Influence of Solvophilic Homopolymers on RAFT Polymerization

3 hours ago - (1−4) Recently, polymerization-induced self-assembly (PISA), especially the formulation of macromolecular RAFT (macro-RAFT) agent ...
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Influence of Solvophilic Homopolymers on RAFT PolymerizationInduced Self-Assembly Yuan Zhang,† Guang Han,§ Mengjiao Cao,† Tianying Guo,*,† and Wangqing Zhang*,†,‡ †

Key Laboratory of Functional Polymer Materials of the Ministry of Education, Institute of Polymer Chemistry, College of Chemistry, and ‡Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China § State Key Laboratory of Special Functional Waterproof Materials, Beijing Oriental Yuhong Waterproof Technology Co., Ltd., Beijing, China S Supporting Information *

ABSTRACT: A new method to modulate RAFT dispersion polymerization by introducing solvophilic homopolymers into the polymerization medium is proposed. It is discovered that in RAFT dispersion polymerization employing poly(4-vinylpyridine) trithiocarbonate macro-RAFT agent, the introduced solvophilic homopolymers of poly(N,N-dimethylacrylamide) (PDMA), poly(ethylene glycol), and poly(4-vinylpyridine) can greatly change both the size/morphology of poly(4-vinylpyridine)-block-polystyrene nanoassemblies and polymerization kinetics. RAFT dispersion polymerization decelerates with the increasing amount of introduced PDMA, and it favors formation of complex block copolymer nanoassemblies in the presence of PDMA. The possible reason for introduced solvophilic homopolymers affecting polymerization kinetics and the block copolymer nanoassemblies is discussed. Adding solvophilic homopolymers into the polymerization medium is believed to be a promising alternative to modulate RAFT dispersion polymerization.

1. INTRODUCTION Amphiphilic block copolymer (BCP) nanoassemblies have attracted a lot of attention in these years.1−4 Recently, polymerization-induced self-assembly (PISA), especially the formulation of macromolecular RAFT (macro-RAFT) agent mediated dispersion polymerization, which is herein briefly called RAFT dispersion polymerization, is proved to be an efficient way to synthesize BCP nanoassemblies with high solid contents up to 50%.5−10 Following this PISA, BCP nanoassemblies with various morphologies have been controllably generated in nonaqueous5−7 and aqueous8−10 dispersions during the RAFT synthesis of amphiphilic BCPs. Several factors listed below are discovered to dictate BCP nanoassemblies. First, the chemical composition of block copolymers, e.g., the polymerization degree (DP) of solvophilic or solvophobic blocks, is an inherent parameter to determine BCP nanoassemblies. Generally, short solvophilic macro-RAFT agents favorably lead to synthesis of worms or vesicles in RAFT dispersion polymerization,11−18 and with DP of the solvophobic block increasing size of BCP nanoassemblies increases or their morphology undergoes a spheres-to-wormsto-vesicles transition.19−33 Second, the concentration of feeding monomer also greatly affects BCP nanoassemblies.34−43 By increasing monomer concentration from 10% to 50%, poly(ethylene glycol)-block-polystyrene nanoassemblies change from nanospheres to entrapped vesicles with almost identical block copolymer composition,42 and poly(2-hydroxypropyl methacrylate-block-benzyl methacrylate) nanoassemblies © XXXX American Chemical Society

change from nanospheres to vesicles by increasing monomer concentration from 10% to 30%.43 Third, solvent character is correlative to block copolymer morphology.44−50 It is found that poly(2-(dimethylamino)ethyl methacrylate)-block-poly(benzyl methacrylate) nanoassemblies synthesized via RAFT dispersion polymerization in the ethanol/water mixture change from vesicles to nanospheres when water content in the alcoholic solvent increases from 0% to 15%.45 Recently, it is found that RAFT dispersion polymerization in viscous poly(ethylene glycol) (PEG) runs much faster than those in alcoholic solvents.51−53 Besides, the BCP nanoassemblies synthesized in PEG generally have high-order morphology in comparison to those synthesized in alcoholic solvents,52,53 and the much higher viscosity of PEG than that of alcohols is ascribed to the difference. Thus, tuning the viscosity of polymerization medium may be an alternative method to modulate BCP nanoassemblies. In this study, several solvophilic homopolymers including poly(N,N-dimethylacrylamide), PEG, and poly(4-vinylpyridine) with different molecular weights were introduced into the polymerization medium to tune polymerization kinetics of RAFT dispersion polymerization and poly(4-vinylpyridine)block-polystyrene (P4VP-b-PS) nanoassemblies. It was discovered that the introduced solvophilic homopolymers changed Received: April 2, 2018 Revised: May 24, 2018

