One-Pot Synthesis of Polyester-Based Linear and Graft Copolymers for Solid Polymer Electrolytes
2021; Chinese Chemical Society; Volume: 4; Issue: 9 Linguagem: Inglês
10.31635/ccschem.021.202101364
ISSN2096-5745
AutoresKairui Guo, Shaoqiao Li, Chen Gong, Jirong Wang, Yong Wang, Xiaolin Xie, Zhigang Xue,
Tópico(s)Conducting polymers and applications
ResumoOpen AccessCCS ChemistryRESEARCH ARTICLE5 Sep 2022One-Pot Synthesis of Polyester-Based Linear and Graft Copolymers for Solid Polymer Electrolytes Kairui Guo, Shaoqiao Li, Gong Chen, Jirong Wang, Yong Wang, Xiaolin Xie and Zhigang Xue Kairui Guo Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 Google Scholar More articles by this author , Shaoqiao Li Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 Google Scholar More articles by this author , Gong Chen Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 Google Scholar More articles by this author , Jirong Wang Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 Google Scholar More articles by this author , Yong Wang Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 Google Scholar More articles by this author , Xiaolin Xie Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 Google Scholar More articles by this author and Zhigang Xue *Corresponding author: E-mail Address: [email protected] Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101364 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Fabrication of a polymer electrolyte with a controllable structure affords a unique opportunity to improve its ionic conductivity and lithium-ion transference number (tLi+) due to the precise design of its polymer chain composition, molecular weight, and molecular weight distribution. Here, we utilized the one-pot combination of reversible addition-fragmentation chain transfer (RAFT) polymerization and organic acid-catalyzed ring-opening polymerization (ROP) of cyclic lactones to design polyester-based linear block and graft copolymers with controlled structures. In the simple but powerful one-pot strategy, the selected bifunctional free radical initiators, bifunctional chain transfer agents (CTAs), and bifunctional monomers were used to obtain several copolymers with different topologies. These copolymers were successfully applied as polymer matrices to solid polymer electrolytes (SPEs), which possessed sufficient ionic conductivities and high tLi+ values. Our strategy provides new insight into the facile fabrication of high-performance SPEs. Download figure Download PowerPoint Introduction Solid polymer electrolyte (SPE) with a controllable structure has been widely used in lithium-ion batteries (LIBs) due to its precisely designed chain and topological structures.1–7 The design and combination of different polymer segments can render the polymer electrolyte good mechanical strength and electrochemical performance and guarantee good cycle performance of LIBs.8–13 Conventional linear poly(ethylene oxide) (PEO)-based SPEs have low ionic conductivity (10−7–10−6 S cm−1) at room temperature.14–19 To improve the electrochemical performance of PEO-based SPEs, the topological structural design of PEO-based matrices has been extensively studied.20–24 The linear block and graft copolymers have been well designed to reduce the regularity of the PEO chain, destroy its crystallinity, and increase the ionic conductivity. However, PEO-based electrolyte with low electrochemically stable window, low room-temperature ionic conductivity, and low lithium-ion transference number makes it still unable to meet the practical application requirements of LIBs.25–28 Polyester with a low glass transition temperature (Tg) and melting temperature is a polymer-type with good biocompatibility and easy degradability,29–32 which is widely used in biomedicine and self-assembly fields.33–36 When applied to the field of SPEs, the polyester-based