Dual Switching in Both RAFT and ROP for Generation of Asymmetric

Nov 29, 2017 - Moreover, in HMCP-PCL-mediated RAFT polymerization, the polymerization system becomes more viscous; polymer chains may experience diffu...
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Dual Switching in Both RAFT and ROP for Generation of Asymmetric A2A1B1B2 Type Tetrablock Quaterpolymers He Dong, Yuejia Zhu, Zhenjiang Li, Jiaxi Xu, Jingjing Liu, Songquan Xu, Haixin Wang, Yu Gao, and Kai Guo* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, 30 Puzhu Road South, Nanjing 211816, China S Supporting Information *

ABSTRACT: In reversible addition−fragmentation chain transfer (RAFT) polymerization, monomers are divided into “moreactivated” monomers (type-A1 monomer) and “less-activated” monomers (type-A2 monomer). In ring-opening polymerization (ROP), monomers are considered to fall into electrophilically polymerizable monomers (lactones and carbonates, type-B1 monomer) and nucleophilically polymerizable monomers (lactides and carbonates, type-B2 monomer). Developing a strategy to copolymerize the four kinds of monomers for formation of asymmetric A2A1B1B2 type tetrablock quaterpolymers by one-pot sequential ROP and RAFT polymerization is a challenge. Herein, we designed and synthesized a molecule, 2-hydroxyethyl 2(methyl(pyridin-4-yl)carbamothioylthio)propanoate, which functioned as a trifunctional initiator, to initiate ROPs and to modulate RAFT polymerizations sequentially in one-pot. We proposed a dual “acid/base switch” strategy in both RAFT polymerizations and ROPs for one-pot generation of asymmetric A2A1B1B2 type tetrablock quaterpolymers. A series of di-, tri-, and tetrablock copolymers were synthesized and showed predicted molar mass and narrow dispersities, manifesting that the ROPs and RAFT polymerizations proceeded independently in controlled manners. The dual “acid/base switch” strategy paved a new avenue to combine RAFT polymerizations and ROPs for synthesis of designed copolymers with advanced functionalities and architectures.

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categories: “more-activated” monomers (MAMs, type-A1 monomer, Figure 1) and “less-activated” monomers (LAMs, type-A2 monomer, Figure 1). MAMs have double bond conjugate to an aromatic ring (e.g., styrenics) or a carbonyl group (e.g., (methyl)acrylates and (meth)acrylamides), while LAMs possess an oxygen or nitrogen lone pair connected to the double bond (e.g., vinyl esters, vinyl amides).24 In the RAFT process, MAMs are not compatible with LAMs, and each one type of the MAMs or LAMs uses its own corresponding one type of RAFT agent.25−29 In parallel, ROPs of cyclic monomers including cyclic esters (lactones and lactides) and carbonates are scarcely copolymerizable under one catalysis mode.30−34 Organocatalyzed ROPs workable in copolymerizations of

INTRODUCTION Block copolymer synthesis incorporating chemically distinct monomers, such as vinyl monomers (type-A monomer) and cyclic carbonyl-containing monomers (type-B monomer), requires integration of mechanically different polymerizations.1−10 Recent development in dual living polymerizations composed of radical polymerization and ring-opening polymerization (ROP) was a success in producing AB type diblock copolymers in one-pot.2,11−17 However, further difficulties are visible.18,19 Even though some monomers A and B are compatible under one-pot processes, the varied monomers of type A or type B are not inherently copolymerizable. Within each type, monomers A or monomers B are not copolymerizable between themselves under identical conditions. A prominent example in radical polymerizations is reversible addition−fragmentation chain transfer (RAFT) process.20−23 Monomers are popularly considered to fall within two main © XXXX American Chemical Society

Received: August 17, 2017 Revised: November 9, 2017

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Figure 1. Classification of four kinds of monomers (the monomers showed here are examples of each type, not represent all).

Scheme 1. (A) “Acid/Base Switch” in RAFT Polymerization for Synthesis of A1A2 Type Diblock Copolymers; (B) “Acid/Base Switch” in ROP for Synthesis of B1B2 Type Diblock Copolymers; (C) Dual “Acid/Base Switch” in Both RAFT and ROP for Generation of a Series of Multiblock Copolymers from AB Type to A2A1B1B2 Type

lactides and carbonates (type-B2 monomer, Figure 1) usually appear to be inactive in lactones, while electrophilic catalysis shows good performances in lactones and carbonates (type-B1 monomer, Figure 1) but not in lactides.35 We wonder whether we could integrate RAFT process and ROP in one-pot to produce diblock copolymers of AB type. Underneath the conditions, we further attempt to synthesize multiblock copolymers of A2A1B1B2 type, in which A1 and A2 are subsets of A, but noncopolymerizable by one type of RAFT agents, and B1 and B2 are subsets of B, but noncopolymerizable by one organocatalysis mode either. To address problems of polymerizing the above-mentioned noncopolymerizable “co-monomers”, an emerging switch methodology inspired us to challenge the task by designing dual switchable RAFT and ROP strategy (Scheme 1) toward A2A1B1B2 type copolymers.18,36 A new strategy combining a switch in RAFT (Scheme 1A) and a switch in ROP (Scheme