A

DOI: 10.1021/acs.macromol.8b00690 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Summary of the Synthesized P4VP-TTC Mn (kg/mol)

a

macro-CTA

[M]0:[CTA]0:[I]0

time (h)

conva (%)

Mn,thb

Mn,GPCc

Mn,NMRd

Đe

P4VP15-TTC P4VP24-TTC P4VP47-TTC P4VP160-TTC

60:4:1 100:4:1 240:4:1 800:4:1

6 6 6 12

98.2 95.5 79.7 80.0

1.98 2.92 5.34 17.20

3.01 3.15 6.02 18.70

2.14 2.60 5.12 18.18

1.02 1.04 1.09 1.11

Monomer conversion determined by 1H NMR. bTheoretical molecular weight according to eq S1. cMolecular weight determined by GPC. Molecular weight determined by 1H NMR. eĐ (Mw/Mn) determined by GPC.

d

solvent was kept at 20%. Herein, a typical RAFT dispersion polymerization in the presence of 20 wt % PDMA582 was introduced. P4VP24-TTC (0.070 g, 0.024 mmol), PDMA582 (0.77 g), St (0.500 g, 4.81 mmol), and AIBN (1.30 mg, 0.0079 mmol) dissolved in the 80/ 20 methanol/water mixture (2.50 g) were weighed into a Schlenk flask with a magnetic bar. The flask content was initially degassed, and then polymerization was run at 70 °C. After a given time, polymerization was stopped by rapid cooling in iced water. St monomer conversion was detected by 1H NMR employing 1,3,5-trioxane as internal standard.58 To detect P4VP-b-PS nanoassemblies, the obtained dispersion was diluted with the 80/20 methanol/H2O mixture, kept at room temperature for about 30 min, and then observed by TEM. To collect P4VP-b-PS for gel permeation chromatography (GPC) and 1 H NMR analysis, the P4VP-b-PS nanoassemblies were separated by centrifugation (10 000 rpm), dissolved in dichloromethane, precipitated into methanol, and finally dried under vacuum. Note: the homopolymers of PDMA, P4VP, and PEG6000 are soluble in methanol and dichloromethane and therefore can be separated from P4VP-b-PS with the procedures introduced above. 2.5. Characterization. Molecular weight and dispersity (Đ, Đ = Mw/Mn) of the prepared polymers were determined by GPC using a Waters 600E GPC system, in which DMF was used as eluent and narrow-polydispersity polystyrene samples were used as standards to calibrate the apparatus. The 1H NMR spectra were recorded on a Bruker Avance III 400 MHz NMR spectrometer using CDCl3 as the solvent. A Tecnai G2 F20 or a Talos F200C electron microscope performed at 200 kV was used for TEM observation. Differential scanning calorimetry (DSC) analysis was performed on a NETZSCH DSC 204 differential scanning calorimeter under a nitrogen atmosphere in which the samples were heated to 250 °C at a heating rate of 10 °C min−1, cooled to 0 °C in 5 min, and then heated to 250 °C at a heating rate of 10 °C min−1.

the polymerization viscosity, changed the polymerization kinetics, and therefore changed the BCP nanoassemblies. Our study demonstrates that adding solvophilic polymers into the polymerization medium for RAFT dispersion polymerization is an alternative way to tune BCP nanoassemblies under PISA conditions.