electrolyte possesses a broader electrochemically stable window and higher lithium-ion transference number,37–42 which is a very promising material and is expected to replace PEO-based SPEs. Notably, polyester has a similar thermal property and semi-crystallinity to PEO.43–45 Building linear block copolymers (BCPs) or graft copolymers and polymers with various topological structures to destroy the crystallinity of polyester has proved to be a good way to meet the practical application requirements of LIBs.46–51 With the development of polymer synthesis technology, various living polymerizations including atom transfer radical polymerization (ATRP),52–56 and reversible addition-fragmentation chain transfer (RAFT) polymerization,57–60 nitroxide-mediated radical polymerization (NMP),61–64 and ring-opening polymerization (ROP)65–68 have become an efficient way to construct polyester-based polymers with different topological structures. The combination of the two living polymerizations allows the molecular weight, molecular weight distribution, and chain structure to be precisely regulated, which is conducive to the realization of fine-tuning the properties of polymers. In practice, a series of attempts have been made to use dual initiators to realize radical polymerization and ROP to build controlled linear BCPs. For example, the two reactions can proceed orthogonally in one pot without being affected, so one-pot synthesis from small molecules to topological polymers can be realized.69–71 The method is expected to be applied to the field of polymer electrolytes, which is more convenient than traditional multi-step synthesis methods. Recently, Coulembier et al.72,73 proposed an ROP method using benzoic acid as a catalyst. They carried out the ring-opening (co)polymerization of L-lactide (L-LA) and ε-caprolactone (ε-CL) in the presence of various alcohol initiators. The polymers obtained have a controllable molar mass, and the distribution was narrow (1.11 < Đ < 1.35). The use of carboxylic acid catalyst avoids the residue of the traditional metal-organic catalytic system in the reaction. Moreover, the carboxylic acid group exists in a large number of commercially available chemical products, and its catalytic effect has always been easily overlooked. Inspired by the carboxylic acid-catalyzed ROP we combined with RAFT polymerization, the one-pot synthesis of controlled polyester-based copolymers is proposed in this work (Figure 1). The purpose is to establish a self-catalyzed carboxylic acid-based ROP reaction and a living radical polymerization in one pot to synthesize a copolymer with a controllable structure. We report that the 4,4′-azobis(4-cyanovaleric acid) (ACVA) as radical initiator and the (2-(benzylsulfanylthiocarbonylsulfanyl)ethanol (BSTSE) as chain transfer agent (CTA) were combined to synthesize a BCP. The special trithioester group of BSTSE can effectively transfer chain growth free radicals and adjust the molecular weight and distribution of free radical polymerization. On the basis of providing free radicals, ACVA containing carboxyl groups could catalyze BSTSE to initiate ROP, thereby completing the synthesis of BCPs in one pot. The CTA (4-(4-cyanopentanoic acid)dithiobenzoate, CPADB) and hydroxyethyl methacrylate (HEMA) were combined in a one-pot system to synthesize a graft copolymer, poly(2-hydroxyethyl methacrylate)-graft-poly(ε-caprolactone (PHEMA-g-PCL) (Figure 1a). The use of chain transfer reagent (CPADB) with carboxylic acid groups effectively controlled the molecular weight and distribution of PHEMA; it could also catalyze the hydroxyl group at the end of HEMA to initiate ROP, thereby completing the synthesis of graft copolymers in one pot. This one-pot strategy rationally combines the carboxylic acid self-catalyzed ROP and RAFT polymerization to construct a complex topological polymer. The