and then blocked with ε-CL. Combination of properties is the driving force of reacting various non-polyamide blocks as activators of anionic lactam polymerization. The toughness improvement of PA6 being a key issue, soft polymers such as polybutadiene [474–477], polyethers [478–483], and polysiloxanes [480, 484, 485] were particularly used. Following a similar approach, Styrene-Butadiene Rubber was also introduced into PA6 with the aim to tune the mechanical properties [486]. Graft copolymers were also designed for compatibilization purpose. Polypropylene or polystyrene grafted with polyamide-6 chains was easily obtained [430, 487–490].

4.3.5 Industrial Processes Using AROP of ε-Caprolactam and ω-Laurolactam

Ring-opening polymerization of ε-caprolactam initiated by water, i.e., the hydrolytic mechanism, is carried out for industrial cast nylon-6. Nowadays, due to fast kinetics of the “activated” anionic ring-opening polymerizations, this approach is more and more envisaged for the preparation of PA6 and PA6-co-PA12 in newly developed industrial applications using mainly powdered materials and molding or extrusion approaches.

4.3.5.1 Powdered Polyamides

As compared to techniques industrially utilized so far, i.e., low-temperature grinding and polymer dissolution/precipitation, AROP yielding PA6 and PA6-co-PA12 offers some advantages such as a much higher particle porosity, a total absence of irregular edges and sintered zones, and a controlled and narrow particle size. Dispersion [491–496], suspension [497], and miniemulsion [498] polymerizations are generally proposed. More recently, the use of phase inversion in PA6/PS blends allowed the preparation of microspheres with controlled diameters [499]. Fast polymerization systems have to be selected for such processes. The suspension method has the advantage to be the faster but suffers from difficult and expensive purification. Such materials are of great interest for cosmetic formulations, coating and graphic art applications, protein or enzyme immobilization techniques, rotational molding and sintering processes, chromatography applications, as well as filtration devices in food and beverage industry.

4.3.5.2 Molding and Extrusion

Reaction injection molding (RIM), resin transfer molding (RTM), rotational molding, and reactive extrusion are the main processes used with an in situ activated anionic polymerization of ε-CL [500, 501]. Due to its high crystallinity and high molar mass, anionic PA6 exhibits, for instance, better thermomechanical properties or lower water uptake as compared to the extruded or molded PA6. The short polymerization times in the order of minutes, as compared to hours for hydrolytic polymerization, the very low cyclic oligomer content, and the much lower initial polymerization temperature (130–170 °C vs. 230–280 °C) are the main advantages of this activated AROP.

Soft polymers bearing terminal N-acyl lactam groups are used in RIM processes as activators of ε-CL polymerization yielding PA6 with good impact strength. Usual initiators such as sodium ε-caprolactamate or magnesium bromide ε-caprolactamate are efficient in that process [502]. RTM enables the injection of the melted monomeric reactants of low viscosity into a mold filled with reinforcing materials like fibers. Reactive extrusion processes regain also attention for the easy preparation of nanocomposites and nanoblends [501, 503–506]. Single- or multiwalled carbon nanotubes and nanosilica are also shown to be dispersed in PA6 modifying its initial properties [507, 508]. Nevertheless, it has to be mentioned that anhydrous conditions are required and may sometimes be considered as a limitation. Deactivation of the anionic groups is known to occur when some clays are used as reinforcing agents.

4.4 Anionic Polymerization of Other Lactams

β-Lactams, 2-pyrrolidone, and 2-piperidone are the three main unsubstituted lactams available and studied by AROP. They are, respectively, yielding polyamide-3, polyamide-4, and polyamide-5. It has to be noticed that N-substituted polyamide-1 as well as polyamide-2 (polypeptide) is not obtained from lactams but from oxadiazolinones and N-carboxyanhydride, respectively.

4.4.1 β-Lactams

Living anionic polymerization can be reached as substituted β-lactams (or β-propiolactams) (Scheme 47) are highly reactive, due to high ring strain, enabling thus the use of low polymerization temperatures and times. The review of Hashimoto published in 2000 describes in detail the specificities of the ring-opening polymerization of such monomers [411].

Šebenda et al. showed first that the activated anionic polymerization of a bulky β-lactam, i.e., 3-butyl-3-methyl-2-azetidinone, has a living character giving a monodisperse polyamide of molar mass very close to the theoretical value [509, 510]. Other substituted monomers were also polymerized in a controlled manner in homogeneous solution, using aprotic and apolar solvents like N,N-dimethylacetamide, DMF, or DMSO in the presence of lithium salts [511–513]. Depolymerization and transamidation both at the acyl lactam chain end and on the polyamide chain are known to occur, therefore broadening the molar mass distribution [411]. Stopping the reaction before complete conversion minimizes the transamidation, enabling the preparation of block and graft copolymers or other structures taking advantage of the living character of the polymerization. The possibilities to play with substituents offer nowadays PA3 materials with amphiphilic character and possibly bioactivity, for instance [514].

4.4.2 2-Pyrrolidone

The particularity of the polymerization of 2-pyrrolidone (Scheme 48), leading to polyamide-4, can be found in a rather low ceiling temperature (70 °C) limiting the reaction temperature to 50 °C [515]. In bulk conditions, economically more interesting than in solution, polymerization rate decreases with time, and partial conversion is obtained due to a phase separation, nucleation, and crystallization with occlusion of the growth centers in this solid phase [516–518]. Despite some potential industrial interests in textile for its good mechanical properties and hydrophilic behavior similar to cotton, synthesis difficulties are one main reason for its non-development.

Similar to ε-caprolactam polymerization, CO₂ was also proposed as activator to successfully prepare PA4 with an improved thermal stability [435, 519]. But depolymerization still remains a major drawback. Using quaternary ammonium salts of 2-pyrrolidone as initiator, instead of sodium or potassium ones, and N-acetyl-2-pyrrolidone as activator, yields up to 80 % could be obtained after 24 h at 30 °C [416]. It is assumed that bulky counterion allows the breaking of hydrogen bonds between polymer chains and creates local irregularities of the crystalline structure, enabling the contact between lactamates and reactive chain ends. Suspension polymerization can also be used [520], and whatever the process used, polyamide-4 was obtained free of structural irregularities thanks to the low polymerization temperature and limited conversions. Block copolymers containing PA4 segments could be obtained using the macroactivator approach [521]. The synthesis of PA4 with a terminal azide function [522] or with ε-CL as co-monomer above the ceiling temperature of 2-pyrrolidone was also performed [523, 524].

4.4.3 2-Piperidone

The ring-opening polymerization of 2-piperidone, also called 2-piperidinone or δ-valerolactam, is kinetically slow due to its stable 6-membered ring [406] (Scheme 49). Moreover, crystallization and side reactions contribute also to the slowness of the reaction. The use of activators is mandatory, and relatively high molar masses of PA5, with a melting temperature of 283 °C, were obtained with quaternary ammonium salts of monomers used as initiators [516, 525].

The use of bicyclic lactams is proposed as an alternative, the ring strain being favorable to a faster AROP.

