Keywords

1 Introduction

Since, the past decades, ILs attracted considerable interest as alternate solvents over conventional organic solvents for a wide range of applications because of their versatile properties such as nonvolatility, nonflammability, high thermal and chemical stability, wide electrochemical window, moderate polarity, non-coordinating nature, non-toxicity, being environment friendly, and reusability. ILs have been reported in catalysis as – the catalyst, cocatalyst, and catalyst activator – the source of a new ligand for a catalytic metal center or just as a solvent medium for the reaction. As most of the ionic liquids are miscible in polar/nonpolar organic and inorganic solvents, catalysts (inorganic or organometallic compounds), monomers, and the polymers in appreciable amount, the significance in polymerization reactions is very high. ILs have been used extensively as solvents in different kinds of polymerization.

In this chapter we discuss various IL-mediated polymerization reactions such as radical polymerization, cationic polymerization, anion polymerization, coordination polymerization, condensation polymerization, and enzymatic polymerization. This chapter represents a summary from the most relevant results of the literature reports up to date. The final part of this chapter dealt about the study on IL-catalyzed polymerization reactions.

2 Radical Polymerization in ILs

For the first time, Rogers et al in 2002 used [BMIM][PF6] as solvent for conventional free radical polymerization of MMA and styrene using Azobisisobutyronitrile (AIBN) initiator [1]. Polymers synthesized in IL media were of high molecular weight with high percentage conversion compared to the polymers synthesized in traditional organic solvents and bulk (Fig. 13.1). This observation could be attributed to the “diffusion control termination” because of high viscosity of ILs and solubility issue of monomers and resulting polymers in ILs [1]. The effect of ILs on MMA radical polymerization kinetics was studied through pulsed laser polymerization (PLP), and the study revealed an increase in propagation rate (k p) and decrease in termination rate (k p) with an increase in IL concentration (see Fig. 13.1) [2, 3]. For instance, relatively a double-fold increase of propagation constant in 50 volume % of [BMIM][PF6] was observed for MMA polymerization when compared to bulk polymerization [2]. On the other hand, as ionic liquid concentration increased to 60 volume %, the rate of termination decreased by one order magnitude. It is also believed that the increase in k p with IL concentration is due to the lowering of the activation energy of polymer propagation step [4]. The rate of polymerization was 79 % faster in [BMIM][PF6], and molecular weight of Polymethyl methacrylate (PMMA) was five times more than that synthesized in organic solvent, benzene [5]. Polymers with enhanced propagation rate, high thermal stability, and low glass transition temperature were reported in free radical homopolymerization of AN in [BMIM][BF4] [6]. Vygodskii research group employed a range of symmetric and asymmetric 1,3-dialkylimidazolium (IL-I), ammonium (IL-II), pyridinium (IL-III), and phosphonium (IL-IV) ILs with different alkyl groups and anions towards free radical polymerization of monomers like MMA, AN, and polymerizable tetrazoles (M-10). The alkyl chain length tethered to cation and the type of anion in IL had a pronounced effect over the molecular weight and properties of synthesized polymers. Imidazolium-based ILs exhibited significant results in polymer synthesis compared to the rest of ILs [7, 8]. ILs with NTf2 anion were noticed to afford polymers with high molecular weight.

Fig. 13.1
figure 1

(i). Size exclusion chromatography (SEC) traces of PMMA (A) polymerized using AIBN in [BMIM][PF6] at 65 °C for 1 h (b) and polymerized using AIBN in benzene at 65 °C for 1 h [1]. (ii). Effect of ionic liquid concentration on propagation rate constant kp (a) and termination rate constant kt [2] (iii) of MMA in the radical polymerization at 25 °C [3]

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As in line with previously observed reports, for MMA radical polymerization with AIBN in [EMIM][EtSO4] with increase in IL concentration in the system, an increase in kp and decrease in kt were noticed [9]. The propagation constant enhanced twofold in glycidyl methacrylate (GMA) polymerization and fourfold in MMA radical polymerization with ILs as solvent media. This is attributed to the decrease in activation energy by the replacement of monomeric species with IL (IL-I, IL-II, & IL-III) [10, 11]. Single-pulse pulsed laser polymerization in conjunction with electron paramagnetic resonance technique (SP-PLP-EPR) studies indicated that the termination rate for the free radical polymerization of PMMA in [EMIM][NTf2] and [BMIM][BF4] is relatively slow in comparison with bulk, which is in line with previous reports [12]. Synthesis of styrene and MMA block copolymers by simple conventional radical polymerization was realized in 2002, by employing [BMIM][PF6] as solvents [4]. ILs facilitated rapid homopolymerization of MMA at high temperatures and the resultant PMMA is insoluble in [BMIM][PF6]. The temperature for the copolymerization of PST-PMMA was decreased in order to minimize the formation of new radicals from initiator residue and also extend the lifetime of growing polymeric radicals. The solubility and insolubility of styrene and polystyrene, respectively, in IL dispersion provide additional advantages facilitating “diffusion-controlled copolymerization.” The polymers were easily separated due to their poor solubility in IL (IL-I) [4].