1B) by a novel trifunctional initiator (Scheme 1C) makes MAMs and LAMs, as well as esters and carbonates, copolymerizable through sequential feeding of the “comonomer” subsets, to produce significant tetrablock quaterpolymers of A2A1B1B2 sequences in one-pot.37−39 In the purpose of combining RAFT process and ROP catalysis into one-pot polymerizations, we design and synthesize a molecule, 2-hydroxyethyl 2-(methyl(pyridin-4yl)carbamothioylthio)propanoate (HMCP, Scheme 2), serving as a trifunctional initiator to initiate ROPs and to modulate RAFT polymerizations. On the switchable trifunctional initiator, the thiocarbonyl mediates RAFT polymerizations, the hydroxyl initiates ROPs, and a third functional group 4pyridyl incorporated will modulate the reactivity of the thiocarbonyl by protonating and deprotonating the 4pyridyl.37,40−42 To the best of our knowledge, there has not yet been any trifunctional initiator reported able to produce B

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polymerization to prepare diblock copolymers of AB type in one-pot. Before attempting one-pot synthesis of block copolymers, kinetics investigations of TfOH and MSA catalyzed respectively ROPs of CL with HMCP as initiator were carried out in acetonitrile at 30 °C. A ratio of [CL]0/ [HMCP]0/[TfOH or MSA]0 = 40/1/2 was used in the polymerizations. Figure 2 shows kinetics investigation of TfOH catalyzed ROP of CL. In the polymerizations, 2 equiv of TfOH was added, 1 equiv of TfOH was used as protonating agent, and another 1 equiv of TfOH acted as acidic catalyst for ROP (all the equivalent values of acid or base is with respect to HMCP in this paper). TfOH-catalyzed ROP of CL proceeded fast and achieved 97% conversion in 6 h (Figure 2C). A linear dependence between ln([M]0/[M]t) and time was observed (Figure 2A), indicating that TfOH-catalyzed ROP of CL was in a controlled manner. When 1 equiv of DMAP was added in the reaction mixture after 140 min, the ROP of CL was completely quenched because 1 equiv of catalyst TfOH was neutralized (Figure 2B). A poly(ε-caprolactone) (PCL) homopolymer with molar mass of Mn = 4.70 kg/mol and dispersity Đ = 1.22 was obtained after 6 h (run 1 in Table 1, Figure 2D). Kinetics investigation of MSA-catalyzed ROP of CL is shown in Figure S4, in which 2 equiv of MSA was introduced in the polymerization, 1 equiv of MSA played the role of protonating agent, and another 1 equiv of MSA was used as catalyst for ROP. The ROP of CL proceeded smoothly and achieved 96% conversion in 11 h (Figure S4C). The value of ln([M]0/[M]t) increased linearly with time, showing a controlled ROP process (Figure S4A). After addition of 1 equiv of DMAP into the reaction mixture at 6 h, 1 equiv of catalyst MSA was neutralized; the ROP of CL was completely quenched (Figure S4B). The MSA-catalyzed PCL homopolymer with Mn of 4.70 kg/mol and narrow dispersity (Đ = 1.08) was obtained after 11 h of polymerization (run 2 in Table 1, Figure S4D). The obtained PCL was characterized by 1H NMR spectroscopy. Among all the signals, the characteristic pyridyl ArH protons at 8.01 ppm demonstrated the incorporation of the protonated initiator in the macromolecular chain (Figure S5). After the addition of DMAP, this pyridyl ArH protons has shifted to 7.27 ppm, indicating that pyridyl end group could deprotonate by DMAP (Figure S6). Additionally, the polymer main chain attributed to the remaining peaks at 4.05, 2.30, 1.64, and 1.37 ppm. All these observations demonstrated that the length of the polyesters blocks could be controlled easily by adjusting the addition time of DMAP. Synthesis of Diblock Copolymers. In the second step, diblock copolymers of A1B1 type and A2B1 type were

Scheme 2. Structure of Acid/Base Switchable Trifunctional Initiator HMCP

tetrablock quaterpolymers of A2A1B1B2 type. In this study, we propose, for the first time, a dual “acid/base switch” (ABS) strategy in both RAFT polymerization and ROP for one-pot generation of asymmetric A2A1B1B2 type tetrablock quaterpolymers (Scheme 3).