4.4.4 Bicyclic Lactams

The key point in the AROP of bicyclic lactams is indeed the ring strain, coming, for instance, from the repulsion of hydrogen atoms, and its release. At first, Hall reported in the 1960s the polymerization of a bicyclic lactam, i.e., the 6-azabicyclo[3.2.1]octan-7-one (Scheme 50a), in the presence of sodium hydride [526, 527]. Hashimoto proposed a detailed review in 2000 relative to bicyclic and heterobicyclic lactams [411]. High temperatures are generally required limiting the livingness of such polymerizations. For the case of bicyclic oxalactams (Scheme 50b), the polymerization could be run at 25 °C in DMSO due to a high kinetic polymerizability related to the high strain of internal bond angles [528, 529]. A living character was observed till 60 % of conversion.

5 Cyclosiloxanes and Other Cyclic Silicon-Based Compounds

5.1 Introduction

Cyclic silicon-containing monomers associated, or not, with oxygen, nitrogen, and carbon represent the main reactants toward the synthesis of silicon-based polymers by anionic polymerization. Their ring-opening leads to polysiloxanes and polycarbosiloxanes, polysilanes and polycarbosilanes, polysilazanes, and a few other silicon-containing polymers. The possibility to vary the molecular structure, of both the main chain and the side groups, enables the modulation of unique physicochemical properties which make them attractive in academic field as well as for industrial applications in some cases. Ring-opening polymerization (ROP) of cyclic oligomers allows in general a better precision in terms of chain lengths and molecular weight distributions than the polycondensation of functional precursors. Both cationic and anionic mechanisms can undergo polymerization of certain monomers, but a stringent control of the reaction conditions is required in order to avoid the formation of by-products, such as short oligomers and rings [530]. Recent reviews [531–534] may add complementary information to this chapter focusing on silicon-containing polymers obtained by anionic polymerization. ROP of strained rings exploits the release of ring strain as the thermodynamic driving force [535]. It proceeds either by kinetic or thermodynamic control, which has noticeable consequences for the product distribution. Under kinetic control, the selective cleavage of the precursors and chain propagation occur almost exclusively, providing high molar mass polymers and barely any by-products. Under thermodynamic reaction control, equilibrium mixtures are obtained which generally consist of low molar mass polymers and high amounts of smaller oligomers and ring species.

5.2 Cyclosiloxanes

5.2.1 Polymerization Generalities

Anionic ring-opening polymerization (AROP) of cyclosiloxanes involves the cleavage of the Si-O bond in the monomer ring and the subsequent regeneration of this bond in a polymer chain. Among the various siloxane monomers, the two most important are hexamethylcyclotrisiloxane (D3) and octamethylcyclotetrasiloxane (D4) (Scheme 51).

Other cyclosiloxanes derived from these monomers are also available by substitution of methyl with various organic groups, such as vinyl, phenyl, fluoroalkyls, etc. Three-dimensional structures, i.e., silsesquioxanes or multicyclic siloxanes, are also available and attractive precursors. Cyclic organosiloxanes are usually prepared by hydrolytic polycondensation of dichlorodialkylsilane (R₂SiCl₂) or a mixture of α,ω-dichlorooligosiloxanes (Cl(R₂SiO)_n-1R₂SiCl) [536, 537]. Other routes are also proposed in the literature [538–540].

Initiation step requires strong bases (inorganic, organic, or organometallic), able to ring-open cyclosiloxanes and form silanolate anion, the active propagating species (Scheme 52). Alkali metals, ammonium, and phosphonium salts are the most used derivatives [533, 541]. The propagation is reversible leading to a back-biting reaction with the formation of cyclic structures of various ring sizes. Chain redistribution also occurs due to the nucleophilic attack of a silanolate to another growing polymer chain (Scheme 53). To get nonequilibrium AROP of cyclosiloxanes in order to minimize those re-equilibration reactions occurring during the final stage of the reaction, the polymerization must be quenched soon after a high monomer conversion is obtained [542].

Polymerization kinetic is dependent on monomer and initiator concentrations as well as experimental conditions. Ion pairs are the main active centers involved in the determination of the polymerization rate [533]. Free silanolate anions are not present in sufficient concentration to play a role, in contrary to aggregated species which are in equilibrium with ion pairs [533, 543, 544]. Fractional order in silanolate is introduced in the kinetic law of AROP of cyclosiloxanes due to the existence of less reactive or inactive aggregates.

The aggregation phenomenon can be minimized when bulky cations or additives are used. The polymerization of D3 initiated by trimethylammonium salts shows a first-order kinetic [545]. The rate of polymerization is directly related to the size of the counterion and increases in the series: Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺ ∼ ⁺NR₄ ∼ ⁺PR₄ [546]. Hexamethylphosphorous triamide, dimethylsulfoxide, dimethylformamide, N-methylpyrrolidone or cryptands, and crown ethers were shown to act as deaggregating agents [547–553]. Ring strain affects also the polymerization rate constants with the following order: D3 > > D4 > D5 > D6. Cyclotrisiloxanes show a remarkable high reactivity thanks to its ring strain and planar conformation [554]. Nevertheless, unexpected enhanced reactivity was observed with unstrained cyclodimethylsiloxanes in the order D4 < D5 < D6 < D7 < D8 when alkali metal were used in bulk or nonpolar solvents [533, 555, 556]. Multidentate interactions of siloxane units of the monomer with the counterion can explain this observation (Scheme 54). Lithium derivatives such as silazane lithium salts ((RMe₂Si)₂NLi), in the presence of promoters such as DMSO, were shown to initiate the AROP of D4 at elevated temperatures in high yields. The resulting polymers exhibited relatively narrow distribution which broaden gradually with time [557]. Propagation in this system is faster than the redistribution reactions, which lead to equilibration.

Organic initiators were more recently proposed for the AROP of cyclosiloxanes. Phosphazene bases, i.e., t-BuP₄, acts as a deprotonating agent of a proton donor molecule such as an alcohol, leading to the formation in that case of an alkoxide of phosphazenium. Its bulkiness and stabilized positive charge, thanks to the resonance effect, enable an instantaneous polymerization of D4 [101, 558]. Similarly to the chemistry developed with cyclic ether monomers, the combination of lithium and phosphazene bases is also very efficient for the polymerization of cyclic siloxanes [559]. Within the same family, the direct use of amino-substituted oligophosphazenium hydroxides (P₅OH) enables to get polydimethylsiloxane in toluene with a first order both in monomer and base, and a faster rate than lithium cryptate systems [549, 560, 561]. Alcohols deprotonated by phosphorus ylides [562] were also proposed as initiators of D4 with the particularity to be thermolabile, facilitating its removal from the final polymer (Scheme 55). N-heterocyclic carbenes expressed also some interests thanks to the presence of alcohols as co-initiator and regulator of chain length [563].

Although polysiloxanes are not ordinarily considered stereoregular, some polymers enriched in stereoregularity are made from the cis-isomers of unsymmetrically substituted strained cyclosiloxanes. The monomers insert randomly at the reactive chain ends with equal probabilities of forming meso or racemic siloxane links while preserving the stereoconfiguration of the original monomer [564]. An advantage of stereoregularity was shown on the mechanical properties of a silicone elastomer [565].

