Poly(Ionic Liquid): A New Phase in a Thermoregulated Phase Separated Catalysis and Catalyst Recycling System of Transition Metal-Mediated ATRP

Poly(ionic liquid)s (PILs) have become the frontier domains in separation science because of the special properties of ionic liquids as well as their corresponding polymers. Considering their function in separation, we designed and synthesized a thermoregulated PIL. That is, this kind of PIL could separate with an organic phase which dissolves the monomers at ambient temperature. When heated to the reaction temperature, they become a homogeneous phase, and they separate again when the temperature falls to the ambient temperature after polymerization. Based on this, a thermoregulated phase separated catalysis (TPSC) system for Cu-based atom transfer radical polymerization (ATRP) was constructed. The copper catalyst (CuBr2) used here is easily separated and recycled in situ just by changing the temperature in this system. Moreover, even when the catalyst had been recycled five times, the controllability over resultant polymers is still satisfying. Finally, only 1~2 ppm metal catalyst was left in the polymer solution phase, which indicates the really high recycling efficiency.


Introduction
Atom transfer radical polymerization (ATRP) is one of the most widely used radical polymerizations, and it is well-known for its numerous advantages, such as a wide range of suitable monomers, basically commercially available initiators, ligands, catalysts, and mild polymerization conditions. The resultant polymers can be synthesized with designed structures, including random, block, graft, star, gradient, hyper-branched (co) polymers, end functional polymers, etc. [1][2][3]. Usually, ATRP is mediated by transition metal catalysts in the presence of heat [4,5], light [6][7][8], electrochemistry [9][10][11][12], or microwave [13][14][15]. However, the transition metal residual in polymer product will pollute/color the polymers, which will not only result in the waste of metal catalysts, charge features, PILs can stabilize catalytically active metal and metal oxide nanoparticles [43]. As a result, PIL-based polymerization systems as supports facilitate the recovery, recycling, and further use of the transition metal catalysts in comparison to molecular ILs [44,45]. Compared with ILs, the viscosities of PILs would not change greatly with the temperature changes, making it a stable phase in polymerization system [46][47][48][49][50].
Combination of the advantages of TPSC, ICAR ATRP, and PILs, in this work, we designed a recycled thermoregulated PIL which are used in the ATRP system as a solvent and constructed a PIL/organic biphasic TPSC system for catalyst separation and recycling. Herein, CuBr 2 was used as the original catalyst, tris ((2-pyridyl) methyl) amine (TPMA) as the ligand, ethyl-2-bromo-2-phenyl acetate (EBPA) as the ATRP initiator, and AIBN as the reducing agent. In addition, methyl methacrylate (MMA) is an important monomer in industry, which plays a vital role in manufacturing of organic glass, coatings, adhesives, and medical polymers [51], therefore MMA was used as a model monomer for this novel polymerization system. In addition, we chose a thermoregulated IL monomer, MPEG350-MI-MA, which was first synthesized in our group to polymerize the aim PIL [30]. The synthetic pathway is shown in Scheme 1. Furthermore, the resultant PIL and benzene were selected as thermoregulated phase and organic phase, respectively. In this novel system, a transition metal catalyst could be easily separated just by changing the polymerization temperature and this has nearly no negative effect on catalytic efficiency, polymer molecular weight, or molecular weight distribution.
as thermoregulated phase and organic phase, respectively. In this novel system, a transition metal catalyst could be easily separated just by changing the polymerization temperature and this has nearly no negative effect on catalytic efficiency, polymer molecular weight, or molecular weight distribution.