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RESULTS AND DISCUSSION In the dual ABS strategy, acid plays the roles of an acid catalyst for ROP and protonating agent for HMCP. To provide effective control over the RAFT polymerizations of MAMs, the acid used here should be strong enough to protonate HMCP completely at the ratio of acid/HMCP = 1:1.37,40,41 Several acid catalysts have been reported to be efficient for ROPs, including sulfonamides,43−46 sulfonic acids (RSO2OH),32,47−49 carboxylic acids,50−56 and phosphoric acids.57−60 It has turned out that the catalysis efficiency does not simply correlate with acidity.61 The acid used here must capable of protonating the bases and highly active in the ring-opening polymerizations. On the basis of our previous experience in acid-catalyzed ROPs,62−65 we choose TfOH and MSA as acidic catalysts and protonating agents. The protonation of HMCP is reversible, and HMCP is stable in the protonation and deprotonation process (Figures S2 and S3). We selected ε-caprolactone (CL) and trimethylene carbonate (TMC) as monomer B1, L-lactide (LLA) as monomer B2, N,Ndimethylacrylamide (DMA) and methyl acrylate (MA) as monomer A1, N-vinylpyrrolidone (NVP) and vinyl acetate (VAc) as monomer A2, trifluoromethanesulfonic acid (TfOH) and methanesulfonic acid (MSA) as acid catalyst and protonating agent, and 4-(dimethylamino)pyridine (DMAP) and 8-diazabicyclo[5.4.0]undec-7-ene (DBU) as base catalyst and deprotonating agent. Ring-Opening Polymerization of ε-Caprolactone Initiated by Trifunctional Initiator HMCP with Strong Acids as Catalysts. At the onset of the investigation on the dual ABS strategy in the preparation of the multiblock copolymers, we first applied this protocol in sequential ROP and RAFT

Scheme 3. Synthesis of Tetrablock Quaterpolymers of A2A1B1B2 Type via the Dual ABS Strategy

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Figure 2. Kinetics plots of TfOH catalyzed ROP of CL: (A) ln([M]0/[M]t) versus polymerization time; (B) ln([M]0/[M]t) versus polymerization time with 1 equiv of DMAP injected into the reaction mixture at 140 min. (C) Conversions of CL versus polymerization time. (D) Mn,sec versus monomer conversion.

Table 1. Experimental Conditions and Characterization of Homopolymers and Diblock Polymersa time (h) runb 1 2 3 4 5 6 7 8 9 10 11 12 13 14

M1

M2

CL CL

CL CL CL CL CL CL CL TMC TMC TMC

DMA NVP MA MA DMA DMA VAc NVP NVP DMA NVP

acid

Qc

ROP

TfOH MSA TfOH

0/2/1 0/2/1 0/1/1 0/0/1 1/2/1 1/2/1 1/2/1 2/2/1 2/2/1 1/2/1 2/2/1 1/2/1 1/2/1 2/2/1

6 11

TfOH MSA TfOH TfOH TfOH TfOH TfOH MSA MSA MSA

6 11 6 6 6 6 6 60 60 60

RAFT

conv (%)d ROP

RAFT

97 96 6 40 8 8 5 5 48 26 26 5 28

95 92 97 97 98 21 86 17 95

97 96 97 97 97 97 97 96 96 96

94 80

Mn,theod (kg/mol)

Mn,NMRe (kg/mol)

Mn,SECf (kg/mol)

Đf

4.70 4.70 4.10 4.40 8.00 7.90 8.60

5.10 4.90 4.50 4.20 9.00 9.20 8.20

4.40 4.30 4.30 4.50 8.60 8.80 9.20

1.22 1.08 1.11 1.13 1.13 1.15 1.13

7.70

8.40

7.30

1.21

8.90 3.90 7.60 7.40

7.70 4.30 8.20 6.80

8.10 4.40 7.90 7.80

1.15 1.09 1.14 1.22

a [HMCP]0 = 0.1 mol/L as a constant in acetonitrile, [M1]0/[M2]0/[HMCP]0 = 40/40/1, ROP at 30 °C, RAFT polymerization at 60 °C. b[M2]0/ [HMCP]0/[AIBN]0 = 40/1/0.25 (entries 3, 5−8, and 13), [M2]0/[HMCP]0/[AIBN]0 = 40/1/0.33 (entries 4, 9−11, and 14); the solution of AIBN in a small amount of acetonitrile was injected into mixture for RAFT polymerization. cQ = [DMAP or DBU]0/[acid]0/[HMCP]0. dMn,theo = ([M1]0/ [HMCP]0) × convROP × MM1 + ([M2]0/[HMCP]0) × convRAFT × MM2 + MHMCP. eCalculated from 1H NMR in CDCl3 by the ratio of integrals of the characteristic signals. fDetermined by SEC in N,N-dimethylformamide (DMF) using polystyrene (PS) standards.

synthesized in one pot on the basis of first block (Scheme 3). The precursor poly(B1) was used as a macro-RAFT agent; monomer of the second block was added into the mixture of the first block precursor. The efficiency of these macro-RAFT agents in controlling polymerizations of MAMs and LAMs was dependent upon the electronic properties of the pyridyl group on the dithiocarbamate nitrogen. Protonated macro-RAFT

agents bearing a pyridinium moiety were effective in controlling polymerization of MAMs but retarded LAMs, while neutral macro-RAFT agents bearing a pyridyl can control RAFT polymerization of LAMs efficiently but inhibit MAMs. To indicate the controlled manner of ABS method in the preparation of A1B1 type diblock copolymers, the kinetics plots of the RAFT polymerization of DMA with protonated HMCP D

