Introduction

Poly(n-butyl acrylate) (PnBA) is a industrially important polymer due to its low glass-transition temperature, durability, and potential use as a soft segment in thermoplastic elastomers [1]. High molecular weight and narrow polydispersity are the essential requirements for synthesis of poly(n-butyl acrylate) [P(nBA)] satisfying the requirements for high performance materials [2]. However, P(nBA) is usually prepared via conventional free-radical polymerization without the precise microstructure control. Living/controlled radical polymerization (L/CRP) techniques provide a facile route to synthesize polymers with predetermined molecular weight and narrow polydispersities [3–11]. Atom transfer radical polymerization (ATRP) is one of the most powerful, versatile, simple, and inexpensive living polymerization techniques [12, 13]. Controlled living free radical polymerization has been successfully used to make n-butyl acrylate (nBA) homopolymer and block copolymers [14–23]. However, there are some drawbacks in conventional ATRP due to the use of low oxidation metals as well as air and moisture free storage [24]. To overcome these drawbacks, some different techniques, for example, reverse ATRP (RATRP) [25–28], activators generated by electron transfer (AGET) [29–32], activators regenerated by electron transfer (ARGET) [33–37] and initiators for continuous initiator regeneration (ICAR) [38–41], have been explored in an attempt to employ high oxidation state metal complexes directly to the reaction instead of the air and moisture sensitive low oxidation state metals.

A mechanism of typical Cu-based ICAR ATRP system is illustrated in Scheme 1 [38]. As shown in Scheme 1, the oxidatively stable state [Cu(II) complexes] is used as a catalyst. The activators [Cu(I) complexes] are produced by the in situ reduction of the copper(II) complexes with AIBN [38]. The repetition reduction cycle enables the amount of catalyst to be decreased significantly. The concentration of copper(I) is allowed to be ppm level, the removal of the residual copper catalyst may not be necessary for some applications.

Scheme 1
scheme 1

Mechanism of Cu-based ICAR ATRP

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ICAR ATRP of styrene, methyl methacrylate, and n-butyl acrylate were very well controlled using between 10 and 50 ppm of copper complexes with Me6TREN and TPMA ligands [39]. In view of the known toxicity of copper compound, iron complexes are more environmentally friendly catalytic systems. Iron complexes have been widely used in controlled radical polymerization [40–43].

Room-temperature ionic liquids have been used as a new generation of green solvents for a number of polymerization reactions, including conventional free radical [44], atom transfer radical polymerization (ATRP) [45–47], reversible addition-fragmentation chain transfer [48, 49], group transfer polymerization [50], and cationic ring opening polymerization [51].

However, to be the best of our knowledge, the ICAR ATRP of nBA with FeCl3·6H2O/Succinic acid has not been reported in ionic liquid yet. In this work, we described the controlled ICAR ATRP of nBA using FeCl3·6H2O/Succinic acid as the catalyst and ethyl 2-bromoisobutyrate (EBriB) as the initiator in ionic liquid. The living characteristics were confirmed by chain extension.

Experimental

Materials

n-Butyl acrylate (nBA, Shanghai Chemical Reagents Co. Ltd., AR grade), was distilled under reduced pressure after removal of inhibitor and stored at low temperature. The ionic liquid [bmim][PF6] was prepared according to a literature procedure [52]. 2-bromoisobutyrate (EBriB, Tianjin Alfa Aesar Chemical Co. Ltd., 99%) was used without further purification. AIBN (AR grade, Wuhan Chemical Co.) was recrystallized from ethanol and dried in vacuo. Ferric chloride hexahydrate (FeCl3·6H2O) (Shanghai Qingfeng Chemical Factory, AR grade), Succinic acid (SA, Wuhan Galaxy Chemical Co. Ltd., AR grade), benzene and other regents were used as received.