Radical copolymerization of AN and St in [BMIM][BF4] and [BMIM][PF6] afforded high molecular weight P(AN-co-St) having bimodal distribution. The high viscosity of ILs and their poor solubility in PST are responsible for the above observation [13]. An intriguing “protected radical” mechanism was proposed in radical copolymerization of styrene and MMA with ILs as solvent media, where the polystyrene radicals are protected from radical termination and subsequently promoted the copolymerization with MMA. Interestingly, no traces of PMMA homopolymer were found after the reaction. The authors emphasized that the high viscosity and solubility of ILs (IL-I) was primarily attributed to the protected radical mechanism which is impossible to reproduce in traditional organic solvents [14]. In another report, imidazolium-based ILs enabled successful copolymerization of two monomers (M-1d & M-3) which strongly differ in polarity that provided amphiphilic copolymers [15]. ILs were also used as plasticizer in the in situ homopolymerization of MMA. The polymers synthesized in IL as plasticizer were found to show less mass loss and high thermal stability compared to the polymer made in dioctyl phthalate (DOP) plasticizer.

ILs (IL-I) also found applications as effective solvent for synthesis of carbon black-grafted polymers through conventional free radical polymerization. Carbon black retards the polymerization of monomers in organic solvents to an extent; but this retardation was reduced drastically in the presence of ILs. This is attributed to the high viscosity of ILs favoring stabilization and prolonging the lifetime of polymer radicals towards grafting over the carbon black surface. Also, the percentage of grafting was higher in ILs compared to organic solvents [16, 17]. In numerous occasions ILs were successfully employed as surfactant as well as solvent in radical dispersion polymerization.

ATRP is one of the widely used controlled polymerization techniques catalyzed by transition-metal complexes. The main drawbacks of this technique are solubility of catalyst in bulk or organic solvents, contamination of polymers with catalyst, and recyclability of catalyst. But, one or more of these drawbacks can be overcome with the application of ILs as solvents in ATRP polymerization. Copper (I)-mediated MMA atom transfer radical polymerization in [BMIM][PF6] resulted in polymers with narrow polydispersity, well-defined architecture, and living characteristics [18]. After this first report of ATRP in ILs, several ILs (IL-I) of the kind [RMIM][PF6] have been extensively used as solvent in transition-metal-mediated polymerization, organostibine-mediated controlled polymerization, and ATRP polymerization of different monomers (M-1b, M1g, M1h, M-2, & M10) such as MMA, acrylates, and acrylonitrile owing to its solubility towards catalyst and monomers [18–24]. The transition-metal catalysts employed in the polymerization were easily separated, reused, and recycled which could be attributed towards their solubility in ILs [18, 24].

The solubility of monomer in ILs is not a critical criterion here. For example, methacrylate, butyl acrylate, and hexyl acrylate are completely miscible, partially miscible, and immiscible in [BMIM][PF6], respectively. But, this limited solubility minimizes the chance of side reactions in such heterogeneous polymerization system [25]. Similarly, the strategy involved in block copolymerization of MA and BA also clearly depends on the solubility of monomers in [BMIM][PF6]. The block copolymers were synthesized in two different ways: first, PBA was synthesized through heterogeneous polymerization in [BMIM][PF6] followed by in situ block copolymerization with MA and second, PMA was synthesized through homogeneous polymerization followed by in situ block polymerization with BA. The former strategy resulted in well-defined block copolymers even after addition of MA after complete conversion of BA, and no homopolymer traces of the latter were observed. But the polymerization in the second way resulted in block copolymers only at the addition of BA before 70 % conversion of MA. No block copolymer was formed when BA was added after complete conversion of MA. This remarkable difference in both strategies is solely attributed towards the solubility of monomers in IL [BMIM][PF6] [26].

The solubility of monomers in IL varies with respect to the cationic and anionic substituents of IL towards the respective monomer. For example, MMA and MA are completely soluble in [BMIM][PF6], and the resulting polymer separates out of IL after homogeneous polymerization leaving the catalyst inside IL thereby preventing the contamination of polymer with monomer and catalyst residues. In copper-mediated ATRP polymerization of MMA, [BMIM] with dibutyl phosphate anion served both as ligand and solvent compared to ILs (IL-I) with other anions such as halides (Br, Cl), chloroaluminates, and dodecyl sulfate which justifies the importance in choosing IL with particular cation and anion [27]. Thus by choosing appropriate IL, one can avoid the usage of external ligands to make ATRP much simpler.

It is observed that the polymerization rate constant increases with a decrease in the length of alkyl chains of IL anions (IL-I). These acid anions were found to chelate with transition-metal catalyst and thereby facilitated ATRP polymerization without use of any ligand which is an added advantage other than easy separation and use of catalyst [28]. N-butyl-N-methyl morpholinium tetrafluoroborate (IL-V) was also used as solvent for ATRP-mediated polymerization of MMA and contributed towards efficient separation and reuse of catalyst [29].

ILs were also used as solvents in reverse ATRP (RATRP) [30–34], as initiators for continuously activator generated ATRP (ICAR ATRP) [35], and as activators generated by electron transfer ATRP (AGET ATRP) [36–43]. ILs of the type 1-alkyl-3-methylimidazolium with different alkyl chains such as butyl, hexyl, octyl, and dodecyl and anions such as tetrafluoroborate and hexafluorophosphate (IL-I) were used extensively as solvents for RATRP polymerization [30–34]. It has been found that ILs with long alkyl chains (octyl, dodecyl) were able to afford well-defined polymers and block copolymers with high polymerization rate constant compared to the ILs with short alkyl chains (butyl and hexyl).