Synthesis of the Thermoregulated Poly(Ionic Liquid) (PIL)
The synthetic pathway of the thermoregulated ionic liquid monomer MPEG350-MI-MA is shown in Scheme 1. Intermediates 1 and 2 were synthesized according to the reported literature [30]. Besides, the intermediate 3 was synthesized according to another previous work [52]. 15 mmol of MPEG350-MI-MA and 0.246 g AIBN (1.5 mmol) were added to a dried Schelenk tube with a stir bar, and then 10 mL of DMSO was also added to the tube. After bubbled with argon for 15 min, the tube was sealed and settled under 80 °C for three days. When the reaction was finished and the mixture was cooled to room temperature, washed the resulted solution with methanol for 3-5 times, then the poly (ionic liquid) (PIL) was obtained. The PIL was determined by gel permeation chromatograph (GPC) with Mn,GPC = 26,300 g/mol and Mw/Mn = 1.46. The sealed ampoule was heated to 70 °C for polymerization. After the desired time of polymerization, the ampoule was taken out and the ampoule was cooled by immersing it into iced water. After the system was completely phase-separated, the ampoule was broken, and transferred the upper polymer solution into THF to get a diluted one, then the mixture was precipitated in methanol. The precipitated product was filtered off with suction from a vacuum distillation flask. To obtain a dried product, the filtered product was placed in a vacuum oven at 30 °C for about 3-4 h. The weight of the dried product was measured and the monomer conversion was calculated according to it. Finally, The PIL phase containing the catalyst was transferred to a new ampoule and fresh EBPA (8.3 µL, 0.047 mmol), MMA (1 mL, 9.4 mmol), AIBN (7.8 mg, 0.047 mmol) and benzene (3 mL) were also added for catalyst recycling experiments. All remaining polymerization operations were the same as above. The polymer obtained was filtered with neutral alumina and reprecipitated, then it was dried in a vacuum oven to a constant weight. A predetermined quantity of PMMA (0.024 mmol) was added in a dry 5 mL ampule with a stir bar as the macro-initiator, then the corresponding amount of CuBr2 (0.047 mmol), PIL (0.108 mmol), MMA (9.4 mmol), AIBN (0.047 mmol), and benzene (3.0 mL) were added. The rest of the procedure was the same as the typical procedure for the TPSC-based ICAR ATRP described above.

Characterizations
The number-average molecular weight (Mn,GPC) and molecular weight distribution (Mw/Mn)

Synthesis of the Thermoregulated Poly(Ionic Liquid) (PIL)
The synthetic pathway of the thermoregulated ionic liquid monomer MPEG 350 -MI-MA is shown in Scheme 1. Intermediates 1 and 2 were synthesized according to the reported literature [30]. Besides, the intermediate 3 was synthesized according to another previous work [52]. 15 mmol of MPEG 350 -MI-MA and 0.246 g AIBN (1.5 mmol) were added to a dried Schelenk tube with a stir bar, and then 10 mL of DMSO was also added to the tube. After bubbled with argon for 15 min, the tube was sealed and settled under 80 • C for three days. When the reaction was finished and the mixture was cooled to room temperature, washed the resulted solution with methanol for 3-5 times, then the poly (ionic liquid) (PIL) was obtained. The PIL was determined by gel permeation chromatograph (GPC) with M n,GPC = 26,300 g/mol and M w /M n = 1.46. The ampoule was bubbled with argon for about 15 min to eliminate the dissolved oxygen, and then flame-sealed. The sealed ampoule was heated to 70 • C for polymerization. After the desired time of polymerization, the ampoule was taken out and the ampoule was cooled by immersing it into iced water. After the system was completely phase-separated, the ampoule was broken, and transferred the upper polymer solution into THF to get a diluted one, then the mixture was precipitated in methanol. The precipitated product was filtered off with suction from a vacuum distillation flask. To obtain a dried product, the filtered product was placed in a vacuum oven at 30 • C for about 3-4 h. The weight of the dried product was measured and the monomer conversion was calculated according to it. Finally, The PIL phase containing the catalyst was transferred to a new ampoule and fresh EBPA (8.3 µL, 0.047 mmol), MMA (1 mL, 9.4 mmol), AIBN (7.8 mg, 0.047 mmol) and benzene (3 mL) were also added for catalyst recycling experiments. All remaining polymerization operations were the same as above. The polymer obtained was filtered with neutral alumina and reprecipitated, then it was dried in a vacuum oven to a constant weight. A predetermined quantity of PMMA (0.024 mmol) was added in a dry 5 mL ampule with a stir bar as the macro-initiator, then the corresponding amount of CuBr 2 (0.047 mmol), PIL (0.108 mmol), MMA (9.4 mmol), AIBN (0.047 mmol), and benzene (3.0 mL) were added. The rest of the procedure was the same as the typical procedure for the TPSC-based ICAR ATRP described above.