Polymerization

A typical example of the general procedure was as follows: n-Butyl acrylate, FeCl3·6H2O, succinic acid, AIBN and [bmim][PF6] were first placed in a three-necked bottle (150 mL). The round flask was placed in an oil bath held by a thermostat at the desired reaction temperature to polymerize under stirring. EBriB was added to initiate the polymerization. After the desired polymerization time, the round flask was cooled by immersing it into iced water. The reactant was pored into a large amount of methanol for precipitation, the obtained poly (nbutyl acrylate) (PnBA) was dried at 60 °C in vacuo for 24 h. Monomer conversion was determined by gravimetry.

Measurements

The number-average molecular weight (Mn,GPC) and molecular weight distribution (Mw/Mn) of the polymer were determined with a Waters 1515 gel permeation chromatography (GPC) equipped with refractive index detector, using HR1, HR3, and HR4 column with molecular weight range 100–500,000 calibrated with polystyrene standard sample. Polystyrene standards were used to calibrate the columns. THF was used as a mobile phase at a flow rate of 1.0 mL/min and with column temperature of 30 °C, and polystyrene standards were used to calibrate the columns. The \( {M_{{n(th)}}} \) of nBA can be calculated by the following equation:

$$ {M_{{n(th)}}} = \left( {{{\left[ {nBA} \right]}_0}/{{\left[ I \right]}_0}} \right) \times {W_{{nBA}}} \times x $$
(1)

Where, [nBA]0 is the initial concentration of nBA, [I]0 is the initial concentration of EBriB and WnBA is the molecular weight of nBA, x is the monomer conversion.

1H NMR spectrum were recorded on a nuclear magnetic resonance (NMR) instrument (Bruker 400 MHz Spectrometer) using CDCl3 as the solvent and tetramethylsilane (TMS) as the internal standard at ambient temperature.

Results and discussion

ICAR ATRP of n-butyl acrylate in ionic liquid

ICAR ATRP of n-butyl acrylate was carried out using ionic liquid with FeCl3·6H2O/succinic acid (SA) as the catalyst and EBriB as the initiator at 80 °C. The process was described in Scheme 2. The molar ratio of [nBA]0/[EBriB]0/[FeCl3·6H2O]0/[SA]0/[AIBN]0 was 300/1/0.01/0.02/0.1, and the amount of Fe was 50 ppm versus monomer. In order to compare the results, control samples were prepared in benzene under the same conditions.

Scheme 2
scheme 2

Preparation of PnBA in inoic liquid by ICAR ATRP

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Figure 1 shows the conversion and kinetic plot of ln([M]0/[M]) versus time for the ICAR ATRP of nBA. In both cases, ln([M]0/[M]) (where M0 is the initial concentration of the monomer, and M is the monomer concentration at any time) increases linearly with increasing time, demonstrating that the number of the active species remains constant during the polymerization process. However, the polymerization rate in ionic liquid was much faster than that in benzene. This was because that FeCl3·6H2O was not soluble in benzene. The conversions were 82.86% in 72 h in ionic liquid and 63.91% in 84 h in benzene, respectively. The apparent rate kapp (kapp = dln([M]0/[M])), that is, the slope of the kinetic plots of ln([M]0/[M]) versus reaction time, were 7.27 × 10−6 s−1 and 3.8 × 10−6 s−1, respectively. In both cases, induction period were observed in Fig. 1. The induction period was about 5 h in ionic liquid and about 13 h in benzene. The induction periods correspond to the decomposition of EBriB and to the establishment of the equilibrium between Fe (III) and Fe (II).

Fig. 1
figure 1

Conversion and Kinetic plots of ln([M]0/[M]) versus time t for the ICAR ATRP of nBA at 80 °C with ionic liquid and benzene as solvent. Polymerization conditions: [nBA]0/[EBriB]0/[FeCl3·6H2O]0/[SA]0/[AIBN]0 = 300/1/0.01/0.02/0.1; [nBA]0 = 6.0 M

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The dependence of Mn and molecular weight distributions (PDI) of the polymers on the monomer conversion was studied. In all cases, the values of Mn increase linearly with increasing monomer conversion demonstrated in Fig. 2 and are very close to the theoretical values throughout the polymerization. However, the values of Mn obtained in benzene were higher than that in ionic liquid. This meant the initiator efficiency, f = Mn,th/Mn,GPC, of EBriB in benzene were lower than that in the ionic liquid. The values of PDI were broader at the beginning of polymerization when the conversion was less than 30% (PDI > 1.25) in ionic liquid. When the conversion is beyond 30%, the values of PDI are narrow (PDI < 1.25), indicating that the polymerization system was a controlled radical polymerization. However, the PDI values were broader in benzene than that in ionic liquid.