2. EXPERIMENTAL SECTION 2.1. Materials. Methyl 2-bromopropanoate (99%, J&K), 4vinylpyridine (4VP, 96%, Alfa), N,N-dimethylacrylamide (DMA, 99.5%, Alfa), and styrene (St, >98%, Tianjin Chemical Company) were distilled under reduced pressure before use. 4-Cyano-4(dodecylsulfanylthiocarbonyl)sulfanylpentanoic acid (CDTPA) was synthesized as discussed.54 2,2′-Azobis(2-methylpropionitrile) (AIBN, >99%, Tianjin Chemical Company, China) was recrystallized from ethanol prior to use. Poly(ethylene glycol) with molecular weight at 6000 Da (PEG6000, Alfa) was used as received. Cupric chloride (CuCl2, 99%, aladdin, China), tris(pyridin-2-ylmethyl)amine (TPMA, 99%, Annaiji, China), and other chemical reagents were used without any further purification. Deionized water was used. 2.2. Synthesis of PDMA and P4VP. Three PDMA samples with different molecular weights were synthesized with initiators for continuous activator regeneration atom transfer radical polymerization (ICAR ATRP, Scheme S1),55−57 and the details are summarized in Table S1. Herein, synthesis of PDMA47 is typically introduced. DMA (6.00 g, 60.61 mmol), methyl 2-bromopropanoate (0.168 g, 1.01 mmol), CuCl2 (0.016 g, 0.12 mmol), TPMA (0.141 g, 0.48 mmol), AIBN (0.199 g, 1.2 mmol), the internal standard of 1,3,5-trioxane for NMR (0.530 g, 5.89 mmol), and 1,4-dioxane (6.00 g) were weighed into a 100 mL Schlenk flask. The flask content was initially degassed by repeated freezing−pumping−thawing cycles, and then polymerization ran at 65 °C and finally quenched after 7.5 h of polymerization with 78.9% monomer conversion by 1H NMR. The polymer solution was passed over a column of basic alumina using dichloromethane as eluent, and then PDMA47 was deposited in iced diethyl ether and finally dried under vacuum. P4VP369 was also synthesized by ICAR ATRP, and the details are summarized in Table S1. 2.3. Synthesis of Poly(4-vinylpyridine) Trithiocarbonate. Four macro-RAFT agents, also named macro-CTAs, of poly(4vinylpyridine) trithiocarbonate, P4VP-TTC, in which TTC represents the RAFT terminal of trithiocarbonate, with different DPs as summarized in Table 1 were prepared by solution RAFT polymerization using CDTPA as the chain transfer agent (CTA). Herein is introduced a typical synthesis of P4VP24-TTC under [4VP]0: [CDTPA]0:[AIBN]0 = 100:4:1. 4VP (8.00 g, 76.19 mmol), CDTPA (1.23 g, 3.05 mmol), AIBN (0.125 g, 0.76 mmol), and ethanol (16.0 g) were weighed into a 100 mL Schlenk flask with a magnetic bar. The flask content was degassed, and then polymerization ran at 70 °C. After 6 h of polymerization, it was quenched by rapid cooling, and 95.5% monomer conversion was determined by 1H NMR. P4VP24TTC was precipitated into cold diethyl ether and dried under vacuum. 2.4. RAFT Dispersion Polymerization and Synthesis of P4VPb-PS Nanoassemblies. RAFT dispersion polymerization of styrene was performed in the methanol/water mixture (80/20 w/w) at 70 °C under [St]0:[P4VP]0:[AIBN]0 = 600:3:1 either in presence or in the absence of PDMA, in which the weight ratio of styrene to the alcoholic