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Figure 3. (A−C) Kinetics plots of RAFT polymerization of DMA using protonated HMCP and HMCP-PCL as RAFT agents. The molar ratio was [DMA]0/[CL]0/[HMCP or HMCP-PCL]0/[TfOH]0/[DMAP]0 = 40/40/1/2/1. (A) ln([M]0/[M]t) of DMA versus polymerization time for HMCP (red line) and HMCP-PCL (black line). (B) Mn and Đ versus monomer conversion. (C) SEC traces for PDMA-b-PCL block copolymers with the elution time. (D, E) Kinetics plots of RAFT polymerization of NVP using HMCP and HMCP-PCL as RAFT agents. The molar ratio was [NVP]0/[CL]0/[HMCP or HMCP-PCL]0/[TfOH]0/[DMAP]0 = 40/40/1/2/2. (D) ln([M]0/[M]t) of DMA versus polymerization time for HMCP (red line) and HMCP-PCL (black line). (E) Mn and Đ versus monomer conversion. (F) SEC traces for PNVP-b-PCL block copolymer with the elution time.

and HMCP-PCL as RAFT agents are plotted at 60 °C at a feed ratio of [DMA]0/[HMCP or HMCP-PCL]0/[AIBN]0 = 40/1/ 0.25 (runs 3 and 7 in Table 1, Figure 3A−C). Linear increases of ln([M]0/[M]t) versus reaction time indicated that RAFT polymerization of DMA proceeded in a controlled manner (Figure 3A). Moreover, Mn develoyped linearly with the increase of monomer conversions; the observed numbers of Mn,NMR and Mn,SEC obtained from NMR and SEC, respectively, fit well with theoretical value (Mn,theo) (Figure 3B). The SEC traces of the block copolymers were monomodal (Đ < 1.2) and shifted toward the higher molar mass region throughout the

RAFT polymerization of DMA (Figure 3C). The successful synthesis of poly(N,N-dimethylacrylamide)-b-poly(ε-caprolactone) (PDMA-b-PCL) diblock copolymers was confirmed by 1 H NMR spectroscopy (Figure S7). All these results are consistent with controlled/living process. In parallel, 2 equiv of DMAP was added to the mixture of PCL precursor; the conversion of DMA was 21% after 5 h at 60 °C (Table 1, run 8). This result was in accordance with our expectationadditional base deprotonated 4-pyridinium moiety of marco-RAFT agent and then deprotonated HMCP-PCL inhibited RAFT polymerization of MAMs. Moreover, we E

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Table 2. Experimental Conditions for RAFT Polymerization of NVP Using Different Macro-RAFT Agents and Characterization of Resulting Triblock Terpolymers runa

macro-RAFT agentb

base

Qd

time (h)

conv (%)e

Mn,theof (kg/mol)

Mn,NMRe (kg/mol)

Mn,SECg (kg/mol)

Đg

1 2 3 4 5

PMA-b-PCL PMA-b-PCL PMA-b-PCLc PMA-b-PCL PMA-b-PCL

DMAP DMAP DBU DBU DBU

40/1/1 40/1/2 40/1/1 40/1/1 40/1/2

36 36 36 36 36

91 83 93 95 17

12.0 11.7 12.1 12.2

13.2 10.3 14.2 13.5

10.9 12.6 13.7 14.0

1.17 1.20 1.19 1.21

a [NVP]0/[HMCP]0/[AIBN]0 = 40/1/0.33. bMacro-RAFT agents used here were prepared previously; the experimental condition and characterization of PMA-b-PCL are shown in Table 1, run 6. cThe experimental condition and characterization of PMA-b-PCL are shown in Table 1, run 5. dQ = [NVP]0/[HMCP]0/[base]0. eCalculated from 1H NMR in CDCl3 by the ratio of integrals of the characteristic signals. fMn,theo = ([NVP]0/[HMCP]0) × convRAFT × MNVP + (Mn,theo of macro-RAFT agent). gDetermined by SEC in DMF using PS standards.