The immiscibility and miscibility of inorganic bases and ILs in organic solvents respectively motivated the researchers towards application of basic ILs (basic ILs) in polymerization process [36, 37]. 1-Butyl-3-methylimidazolium ILs with basic anions such as hydroxide, carbonate, bicarbonate, and phosphate were used as additives replacing inorganic bases such as NaOH, Fe(OH)3, and basic Al2O3 in AGET ATRP polymerization of MMA [39, 40]. Generally, AGET ATRP polymerization takes place in the presence of a reducing agent like vitamin C which reacts with higher oxidation state catalyst (deactivator) and generates lower oxidation state catalyst (activator) in situ. The main advantage of AGET ATRP over the conventional one is that the former reduces the amount of catalyst to be added and thus benefits the environment. Apart from being used as additives, ILs were also used as solvents for AGET ATRP-mediated polymerization of AN and MMA [38, 41–43]. The dependence of polymerization rate over alkyl chain length of ILs followed expected trend as discussed earlier, i.e., polymerization rate increased with increase in IL cation’s alkyl chain length [38]. The biphasic ATRP polymerization of MMA catalyzed by an ionic liquid tethered to copper complex facilitating efficient separation and reusability of the catalyst resulting in well-defined polymers with narrow molecular weight distribution and low polydispersity index (PDI) (~1.2–1.4). The catalyst settled at the bottom of the reactor allowing an easier separation from the polymer solution, and the recycled catalyst exhibited good control and catalytic activity as fresh catalyst [44]. Achiral IL such as [BMIM][PF6]-mediated ATRP produces atactic polymers, whereas chiral ILs (see CILs in Fig. 13.2 then remove the following figure) yield rise to well-defined polymers with specific tacticity [45–48].

Fig. 13.2
figure 2

List of ILs used in radical polymerization (a) of a different kind of monomers (b)

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The nitroxide mediated radical polymerization (NMP) polymerization of MA in [BMIM][PF6] and [HMIM][PF6] is observed to be faster compared to organic solvent, anisole. Surprisingly, the polymerization of MA took place both in bulk and IL without 4-oxo-TEMPO which may be due to spontaneous polymerization [49]. But, the attempt towards BPO/TEMPO- and TMPPAH-mediated NMP polymerization of MMA and styrene failed probably due to the slow degradation of TEMPO in IL. Instead, an uncontrolled polymerization of the monomers took place without any reactivation of monomeric chains [50]. With an attempt to overcome this drawback, it was tried to copolymerize MMA with small amount of styrene in [BMIM][NTf2]. The copolymerization took place faster in IL within 2 h with high conversions compared to the organic solvent, dioxane [51].

ILs of the kind 1-alkyl-3-methylimidazolium salts (IL-I) were reported as solvents for the first RAFT homopolymerization of MMA, MA, AA, and ST. The length of alkyl chains in ILs did not follow a particular trend over the polymerization kinetics which is contrary to the ATRP polymerization and free radical polymerization. Interestingly, there was a clear difference between the theoretical and the observed molecular weight which could be attributed to the poor solubility of RAFT agent in IL. All the monomers gave high conversions with linear increase in molecular weight except styrene due to the insolubility of PST in IL, whose conversions are less than 2 % [52, 53]. But, pyridinium-based ILs (IL-IV) when used for the homopolymerization of ST, afforded more than 80 % conversion after 10 h. In coincidence with the previous observations in free radical polymerization, the IL with more viscosity exhibited high polymerization rate and slightly higher compared to toluene. Initially, the reaction mixture was clear and homogeneous and eventually turned cloudy with the progress of polymerization indicating a precipitation/heterogeneous polymerization. Though there was a lot of debate on trapped radical mechanism in such kind of heterogeneous free radical polymerization system, the authors stated that no such mechanism took place in controlled RAFT homopolymerization of ST. The constant rate of propagation and rate of termination throughout the experiment supports their statement [54].

When ILs with different anions were employed in RAFT homopolymerization of MMA, ILs with EtSO4 and BF4 anions resulted in heterogeneous polymerization with low conversions and low molecular weight polymers, and on the other hand, ILs with NTf2 and PF6 anion successfully afforded solution-type polymerization with high conversion and good control over molecular weight distribution. Room temperature ionic liquid (RTIL) tethered chain transfer agent (CTA) gave better results than literature CTAs due to their difference in solubility in ILs [55]. The same research group has explained the evidence for the difference in polymerization rate and solubility of monomers in different ILs (IL-I) through rotating frame nuclear overhauser effect spectroscopy (ROESY) technique. The protons of CTA and IL were labeled, and the effect of polar and nonpolar domain of ILs over the growing polymeric chain was studied. These investigations revealed that viscosity is not the only factor but also the solvation environment of the monomeric species and the type of IL employed plays a major role in the polymerization kinetics. The continued solubility of growing reactive species in IL depends upon the domains of IL which could rearrange around them. For example, the growing RAFT end-capped MMA chain in [BMIM][BF4] separates out of IL after reaching a certain length leaving the monomer residues and unreacted initiator which further undergoes uncontrolled polymerization affording high molecular weight and multimodal molecular weight distribution curve [56]. ILs were also used as porogen in composition with organic solvent facilitating high column efficiency and high permeability which are the major challenges in synthesis of molecularly imprinted polymers (MIPs) [57].

The reactivity ratios depend on the monomers being polymerized, polymerization temperature, initiation, and solvent. These reactivity ratios could be tuned by changing the initiation mechanism from radical to charge transfer (CT) [58]. The reactivity of monomers was found to be enhanced in CT polymerization mediated by ILs as solvents compared to conventional free radical polymerization [59, 60].