Characterizations
The number-average molecular weight (M n,GPC ) and molecular weight distribution (M w /M n ) values of PMMA were determined using a TOSOH HLC-8320 gel permeation chromatograph (GPC) equipped with a refractive-index detector (TOSOH), using TSKgel guardcolumn SuperMP-N (4.6 × 20 mm) and two TSKgel SupermultiporeHZ-N (4.6 × 150 mm) with measurable molecular weights ranging from 10 3 to 1 × 10 6 g·mol −1 . THF was employed as the eluent at a flow rate of 0.35 mL·min −1 at 40 • C. The GPC samples were injected using a TOSOH plus auto-sampler and calibrated with PMMA standards purchased from TOSOH. 1 H NMR spectra were recorded on an INOVA 400 MHz nuclear magnetic resonance (NMR) instrument using CDCl 3 and DMSO-d 6 as the solvents and tetramethylsilane (TMS) as an internal standard at ambient temperature.

Selection of Solvent for TPSC-Based ICAR (Initiators for Continuous Activator Regeneration) ATRP System
The PIL (M n,GPC = 26,300 g/mol, M w /M n =1.46) was confirmed with 1H NMR spectroscopy. From Figure 1, it can be seen that the monomer's double bond disappeared at δ = 5.84-6.47 ppm and a new group of chemical shifts were found at δ = 2.65-2.95 ppm (f in Figure 1) assigned to polymer backbone hydrogen bonds, which indicated that the PIL was obtained successfully. In order to construct the TPSC-based ATRP system, we screened out the optimal solvent system firstly. The results are shown in Table 1. Six organic solvents including p-xylene, o-xylene, toluene, benzene, cyclohexane, and n-hexane were investigated to form different mixed solvents with the synthesized PIL. At room temperature, only cyclohexane/PIL solvent pair was miscible, which indicating that it did not meet the biphasic solvent requirement of TPSC system. After increasing the temperature to 70-90 • C, it is found that only benzene/PIL solvent pair became miscible completely, namely forming a homogeneous solution. Therefore, benzene was selected as the optimal organic solvent applied in this TPSC system. Actually, as shown in Figure 2, the homogeneous polymerization and heterogeneous separation process could be easily realized just by changing the reaction temperature with benzene/PIL solvent system. equipped with a refractive-index detector (TOSOH), using TSKgel guardcolumn SuperMP-N (4.6 × 20 mm) and two TSKgel SupermultiporeHZ-N (4.6 × 150 mm) with measurable molecular weights ranging from 10 3 to 1 × 10 6 g·mol −1 . THF was employed as the eluent at a flow rate of 0.35 mL·min −1 at 40 °C. The GPC samples were injected using a TOSOH plus auto-sampler and calibrated with PMMA standards purchased from TOSOH. 1 H NMR spectra were recorded on an INOVA 400 MHz nuclear magnetic resonance (NMR) instrument using CDCl3 and DMSO-d6 as the solvents and tetramethylsilane (TMS) as an internal standard at ambient temperature.

Selection of Solvent for TPSC-Based ICAR (Initiators for Continuous Activator Regeneration) ATRP System
The PIL (Mn,GPC = 26,300 g/mol, Mw/Mn =1.46) was confirmed with 1H NMR spectroscopy. From Figure 1, it can be seen that the monomer's double bond disappeared at δ = 5.84-6.47 ppm and a new group of chemical shifts were found at δ = 2.65-2.95 ppm (f in Figure 1) assigned to polymer backbone hydrogen bonds, which indicated that the PIL was obtained successfully. In order to construct the TPSC-based ATRP system, we screened out the optimal solvent system firstly. The results are shown in Table 1. Six organic solvents including p-xylene, o-xylene, toluene, benzene, cyclohexane, and nhexane were investigated to form different mixed solvents with the synthesized PIL. At room temperature, only cyclohexane/PIL solvent pair was miscible, which indicating that it did not meet the biphasic solvent requirement of TPSC system. After increasing the temperature to 70-90 °C, it is found that only benzene/PIL solvent pair became miscible completely, namely forming a homogeneous solution. Therefore, benzene was selected as the optimal organic solvent applied in this TPSC system. Actually, as shown in Figure 2, the homogeneous polymerization and heterogeneous separation process could be easily realized just by changing the reaction temperature with benzene/PIL solvent system.       Subsequently, we investigated the effect of types of ligands (Me6TRAN, PMDETA, TPMA, and TDA-1), reducing agents (AsAc, AsAc-Na, glucose, and AIBN) on polymerization. As listed in Table 2, all the polymerizations could be performed smoothly (Entries 1-8, Table 2). However, TPMA and AIBN were selected as the ligand and reducing agent, respectively, by considering polymerization rate and controllability over PMMA molecular weight and its distribution. Therefore, an optimal ICAR ATRP system could be constructed by using MMA as the model monomer, EBPA as the ATRP initiator, CuBr 2 as the catalyst, TPMA as the ligand, AIBN as the reducing agent, and benzene/PIL as the solvent pair system. In addition, we also investigated the effect of molar ratio of CuBr 2 to AIBN, and the results are shown in Table 3. It can be seen from Table 3 that the polymerization could be successfully carried out with a wide range of [AIBN] 0 :[CuBr 2 ] 0 ([AIBN] 0 :[CuBr 2 ] 0 = 0.5-2:1). However, the polymerization rate increased with the increase of the amount of AIBN since more active catalyst CuBr could be generated correspondingly as expected by ICAR ATRP mechanism [37].