SPEs based on the polyesters (Figure 1b) with controlled structure display high oxidative stability (>5.0 V vs Li+/Li at 60 °C) and high tLi+ values (>0.7 at 60 °C). Figure 1 | One-pot combination of RAFT polymerization and ROP. (a) Synthesis of linear block and graft polyester-based copolymers. (b) Schematic illustration of the preparation of brush-shaped PESPEs. Download figure Download PowerPoint Experimental Methods A typical procedure for ROP of ε-CL with a carboxylic acid catalyst In a typical reaction, glycolic acid (38.0 mg, 0.50 mmol) and ε-CL (3.42 g, 30.00 mmol) were placed in a dry Schlenk flask (25 mL). The oxygen in the flask was removed via three freeze/pump/thaw cycles. The flask with argon gas was immersed in the oil bath at 100 °C. At timed intervals, polymer samples were withdrawn from the flask for gel permeation chromatography (GPC) and nuclear magnetic resonance (NMR) measurements to determine the molecular weight, molecular weight distribution, and monomer conversion. The polymer solution was diluted with tetrahydrofuran (THF), and the final polymer was obtained after precipitation in excess methanol. These polymers were dried under vacuum for 1 day. A typical procedure for one-pot synthesis of PCL-based linear PCP Radical initiator and ROP catalyst (ACVA, 5.6 mg, 0.02 mmol), the RAFT agent and ROP initiator (BSTSE, 14.7 mg, 0.06 mmol), ε-CL (0.685 g, 6.00 mmol), methyl methacrylate (MMA, 0.150 g, 1.50 mmol), and mesitylene (50 vol %/MMA, internal reference for NMR) were added into a dry 25 mL of Schlenk flask. The oxygen in the flask was removed via three freeze/pump/thaw cycles. The flask with argon gas was immersed in the oil bath at 70 °C to carry out the RAFT polymerization of MMA for 10 h. After the MMA polymerization was completed, the temperature was raised to 130 °C for ROP of ε-CL for 60 h. At time intervals, the polymer samples were withdrawn from the flask for GPC and NMR measurements. The polymer solution was diluted with THF, and the final polymer was obtained after precipitation in excess methanol. These polymers were dried under vacuum for 1 day. All other BCPs ( Supporting Information Figures S1–S4) were synthesized via the same procedure. A typical procedure for one-pot synthesis of PCL-based graft copolymer Radical initiator 2,2′-azobis(2-methylpropionitrile) (AIBN, 3.28 mg, 0.02 mmol), the RAFT agent and ROP catalyst (CPADB, 16.8 mg, 0.06 mmol), HEMA (0.312 g, 2.40 mmol), ε-CL (5.478 g, 48.00 mmol), and mesitylene (50 vol %/HEMA) were added into a dry 25 mL of Schlenk flask. The oxygen in the flask was removed via three freeze/pump/thaw cycles. The flask with argon gas was immersed in the oil bath at 60 °C to run the RAFT polymerization of HEMA for 24 h. After the RAFT polymerization was completed, the temperature was raised to 130 °C for ROP for 72 h. At time intervals, polymer samples were withdrawn from the flask for GPC and NMR measurements. The polymer solution was diluted with THF, and the final polymer was obtained after precipitation in excess methanol. These polymers were dried under vacuum for 1 day. All other graft copolymers ( Supporting Information Figures S5–S9) were synthesized via the same procedure. Fabrication of brush-shaped PCL-based SPEs PHEMA-g-PCL was dissolved into acetonitrile for 2 h at 60 °C. Then LiClO4 was added into the homogeneous polymer solution and stirred for 12 h. The polymer electrolyte precursor was injected into the polyacrylonitrile (PAN) electrospinning film. The polyester-based SPE (PESPE) was obtained by evaporating the solvent under vacuum at 60 °C for 24 h. The thickness of the PESPE film was ∼50 μm. Results and Discussion Synthesis of polyester-based homopolymer Our first attempt was to synthesize the homopolymer in the presence of glycolic acid or 4-hydroxybenzoic acid as a bifunctional initiator to investigate the self-catalyzed