catalyzed PCL as precursor; the characterization of PVAc-bPCL indicated that the process of RAFT was well-controlled (run 9 in Table 1, Figure S10). It is worth noting that a much shorter induction period was observed in the RAFT polymerizations with macro-RAFT agents (Figure 3A,D), and the rate of polymerization became moderately fast in the presence of macro-RAFT agents. Similar results have also been reported from the RAFT polymerizations using other dithiobenzoate-based agents.70,71 The crosstermination between short radicals and the intermediate radical formed by the RAFT process was thought to be the reason for retardation in dithiobenzoate-mediated RAFT polymerization. The exact meaning of short radicals has been estimated to be radicals including initiator fragments, RAFT leaving groups, and dimeric chains or shorter. In contrast to macro-RAFT agent such as HMCP-PCL with a long chain R group, low molar mass RAFT agent such as HMCP generate more short radicals and lead to a longer induction period with a lower polymerization rate. Moreover, in HMCP-PCL-mediated RAFT polymerization, the polymerization system becomes more viscous; polymer chains may experience diffusion limitations and the radical reactions (radical addition and termination) readily become diffusion controlled.72 When reactions became diffusion controlled, the activation/deactivation equilibrium between propagating chain and dormant chain started to shift to the direction of the propagating chain, which yielded a high radical concentration and thus enhanced polymerization rate. In addition, TMC was selected as B1 monomer to prepare 1 1 A B and A2B1 type diblock copolymers. MSA was used as the catalyst for ROP of TMC, since decarboxylation often occurs in TfOH-catalyzed ROPs of TMC. MSA-catalyzed ROP of TMC proceeded in the ratio of [TMC]0/[MSA]0/[HMCP]0 = 40/2/ 1; the conversion of TMC reached to 96% after 60 h (Table 1, run 12). The poly(trimethylene carbonate) (PTMC) was characterized by 1H NMR (Figure S11) and SEC. After that the living PTMC precursor was used as macro-RAFT agent to prepare diblock copolymers poly(N,N-dimethylacrylamide)-bpoly(trimethylene carbonate) (PDMA-b-PTMC) and poly(Nvinylpyrrolidone)-b-poly(trimethylene carbonate) (PNVP-bPTMC) (runs 13 and 14 in Table 1, Figures S12 and S13). The resulting diblock copolymers showed narrow dispersity, indicating that the one-pot synthesis proceeded successfully in a controlled manner. Those entire evidence manifest the welldefined diblock copolymers of AB type were successfully prepared through the ABS strategy in one-pot sequential ROP and RAFT polymerizations. Synthesis of Triblock Terpolymers. The significance of the ABS method combining ROP and RAFT polymerizations is the ability to control a much wider range of monomers than the

investigated the effects of marco-RAFT agent protonation by TfOH and MSA on controlling RAFT polymerization of MA. As shown in Table 1, runs 5 and 6, experimental conditions of two polymerizations were the same, except for acid and PCL precursors. After 8 h, the conversions of MA were approximately equal and all high up to 94%. Observed Mn,NMR and Mn,SEC of the two copolymers both agreed with the theoretical value Mn,theo, and the values of dispersity were both low (99%, TCI), methyl acrylate (MA; >99%, TCI), N-vinylpyrrolidone (NVP; >99%, TCI), and vinyl acetate (VAc; >99%, TCI) were filtered through a neutral alumina column (70−230 mesh), dried over calcium hydride, and distilled under reduced pressure. ε-Caprolactone (CL, 99.0%, TCI) was dried over calcium hydride and was purified by vacuum distillation. Trimethylene carbonate (TMC, 99%, Daigang Biomaterial Co.) was recrystallized from benzene/n-hexane three times to obtain white crystals. L -Lactide (LLA; >98%, TCI) was purified through recrystallization using dry toluene at least three times and dried under high vacuum for 72 h. 2,2′-Azoisobutyronitrile (AIBN, 98%, Energy Chemical) was recrystallized three times from methanol before use. 4-(Dimethylamino)pyridine (DMAP, 99%, J&K Scientific Co.), 2-

HMCP-poly(B1) mediates RAFT polymerization of A1 to afford A1B1 type diblock copolymer. Third (step III), introduce another 1 equiv of base to deprotonate the 4-pyridinium moiety; a neutral marco-RAFT agent controls RAFT polymerization of monomer A2 to produce A2A1B1 type triblock terpolymer. Finally (step IV), add 1 equiv of DBU to catalyze ROP of B2 to synthesize tetrablock quaterpolymer in A2A1B1B2 sequences. To further prove the reproducibility of the dual ABS strategy for synthesis of A2A1B1B2 type tetrablock quaterpolH