Polymerization Kinetics of MMA
To make a relatively low viscosity reaction condition and a fit polymerization rate as well as the better controllability of polymerization, we finally chose the molar ratio of

Polymerization Kinetics of MMA
To make a relatively low viscosity reaction condition and a fit polymerization rate as well as the better controllability of polymerization, we finally chose the molar ratio of

Chain-End Analysis and Chain Extension
In order to verify the structure of the resultant polymer and the chain-end functionality, we made analysis of a resultant PMMA (Mn,GPC = 4250 g/mol, Mw/Mn = 1.22) by 1 H NMR spectroscopy. From the 1 H NMR spectrum of the polymer (Figure 4), it can be seen that the chemical shifts at δ = 4.0-4.1 ppm (e in Figure 4) and δ = 7.15-7.35 ppm (as in Figure 4) are attributed to the methyl of initiator EBPA and hydrogen of aromatic rings, respectively. This indicated that the initiator EBPA moieties had successfully attached on the structure of the polymers. The chemical shifts at δ = 3.78 ppm (c in Figure 4) are attributed to the bromine-terminated methyl ester group at the chain end [13]. In addition, to further demonstrate the "living" feature of the resulting polymers, we used the resultant PMMA as a macroinitiator for the chain extension via TPSC-based ICAR ATRP method. The molecular weight increased to Mn,GPC = 39,000 g/mol from Mn,GPC = 6600 g/mol after chain extension, while the molecular weight distribution kept relatively narrow (Mw/Mn = 1.36) ( Figure 5). The successful chain extension further demonstrated the "living" character of this novel ATRP catalyst separation and recycling system.

Chain-End Analysis and Chain Extension
In order to verify the structure of the resultant polymer and the chain-end functionality, we made analysis of a resultant PMMA (M n,GPC = 4250 g/mol, M w /M n = 1.22) by 1 H NMR spectroscopy. From the 1 H NMR spectrum of the polymer (Figure 4), it can be seen that the chemical shifts at δ = 4.0-4.1 ppm (e in Figure 4) and δ = 7.15-7.35 ppm (as in Figure 4) are attributed to the methyl of initiator EBPA and hydrogen of aromatic rings, respectively. This indicated that the initiator EBPA moieties had successfully attached on the structure of the polymers. The chemical shifts at δ = 3.78 ppm (c in Figure 4) are attributed to the bromine-terminated methyl ester group at the chain end [13]. In addition, to further demonstrate the "living" feature of the resulting polymers, we used the resultant PMMA as a macroinitiator for the chain extension via TPSC-based ICAR ATRP method. The molecular weight increased to M n,GPC = 39,000 g/mol from M n,GPC = 6600 g/mol after chain extension, while the molecular weight distribution kept relatively narrow (M w /M n = 1.36) ( Figure 5). The successful chain extension further demonstrated the "living" character of this novel ATRP catalyst separation and recycling system.