Fig. 2
figure 2

Dependence of the molecular weights and molecular weight distributions on the monomer conversion for the ICAR ATRP of nBA at 80 °C with ionic liquid and benzene as solvent. [nBA]0/[EBriB]0/[FeCl3·6H2O]0/[SA]0/[AIBN]0 = 300/1/0.01/0.02/0.1; [nBA]0 = 6.0 M

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Effects of the amount of catalyst on the polymerization

In polymerization, the amount of catalyst has a strong effect on the polymerization rate and the level of control attained in the polymerization. In this work, the effects of the concentrations of catalyst on the polymerization were investigated. Polymerizations were run by ICAR ATRP with varied amounts of Fe catalyst in ionic liquid with molar ratio of [nBA]:[EBriB]:[SA]:[AIBN] at 300:1:0.2:0.1. The results are shown in Fig. 3.

Fig. 3
figure 3

Kinetic plots of ln([M]0/[M]) versus reaction time t for the ICAR ATRP of nBA with 10, 50, and 100 ppm of Fe at 80 °C with ionic liquid as solvent. Polymerization conditions: [nBA]0/[EBriB]0/[SA]0/[AIBN]0 = 300/1/0.02/0.1; [nBA]0 = 6.0 M

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As shown in Fig. 3, the polymerization rate increases with the amount of iron increasing from the 10 ppm to 100 ppm versus monomer. First-order kinetics was observed, indicating the propagating radical concentration was kept constant. The amount of iron was found to have influence on the polymerization rate as well as equilibrium of ICAR ATRP. The apparent rate constants are 5.01 × 10−6 s−1, 7.27 × 10−6 s−1, and 1.01 × 10−5 s−1 with different Fe concentrations 10 ppm, 50 ppm, 100 ppm, respectively.

Figure 4 shows the effects of amounts of iron on Mn and PDI. For the ICAR ATRP systems with different amount of iron, the Mn increases linearly with the monomer conversion and is very close to the corresponding theoretical values, indicating good controllability over the polymerizations in this study. The values of PDI decrease slowly when the conversion increases. When the conversion was beyond 30%, the values of PDI were low even if 10 ppm of iron was used. The advantage of low amounts of catalyst reduced side reactions between the growing radical and iron species, leading to well-defined poly(nBA). On the other hand, the metal residue was reduced in the products.

Fig. 4
figure 4

Dependence of the molecular weights and molecular weight distributions on the monomer conversion for the ICAR ATRP of nBA at 80 °C with ionic liquid as solvent. [nBA]0/[EBriB]0/[SA]0/[AIBN]0 = 300/1/0.02/0.1; [nBA]0 = 6.0 M

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Effects of temperature on the polymerization

The effect of temperature on the rate of ICAR ATRP of nBA was investigated, and the results are shown in Fig. 5. In all cases, first-order linear plots are observed, and the apparent rate constant increases with increasing temperature, suggesting that the radical concentration is constant. The induction period was reduced at 90 °C. The apparent rate constants were 1.07 × 10−6 s−1, 7.27 × 10−6 s−1, and 3.61 × 10−5 s−1 at 70, 80, and 90 °C, respectively. The Arrhennius plot of lnkapp versus 1/T obtained from the experimental results in Fig. 5 was given in Fig. 6. The apparent activation energy calculated on the slope of the plot was 50.44 kJ mol−1. According to the Eq. 2, \( \Delta {H_{{prop}}} \)=17.6 kJ mol−1 for nBA [53],

Fig. 5
figure 5

Dependence of ln([M]0/[M]) on the reaction time at different temperatures

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Fig. 6
figure 6

Effect of temperature on kapp

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$$ \Delta H_{{eq}}^0 = \Delta {H_{{app}}} - \Delta {H_{{prop}}} $$
(2)

Where, \( \Delta H_{{eq}}^0 \) is the enthalpy of the equilibrium, \( \Delta {H_{{app}}} \) is the apparent enthalpy of activation, and \( \Delta {H_{{prop}}} \) is the activation enthalpy of propagation. Then \( \Delta H_{{eq}}^0 \) is 32.84 kJ mol−1. The value is smaller than the value reported [2].