3. RESULTS AND DISCUSSION 3.1. Synthesis of PDMA and P4VP by ICAR ATRP. The PDMA and P4VP homopolymers were prepared by ICAR ATRP under [monomer]0:[methyl 2-bromopropanoate]0: [CuCl2 ]0 :[TPMA] 0:[AIBN]0 = DP:1:0.12:0.48:1.2. This ICAR ATRP was used, since it afforded controllable synthesis of well-defined polymers.55−57 Herein, ICAR ATRP but not RAFT is used, since homopolymers synthesized by RAFT can be further block extended in the subsequent RAFT dispersion polymerization. Herein, three PDMA samples with different DPs, e.g., PDMA47, PDMA107, and PDMA582, were synthesized by ICAR ATRP by targeting DP at 60, 120, and 600, respectively. The synthesized PDMA samples were analyzed by GPC and 1H NMR (Figures S1 and S2, Table S1). Besides, P4VP369 was also synthesized by this ICAR ATRP at 61.5% monomer conversion by targeting DP at 600. Note: herein the DP values of PDMA47, PDMA107, PDMA582, and P4VP369 are calculated according to the monomer conversions. In this study, three homopolymers of PDMA, P4VP, and PEG were introduced into RAFT dispersion polymerization to tune P4VP-b-PS nanoassemblies. Selection of these homopolymers is based on two concerns. First, these homopolymers are B

DOI: 10.1021/acs.macromol.8b00690 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of P4VP-b-PS Nanoassemblies in the Absence or Presence of PDMA

dispersion polymerizations employing a typical P4VP24-TTC macro-CTA in the absence or presence of 20 wt % PDMA with different molecular weights were performed as shown in Scheme 1. Clearly, these RAFT dispersion polymerizations afford the P4VP-b-PS nanoassemblies just as discussed elsewhere.63−65 To minimize the effect of the DP of the PS block on P4VP-b-PS nanoassemblies, all RAFT dispersion polymerizations were stopped at similarly monomer conversion at 91−95%, and then P4VP-b-PS nanoassemblies were detected by TEM. Figure 2 indicates that the P4VP24-b-PS nanoassemblies synthesized in the absence of PDMA is much different from

soluble in the 80/20 methanol/water mixture, which ensures P4VP-b-PS nanoassemblies being synthesized following RAFT dispersion polymerization. Second, interaction between the introduced homopolymers and the P4VP-TTC macro-CTA is relatively weak compared with hydrogen bonding. This weak interaction can simplify our study, since strong interaction such as static interaction and hydrogen bonding between the introduced homopolymers and block copolymers can greatly change the nucleation of block copolymers as discussed elsewhere,59−62 and this is beyond the present study. 3.2. Synthesis of the P4VP-TTC Macro-CTA. The P4VPTTC macro-CTA was prepared by RAFT solution polymerization following Scheme 1. As discussed previously,11−18 the DP of macro-CTAs can greatly affect the size/morphology of BCP nanoassemblies. Herein, four macro-CTAs with different DPs, e.g., P4VP15-TTC, P4VP24-TTC, P4VP47-TTC, and P4VP160-TTC, were prepared by altering the 4VP/CDTPA/ AIBN molar ratio. These P4VP-TTC macro-CTAs were characterized by 1H NMR (Figure S3) and GPC (Figure 1).

Figure 1. GPC traces of the P4VP-TTC macro-CTAs.

By comparing chemical shifts at 8.33 and 0.88 ppm of protons corresponding to pyridine ring in P4VP block and the RAFT terminal, respectively, the molecular weight of P4VPTTC, Mn,NMR, is obtained. The GPC-determined molecular weight of Mn,GPC, Mn,NMR, and theoretical molecular weight Mn,th calculated by monomer conversion following eq S1 are close to each other (Table 1). Furthermore, the Đ values (Mw/ Mn) are relatively low. These results demonstrate a controlled synthesis of the P4VP-TTC macro-CTAs. 3.3. Primary Evaluation of the Introduced PDMA Affecting BCP Nanoassemblies. To check the introduced homopolymers affecting P4VP-b-PS nanoassemblies, RAFT

Figure 2. TEM images of P4VP24-b-PS nanoassemblies prepared by RAFT dispersion polymerization in the absence of PDMA (A) and in the presence of 20 wt % PDMA47 (B), 20 wt % PDMA107 (C), and 20 wt % PDMA582 (D).