As a general observation, when radical polymerization was carried out in ILs, higher molecular weight polymers with remarkably faster reaction rates in good conversions were noticed over conventional solvents. An increase in Kp/Kt with an increase in IL concentration is another advantage in IL-mediated radical polymerization. Block copolymer can be achieved by simple conventional radical polymerization in IL. In IL-mediated ATRP, not only the isolation of the products is easy but also the recovery and reuse of catalysts is possible. It is also interesting to observe that the disproportionation of Cu(0) is highly favored in IL-mediated ATRP.

3 Ionic Polymerization in ILs

3.1 Cationic Polymerization in ILs

Cationic living polymerization of styrene was achieved in ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide, [BMPyr][NTf2], which acted as a strong proton donor catalyzed by bis(oxalato) boric acid (HBOB) [61]. The cationic polymerization in ionic liquid strongly depends on the nature of anion. It was observed that organoborate acid-catalyzed styrene polymerization progressed smoothly in ILs with NTf2 anion, irrespective of the cation, but when counter anion in the IL changed from NTf2, no polymerization was observed. This clearly indicates the importance of the weekly basic counter anion such as NTf2 in this cationic polymerization. Relatively higher basic counter anions in ILs completely deprotonate the borate catalyst thus preventing the polymerization of monomers through protonation [62]. Cationic styrene polymerization carried out in neutral ionic liquid [MIM][PF6] with RCl/TiCl4 provided good conversions, but with less control over molecular weight and PDI [63] Kubisa et al demonstrated that C–Cl bond in phenylethyl chloride can be ionized in ILs in the absence of any co-initiator, to be used for styrene polymerization [63]. They observed an increase in the ionization of C–Cl bond in IL/SO2 with more efficacy. However, in none of these cases, ionization is not sufficient enough to obtain controlled styrene polymerization [64]. It was observed that styrene polymerization initiated by AlCl3 in [BMIM][PF6] produced higher molecular weights PS in quantitative conversions in comparison with other solvent systems DCM, scCO2, or [BMIM][PF6] + scCO2 that resulted in oligomers [65]. Very recently, vinyl ethers were polymerized via controlled manner in the absence of any Lewis acid catalyst, by in situ generation of vinyl ether–hydrogen halide adduct through simple mixing of CF3SO3H and Bu4NI to vinyl ether monomer. It was shown that these vinyl ether–hydrogen halide adducts act as cationogens for living cationic polymerization catalyzed by metallic Lewis acids [66].

There are an increasing number of reports on cationic ring-opening polymerization of a different kind of monomers in ionic liquids (Table 13.1). [BMIM][BF4]-mediated cationic ring-opening polymerization of 3-ethyl-3-hydroxymethyloxetane (EOX) afforded low molecular weight polymers with high conversions. The intramolecular hydrogen bonding which is believed to be the limiting factor for lower molecular weights was not influenced by relatively polar ILs at higher temperatures in order to get the higher molecular weight polymers [67]. These limitations were also visible in the cationic ring-opening polymerization of 3,3-bis(chloromethyl)oxacyclobutane in ILs [68]. Rare-earth metal triflates in ILs were used as an effective reusable catalyst at least for three cycles in the synthesis of poly(ε-caprolactone) (PCL) by ring-opening polymerization of lactones without significant loss of catalyst activity [69]. After completion of the reaction, the metal catalyst which is soluble in ILs will settle down at the bottom which can easily be separated from a polymer by simple decantation and reused for further polymerization reactions as shown in Fig. 13.3.

Fig. 13.3
figure 3

Schematic ROP of CL using reusable RE(OTf)3 in an IL [69]

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Table 13.1 List of catalysts and ionic liquids used in the cationic polymerization
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ROP of propene oxide and cyclohexene was carried out successfully in imidazolium-based ILs catalyzed by double metal cyanide and recyclable scandium triflate, respectively [70, 71]. ROP of ε-caprolactone catalyzed by polymer-anchored scandium triflate showed living nature, and the authors observed acceleration in polymerization when they used [EMIM][PF6] and [BMIM][PF6] ionic liquids over toluene [72]. The solubility of polysaccharides (such as cellulose, chitosan, chitin, starch, etc.) in ILs [73] was exploited to prepare a variety of polysaccharide-grafted polymers.

3.2 Anionic Polymerization in ILs

1,3-Dialkyl-substituted imidazolium-based ILs are vulnerable in basic conditions due to the presence of acidic proton at second position of imidazolium ring. When anionic polymerization reactions were carried out in imidazolium-based ionic liquids, either initiator deactivation or chain transfer to imidazolium ring of the ionic liquid was also observed. For instance, when MMA was polymerized in [BMIM][PF6] using alkyl lithium (anionic initiator), very poor yields (1–9 %) were observed due to decomposition of the anionic initiator [74]. Kubisa et al noticed the presence of ILs as chain ends when MMA was polymerized in [RMIM][PF6] or [RMIM][BF4] initiated by BuLi [75]. The chain transfer to imidazolium-based ILs in anionic polymerization is shown in Fig. 13.4. These studies suggest that imidazolium-based ILs are not suitable solvents for controlled anionic polymerization.

Fig. 13.4
figure 4

Chain transfer to imidazolium ionic liquid in anionic polymerization of MMA

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Sodium salt of hydroxylated imidazolium IL at second position initiated the anionic ring-opening polymerization of ethylene oxide that produced polyethylene oxide with degree of polymerization (DP) around 30 without any side reactions [76]. Under relatively mild reaction conditions, anionic polymerization of styrene in phosphonium-based IL was reported. Upon addition of imidazolium-based zwitterion, fourfold increase in yields was noticed with decrease in molecular weight with slightly higher PDI values [77].