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the conversion of B1 monomer was determined by 1H NMR measurement. The resulting poly(B1) was precipitated in an excess of cold methanol, filtered, and dried under vacuum for 48 h. CL was chosen as B1 monomer here. RAFT Polymerizations of A1 Monomers Controlled by HMCP. A general procedure for RAFT polymerization of A1 monomer was carried out at a ratio of [A1]0/[HMCP]0/[acid]0/[AIBN]0 = 40/ 1/1/0.25. A1 (16.0 mmol), HMCP (0.120 g, 0.40 mmol), acid (0.40 mmol), and a solution of AIBN (16.4 mg, 0.10 mmol) in dry acetonitrile (0.2 mL) were dissolved in 4.0 mL of dry acetonitrile in an ampule. The ampule was degassed by three repeated freeze− evacuate−thaw cycles and sealed. The mixture was heated at 60 °C for a predetermined time. During polymerization, 0.1 mL of the mixture was withdrawn periodically to determine the monomer conversion by 1H NMR measurement. The resulting poly(A1) was precipitated in an excess of cold methanol, filtered, and dried under vacuum for 48 h. DMA and MA were chosen as A1 monomer; MSA and TfOH were chosen as acid here. RAFT Polymerizations A2 Monomers Controlled by HMCP. A general procedure for RAFT polymerization of A1 monomer was carried out at a ratio of [A1]0/[HMCP]0/[acid]0/[AIBN]0 = 40/1/0/ 0.33. A1 (16.0 mmol), HMCP (0.120 g, 0.40 mmol), and a solution of AIBN (21.9 mg, 0.13 mmol) in dry acetonitrile (0.2 mL) were dissolved in 4.0 mL of acetonitrile in an ampule. The ampule was degassed by three repeated freeze−evacuate−thaw cycles and sealed. The mixture was heated at 60 °C for a predetermined time. During polymerization, 0.1 mL of the mixture was withdrawn periodically to determine the monomer conversion by 1H NMR measurement. The resulting poly(A2) was precipitated in an excess of cold methanol, filtered, and dried under vacuum for 48 h. NVP and VAc were chosen as A2 monomer; MSA and TfOH were chosen as acid here. One-Pot Synthesis of Diblock Copolymers of A1B1 Type. The mixed solution of base (0.40 mmol), A1 (16.0 mmol), AIBN (16.4 mg, 0.10 mmol), and acetonitrile (0.2 mL) was added into the reaction mixture of poly(B1) in an ampule. The ampule was degassed by three repeated freeze−evacuate−thaw cycles and sealed. The mixture was heated at 60 °C for a predetermined time. During polymerization, 0.1 mL of the mixture was withdrawn periodically to determine the monomer conversion by 1H NMR measurement. The resulting A1B1 type diblock copolymers were precipitated in an excess of cold methanol, filtered, and dried under vacuum for 48 h. DMA and MA were chosen as A1 monomer; DMAP and DBU were chosen as base here. One-Pot Synthesis of Diblock Copolymers of A2B1 Type. The mixed solution of base (0.80 mmol), A2 (16.0 mmol), AIBN (21.9 mg, 0.13 mmol), and acetonitrile (0.2 mL) was added into the reaction mixture of poly(B1) in an ampule. The ampule was degassed by three repeated freeze−evacuate−thaw cycles and sealed. The mixture was heated at 60 °C for a predetermined time. During polymerization, 0.1 mL of the mixture was withdrawn periodically to determine the monomer conversion by 1H NMR measurement. The resulting A2B1 type diblock copolymers were precipitated in an excess of diethyl ether, filtered, and dried under vacuum for 48 h. NVP and VAc were chosen as A2 monomer; DMAP and DBU were chosen as base here. One-Pot Synthesis of Triblock Terpolymers of A2A1B1 Type. The mixed solution of base (0.40 mmol), A2 (16.0 mmol), AIBN (21.9 mg, 0.13 mmol), and acetonitrile (0.2 mL) was added into the reaction mixture of A1B1 type diblock copolymer in an ampule. The ampule was degassed by three repeated freeze−evacuate−thaw cycles and sealed. The mixture was heated at 60 °C for a predetermined time. During polymerizations, 0.1 mL of the mixture was withdrawn periodically to determine the monomer conversion by 1H NMR measurement. The resulting A2A1B1 type triblock terpolymers were precipitated in an excess of diethyl ether, filtered, and dried under vacuum for 48 h. The kind of base used here was same with the base used in A1B1 type diblock copolymer. One-Pot Synthesis of Tetrablock Quaterpolymers of A2A1B1B2 Type. In a glovebox, B2 (20.0 mmol) and DBU (59.8 μL, 0.40 mmol) were added in the reaction mixture of A2A1B1 type triblock terpolymers in an ampule. The mixture was heated at room