those synthesized in the presence of PDMA, and the DP of PDMA also exerts somewhat influence on the P4VP24-b-PS nanoassemblies. For examples, 42 ± 4 nm nanospheres are formed in the absence of PDMA (Figure 2A), and with the DP of PDMA increasing from 47 to 582, the mixture of 157 ± 32 nm nanospheres, lamellae and vesicles (Figure 2B), the mixture C

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Figure 3. 1H NMR spectra (A) and the GPC traces (B) of the P4VP24-b-PS186/PDMA582 mixture (black) before separation and the separated P4VP24-b-PS186 (red) and PDMA582 (blue) after separation.

of 159 ± 35 nm nanospheres and vesicles (Figure 2C), and the 159 ± 59 nm vesicles (Figure 2D) are formed. The P4VP24-bPS BCPs prepared in the absence of PDMA or in the presence of 20 wt % PDMA were collected, characterized by NMR and GPC (Figures S4 and S5), and all P4VP24-b-PS BCPs were found to have a similar chemical composition (Table S2). Note: Mn,NMR of P4VP24-b-PS is calculated by comparing the chemical shifts at δ = 8.33 ppm of the proton corresponding to pyridine ring in the P4VP block and at δ = 6.20−7.20 ppm corresponding to benzene ring in the PS block following eq S3. Thus, it is believed that the different size/morphology of P4VP24-b-PS nanoassemblies is just ascribed to the introduced PDMA homopolymers. To investigate the reason that the introduced homopolymers affect the P4VP24-b-PS nanoassemblies, the BCP colloids were collected by centrifugation, then rapidly washed with methanol, and finally dried and characterized by 1H NMR and GPC (Figure 3). In 1H NMR spectra, both signals assigned to the P4VP24-b-PS BCPs and the introduced PDMA582 homopolymer are observed. Also, two peaks in GPC traces are observed. To further check ingredients in colloids, the colloids were initially dissolved in CH2Cl2 and then precipitated in methanol. Both the precipitates and the ingredients dissolved in methanol were collected and characterized by 1H NMR and GPC (Figure 3). By comparing the 1H NMR spectra and GPC traces before and after separation, it is concluded that PDMA is really included in the P4VP24-b-PS nanoassemblies. The PDMA582/ P4VP24-b-PS186 weight ratio in the colloids, 8.8%, can be obtained by comparing the chemical shifts at 6.19−7.20 ppm and 2.76−3.19 ppm in the samples before separation (Figure 3A). Note: the PDMA582 included in the colloids can be partly washed out with methanol even with a quick washing, and this will lead to an underestimated ratio of PDMA582/P4VP24-bPS186. Despite this, from the ratio it is concluded that about 6 wt % PDMA is included in the P4VP24-b-PS nanoassemblies and 94 wt % PDMA582 is soluble in the polymerization medium. It is further found that the PDMA with larger DP could be included in the P4VP24-b-PS nanoassemblies more than those of short PDMA under other similar conditions. For examples, when 20 wt % PDMA582, PDMA107, and PDMA47 are introduced into the RAFT dispersion polymerization, the weight ratio of the included PDMA to P4VP24-b-PS, PDMA/ P4VP24-b-PS, is 8.8%, 8.3%, and 5.6%, respectively. General dispersion polymerization to synthesize microsized particles is widely reported, and in this dispersion polymerization a suitable stabilizer usually a solvophilic homopolymer is