5.2.2 AROP in Solid State and Emulsion

These two processes can be used both in anionic and cationic ROP of cyclosiloxanes. The discussion will focus on the parameters and consequences of the anionic route.

A simple approach based on crushed potassium hydroxide or potassium silanolates added onto a cyclosiloxane gives high molar mass polymers with high dispersity and high yields [566–569]. Polymerization proceeds inward from the surface of the monomer crystals, producing a highly crystalline material. The highly ordered crystalline state of hydroxycyclosiloxanes provides a possibility of solid-state synthesis of stereoregular polysiloxanes.

Polymerization in emulsion is also proposed to conduct anionic polymerization of cyclosiloxanes [532]. The synthesis of poly(dimethylsiloxane) from D4 in aqueous emulsion using an emulsifying agent acting also as initiator (benzyldimethyldodecylammonium hydroxide) gives controlled molar mass, a low dispersity and high yields [570, 571]. The amount of cyclics formed (essentially D4–D7) is lower than that observed in bulk. Polymerization proceeds by a combination of the addition and condensation mechanism involving redistribution reactions. The first stage of the anionic polymerization process occurs at the siloxane-water interface or in the siloxane phase close to the surface. Once the chains reach a critical degree of polymerization corresponding to their loss of surface tension activity, they penetrate into the particles where side reactions such as redistribution and condensation occur. The rate is strongly dependent on the size of the surface, which is function of the concentration of emulsifier. Polycondensation is responsible for a rapid increase in molar mass observed at high monomer conversions. Another α,ω-dihydroxy-terminated polysiloxane of low molar mass, issued from the polymerization of 2,4,6-trimethyl-2,4,6-tris(3,3,3-trifluoropropyl)cyclotrisiloxane with an anionic miniemulsion process, was also obtained [572]. The kinetic study showed that polymerization occurs in two stages. During the first stage, which corresponds to the nonequilibrium AROP, the maximum yield is close to 100 %, and the dispersity remains narrow (1.3). The second stage involves condensation and back-biting reactions leading to an increase of both molar masses, up to 60,000/mol, and dispersity (2.0). This approach was developed for other homopolymers [573] and copolymers [574].

5.2.3 Copolymerization and Functionalization

Anionic ring-opening polymerization offers possibilities in the controlled synthesis of functionalized polysiloxane polymers and copolymers. Functionalized initiators and terminators are currently used in nonequilibrium polymerization to introduce functional groups to one or both ends [548, 575–581]. The AROP allows the synthesis of block copolymers [550, 582–588], graft copolymers [589, 590], star polymers [544, 548, 591, 592], and polymeric networks [577, 581]. Alternating copolysiloxanes were also prepared by a regioselective polymerization of cyclosiloxanes containing different siloxane units. It depends strongly on the nature of counterion [593, 594]. Simultaneous polymerization of a mixture of cyclosiloxanes gives polymers with a composition depending on Mayo-Lewis reactivity ratios only when the propagation reactions are irreversible. Gradient copolysiloxanes can be obtained starting from cyclotrisiloxane monomers [595]. Equilibrium copolymerization of cyclotetrasiloxanes leads to random structures [596, 597]. As usual, copolymers aim at broadening the scope of properties and applications. For instance, the introduction of methylphenyl or diphenylsiloxane units to PDMS helps to improve thermal, oxidation, or radiation stability, whereas fluoroalkyl groups enhance their resistance to fuel and oils.

5.3 Other Cyclic Organosilicon Monomers

5.3.1 Silsesquioxanes, Cyclic Carbosiloxanes, and Cyclic Silaethers

These three monomers are very similar to the cyclosiloxanes family as they can be polymerized anionically by breaking a siloxane bond.

Silsesquioxanes, of empirical formula RSiO_3/2, represent a wide class of more or less ordered three-dimensional structures (Scheme 56). They are intermediate structures between siloxanes (O/Si = 1) and silica (O/Si = 2). They are usually generated by hydrolytic condensation of trialkoxy- or trichlorosilanes. Numbers of reviews may give additional and detailed information about this compound and its properties and applications [598–601]. As an example, the anionic ring-opening copolymerization of D4 with polyhedral oligomeric silsesquioxanes (POSS) derivatives leads to cross-linked polysiloxanes exhibiting good thermal stability [602, 603].

Poly(carbosiloxane)s are obtained from high ring strain cyclic monomers, i.e., 1-oxa-2,5-disilacyclopentanes, having both carbosilane and silyloxy linkages (Scheme 57). Lithium or sodium silanolates were shown to initiate the polymerization in the presence of a polar solvating agent such as THF or dioxane to avoid aggregation of active centers [543, 604–606]. Strongly basic N-heterocyclic carbenes and guanidine derivatives in the presence of alcohols or other hydrogen bond donors were shown to allow the synthesis of poly(carbosiloxane)s with controlled molar masses [607] and also to cyclic poly(carbosiloxane)s in the absence of alcohol [608]. Monomers bearing a chiral center could be synthesized and led to optically active polymers by AROP [609, 610].

The anionic polymerization of cyclic silaethers, or oxysilylenes, enables the cleavage of both the Si–Si and Si-O bonds, and lead to a polysilaether with an irregular structure and, at equilibrium, a mixture of polysiloxanes and polysilanes by rearrangement (Scheme 58) [611–613]. A silyl anion, as compared to a silanolate one, is a nucleophile able to initiate the polymerization of an ethylenic monomer. It was used for instance to change an alkoxide into a carbanion active center [613].

5.3.2 Cyclosilanes

Strained cyclosilanes were shown to ring-open anionically yielding high molar masses polysilanes (Scheme 59). Initiators such as butyllithium, silylpotassium, or lithium silyl cuprates were used with cyclotetrasilane [614–617] bearing methyl and/or phenyl groups. Diblock polystyrene-polysilane copolymers exhibiting a phase separation could be prepared using polystyryllithium to initiate the ROP of 1,2,3,4-tetramethyl-1,2,3,4-tetraphenylcyclotetrasilane in the presence of 12-crown-4 to enhance the polymerization [618]. Tetrabutylammonium fluoride and silyl potassium appeared efficient initiators for nonamethyl(phenyl)cyclopentasilane [619]. The strong affinity of fluoride anion to Si atom promoted the generation of silyl anion without any additives. The potassium initiator required the use of hexamethylphosphoramide or crown ethers promoters capable to solvate the potassium cation in order to enhance the reactivity of the silyl anion. Low temperature was needed (−78 °C) to reach high polymer yield (80 %), as well as quenching to prevent the back-biting reaction when temperature increases. Such a polymer is a kinetic product and cyclic oligosilanes are thermodynamically more stable.

5.3.3 Cyclocarbosilanes

Polycarbosilanes are attractive materials as they contain only Si-C bonds in the backbone making them of interest as silicon carbide precursors used for the preparation of ceramic fibers. The anionic route offers an attractive way to ring-open strained silacyclobutane monomers using organolithium as initiators (Scheme 60) [620–625]. The polymerization yields high molar mass poly(silanediylmethylene)s with a strictly alternating SiR₂/CH₂ backbone structure. Depending on substituents in the ring and on the initiator, polymerizations may proceed in a controlled and living manner [626, 627]. Optically active polymers [628] as well as block copolymers based on silacyclobutane [629, 630] were also described. As cyclic silaethers, silacyclobutane may be used to transform a weak nucleophilic center into a more nucleophilic one. This makes possible the copolymerization of heterocyclics with vinylic monomers [631].