ROP (Table 1). The ROP of ε-CL initiated and catalyzed by glycolic acid was studied in bulk polymerization at 130 °C. A molar ratio of [ε-CL]0/[glycolic acid]0 = 100/1 was used in the polymerization. Supporting Information Figure S10 shows the kinetic study of the ROP of ε-CL catalyzed by glycolic acid. The ROP of ε-CL achieved a conversion of 73% within 30 h. After 30 h, the PCL homopolymer with a molar mass of Mn = 8.8 kDa and a degree of dispersion of Đ = 1.43 was obtained. During the reaction process, the Mn,NMR and Mn,GPC were detected and found to be in good agreement with the theoretical molecular weight (Mn,theo). As shown in Table 1, high temperature promoted the ability of bifunctional initiators to catalyze the ROPs of ε-CL or δ-valerolactone (δ-VL), and the ROPs of cyclic lactones initiated by 4-hydroxybenzoic acid and glycolic acid enabled polyesters with a relatively low Đ value (<1.43) and controlled molecular weight to be obtained ( Supporting Information Figure S11). Table 1 | Results of the ROPs of ε-CL and δ-VL in Bulk Initiated by Glycolic Acid and p-Hydroxybenzoic Acida Entry [M]0 [I]0 [M]0/[I]0 T(°C) T(h) Conv. (%)b Mn,theo (kDa)c Mn,NMR (kDa)d Mn,GPC (kDa)e Ðe 1 ε-CL Glycolic acid 60 100 12 76 5.3 5.2 4.7 1.20 2 Glycolic acid 150 120 24 58 10.0 9.8 9.6 1.23 3 Glycolic acid 150 130 30 68 11.7 11.5 12.0 1.23 4 Glycolic acid 200 150 30 67 15.3 15.1 14.5 1.34 5 4-Hydroxybenzoic acid 40 100 30 68 3.2 3.1 3.2 1.23 6 4-Hydroxybenzoic acid 100 120 32 61 7.1 6.9 8.3 1.24 7 δ-VL Glycolic acid 70 120 24 61 4.3 4.2 4.0 1.30 8 Glycolic acid 100 120 30 64 6.5 6.4 7.1 1.43 aReactions were performed in bulk under argon atmosphere with reaction conditions: m I0= 0.5 mmol. bε-CL conversions were calculated from 1H NMR in CDCl3 by the ratio of integrals of the characteristic signals. cMn,theo = ([M]0/[I]0) × ConvROP × MM + MI. dCalculated from 1H NMR in CDCl3 by the ratio of integrals of the characteristic signals. eDetermined by GPC in THF as eluent using polystyrene (PS) standards. Synthesis of polyester-based BCP The bifunctional initiator-catalyzed ROPs proved to be a good strategy to obtain linear polyesters. Self-catalyzed ROP was expected to combine with other living polymerization reactions to synthesize polymers with various topological structures. Based on the self-catalyzed ROPs of cyclic lactones, we focused on more complex topological structures. The use of BSTSE with a hydroxyl group was preferred as a bifunctional CTA because it could organically link vinyl monomers and cyclic lactones, and ACVA was selected as a bifunctional initiator (Figure 1a). In the first step of low-temperature RAFT polymerization, ACVA decomposed into free radicals and promoted the polymerization of vinyl monomers (Figure 2). In the second step of high-temperature ROP reaction, the carboxyl group of the remaining ACVA in the system acted as an organic catalyst for the ring-opening reaction, avoiding the introduction of additional catalysts for ROP reactions and combining two bifunctional agents (BSTSE and ACVA) to complete the one-pot synthesis of polyester-based BCPs. The BSTSE containing hydroxyl functionality was exploited as a bifunctional initiator to allow the subsequent realization of RAFT polymerization and ROP. Figure 2 | Mechanism of one-pot synthesis of BCPs based on RAFT polymerization and self-catalyzed ROP via a carboxylic acid group. Download figure Download PowerPoint Using the two-gradient heating method, RAFT polymerization was run at 70 °C,74 and ROP at 130 °C with [MMA]0/[ε-CL]0/[BSTSE]0/[ACVA]0 = 80/500/1/0.333 (Figure 3). The linear increase of ln([M]0/[M]t) with respect to the polymerization time indicated that the MMA polymerization proceeded in a controlled manner ( Supporting Information Figure S12). In addition, with an increase in the conversion rate, Mn,GPC, Mn,NMR, and Mn,theo were highly