ILs of the types [BMIM][PF6] and [BMIM][NTf2] were used as reaction medium for the ring-opening polymerization of g-benzyl-l-glutamate-N-carboxyanhydride (BLG-NCA) initiated by butylamine. When compared with common organic solvents, the polymer obtained in ILs showed same molecular weight and PDI values except small decrease in yields; this is due to heterogenized reactions in ILs [78]. But the group transfer polymerization of MMA carried out at ambient temperatures in ionic liquids gave relatively good yields with or without catalyst [79].

3.3 Transition-Metal-Catalyzed Polymerization in ILs

The poor solubility of highly active transition-metal complexes in organic solvents can be overcome by choosing appropriate ionic liquids with relatively non-coordinating counter anions such as NTf2, PF6, etc (Table 13.2). The first report on ILs in the homogeneous oligomerization [80] and polymerization reactions [81, 82] appeared in 1990. Later several research groups successfully carried out the oligomerization (ethylene and higher olefins) [83–92] and polymerization reactions in ILs [92, 91, 93–99]. The major concern in the homogeneous polymerization is the recovery of costly catalyst system. IL-based biphasic reactions facilitate the easy separation of catalyst system for further usage. Better results were observed in 2001 by Wasserscheid et al in biphasic oligomerization of ethylene to higher olefins with cationic nickel complexes in ILs of the type [RMIM][PF6]. The reactivity and selectivity in ILs were noticed to be better compared to conventional organic solvents [86–88].

It was observed that propene and 1-butene dimerizations with Ni(II) heterocyclic complexes were inactive or sparingly active in toluene. Upon changing the solvent to imidazolium-based ionic liquids, the same catalyst system showed considerable activity towards dimerization of propene and 1-butene. The catalyst inability towards oligomerization of propene and butene in toluene was attributed to rapid decomposition of Ni(II) complex via reductive elimination. But such decomposition was not observed in imidazolium-based ILs [88]. Biphasic ethylene oligomerization was carried out with Ni(II)–diiminophosphorane complexes in organochloroaluminate IL. The recovered catalyst system in repeated recycling showed an increase in activity with the expense of its selectivity (Fig. 13.5). This decrease in selectivity was attributed to the formation of a more active and new kind of Ni complexes due to ligand displacement of diiminophosphoranes with chloroaluminates [93].

Fig. 13.5
figure 5

Effect of selectivity in C4 upon repeating cycles in organochloroaluminate IL biphasic oligomerization of ethylene with four Ni complexes 1–4 (see Table 13.2) [89]

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Recently, it has been reported that buffered chloroaluminate ILs can activate Ni catalysts in the biphasic dimerization of propene with 84 % selectivity [98, 97]. IL-mediated biphasic ethylene polymerization was carried out successfully with 1,4-bis(2,6-diisopropylphenyl) acenaphthenediiminenickel(II) dichloride (catalyst) in [BMIM] organoaluminate (solvent as well as cocatalyst) and toluene at mild polymerization conditions such as 1,050 mbar pressure and temperature between −10 °C and +10 °C. For further reuse, the recycled catalyst needed addition of fresh cocatalyst trimethylaluminum (TMA) that leads to the formation of multiple active sites and eventually restricted the single-site polymerization process [93].

It was even demonstrated that ethylene polymerization can be successfully carried out in ionic liquid (imidazolium-based or pyridinium-based) hexane biphasic system using metallocene catalyst Cp2TiCl2 activated by alkylaluminum compounds such as MAO, Et2AlCl EtAlCl2, or Et3Al [96, 95]. By selecting suitable solvent and buffer, the same Ni catalyst (which was used for ethylene dimerization) was effective in producing 38 % polyethylene with Mn of 140 Kg/mol. Recently, ethylene polymerization was carried out homogeneously in ionic liquid and an appropriate organic solvent. After completion of the reaction, addition of nonpolar solvent to the polymerization mixture facilitates the precipitation of ionic liquid with catalyst at the bottom by leaving the polymer and organic solvent above as shown in Fig. 13.6 [92]. This is a very effective way of recovering the catalyst system using ionic liquid-based homogeneous polymerization.

Fig. 13.6
figure 6

Homogeneous IL-based ethylene polymerization reactions and subsequent catalyst recovery by precipitation [92]

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Our attempts to polymerize 1-hexene with non-metallocene catalysts [100, 101] in [BMIM][BF4] and [BMIM][PF6] did not afford any polymer (unpublished results). Even the absence of literature reports on higher α-olefin polymerization in ILs suggests that ILs are not suitable solvents for α-olefin polymerization.

Poly(phenylacetylene) was prepared in high yields with high cis percentage (95–100 %) by Rh(I)/triethylamine catalyst system in [BMIM][BF4] and [BPy][BF4]. The recovered catalyst system in IL was effective for further use without any significant loss in activity [97]. An increase in molecular weights and yields was reported when Pd-catalyzed copolymerization of styrene and CO was carried out in [HPy][NTf2]/methanol in comparison with methanol-mediated polymerization. The catalyst productivity in [HPy][NTf2] was comparable to that of highly polar non-coordinating solvent trifluoroethanol [99, 98] (Table 13.2).