5.3.4 Cyclosilazanes

Despite the high reactivity with water, oxygen, etc., of Si-N bonds present in polysilazanes, obtained by ROP of cyclosilazanes (Scheme 61), these materials gained interest as precursors of Si-N and Si-CN ceramics through pyrolysis. Organolithium and organosodium are the typical initiators used in AROP leading to high molar masses in a living manner [632, 633]. The polymerization is kinetically controlled by the ring strain and by the steric hindrance around the nitrogen atom and/or the electronic effects of the R substituent on the Si-N bond [634, 635]. As a possible example, a pendant double bond could be introduced into a polystyrene-polysilazane block copolymer using 1,1,3,N,N’-pentamethyl-3-vinylcyclodisilazane as co-monomer added to living polystyryllithium [636]. Such a copolymer enabled the formation of cross-linked micelles and ceramic nanoparticles after pyrolysis.

5.3.5 Ferrocenylsilanes

Polyferrocenylsilanes (PFS) and polyferrocenylsilane block copolymers, where iron and silicon are present in the main chain, are obtained from AROP of strained ferrocenylsilanes. The first report of living carbanionic ROP appeared in 1994, and this process permitted the synthesis of PFS with predictable molar masses and narrow dispersity [637]. The mechanism is based on a Cp-Si bond cleavage in the presence of lithium-based initiators (Scheme 62). Reviews published by Rider and Manners [638] and Bellas and Rehahn [639] propose details in their preparation as well as other polymerization routes or self-assembly toward nanostructured materials.

6 Cyclic Carbonates

6.1 Introduction

The polymerization of aliphatic or aromatic cyclic carbonates was highly investigated and recently reviewed [640–644]. Indeed, due to transparency, good heat resistance (up to 130 °C), high toughness, and excellent dimensional stability, polycarbonates (PC) are used in a broad range of applications like elastomers, sealants, foams, coatings, adhesives, etc. Aliphatic polycarbonates and copolycarbonates are also valuable biomaterials thanks to their biocompatibility and biodegradability.

6.2 5-Membered Cyclic Carbonates

The polymerization of 5-membered cyclic carbonates follows a peculiar behavior as their ceiling temperatures are below 25 °C. As a consequence, no ROP should be possible to yield poly(alkylene carbonate). Nevertheless, they can be polymerized at high temperatures (above 150 °C) resulting in poly(ether-carbonate)s (path A, Scheme 63), the repeating units being a mixture of alkylene carbonate (content generally lower than 50 mol%) and the corresponding alkylene oxide units coming from decarboxylation reactions during the polymerization with organometallics [645, 646], metal alkoxide [647–651], or organic initiating systems [96, 652]. Rokicki developed also the synthesis of poly(ether-carbonate)s through the combination of AROP of 5-membered cyclic carbonates initiated by bisphenolates leading to reactive difunctional species and their coupling reactions with dihalo compounds (path B, Scheme 63) [653, 654].

Detailed mechanistic studies of the polymerization of ethylene carbonate (EC) with KOH as the initiator performed at 150–200 °C in bulk suggested that, in the early stage of the polymerization, a major polymer structure comprises one EC unit per two ethylene oxide (EO) units (the content of EC units, even in the earliest stage of the reaction, was not higher than 32 mol-%). For longer reaction times, the content of EO units increases, through hydrolysis of the carbonate units [650]. Polymerization proceeded thus in two stages: during the first stage, EC conversion took place with an increase of molar masses, while in the second stage, when EC was completely consumed, a decrease of both the number of EC units and molar mass was noticed, indicating the occurrence of chain cleavage and decarboxylation reactions. During propagation, the alkoxide propagating species can attack the carbon atom of the carbonyl group. In this case, the reaction is reversible, but the new alkoxide is not able to attack again on the carbon atom of the carbonyl group of another EC monomer as it would yield an EC-EC sequence which is thermodynamically not possible. An alkoxide species can also attack the carbon atom of a methylene group, followed by decarboxylation and irreversible formation of an ethylene oxide unit (Scheme 64). The most probable EC polymerization mechanism should thus be a combination of methylene and carbonyl carbon attack. Finally, after total monomer consumption, elimination reactions were detected, yielding vinyl end groups. Similar results were observed with other initiating systems for the AROP of EC. With butyllithium, the resulting polymers contained only 10 mol-% of carbonated units [645], and with potassium methoxide [647] and phosphazene [96], 28 and 20–25 mol-% of EC units were conserved, respectively. The AROP of propylenecarbonate yielded also poly(propylene carbonate-co-propylene oxide) copolymers whatever the initiating system [648, 649, 652].

Rokicki took advantage of these side decarboxylation reactions in order to produce hyperbranched aliphatic polyether through the AROP of glycerol carbonate (1, Scheme 65) conducted at 170 °C using trimethylolpropane/potassium methanolate as the initiating system [651]. Attempts to polymerize aromatic five-membered cyclic carbonate with sec-BuLi and potassium dihydronaphthalide revealed unsuccessful [655]. In contrast to other five-membered cyclic carbonates, five-membered cyclic carbonates obtained from methyl 4,6-O-benzylidene-glucopyranoside (2, Scheme 65) can be polymerized at relatively low temperatures (<60 °C) with alkali metal alkoxides or organic initiator, without elimination of carbon dioxide, to produce polycarbonates consisting exclusively of carbonate repeating units [656–658]. Such a behavior was suggested to be due to the ring strain which may result from the connection of two hydroxyl groups in E (trans) position by the carbonate linkage.

6.3 6-Membered Cyclic Carbonates

In contrast to thermodynamically unfavorable 5-membered cyclic carbonates, 6-membered cyclic carbonates easily polymerize with anionic initiators to afford PCs without ether sequences and generally high molar masses [659–663]. The AROP of trimethylene carbonate (TMC) was first reported in the 1930s using K₂CO₃ [664, 665]. Since then, many other initiators were able to polymerize 6-membered cyclic carbonates (Scheme 66), like butyllithium [666–672], alkali metal alkoxides [667, 672–676], naphthalene potassium [667], sodium hydride [673], and pure organic nucleophiles [379, 673, 677–688].

An important feature of AROP of 6-membered cyclic carbonates is its equilibrium character. Indeed, polymerization did not go to completion with the presence of residual monomer. Nevertheless, this drawback could be taken as an advantage as it allows polycarbonates recycling. It was shown that the monomer substitution had a strong effect on the equilibrium monomer concentration. For example, the AROP of TMC, 2,2-dimethyl trimethylene carbonate (DTC) and CC1–3 (Scheme 66) in THF solution using potassium tert-butoxide as the initiator exhibited an increasing monomer concentration at equilibrium, with an increasing bulkiness of the substituents, CC3 monomer being almost not polymerized [671, 675]. It was assessed that the decrease in polymerizability of the 6-membered cyclic carbonates with increasing bulkiness of the substituents was due to the conformational distortion of the polymer backbone, rather than in the change of conformation of the monomer caused by the substituents [641]. Several other parameters may also influence the polymerization rate. For example, the polymerization of DTC in toluene with lithium as a counterion was slower than that with potassium one due to the covalent character of the lithium-oxygen bond compared with the potassium-oxygen bond leading to a lower nucleophilicity of the lithium alkoxide. In the case of monomer CC4, the back-biting reaction was restricted due to the stiffness of the polymeric chain [670].