consistent ( Supporting Information Figures S13 and S14). It should be mentioned that at 70 °C, the optimal temperature of ROP was not reached at this time; thus, ε-CL was used as a solvent in the MMA polymerization system to promote temperature rise. After 10 h, the MMA polymerization conversion rate reached more than 99% ( Supporting Information Figure S15). At this time, the system had heated to 130 °C to induce the ROP of ε-CL (Figure 3a). Notably, ln([M]0/[M]t) increased linearly relative to the reaction time of ε-CL ROP ( Supporting Information Figure S16). Figure 3 | Analysis for the one-pot synthesis of PCL-based BCPs. (a) 1H NMR spectra (400 MHz, CDCl3) of ε-CL ROP system. (b) GPC traces for BCP (PMMA-b-PCL) with the retention time. Download figure Download PowerPoint As shown in Supporting Information Figure S13, Mn,GPC increased with the increase in ε-CL conversion rate. Similarly, Mn,GPC, Mn,NMR, and Mn,theo were highly consistent ( Supporting Information Figure S17). Notably, the molecular weight distributions of all investigated BCPs (PMMA-b-PCL) were controlled in an extremely narrow Đ range (Đ < 1.17, Figure 3b), indicating that the carboxyl group-catalyzed ROP proceeded a living process. Thus, the combination of self-catalyzed ROP and RAFT polymerization was proven to be a suitable and convenient method for the one-pot synthesis of polyester-based linear BCPs with controlled structures. In the one-pot synthesis of BCPs, we examined the combination of RAFT polymerizations of methacrylate monomers (MMA, n-butyl methacrylate [BMA], and poly(ethylene glycol)methacrylate [PEGMA]) or styrene and ROP of ε-CL. Considering that high temperature could increase the reaction rate of ROP, we controlled it as a two gradient temperature reaction. The temperature of RAFT polymerization was controlled at 70 °C, and the ROP reaction was run at 130 °C. Overall, the RAFT polymerization conversion rate of methacrylate monomers was close to 99% (Table 2, entries 1–6). The length of the two blocks could be adjusted precisely to obtain different PCL-based BCPs with controlled molecular weights and narrow molecular weights’ distributions by controlling the ratio of monomer to BSTSE and the reaction time. Compared with the traditional one-pot synthesis of block polymers,69,70 our one-pot synthesis method used ACVA bifunctional free radical initiator instead of traditional free radical initiators combined with ROP catalysts (Figures 4a–4c). We focused on improving novel polyester-based copolymers to fabricate state-of-the-art SPEs with greatly enhanced electrochemical performances. For this purpose, the metal-free green ROP reaction was exploited on ε-CL directly using BSTSE with hydroxyl terminal group as an initiator and ACVA as a catalyst. Table 2 | Summarized Results of the One-Pot Synthesis of PCL-Based BCPsa Entry M1 M2 Molar ratiob Time (h) of RAFT Time (h) of ROP Conv. (%) of RAFT Conv. (%) of ROP Mn,theo (kDa)c Mn,NMR (kDa)d Mn,GPC (kDa)e Đe 1 MMA ε-CL 25/100/1 10 60 >99 89 12.8 11.4 10.6 1.26 2 MMA ε-CL 100/400/1 24 72 >99 81 48.1 45.3 39.1 1.13 3 MMA ε-CL 250/750/1 24 72 >99 83 96.2 93.2 83.6 1.11 4 BMA ε-CL 25/100/1 10 60 >99 85 13.4 12.8 15.9 1.20 5 BMA ε-CL 100/400/1 24 72 >99 78 50.1 53.1 64.2 1.10 6 BMA ε-CL 250/750/1 24 72 >99 86 109.4 105.2 86.9 1.13 7 St ε-CL 25/100/1 16 60 71 88 12.1 10.2 9.9 1.17 8 St ε-CL 250/750/1 24 72 67 72 79.3 77.3 76.2 1.26 9 St/PEGMA ε-CL 250/100/750/1 24 72 62/>99 62 116.9 101.5 103.9 1.09 aReaction conditions: ACVA and BSTSE are employed as initiator and CTA, respectively. mACVA = 5.6 mg, [BSTSE]0/[ACVA]0 = 3/1 (molar ratio), ROP is run at 130 °C, RAFT polymerization is controlled at 70 °C. MMA = methyl methacrylate, BMA = n-butyl methacrylate, St = styrene, and PEGMA = poly(ethylene glycol)methacrylate (Mn = 475 g mol−1). b[M1]0/[M2]0/[BSTSE]0. cMn,theo = ([M1]0/[BSTSE]0) × ConvRAFT × MM1 + ([M2]0/[BSTSE]0) × ConvROP × MM2 + MBSTSE. dCalculated from 1H NMR in CDCl3 by the ratio of integrals of the characteristic signals. eDetermined by GPC in THF as eluent using polystyrene (PS) standards. Figure 4 | (a and b) One-pot synthesis of BCPs by a combination of RAFT initiation system and ROP catalyst in reported works. (c) One-pot synthesis of PCL-based BCPs via carboxylic acid-catalyzed ROP and RAFT polymerization in this work. Download figure Download PowerPoint The self-catalyzed strategy avoided the introduction of metal catalysts and controlled the molecular weight distribution of BCPs in a very narrow range. Hence, we could freely choose the copolymers with different chain segments and chain lengths to fine-tune the electrochemical performances of SPEs. We selected the PCL-based BCP, PMMA-b-PCL (Table 2, entry 3), PBMA-b-PCL (Table 2, entry 6), PS-b-PCL (Table 2, entry 8), or P(St-r-PEGMA)-b-PCL (Table 2, entry 9) as the polymer matrix to preliminarily study the ionic conductivity of BCP-based SPEs with 25 wt % of LiClO4. We found that the ionic conductivity of SPEs based on PMMA-b-PCL, PBMA-b-PCL, and PS-b-PCL based were relatively similar. PMMA, PBMA, and PS segments efficiently improved the mechanical properties of SPEs and inhibited the semi-crystallization of PCL. The PCL segment was responsible for the conduction of lithium ions. When PEGMA was incorporated into the polymerization system, the ionic conductivity of the obtained P(St-r-PEGMA)-b-PCL-based SPE was significantly improved compared with PS-b-PCL, reaching 2.73 × 10−5 S cm−1 at 30 °C ( Supporting Information Figure S18). Random RAFT copolymerization technique was employed to synthesize the PCL-based BCP with the poly(St-r-PEGMA) block. The presence of oxyethylene units in the side chain of PEGMA significantly improved the solubility of lithium salts; thus, facilitating the dissociation of Li+ and oxyethylene units and increasing the ionic conductivity.75 Notably, the choice of the optimal PCL-based BCP composition for further electrochemical tests was made based on its film-forming ability and mechanical properties. This also validated the characteristics of our reported one-pot synthesis of polymer electrolytes, which could be implemented to freely design polymerized monomers, control the feed ratio, and screen for the best electrochemical performance. Synthesis of a polyester-based graft copolymer Given the enhancement on the ionic conductivity of PCL-based BCPs, the next improvement was attained either by the graft copolymerization of vinyl monomer with a hydroxyl group having the ability to initiate ROPs, such as HEMA or by an introduction of side chains with lithium-ion conduction. The CTA (CPADB) with a carboxyl group possesses the ability to catalyze the ROPs of cyclic lactones, and the PHEMA polymer obtained via the RAFT polymerization of HEMA was employed as a polyhydroxy macroinitiator to initiate the ROPs of cyclic lactones. Combining two bifunctional reagents (CPADB and HEMA) achieved the one-pot synthesis of the polyester-based graft copolymer (Figure 5). We also used the two-gradient temperature reaction method to control the RAFT polymerization to 60 °C76 and ROP to 130 °C. We added the free radical initiator (AIBN), bifunctional monomer (HEMA), self-catalyzed RAFT agent (CPADB), and ε-CL into the one-pot polymerization system. At the RAFT polymerization temperature, ε-CL did not undergo ROP due to temperature limitation, but it could be used as a suitable solvent to dissolve the HEMA polymerization mixture well. After completing the RAFT polymerization, PHEMA was employed as a polyhydroxy macroinitiator; with a carboxyl group derived from CPADB at the tail of the polymer, it could initiate and catalyze the ROP of ε-CL at 130 °C, resulting in the formation of PCL-based graft copolymer, PHEMA-g-PCL. Figure 6 presents