bromopropionyl bromide (99%, J&K Scientific Co.), n-butyllithium (n-BuLi, 1.6 M in hexanes, J&K Scientific Co.), 4-(methylamino)pyridine (>98%, TCI), trifluoromethanesulfonic acid (TfOH, 99.8%, Alfa Aesar), methanesulfonic acid (MSA, 99.5%, TCI), ethylene glycol (anhydrous, 98%, J&K Scientific Co.), and 8-diazabicyclo[5.4.0]undec7-ene (DBU, 98%, TCI) were used as received. Instrumentation and Measurements. The number-average molecular weight (M n,NMR ) and monomer conversion were determined by 1H nuclear magnetic resonance (1H NMR, Bruker ARX-250 spectrometer, 300 MHz) spectroscopy in CDCl3 at ambient temperature. The delay time used here for 1H NMR spectra is 1 s. The number of scans for 1H NMR spectra is 32 times. DMA monomer conversion (%) was determined by comparing the integrated peak area of the signals at 6.4−6.6 ppm (1H of the monomer) with that of the peak at 2.6−3.1 ppm (6H of the monomer and 6H of the corresponding polymer). NVP monomer conversion (%) was determined by comparing the integrated peak area of the signals at 6.8−7.0 ppm (1H of the monomer) with that of the peak at 2.9−3.4 ppm (2H of the corresponding polymer). CL monomer conversion (%) was determined by comparing the integrated peak area of the signals at 4.20−4.30 ppm (2H of the monomer) with that of the peak at 4.04−4.12 ppm (2H of the corresponding polymer). The numberaverage molecular weight (Mn,SEC) and dispersity (Đ) were determined by size exclusion chromatography (SEC) using a SSI 1500 pump coupled with a column (Waters Styragel HR 2, 5.0 μm, 300 × 7.8 mm) and a differential refractive index detector (DRI detector, Wyatt Optilab rEX, 658 nm).The column was calibrated with narrow polydispersity PMMA standards. The eluent used was 0.010 M LiBr/ DMF at a flow rate of 0.70 mL/min at room temperature. Synthesis of 2-Hydroxyethyl 2-(Methyl(pyridin-4-yl)carbamothioylthio)propanoate (HMCP). 2-Hydroxyethyl 2-bromopropionate was synthesized using the method described in the literature.10 n-BuLi (6.25 mL, 1.6 M in hexane, 10 mmol) was added to the solution of 4-(methylamino)pyridine (1.08 g, 10 mmol) in dry THF (100 mL) under nitrogen. The mixture was left at ambient temperature for 1 h. Afterward, carbon disulfide (0.90 mL, 1.14 g, 15 mmol) was added dropwise, and the reaction mixture was stirred at room temperature for 3 h under nitrogen. Subsequently, 2hydroxyethyl 2-bromopropionate (2.17 g, 11 mmol, 1.1 equiv) was injected in. The mixture was stirred at room temperature for 6 h, and then saturated sodium bicarbonate solution NaHCO3 (50 mL) was added. The solution was extracted with dichloromethane (100 mL × 3) and saturated brine (100 mL × 2), and the organics were combined. After the removing of solvent under reduced pressure, the viscous yellow liquid was obtained. The resulting mixture was purified by column chromatography using hexane/ethyl acetate (2:5 v/v) as eluent. The product was a yellow solid (2.17 g, yield = 72.3%). 1H NMR (300 MHz, CDCl3) (Figure S1): δ 1.54 (d, 3H, CH3), 3.72 (s, 3H, NCH3), 3.83 (t, 2H, CH2OH), 4.30 (m, 2H, OCH2CH2OH), 4.62 (q, 1H, CH), 7.27 (m, 2H, m-ArH), 8.75 (m, 2H, o-ArH). Ring-Opening Polymerizations of B1 Monomers Catalyzed by MSA. A general procedure for ROP of B1 monomer was carried out at a ratio of [B1]0/[HMCP]0/[MSA]0 = 40/1/2. In a glovebox, B1 (16.0 mmol), HMCP (0.120 g, 0.40 mmol), and MSA (52 μL, 0.80 mmol) were dissolved in 4.0 mL of acetonitrile in an ampule. The mixture was stirred at 30 °C for a predetermined time under an argon atmosphere. During polymerizations, 0.10 mL of the reaction mixtures was taken out periodically, an excess of triethylamine was added, and the conversion of B1 monomer was determined by 1H NMR measurement. The resulting poly(B1) was precipitated in an excess of cold methanol, filtered, and dried under vacuum for 48 h. CL and TMC were chosen as B1 monomer here. Ring-Opening Polymerizations of B1 Monomers Catalyzed by TfOH. A general procedure for ROP of B1 monomer was carried out at a ratio of [B1]0/[HMCP]0/[TfOH]0 = 40/1/2. In a glovebox, B1 (16.0 mmol), HMCP (0.120 g, 0.40 mmol), and TfOH (70.8 μL, 0.80 mmol) were dissolved in 4.0 mL of acetonitrile in an ampule. The mixture was stirred at 30 °C for a predetermined time under an argon atmosphere. During polymerizations, 0.10 mL of the reaction mixtures was taken out periodically, an excess of triethylamine was added, and I

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Macromolecules temperature for a predetermined time. During polymerization, 0.1 mL of the mixture was withdrawn periodically to determine the monomer conversion by 1H NMR measurement. The resulting A2A1B1B2 type tetrablock quaterpolymers were precipitated in an excess of cold methanol, filtered, and dried under vacuum for 48 h. LLA was chosen as B2 monomer here.