The AROP of 6-membered cyclic carbonates presents transesterification reactions, besides initiation and propagation reactions (Scheme 67). The initiation reaction comprises the nucleophilic attack of the initiator on the carbonyl carbon atom, followed by an acyl-oxygen cleavage and formation of the active species, an alkoxide. Peculiar initiation behaviors were also observed. When the ROP was initiated by naphthalene potassium, this latter did not act as an electron-transfer reagent (e.g., like for styrene polymerization) but as a nucleophile, naphthalene being incorporated in the polymeric chain [667]. Intramolecular nucleophilic attacks on carbonyl carbon atom (back-biting) lead to cyclic oligomers, while intermolecular transesterification leads to a change of the macromolecule length. As a consequence, the control of the polymerization was poor, and bimodal distribution of molar masses was generally observed.

Instead of using metallic initiators, it is possible to use organic ones. Murayama et al. were the first to show that tertiary amines such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), 1,4-diazabicyclo[2.2.2]octane (DABCO), and 4-dimethylaminopyridine (DMAP) (Scheme 68) were able to achieve the AROP of a 6-membered cyclic carbonate (CC4, Scheme 66) in bulk at 120 °C, whereas no polymer was obtained with triethylamine, aniline, N,N-dimethylaniline, or pyridine [679]. It was suggested a zwitterionic polymerization mechanism, which was confirmed by mass spectrum analysis of the products. Tapered copolymers were also obtained when monomer CC4 was simultaneously polymerized with glycidyl naphthyl ether.

The ROP of TMC was also performed with N-heterocyclic carbenes, guanidine, and amidine bases in bulk at 65 °C with a good control, yielding well-defined polycarbonates with molar masses up to 50,000 g/mol, dispersity index below 1.08, and high end-group fidelity [680]. Similarly, the tertiary amine 2-(dimethylamino) ethanol (DMAE) was used as an efficient initiator/catalyst for the ROP of TMC in bulk at 50 °C leading to α,ω-heterotelechelic PTMC [682]. In this case, the mechanism could be either an activated monomer or an activated chain-end one. Phosphazenes revealed also efficient deprotonating agents of alcohols for the polymerization of TMC [688, 689]. Recently, several carbenes were used for the controlled polymerization of DTC [379, 685].

Some examples of initiator-free polymerization of cyclic carbonates were also described in the literature, assuming an anionic mechanism [690–693]. TMC can undergo spontaneous polymerization in bulk above 100 °C, with the formation of a zwitterion intermediate with a well-stabilized trialkoxycarbenium ion and on alkoxide (Scheme 69), whereas DTC cannot [691]. Initiator-free polymerizations were also observed for the thermal ROP of 5-benzyloxy-trimethylene carbonate (BTMC) in bulk at 150 °C or the microwave-assisted ROP of TMC. Molar masses were generally high.

In spite of some side reactions, the AROP of functional cyclic carbonates remains the preferable way to prepare functional polycarbonates. Several pathways permit functional monomer synthesis: from 2,2-bis(hydroxymethyl)propionic acid, glycerol, or alkyl malonates [640, 641, 643]. A number of functionalized PCs and copolycarbonates can be obtained by direct polymerization of cyclic monomers bearing functional groups. Functional side-chain groups introduced into PCs are carboxylic group and their derivatives, hydroxyl, allyl, acrylate, methacrylate, styrene, and stilbene derivatives, and even five-membered cyclic carbonates (Scheme 70).

Polycarbonates with carboxylic side groups could be synthesized through the ROP of CC5-type monomers (Scheme 70) with DBU [684] or sec-butyllithium [694] at room temperature in solution. With sec-butyllithium, bimodal distribution of the molar masses was observed, whereas the polymers exhibited low dispersity with DBU. Aliphatic amines with different chain lengths were easily conjugated onto the polymer backbone in order to form nanoparticles [684].

6-Membered cyclic carbonate bearing free hydroxyl group attached to the ring via aliphatic spacer (CC6, Scheme 70) was polymerized with DBU in bulk or in solution at temperatures ranging from 60 to 110 °C to yield hyperbranched polycarbonates composed of carbonate and glycerol units [683]. The linear equivalent of poly(CC6) was obtained through the polymerization of CC7 followed by free radical addition of mercaptoethanol to the pendent allyl groups. Attempts to polymerize CC8 with sec-butyllithium in solution resulted in a mixture of polymers, cyclic oligomers, and unreacted monomer [668, 669]. The hydroxyl function of monomer CC9 was first protected by reaction with trimethylsilyl chloride and benzyl chloroformate of phenyl isocyanate and then polymerized with lithium alkoxide in solution at low temperature to yield bimodal distributions or even cross-linking [674]. After deprotection, polycarbonates with one hydroxyl group per repeating units were obtained. Amino acid functionalized polycarbonates were also synthesized through the ROP of CC10-type monomers with alkali metal alkoxides or n-butyllithium in solution at low temperature followed by deprotection [672]. Monomodal distributions were obtained, and the configuration of the monomer was inverted during the polymerization.

Bifunctional cyclic carbonate consisting of both 5- and 6-membered rings (CC11, Scheme 70) was polymerized with DBU at 60 °C in solution to afford a polycarbonate with remaining 5-membered cyclic carbonate group in the side-chain, as this latter did not polymerize in these conditions [677]. At such elevated temperature, conversion stopped around 50 % due to the equilibrium nature of the polymerization.

Styrene side groups were also introduced onto polycarbonates through the polymerization of monomer CC12 (Scheme 70) with potassium tert-butoxide as the initiator in THF at 0 °C [676]. Subsequent radical cross-linking of styrenic groups and anionic de-cross-linking of the carbonate units was performed. Aromatic cyclic carbonate CC13 (Scheme 70) was polymerized by sec-butyllithium or dihydronaphthalene potassium, but it was evidenced the presence of decarboxylation reactions to a great extent [655].

Macroinitiators such as polymeric Li, Na, and K alkoxides can also be used for the initiation of the 6-membered cyclic carbonate polymerization. Thus, living vinyl polymers [695], hydroxyl group-terminated polymers of poly(tetrahydrofuran) (PTHF) [696, 697], poly(ethylene oxide) (PEO) [697–699] and poly(dimethylsiloxane) (PDMS) [697, 700] were transformed to alkoxides by treatment with sec-BuLi or K-naphthalene and used as initiators for AROP of DTC allowing the synthesis of di- and triblocks copolymers. The polymerization initiated by PTHF alkoxides with different counterions was slower than that initiated by PEO alkoxides, because of the lower solvation ability of PTHF. It was also shown that the polymerization rate was highly dependent on the counterion, potassium alkoxides being more reactive than lithium alkoxides. Besides, higher molar mass PDMS macroinitiators exhibited lower polymerization rate. In the same vein, living poly(methyl methacrylate) (PMMA) prepared by Group Transfer Polymerization (GTP) was used as a macroinitiator for the ROP of DTC after transformation of the silyl ketene acetal into an alkoxide [701]. PMMA-b-PDTC block copolymers were thus obtained.