Effect of AIBN

In an ICAR ATRP, the active Fe (II) complexes were produced by the in situ reduction between the reducing agent AIBN and the Fe (III) complexes. Therefore, the reducing agent AIBN plays an important role in the ICAR ATRP process [38]. The effect of ICAR on the solution polymerization of nBA was investigated in this work. The results are listed in Table 1. From Table 1, it can be seen that the conversion increases from 32.18 to 77.82% with increasing the amount of AIBN. This was because the more concentration of AIBN was increased, the more Fe (III) complexes were reduced to the Fe (II) complexes, and the concentration of catalyst of the Fe (II) complexes increased. Hence, the polymerization rate increased with increasing the concentration of AIBN. The Mn increased when the concentration of AIBN was increased, but the Mn was close to theoretical values and the Mw/Mn values were also low (PDI < 1.21).

Table 1 Effects of the concentration of AIBN on ICAR ATRP of nBA at 80 °C in ionic liquid
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1H NMR spectrum of PnBA

The structure of the synthesized PnBA was confirmed by 1H-NMR spectroscopy. As shown in Fig. 7, the resonance signals of methyl protons at δ = 0.9 ppm, and methene protons at δ = 1.1 ppm-1.7 ppm were observed. The chemical shift for the methene protons on backbone were appeared at δ = 1.8-1.9 ppm. The chemical shift δ = 3.6 ppm was attributed to the CHCl. The chemical shift δ = 3.9 ppm was attributed to the OCH2. The signal at δ = 4.09 ppm was assigned to the methyne group in the chain ends of PnBA. It indicated that the polymerization was carried out via ICAR ATRP process, which is living radical polymerization and a halogen-containing living end group was observed.

Fig. 7
figure 7

1H-NMR spectrum of P(nBA) (Mn = 16512 g/mol, PDI = 1.27, conversion = 38.9%). a δ = 3.6 ppm; b δ = 1.8–1.9 ppm; c δ–0.9 ppm; d δ = 4.09 ppm; and e δ = 3.9 ppm

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Chain extension of P(nBA)

The chain-end functionality of the obtained poly(nBA) was investigated by a chain extension experiment with nBA via ICAR ATRP. The obtained poly(nBA) (conversion is 38.9%, Mn = 16512 g/mol, PDI = 1.27) was used as a macroinitiator for the ICAR ATRP of nBA in ionic liquid at 80 °C, using a FeCl3·6H2O/SA catalyst system in the presence of the reducing agent, AIBN. The overall polymerization conditions were [nBA]/[macroinitiator]/[FeCl3·6H2O]/[SA]/[AIBN] = 300:1:0.01:0.02:0.1. The chain extensions were successful. The results are shown in Fig. 8. It was clear from the result that the end groups of the polymers were active in the chain extension reaction.

Fig. 8
figure 8

GPC curves before and after chain extension with P(nBA) as a macroinitiator

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Conclusions

The ICAR ATRP of nBA, with EBriB as an initiator and FeCl3·6H2O/SA as a catalyst, AIBN as a reducing agent, can be successfully performed at 80 °C in ionic liquid. The ICAR ATRP of nBA in ionic liquid appears to give better results in terms of the polymerization rate and PDI values than that in benzene. The polymerization rate increased with temperature and the apparent activation energy was found to be 32.84 kJ mol−1. The kinetics experiment indicated that the polymerization of nBA is a ‘living’/controlled polymerization. The obtained poly (nBA) possessed a chlorine-terminated atom, as was proved by the chain extension reaction a macroinitiator to confirm the ‘living’/controlled nature of the polymerization process.