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(7) Jakubowski, W.; Matyjaszewski, K. Activator generated by electron transfer for atom transfer radical polymerization. Macromolecules 2005, 38, 4139−4146. (8) Bernaerts, K. V.; Du Prez, F. E. Dual/heterofunctional initiators for the combination of mechanistically distinct polymerization techniques. Prog. Polym. Sci. 2006, 31, 671−722. (9) Lattuada, M.; Hatton, T. A. Functionalization of monodisperse magnetic nanoparticles. Langmuir 2007, 23, 2158−2168. (10) Kang, H. U.; Yu, Y. C.; Shin, S. J.; Kim, J.; Youk, J. H. One-Pot Synthesis of Poly(N-vinylpyrrolidone)-b-poly(epsilon-caprolactone) Block Copolymers Using a Dual Initiator for RAFT Polymerization and ROP. Macromolecules 2013, 46, 1291−1295. (11) Degirmenci, M.; Gokkaya, C.; Durgun, M. One-step synthesis of a mid-chain functional macrophotoinitiator of a polystyrene-poly(epsilon-caprolactone) diblock copolymer via simultaneous ATRP and ROP using a dual-functional photoinitiator. Polym. J. 2016, 48, 139− 145. (12) Fu, C.; Xu, J.; Kokotovic, M.; Boyer, C. One-Pot Synthesis of Block Copolymers by Orthogonal Ring-Opening Polymerization and PET-RAFT Polymerization at Ambient Temperature. ACS Macro Lett. 2016, 5, 444−449. (13) Aydogan, C.; Kutahya, C.; Allushi, A.; Yilmaz, G.; Yagci, Y. Block copolymer synthesis in one shot: concurrent metal-free ATRP and ROP processes under sunlight. Polym. Chem. 2017, 8, 2899−2903. (14) Fournier, D.; Hoogenboom, R.; Schubert, U. S. Clicking polymers: a straightforward approach to novel macromolecular architectures. Chem. Soc. Rev. 2007, 36, 1369−1380. (15) Durmaz, H.; Hizal, G.; Tunca, U. Linear Tetrablock Quaterpolymers via Triple Click Reactions, Azide-Alkyne, DielsAlder, and Nitroxide Radical Coupling in a One-Pot Fashion. J. Polym. Sci., Part A: Polym. Chem. 2011, 49, 1962−1968. (16) Altintas, O.; Tunca, U. Synthesis of Terpolymers by Click Reactions. Chem. - Asian J. 2011, 6, 2584−2591. (17) Gregory, A.; Stenzel, M. H. Complex polymer architectures via RAFT polymerization: From fundamental process to extending the scope using click chemistry and nature’s building blocks. Prog. Polym. Sci. 2012, 37, 38−105. (18) Guillaume, S. M.; Kirillov, E.; Sarazin, Y.; Carpentier, J.-F. Beyond Stereoselectivity, Switchable Catalysis: Some of the Last Frontier Challenges in Ring-Opening Polymerization of Cyclic Esters. Chem. - Eur. J. 2015, 21, 7988−8003. (19) Ottou, W. N.; Sardon, H.; Mecerreyes, D.; Vignolle, J.; Taton, D. Update and challenges in organo-mediated polymerization reactions. Prog. Polym. Sci. 2016, 56, 64−115. (20) Moad, G.; Rizzardo, E.; Thang, S. H. Living radical polymerization by the RAFT process. Aust. J. Chem. 2005, 58, 379− 410. (21) Moad, G.; Rizzardo, E.; Thang, S. H. Living radical polymerization by the RAFT process - A first update. Aust. J. Chem. 2006, 59, 669−692. (22) Moad, G.; Rizzardo, E.; Thang, S. H. Living Radical Polymerization by the RAFT Process - A Second Update. Aust. J. Chem. 2009, 62, 1402−1472. (23) Moad, G.; Rizzardo, E.; Thang, S. H. Living Radical Polymerization by the RAFT Process - A Third Update. Aust. J. Chem. 2012, 65, 985−1076. (24) Moad, G.; Keddie, D.; Guerrero-Sanchez, C.; Rizzardo, E.; Thang, S. H. Advances in Switchable RAFT Polymerization. Macromol. Symp. 2015, 350, 34−42. (25) Keddie, D. J.; Moad, G.; Rizzardo, E.; Thang, S. H. RAFT Agent Design and Synthesis. Macromolecules 2012, 45, 5321−5342. (26) Keddie, D. J. A guide to the synthesis of block copolymers using reversible-addition fragmentation chain transfer (RAFT) polymerization. Chem. Soc. Rev. 2014, 43, 496−505. (27) Martens, S.; Driessen, F.; Wallyn, S.; Turunc, O.; Du Prez, F. E.; Espeel, P. One-Pot Modular Synthesis of Functionalized RAFT Agents Derived from a Single Thiolactone Precursor. ACS Macro Lett. 2016, 5, 942−945.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01784.

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Figures S1−S16 and Table S1 (PDF)

AUTHOR INFORMATION

Corresponding Author

*Tel +86 25 5813 9926; Fax +86 25 5813 9935; e-mail guok@ njtech.edu.cn (K.G.). ORCID

Zhenjiang Li: 0000-0002-1100-7297 Kai Guo: 0000-0002-0013-3263 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (U1463201, 21522604), the National Key Research and Development Program of China (2017YFC1104802), Natural Science Foundation of Jiangsu Province, China (BK20150031), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and the Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (TAPP).

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REFERENCES

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