The simultaneous or sequential polymerization of DTC with several cyclic esters or other cyclic carbonates (CC4, CC5, and CC8, Scheme 70), initiated with butyllithium, potassium dihydronaphthalene, or organic initiators, was performed in solution or in bulk [379, 668–670, 694, 702–704]. With ε-caprolactone (CL), tapered copolymers were obtained as DTC was more reactive than CL. Triblock copolymers with tapered DTC/CL outer blocks could also be obtained using macroinitiators (PEO or PTHF based) [699]. With pivalolactone, only block copolymers were synthesized; DTC was first reacted by alkoxide active species, followed by the reaction of pivalolactone through carboxylate active species. With the other cyclic carbonates, statistical or block copolymers were obtained from simultaneous or sequential polymerization, respectively.

6-Membered cyclic carbonates were also copolymerized with oxiranes [678, 705] and anhydride [706]. DTC was copolymerized with glycidyl phenyl ether (GPE) with DBU in bulk at 90 °C. An acceleration of GPE polymerization was observed, and quantitative yields were obtained. PGPE-b-PTMC copolymers were also successfully synthesized through the sequential polymerization of GPE and TMC with tetrabutylammonium fluoride. Attempts to copolymerize TMC and adipic anhydride with sec-butyllithium in several conditions revealed unfruitful as mixture of homopolymers were detected [706].

It was demonstrated that cyclic monothiocarbonate [707] and thiocarbonate [708, 709] (Scheme 71) could be polymerized by AROP. For the cyclic monothiocarbonate, potassium tert-butoxide revealed a good initiator yielding a polymer that precipitates during the course of the polymerization. It was shown that the propagating species was not an alkoxide but a thiolate as the monomer ring-opens exclusively through the carbonyl sulfur bond cleavage. Thiocarbonate was polymerized by n-butyllithium or potassium alkoxides in solution at room temperature or by DBU in bulk or in solution at 120 °C. Polymerization was pretty slow and proceeded with an isomerization of the thiocarbonate group.

6.4 Larger-Ring Cyclic Carbonates

The ROP of 7-membered cyclic carbonate (tetramethylene carbonate, TeMC, Scheme 72) is generally faster than that of the six-membered one due to relatively high ring strain. However, the polymerization of 7-membered cyclic carbonates was scarcely investigated because of the difficulty to synthesize the monomers. Indeed, TeMC is thermally unstable and difficult to isolate and purify. The polymerization of TeMC initiated with sec-butyllithium was carried out in THF to yield the corresponding polycarbonate in a relatively high yield in a short time [710]. Like for 6-membered cyclic carbonates, an important residual monomer concentration was observed with the formation of cyclic oligomers via back-biting reaction, which is characteristic for equilibrium polymerizations. However, the relative polymerization rate of TeMC is about 35 times faster than that of TMC.

Another 7-membered cyclic carbonate (β-Me7CC, Scheme 72) was polymerized in bulk at elevated temperature only (100 °C) with organic compounds (DMAP, phosphazene) with very good yields [711]. No regioselectivity was observed during ring-opening of the monomer.

The ROP of large-ring aromatic cyclic carbonates was also studied. It was shown that the polymerization of monomer CC14 with alkoxide or alkyllithium [712] or CC15 with sec-butyllithium or dihydronaphthalene lithium [713] failed, but CC15 was easily polymerized by dihydronaphthalene potassium or potassium tert-butoxide. Monomer CC16 was easily polymerized with potassium tert-butoxide in THF at room temperature to afford the corresponding polycarbonate in high yield [714]. Cyclic oligomeric carbonates of bisphenol A (CC17, Scheme 72) were polymerized by potassium dihydronaphthalene in THF [713] or in bulk at 250 °C [715]. The polymerization and copolymerization of cyclo bis(hexamethylene carbonate) and its fluorinated analog (CC18 and CC19, Scheme 72) were also successfully performed using sec-butyllithium in toluene [716].

7 Cycloalkanes, Cyclic Sulfides and Amines, Cyclic Ureas, Depsipeptides, and Cyclic Phosphorous Monomers

7.1 Introduction

The successful synthesis of polymers and copolymers issued from cyclic ethers, esters, lactams, carbonates, or siloxanes through anionic ring-opening polymerization triggered researches in cyclic monomers containing, or not, other heteroatoms or combination of several heteroatoms. New properties were expected for novel uses.

7.2 Cycloalkanes

Cycloalkanes are expected to polymerize by breaking a carbon-carbon single bond of a monomer ring. Such a bond does not generally react with free radicals and rarely participate in reactions with electrophiles and nucleophiles [717]. In addition, as the two atoms making the bond are identical, no polarization is introduced into the monomer, making ionic reactions with nucleophiles or electrophiles difficult. Reactivity can only be expected when monomer substituents are introduced on at least one of the two carbons, thereby increasing the bond polarity and introducing some zwitterionic nature into the bond, or when the overlap of atomic orbitals in the carbon-carbon bond is disturbed by geometric parameters, particularly observed with highly strained polycyclic systems [717, 718]. Cycloalkanes polymerized by anionic ring-opening polymerization are composed of functionalized cyclopropanes, cyclobutanes, and polycyclic molecules with high intrinsic polymerizabilities (e.g., bicycloalkanes and propellanes). Detailed reviews may give additional information in anionic polymerization as well as other polymerization methods used [719–723].

The AROP of cyclopropanes activated by various substituents (Scheme 73) was effective using mainly alkali metal derivatives as initiators. Two electron-withdrawing substituents on the same carbon are often needed for the polymerization to be efficient but still drastically less reactive than the corresponding vinyl monomers [724]. Sodium thiophenolate is shown to initiate the polymerization of cyclopropane-1,1-dicarboxylates with a living character in some cases [725–728]. Phosphazenium thiophenol or bisthiols were also proposed for the successful AROP of di-n-propyl cyclopropane-1,1-dicarboxylate [729, 730]. Well-defined monofunctional or difunctional polymers with a low dispersity were obtained through a living process in THF between 30 and 60 °C or in toluene between 30 and 100 °C. A much higher reactivity is noticed as compared to the alkali metal thiophenolate initiator used in DMSO at higher temperature. The polymerization of cyclopropanes bearing cyano [724, 731] or fluorine [732] groups initiated with sodium thiophenolate or fluorenyl lithium, respectively, was also observed. Sodium cyanide was particularly effective as anionic initiator of various trisubstituted cyclopropanes in DMF [733–737].

Cyclobutanes exhibit a much lower tendency to anionically ring-open as compared to cyclopropanes and as also observed for heterocyclic rings. Reasonable evidences for an AROP were only reported in highly activated monomers. The polymerizations of cyclobutanes substituted by nitrile groups on one or two carbons, and further substituted by an ether group on a neighboring carbon, are the most efficient (Scheme 74) [738, 739].