kinetic graphs of RAFT polymerization of HEMA and ROP of ε-CL for the one-pot synthesis of graft copolymers. The kinetic study of graft copolymers synthesis starts with the RAFT polymerization of HEMA using CPADB as the CTA and AIBN as initiator. For observation convenience, we increased the molar ratio of HEMA in the polymerization system and carried out the RAFT polymerization of HEMA at 60 °C. The feed ratio was [ε-CL]:[HEMA]:[CPADB]:[AIBN] = 400:200:1:0.333. Figure 5 | Mechanism of one-pot synthesis of graft copolymers based on RAFT polymerization and self-catalyzed ROP via a carboxylic acid group. Download figure Download PowerPoint It could be seen that under the solvation effect of ε-CL, the polymerization reaction of HEMA proceeded quickly and reached a conversion rate of 99% within 12 h ( Supporting Information Figure S19). A linear relationship between ln([M]0/[M]t) and the polymerization time was noted ( Supporting Information Figure S20). Mn,GPC, Mn,NMR, and Mn,theo were highly consistent with increased HEMA conversion (Figure 6a), and PHEMA always maintained a narrow molecular weight distribution ( Supporting Information Figure S21). It should be pointed out that the polymerization temperature at this time did not reach the reaction temperature of ROP initiation. ε-CL only served as the solvent for the HEMA RAFT polymerization owing to its ability to dissolve PHEMA. Using the optimal conditions determined previously for RAFT polymerization of methacrylate monomers (Table 2), namely ([CTA]:[free radical initiator] = 3:1 by mol), a set of graft copolymers targeting different molecular weight and types of side chains could be synthesized via the one-pot method. Further, we explored the effect of feeding ratio ([ε-CL]:[HEMA]:[CPADB]:[AIBN]=500:25:1:0.333) on RAFT polymerization of HEMA. Figure 6 | Analysis for the one-pot synthesis of PCL-based graft copolymers. (a) Mn and Đ versus monomer conversion for the RAFT polymerization of HEMA at 60 °C. (b) 1H NMR spectra (400 MHz, CDCl3) of ε-CL ROP system. (c) Mn and Đ versus monomer conversion for the ROP of ε-CL at 130 °C. (d) GPC traces for graft copolymer (PHEMA-g-PCL) with the retention time. Download figure Download PowerPoint To closely monitor the ROP reaction progress, we used 1H NMR spectra to investigate the extent of polymerization (Figure 6b). After 24 h of RAFT polymerization at 60 °C, the reaction temperature was raised to 130 °C for the induction of ROP of ε-CL (Figures 6b and 6c). At this time, the double bond of HEMA had disappeared entirely, with above 99% conversion. At 130 °C, the PHEMA macroinitiator with the carboxyl-terminal group could initiate and self-catalyze the ROP of ε-CL in the absence of any additional catalysts, and the ε-CL conversion reached 82% in 72 h. Except for high conversion of ε-CL obtained from the 1H NMR, the 1H NMR revealed signals assigned to PCL side chains, namely at 4.05 (-O-CH2CH2-), 2.30 (-O-CH2CH2CH2CH2CH2-), 1.64 (-O-CH2CH2CH2CH2CH2-), and 1.37 ppm (-O-CH2CH2CH2CH2CH2-). Similarly, Mn,GPC, Mn,NMR, and Mn,theo were highly consistent with an increase in ε-CL conversion (Figure 6c). The molecular weight distribution gradually decreased with increasing molecular weight (Đ < 1.12, Figure 6d). The GPC chromatograms of all investigated graft copolymers exhibited a single symmetrical peak and showed no competitive side reactions. All these characterizations proved that the one-pot method based on the self-catalytic effect of CPADB could synthesize a polyester-based graft copolymer with various lengths of main or side chains (PHEMA-g-PCL) by adjusting the polymerization temperature and time. As indicated above, the random RAFT copolymerization for the one-pot synthesis of the PCL-based BCPs approach could be used to prepare high-performance SPEs. Thus, we utiliz
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