Table 13.2 Different transition-metal complex and ILs used for oligomerization and polymerization reactions
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Compared to anionic polymerization, cationic polymerizations in IL media are more feasible. This is because under basic conditions imidazolium-based ILs are less stable and it leads to deactivation of the initiator and/or chain transfer to IL. An increase in catalyst activity was observed in some IL-mediated cationic polymerization. Though the mechanism is not clearly known, it is believed that the ILs can stabilize the charged intermediates. This can be attributed to the moderately polar and non-coordinating nature of ILs.

4 Polycondensation in ILs

In general, polycondensation processes require harsh conditions such as high temperatures and vacuum conditions for long reaction periods using protic acid or organometallic catalysts. These harsh reaction conditions not only pose environmental problems but also lead to side reactions resulting in polymers with undesired molar masses with varying microstructures. So there is a great demand for the replacement of toxic reagents and volatile solvents; ILs are being found as an efficient replacement for organic solvents owing to their unique properties like nonvolatile, thermally stable, nonflammable, and chemically inert. For the first time in 2002, different 1,3-dialkyl imidazolium-based ILs were used as reaction media successfully for the synthesis of high molecular weight aromatic polyamides and polyimides in the absence of any added catalyst [102, 103]. Synthesis of high molecular weight polyimides with quantitative conversions was realized in ILs/triphenyl phosphite (TPP) by direct polycondensation of different monomers such as diacids and diamides [104].

The addition of 40 % zwitterion IL () improved the solubility of the monomers such as aromatic amines and di-anhydrides into 1,3-dialkyl imidazolium ILs, and eventually their polycondensation resulted in high molecular weight polyimides [105]. Poly(1,3,4-oxadiazole)s (PODs) and phosphonic acid-functionalized PODs were prepared in [PMIM]Br/TPP by direct polycondensation of corresponding dicarboxylic acids and their dihydrazides [106, 107] (Fig. 13.7).

Fig. 13.7
figure 7

Synthesis of various phosphonated PODs in ILs [107]

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Macroporous polyureas were prepared by polycondensation reaction at the interface of hexane and IL. FT-IR studies suggest the surface interactions between ILs and polyurea, which are responsible for the formation of macroporous polyureas [108]. The direct polycondensation of glycolic acid in imidazolium-based ILs at 200–240 °C afforded only poly(glycolic acid) (PGA) oligomers due to evaporation of glycolic acid at this temperature. Even the post polycondensation could produce PGA of only a maximum of 45 DP. Here, the solubility of PGA in IL is the limiting factor for higher molar mass. Another drawback observed in this kind of polyesterification in ILs is the less activity of Zn(OAc)2, and authors explained this as a consequence of its preferential interaction with ILs over acidic end groups [109] (Fig. 13.8).

Fig. 13.8
figure 8

Polycondensation of glycolic acid in ILs [109]

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In another aliphatic polyester synthesis (from diols and diacids) in ionic liquid [HMIM]PF6 catalyzed by SnCl2, same kind of catalyst deactivation was noticed, with lower molecular weights (Mw <1.8 × 104) as that of catalyst-free bulk polymerization [110]. An increase in molecular weights of polyesters was noticed upon changing the counter anion from PF6 to NTf2 in the ionic liquid. The mechanism of this metal activation/deactivation in ILs was explained based on hard and soft acid-base principle [111]. It is believed that Sn+2 which is a hard acid will totally fluorinate to give metal fluoride complex in IL with PF6 counter anion and become inactive for the polymerization. But such a fluoride complexation is unlikely in NTf2 (due to strong covalent bonded C–Fs); this may be the reason why Sn+2 is catalytically active and provided relatively higher molecular weight polyesters in NTf2-based ILs [110].

In IL-mediated polyphenol synthesis, the nature of ionic liquid largely influenced the outcome of the polymer. When hydrophilic ILs ([HMIM][Cl] and [HMIM][Br]) were employed as solvents, they produced low molecular weight polyphenols with low PDI. On the other hand, relatively higher molecular weight polyphenols with very high PDI were noticed in hydrophobic ILs ([HMIM][BF4] and [HMIM][CF3SO3]). Depending upon the type of ILs (hydrophilic or hydrophobic), the interaction between imidazolium ring in ILs and phenol monomer varies thus affecting the molecular weight of the polymer (Fig. 13.9) [112].

Fig. 13.9
figure 9

Polymerization of phenol in (a) hydrophilic, hydrophobic, and (b) Bronsted acid ILs (ii). Interaction of phenol with (a) hydrophilic IL [HMIM][Br] and (b) hydrophobic IL [HMIM][BF4] [112]

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Higher molar mass polyethers were prepared using BAILs [BBSIm][NTf2] and [OBSIm][NTf2] by polycondensation of aliphatic diols at relatively lower temperatures 130 °C [113]. Recently high-performance polyetheretherketone (PEEK) was prepared in [BMIM][NTf2]/K2CO3 by polycondensation between hydroquinone and 4,4′-dihalobenzophenones at temperature around 320 °C, and the obtained PEEK showed properties same as that of PEEK synthesized in commercially used solvent diphenylsulfone under identical conditions. The major advantage of using ILs over commercial diphenylsulfone as a solvent in this polycondensation is due to the absence of solvent occlusion and the ease of polymer recovery [114].

Because of their higher thermal stability and nonvolatile nature, ILs are supposed to be the ideal media for the polycondensation. The increasing number of reports in this field indicates that ILs are promising solvents in the polycondensation over conventional solvents.