Activated bicyclobutanes [740–745] or other bicycloalkanes [746] and [1.1.1]propellanes [747–749] were observed to give oligomers or polymers using conventional anionic initiators, i.e., alkyllithium (Scheme 75).

7.3 Cyclic Sulfides and Amines

The anionic route is proposed, in addition to cationic and coordinative ones [750], as an efficient approach for the polymerization of cyclic sulfides (Scheme 76), in particular for thiirane (ethylene sulfide) and various substituted thiiranes. Thiolates are commonly used to attack the monomer, proceeding exclusively at the methylene carbon and leading to pure head-to-tail structures [751–753]. Naphthylsodium was found to act as a bifunctional initiator and to give a living character to the polymerization. The initiation reaction was proposed to consist of a desulfurization process producing ion radicals that combine to form dithiolates [754]. In a similar way to epoxide polymerization, Inoue experimented with success (N-methyl-5,10,15,20-tetraphenylporphinato)zinc propanethiolate as initiator of propylene sulfide (or methylthiirane MT) [755].

Organic initiators were also proposed. Tertiary amines such as 1,4-diazabicyclo[2.2.2]octane (DABCO) enable the polymerization of ethylene sulfide to high molar mass polymers through a zwitterionic mechanism [756]. In contrast, MT was only slowly polymerized by polyamines to give low molar mass materials. Nicol et al. described the living polymerization of MT initiated by various mono- and dithiolates [757]. The deprotonation of the thiols was carried out by addition of a strict stoichiometric amount of a bulky strong organic base such as 1,8-diazabicyclo[5,4,0]undec7-ene (DBU) (Scheme 77).

Advantages of thiols are based on low pKa values ranging from 7 to 11 in water, excluding substantially deactivation due to protonation in environments with pH ≥10. That allows a living character of the polymerization at not excessively basic pH and under non-anhydrous conditions, which is different from the structurally similar epoxide polymerization. It is possible to polymerize hydroxyl-containing monomers such as hydroxymethyl thiirane in a living manner [758] or to work in emulsion in water with restrictions, such as limited conversions producing polymers with molar masses lower than predicted ones due to physical reasons and not chemical [759]. On the other hand, the use of thiols is often complicated by the presence of disulfide impurities coming from oxidation of the initiating thiol, which results in transfer reactions [760]. Protected thiols which are deprotected right before polymerization may be proposed. Examples are the use of thioester, which is transformed into thiolate by the addition of sodium methanolate [761, 762] and the ring-opening of cyclic dithiocarbonates by an amine [763].

Thanks to the livingness of the polymerization of thiiranes, di- and triblock copolymers were prepared, marrying mainly polythiirane or poly(methylthiirane) (PMT) with polystyrene and derivatives, poly(methyl methacrylate), polyethers, and polydienes [762, 764–770]. A macromonomer approach was also helpful to obtain comb-like polymers with polythiirane main chains and various side chains [771–774]. The synthesis of star-shaped PMT by polymerization with tri- and tetrathiol initiators was also investigated [761–763, 775, 776].

The AROP of the four-membered rings family of cyclic sulfides (Scheme 76) is much more limited. Thiethane and 3,3-dimethylthietane were polymerized to high molar mass polymers by initiation with naphthylsodium or butyllithium [777–780]. The polymerizations were shown to occur with carbanions as active species instead of thiolates. The sulfur atom is attacked due to severe bond angle distortions forced upon the atom by the geometry of the molecule.

The conversion of cyclic amines into linear polyamines is much more limited by AROP, despite potential utilities, e.g., ion exchange chromatography or biomedicine [781, 782]. The polymerization of sulfonylaziridines was only effective in the presence of amide initiator, generated by the deprotonation of a primary sulfonamide (N-benzyl methanesulfonamide) by potassium bis(trimethylsilyl)amide (KHMDS), leading to polymers with a low dispersity (Scheme 78) [783].

7.4 Cyclic Ureas and Depsipeptides

The ROP of cyclic ureas has attracted only minor interest. Dimethylene urea and trimethylene urea can be successfully ring-open using sodium hydride as initiator leading to polyureas [784]. But the synthesis of polyurethanes, starting from tetramethylene urea (TeU) and cyclic carbonates, was particularly investigated by Keul and Höcker [785–788]. It was shown that first the cyclic carbonate polymerizes, and then TeU is formally inserted into the polycarbonate chain after deprotonation of the amine by dibutylmagnesium (Bu₂Mg) (Scheme 79).

Using γ-butyrolactone (γ-BL) instead of cyclic carbonates in the presence of Bu₂Mg, alternating poly(amide urethane)s were achieved [789]. The homopolymerizations of TeU and γ-BL were not observed. TeU reacts initially with Bu₂Mg to form the salt in which the nucleophilicity of the nitrogen is enhanced and the reaction between activated TeU and γ-BL is made possible. Ring-opening leads to the AB monomer. It is followed by the nucleophilic attack of the alkoxide at the endocyclic carbonyl carbon atom, resulting in polymer after ring-opening (Scheme 80).

Polydepsipeptides, alternated copolymers of α-hydroxy acids and α-amino acids, belong to the poly(ester-amide) family and are interesting for their degradable character. Ring-opening polymerization of morpholine-2,5-dione (MD) and its derivatives (Scheme 81), in the presence of stannous catalyst, is the main way to obtain such polymers [790]. Detailed information can be found in the review of Dijkstra [791]. AROP using potassium alkoxides was also applied to provide polymers with limited conversions and molar masses [792]. The lack of control may be explained by the presence of a proton on the amine. Block copolymers such as polymorpholine-2,5-dione-b-polylactide were nevertheless prepared by a two-step procedure for surfactants applications [793, 794].

7.5 Cyclic Phosphorus Monomers

Cyclic phosphorus monomer family gives rise to polymers of interest in particular in biomedical field, due to biocompatibility, biodegradability, and structural similarities to naturally occurring nucleic acids, or in flame retardant applications. Lapienis reviewed recently all ring-opening polymerizations leading to polymers containing phosphorus atoms [795]. Only few monomers are polymerized by an anionic route. Poly(phosphate esters) can be prepared from cyclic phosphoesters (Scheme 82) either with alkali metal initiators [796–800] or organic initiators (tertiary amines, TBD, or DBU) [801–805]. 5-Membered phosphoesters were by far the most studied monomers. The presence of a substituent on the ring decreases the polymerizability of the monomer as high polymerization temperatures are needed to get only oligomers. 6-Membered phosphoesters are also difficult to polymerize [799]. Fluorosubstituted phosphoesters (Scheme 82) can undergo AROP with KOH, butyllithium, or triethylamine, in bulk at 220–270 °C, giving rubber-like polymers, the molar mass being highly dependent on the initiator [800]. The presence of polar agents (e.g., THF, diethyl ether, and dimethylformamide (DMF)) considerably lowers the required polymerization temperature (100 °C). The polymerization presents a living character with organic initiators, enabling the formation of random and block copolymers [802, 803].