Catalyst Recycling and Reuse
In the constructed TPSC-based ICAR ATRP, the recycling efficiency is the most important parameter. Therefore, we conducted the following experiments using the recovered PIL phase. The monomer (MMA), ligand (TPMA), reducing agent (AIBN), and organic solvent (benzene) needed for the typical MMA polymerization were respectively added to the recovered PIL phase and carried out the subsequent recovered polymerization. When the polymerization was completed, a part of the polymer phase was taken out to have an inductively coupled plasma (ICP) test to determine the amount of transition metal remaining in the polymer solution. The recycling experiments and corresponding results are shown in Table 4. It can be seen that after five recovery experiments, the catalysis efficiency of the catalyst was still maintained at a high level, and the resulting polymers kept narrow molecular weight distributions. Importantly, the residual Cu catalyst in polymer solution phase was less than 2.2 ppm in every recycling experiment. That is to say even after five recycling polymerizations the catalyst recycling efficiency still remained high (more than 94%, as is shown in Figure 6). These results was much better than that (less than 90%) reported by our previous similar

Catalyst Recycling and Reuse
In the constructed TPSC-based ICAR ATRP, the recycling efficiency is the most important parameter. Therefore, we conducted the following experiments using the recovered PIL phase. The monomer (MMA), ligand (TPMA), reducing agent (AIBN), and organic solvent (benzene) needed for the typical MMA polymerization were respectively added to the recovered PIL phase and carried out the subsequent recovered polymerization. When the polymerization was completed, a part of the polymer phase was taken out to have an inductively coupled plasma (ICP) test to determine the amount of transition metal remaining in the polymer solution. The recycling experiments and corresponding results are shown in Table 4. It can be seen that after five recovery experiments, the catalysis efficiency of the catalyst was still maintained at a high level, and the resulting polymers kept narrow molecular weight distributions. Importantly, the residual Cu catalyst in polymer solution phase was less than 2.2 ppm in every recycling experiment. That is to say even after five recycling polymerizations the catalyst recycling efficiency still remained high (more than 94%, as is shown in Figure 6). These results was much better than that (less than 90%) reported by our previous similar

Catalyst Recycling and Reuse
In the constructed TPSC-based ICAR ATRP, the recycling efficiency is the most important parameter. Therefore, we conducted the following experiments using the recovered PIL phase. The monomer (MMA), ligand (TPMA), reducing agent (AIBN), and organic solvent (benzene) needed for the typical MMA polymerization were respectively added to the recovered PIL phase and carried out the subsequent recovered polymerization. When the polymerization was completed, a part of the polymer phase was taken out to have an inductively coupled plasma (ICP) test to determine the amount of transition metal remaining in the polymer solution. The recycling experiments and  Table 4. It can be seen that after five recovery experiments, the catalysis efficiency of the catalyst was still maintained at a high level, and the resulting polymers kept narrow molecular weight distributions. Importantly, the residual Cu catalyst in polymer solution phase was less than 2.2 ppm in every recycling experiment. That is to say even after five recycling polymerizations the catalyst recycling efficiency still remained high (more than 94%, as is shown in Figure 6). These results was much better than that (less than 90%) reported by our previous similar TPSC-based ICAR ATRP in p-xylene/PEG-200 biphasic system [27].

Conclusions
A novel TPSC-based ICAR ATRP system with high catalyst recycling efficiency was successfully constructed via a thermoregulated PIL/benzene as the solvent pair strategy. In this system, the Cu catalyst is miscible with the monomer/polymer at the polymerization temperature (70 °C); when cooled to room temperature, the ATRP catalyst dissolved in PIL was in situ separated from the polymer organic solution (benzene solution) easily and recycled for the next polymerization facilely. Therefore, this strategy can avoid the conventional tedious post-treatment steps of recycling catalyst, which will be much beneficial for the industrial process of ATRP.

Conclusions
A novel TPSC-based ICAR ATRP system with high catalyst recycling efficiency was successfully constructed via a thermoregulated PIL/benzene as the solvent pair strategy. In this system, the Cu catalyst is miscible with the monomer/polymer at the polymerization temperature (70 • C); when cooled to room temperature, the ATRP catalyst dissolved in PIL was in situ separated from the polymer organic solution (benzene solution) easily and recycled for the next polymerization facilely. Therefore, this strategy can avoid the conventional tedious post-treatment steps of recycling catalyst, which will be much beneficial for the industrial process of ATRP.