5 Enzymatic Polymerization in ILs

The crucial advantage of enzymatic catalysis is its inherent chiral discrimination ability and to proceed at milder conditions in the absence of any metal or organometallic compound. Enzymatic catalysis in ionic liquids – as an alternative for organic solvent – has gained considerable attention of the researchers [115, 116]. There are plenty of reports on the enzymatic polymerization in organic solvents [117, 118]. It is observed that enzymes are more stable in ILs than in conventional organic solvents where their decomposition was noticed [119–121]. There is limited number of reports on enzymatic polymerization reactions in IL medium. In general, lipases immobilized on porous polymer beads were used as catalysts in IL-mediated polymerization reactions. So far these enzymatic polymerization reactions in IL medium were restricted largely towards polyester synthesis either by ring-opening polymerization (ROP) of lactones or by polycondensation of hydroxy acids [122–133]. In most of these polymerization reactions, lipase B from Candida antarctica (Novozyme 435) in imidazolium-based ILs (of the kind [RMIM] where R = alkyl group) with PF6 as counter anion was used as solvent. Less monomer conversions and lower molecular weights were observed when ROP of ε-caprolactone was carried out with Novozyme 435 in ILs such as [BMIM][PF6], [BMIM][BF4], and [BMIM][NTf2] over toluene. Apparently this is due to the limited miscibility of the polymer in ILs [122].

IL-coated enzymes showed higher catalytic activity than that of IL-mediated reactions. [BMIM][PF6]-coated Novozyme 435 was very effective in synthesizing high molecular weight (Mw = 182 k g/mol) poly(1,4-dioxan-2-one) (PPDO) [130]. When enzymes are coated with ILs, they protect from bound water, and in the presence of bound water, activation and propagation reactions will be more feasible (Fig. 13.10) [130].

Fig. 13.10
figure 10

Schematic representation of IL-coated lipase particles for lactide polymerization in solvent-free conditions [132]

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Apart from polyester synthesis, the other kinds of polymerization reactions under this category are soybean peroxidase-catalyzed polymerization of phenols in [BMIM][BF4]/[BMPy][BF4] [134] and horseradish peroxidase-catalyzed aniline polymerization using anionic surfactant sodium dodecylbenzenesulfonate (SDBS) in [BMIM][NTf2] [135]. It was observed that the deactivation of horseradish peroxidase by SDBS was minimized in the presence of ionic liquid [BMIM][OTf] [135].

Long reaction time and lower molecular weights are major disadvantages that were noticed in this type of polymerization reactions. Through the chiral discrimination capacity, enzymes facilitate the synthesis of chiral polymers from racemic monomer, but this advantage could not be explored further due to abovementioned limitations.

6 Ionic Liquids as Catalysts for Polymerization

As discussed earlier in this chapter, ionic liquids have been extensively used as solvents in different kinds of polymerization. Recently, researchers are finding applications of ionic liquids as catalysts in ring-opening polymerization, photopolymerization, ATRP polymerization, and proton-mediated polymerization.

Acidic and basic ILs were employed as an effective and reusable catalyst in some polymerization reactions. The acidic ionic liquids can be Lewis acid ionic liquids (LAILs), Bronsted acid ionic liquids (BAILs) (e.g., see IL-1 to IL-12 in Fig. 13.8), or Bronsted-Lewis acid ionic liquids (BA-LA ILs) (IL-13 in Fig 13.8). In Bronsted acid ionic liquids (BAILs), the Bronsted acid group can be introduced at cationic part, at anionic part, or at both cationic and anionic parts of the ionic liquid as shown in Fig. 13.11.

Fig. 13.11
figure 11

List of acid and basic ILs used as catalyst in polymerization reactions

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The catalytic activity of acidic ILs such as pyrrolidinium bisulfate [H-NMP][HSO4], pyrrolidinium chloride [H-NMP][Cl], and morpholinium bisulfate [H-Mor][HSO4] for the ring-opening polymerization of ε-caprolactones in toluene with benzyl alcohol as initiator was investigated recently [136]. A decrease in inherent viscosity with progress in reaction time was observed which was attributed to the increase in intramolecular transesterification. Pyrrolidinium chloride [H-NMP][Cl] was successful in catalyzing starch-grafted-PCL copolymerization; but pyrrolidinium bisulfate [H-NMP][HSO4] and morpholinium bisulfate [H-Mor][HSO4] could not catalyze the same, as starch gets easily hydrolyzed by the two ILs.

The direct polyesterification of 12-hydroxydodecanoic acid in Bronsted acid ionic liquids that acted as media as well as catalyst afforded high molar mass polyesters with Mn up to 50,000 g/mol in good yields (around 90 %) at atmospheric pressure and moderate temperatures (90–130 °C) which are milder conditions as compared with conventional ones (>200 °C, low pressure) [137]. The advantages of BAIL-catalyzed polyesterification which are worth mentioning include mild conditions, short reaction periods, good yields, and high molar mass. The effective polyesterification is largely influenced by cationic and anionic parts of the BAILs. The BAIL [BBSIm][NTf2] exhibited high activity; within 5 min, Mw reached close to 35 k g/mol, but during prolonged reaction time, side reactions such as etherification of OH end groups and degradation (acid-catalyzed ester and ether scissions) were reported [137, 138] (Fig. 13.12).