5-, 6-, 7-, and 8-Membered cyclic phosphonates (Scheme 83) can also undergo anionic polymerization at high temperatures [806]. Very recently, it was shown that DBU can perform the synthesis of poly(ethylene methylphosphonate) with an excellent control with molar masses up to 20,000 g/mol [807]. The polymerization of cyclic [808] or bicyclic [809] H-phosphonate was also performed with an alkyllithium. As an example, the polymerization of 2-hydro-2-oxo-1,3,2-dioxaphosphorinane was achieved in bulk or in dichloromethane solution, initiated by n-BuLi, but also EtONa and t-BuOK, at 25–45 °C, to give a high molar mass polymer (route a, Scheme 84) [808]. Some trivalent phosphorus cyclic compounds were also polymerized by AROP with potassium or cesium alkoxides (no polymerization occurred with lithium or sodium) or potassium trimethylsilanolate (route b, Scheme 84) [810–812]. After acetolysis, poly(2-diethylamino-1,3,2-dioxaphosphorinane) gave the same polymer as the one obtained from 2-hydro-2-oxo-1,3,2-dioxaphosphorinane, which can be easily converted to polyacid after oxidation.

8 Conclusion

A large variety of cyclic monomers can be polymerized by anionic ring-opening polymerization. In spite of required rigorous experimental procedures as compared to some other polymerization routes, some industries and academic researchers used to play with such chemistry and already take advantages of some polymeric structures prepared by AROP. Indeed, thanks to the recent progress in the control of the polymerizations, functionalized polymers and copolymers (block in particular) are now available, broadening the scope of properties and thus opening many perspectives in various applications.

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Authors and Affiliations

University of Bordeaux, LCPO, UMR 5629, Pessac, F-33600, France
Stéphane Carlotti & Frédéric Peruch
CNRS, LCPO, UMR 5629, Pessac, F-33600, France
Stéphane Carlotti & Frédéric Peruch

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Stéphane Carlotti
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Frédéric Peruch
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Correspondence to Stéphane Carlotti .

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King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
Nikos Hadjichristidis
Tokyo Institute of Technology, Tokyo, Japan
Akira Hirao

Abbreviations

18C6:: 1,4,7,10,13,16-Hexaoxacyclooctadecane (18-crown-6 ether)
ACE:: Active chain end
AGE:: Allyl glycidyl ether
AM:: Activated monomer
AROP:: Anionic ring-opening polymerization
BEMP:: 2-tert-butylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine
BL:: β-butyrolactone
γ-BL:: γ-butyrolactone
BO:: Butylene oxide
BTMC:: 5-Benzyloxy-trimethylene carbonate
Bu₂Mg:: Dibutylmagnesium
i-Bu₃Al:: Triisobutylaluminum
BuLi:: Butyllithium
CC:: Cyclic carbonate
CL:: ε-Caprolactone
ε-CL:: ε-Caprolactam
Cp:: Cyclopentadienyl
D3:: Hexamethylcyclotrisiloxane
D4:: Octamethylcyclotetrasiloxane
D5:: Decamethylcyclopentasiloxane
D6:: Dodecamethylcyclohexasiloxane
DABCO:: 1,4-Diazabicyclo[2.2.2]octane
DBU:: 1,8-Diazabicyclo[5.4.0]undec-7-ene
DHCM:: 3,4-Dihydrocoumarin
DMAE:: 2-(Dimethylamino)ethanol
DMAP:: N,N-Dimethylamino pyridine
DMF:: Dimethylformamide
DMO:: 2,2-Dimethyloxirane
DMSO:: Dimethylsulfoxide
DP:: Degree of polymerization
DTC:: 2,2-dimethyl trimethylene carbonate
EC:: Ethylene carbonate
ECH:: Epichlorohydrin
EEGE:: 2-Ethoxyethyl glycidyl ether
EO:: Ethylene oxide
EtONa:: Sodium ethoxide
EVGE:: Ethoxy vinyl glycidyl ether
GME:: Glycidyl methyl ether
GPE:: Glycidyl phenyl ether
GTP:: Group transfer polymerization
HMPA:: Hexamethylphosphoramide
KHMDS:: Potassium bis(trimethylsilyl)amide
LA:: Lactide
ω-LL:: ω-Lauryllactam
MAlBP:: Methylaluminum bis(2,4,6-tri-tert-butylphenolate)
MD:: Morpholine-2,5-dione
MLABz:: Benzyl-β-malolactonate
MT:: Methylthiirane
NHC:: N-heterocyclic carbene
NOct₄Br:: Tetraoctylammonium chloride
PA3:: Polyamide 3
PA5:: Polyamide 5
PA6:: Polyamide 6
PA10:: Polyamide 10
PA12:: Polyamide 12
PBL:: Poly(β-butyrolactone)
PBO:: Poly(butylene oxide)
PC:: Polycarbonate
PCL:: Poly(ε-caprolactone)
PDMS:: Poly(dimethyl siloxane)
PEEGE:: Poly(2-ethoxyethyl glycidyl ether)
PEO:: Poly(ethylene oxide)
PGME:: Poly(glycidyl methyl ether)
PGPE:: Poly(glycidyl phenyl ether)
PHA:: Polyhydroxyalkanoate
PL:: β-Propiolactone
PLA:: Polylactide
PMMA:: Poly(methyl methacrylate)
PMT:: Polymethylthiirane
POSS:: Oligomeric silsesquioxane
PO:: Propylene oxide
PPL:: Poly(β-propiolactone)
PPO:: Poly(propylene oxide)
PPY:: 4-Pyrrolidinopyridine
PS:: Polystyrene
PTHF:: Polytetrahydrofuran
PTMC:: Poly(trimethylene carbonate)
PVL:: Pivalolactone (α,α-dimethyl-β-propiolactone)
PtBuGE:: Poly(tert-butyl glycidyl ether)
RIM:: Reaction injection molding
ROP:: Ring-opening polymerization
RTM:: Resin transfer molding
TBD:: 1,5,7-Triazabicyclo[4.4.0]dec-5-ene
t-BuGE:: tert-butyl glycidyl ether
t-BuOK:: Potassium tert-butoxide
tBuP₁ :: N′-tert-butyl-N,N,N′,N′,N″,N″-hexamethylphosphorimidic triamide
tBuP₂ :: 1-tert-Butyl-2,2,4,4,4-pentakis(dimethylamino)-2λ⁵,4λ⁵-catenadi(phosphazene)
tBuP₄ :: 1-tert-Butyl-4,4,4-tris(dimethylamino)-2,2-bis[tris(dimethylamino)-phosphoranylidenamino]-2-λ⁵,4-λ⁵-catenadi (phosphazene) (phosphazene base)
TeMC:: Tetramethylene carbonate
TeU:: Tetramethylene urea
THF:: Tetrahydrofuran
TMC:: Trimethylene carbonate
TPPAlCl:: α,β,γ,δ-Tetraphenylporphyrin aluminum chloride
δ-VL:: δ-Valerolactone

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Carlotti, S., Peruch, F. (2015). Cyclic Monomers: Epoxides, Lactide, Lactones, Lactams, Cyclic Silicon-Containing Monomers, Cyclic Carbonates, and Others. In: Hadjichristidis, N., Hirao, A. (eds) Anionic Polymerization. Springer, Tokyo. https://doi.org/10.1007/978-4-431-54186-8_5

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DOI: https://doi.org/10.1007/978-4-431-54186-8_5
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