Fig. 13.12
figure 12

Two different side reactions, namely, (i) etherification, (ii) dehydration reactions in [BBSIm][NTf2]-catalyzed polyesterification of 12-hydroxydodecanoic acid and (iii) is ether scission

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Other BAILs, namely, [BBSIm][NTf2] and [OBSIm][NTf2], were effective in catalyzing the polyetherification of aliphatic diols with n = 7–12 methylene units at around 130 °C [113]. 1H NMR studies revealed that the OH groups underwent H+-catalyzed dehydration followed by carbocation migration to give double bonds at α, β, and γ position to the original OH group. Further, these double bonds at γ and/or δ position got hydroaminated by NHTf2 in acidic conditions. But these two BAILs were not effective in polyetherification of 1,4-butanediol and 1,6-hexane diol; instead it resulted in cyclic ethers such as tetrahydrofuran and oxapene, respectively [113] (Fig. 13.13).

Fig. 13.13
figure 13

Polyetherification of aliphatic diols using in BAILs

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Sulfonic acid-functionalized BAILs are effective in copolymerization of L-lactic acid and ε-caprolactones [139]. These BAILs could be easily separable by phase separation and recyclable for a minimum of four times without any decrease in catalytic efficiency. Controlled kinetic studies revealed that the copolymerization is a combination of polycondensation and simultaneous transesterification immediately after fast ring-opening polymerization of ε-caprolactone.

Polyphenols were synthesized using BAILs without any other additives. In this process they have isolated the polyphenol and recovered the catalyst BAIL by simple reprecipitation in water and filtration. This is a complete green polymerization process free from any organic solvents and free acid [112]. Lewis acid ionic liquids (LAILs) of the type [EtOCOCH2-mim][Cl-AlCl3] in combination of SbCl3 (cocatalyst) were used in the polymerization of α-pinene in toluene solvent [140, 141]. As compared with commonly employed catalyst AlCl3, these LAILs are more effective in polymerizing α-pinene, and also in the latter case, the polymer separation is easy, and the catalyst can be effectively reused for several cycles. It was noticed that trace amounts of water was required for generation of activated proton which is responsible for higher catalytic activity (Fig. 13.14). The polymerization α-pinene with [EtOCOCH2-mim][Cl-CuCl2] could not proceed due to its inability to give activated proton that supports the proposed mechanism.

Fig. 13.14
figure 14

Mechanism of α-pinene polymerization with LAILs

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Bronsted-Lewis acid ILs (BA-LA ILs) are also reported to be effective catalysts in dimerization of fatty acid methyl esters [142]. Basic ILs (see IL-14 in Fig. 13.8) such as [Bmim][OH], [Bmim][PO4], [Bmim][CO3], and [Bmim][HCO3] were used as ligand and catalyst in iron-mediated AGET atom transfer radical polymerization [39]. The catalytic activity of (BMIm)2(DMIm)PW12O40 was investigated for photopolymerization of styrene, methyl methacrylate, butyl methacrylate, and vinyl acetate in the absence of any additional co-initiators. And it was reported to be efficient recovery and reusability of catalyst [143]. Xanthenyl phosphonium salts were used as thermo-latent initiator in the bulk polymerization of glycidyl phenyl ether that afforded polymer with a maximum Mn of 3,400 in quantitative conversions [144].

Hydroxylated imidazolium-based ILs were used as initiators with trifluoromethanesulfonic acid in cationic ring-opening polymerization of LA to synthesize medium molecular weight PLA with IL end groups. These IL functionalized polylactides were further used to stabilize the carbon nanotube suspensions. The strong interactions between IL part of PLA and carbon nanotube surfaces are the key in the stabilization of its suspension, and this interactions were noticed through SEM images [145]. ILs were also used as ligands towards metal complexes (catalysts) for polymerization reactions such as ATRP [44].

In polymerization reactions, when compared with traditional catalysts, some ILs showed better catalytic performance and recyclability. The adjustable acidity/basicity to desired level, the easier separation and isolation of polymer solution from catalyst, and the recyclability and reusability of catalyst are the predominant advantages of ILs over other commercial catalysts/ligands.

7 Conclusion

In general, higher molecular weight polymers with remarkably faster reaction rates in good conversions were noticed in IL-mediated radical polymerization reactions over conventional solvents. When polymerization is carried out in ILs, the observed advantages are increase in Kp/Kt with an increase in IL concentration (in radical polymerization), block copolymer synthesis by simple conventional radical polymerization, easy isolation of the products, and recovery and reuse of catalysts (ATRP, RAFT, Z-N system, etc.). It is also interesting to observe that the disproportionation of Cu(0) is highly favored in IL-mediated ATRP. Radical polymerization in ILs is very promising towards the commercialization of some of these techniques for the production of commodity polymers (homo- and block copolymers) in living manner.

Compared to anionic polymerization, cationic polymerizations in IL media are more feasible. This is because under basic conditions imidazolium-based ILs are less stable and it leads to deactivation of the initiator and/or chain transfer to IL. An increase in catalyst activity was observed in some IL-mediated cationic polymerization. Though the mechanism is not clearly known, it is believed that the ILs can stabilize the charged intermediates. This can be attributed to the moderately polar and non-coordinating nature of ILs.

Because of their higher thermal stability and nonvolatile nature, ILs are supposed to be the ideal media for the polycondensation. Though there are increasing reports in this field, much more is anticipated. Compared to other processes, the IL-mediated enzymatic polymerization suffers from the disadvantages such as long reaction time and lower molecular weights. Through the chiral discrimination capacity, enzymes facilitate the synthesis of chiral polymers from racemic monomer, but this advantage could not be explored further due to abovementioned limitations.

By selective choice of cationic and counter anionic part of ILs, the polymerization kinetics and the polymer properties can be tuned. There are more added advantages with positive impact on the environment when ILs are used as solvents in polymerization process. This is a promising research area that can be explored further for the better generation of tomorrow.