needed to stabilize the microsized particles.66−68 Herein, control experiments of dispersion polymerization of styrene in the 80/20 methanol/water mixture in the presence and absence of PDMA582 were conducted under other similar conditions with the aforementioned RAFT dispersion polymerization. In the control experiments, the dispersion polymerization in the presence of PDMA582 led to 470 ± 70 nm stable particles of polystyrene (PS), whereas the PS aggregates synthesized in the absence of PDMA582 had a much larger size of 2200 ± 360 nm, and they tended to deposit in the solvent (Figure S6). This indicates that the included PDMA582 stabilizer greatly decreases the size of the PS particles and can help the PS particles to keep suspending in the polymerization medium. The 470 ± 70 nm particles were collected and checked by 1H NMR (Figure S7), and 29.1% PDMA582 was found to be included in the particles. DSC analysis shows that PDMA582 is incompatible with PS (Mw = 82.01 kg/mol, Đ = 1.83), since the PS/PDMA mixture exhibits two glass transition temperatures very close to those of the individual homopolymers (Figure S8). This suggests that although the two polymers of PS and PDMA are somewhat incompatible, the PDMA582 chains can be included within the particles, more possibly on the particle surface, during the gradual growth of the particles in dispersion polymerization. Inclusion of PDMA582 in the PS particles may be due to the long chains of PDMA582 and PS being entangled during dispersion polymerization, and this hypothesis can be partly confirmed by the less included PDMA at cases of low DP as aforementioned. Herein, it is deemed that the inclusion of the PDMA homopolymer in P4VP24-b-PS nanoassemblies is just as similar as those in synthesis of PS particles by dispersion polymerization. Furthermore, it is noticed that the fraction of the included PDMA582 in the P4VP24-b-PS186 nanoassemblies is lower than that in the microsized PS particles (8.8% vs 29.1%). This is possibly due to the tethered P4VP block, which decreases inclusion of PDMA582 into the P4VP24-b-PS186 nanoassemblies due to steric repulsion between the solvophilic P4VP block and PDMA582, although PDMA582 is somewhat compatible with P4VP24-b-PS186 (Figure S8). As is known, the morphology of amphiphilic AB diblock copolymer nanoassemblies is correlative to the balance of A solvophilic block and B solvophobic block in solvent.1−4 It was reported that when a solvophobic homopolymer B is mixed with an amphiphilic diblock copolymer AB, the size and/or morphology of the self-assembled AB/B is much different from that of pure AB.69−72 Besides, by simultaneously using a macroD

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above 20 wt % is not checked, since the polymerization mixture becomes too viscous. As discussed previously,11−18 BCP nanoassemblies undergo a vesicles-to-nanospheres transition with increasing DP of macroCTAs in RAFT dispersion polymerization. In the present RAFT dispersion polymerization in the absence of PDMA582, P4VP-b-PS nanoassemblies follow this rule, and they change from 220 ± 58 nm vesicles to 22 ± 2 nm spheres with DP of the P4VP-TTC macro-CTA increasing from 15 to 160, in which the PS block keeps a constant DP at about 185 (Figure 5). Into this RAFT dispersion polymerization, 20 wt % PDMA582 was added. After quenching the RAFT dispersion polymerizations at similar monomer conversion of 91−94.5% to ensure P4VP-b-PS having similar DP of the PS block, P4VPb-PS nanoassemblies were checked. As shown in Figure 5, 20 wt % PDMA582 can alter the P4VP-b-PS nanoassemblies whether employing a short or a long P4VP-TTC macro-CTA. When 20 wt % PDMA582 is introduced into the RAFT dispersion polymerization, for P4VP-b-PS including the shortest P4VP15 block 220 ± 58 nm vesicles convert into 666 ± 56 nm porous particles (Figure 5A and Figure S10, in which the porous particles are stained by pH = 1 HCl aqueous solution), and the formation of porous particles is possibly ascribed to vesicle fusion as reported by Pan and co-workers;74 for P4VP-bPS including a moderate P4VP24 block, 42 ± 4 nm nanospheres convert into 159 ± 59 nm vesicles (Figure 5B); for P4VP-b-PS including a long P4VP47 or P4VP160 block, the nanosphere size increases from 32 ± 3 nm to 58 ± 5 nm (Figure 5C) and from 22 ± 2 nm to 39 ± 4 nm (Figure 5D), respectively. The amount of introduced PDMA582 affecting RAFT dispersion polymerization employing the typical P4VP24-TTC was further investigated. The polymerization rate decreases with PDMA582 increasing (Figure 6A), and a two-stage plot of ln([M]0/[M]) vs polymerization time is observed in RAFT dispersion polymerization either in the absence of PDMA582 or in the presence of 20 wt % PDMA582 (Figure 6B and Figure S11), which indicates that the present RAFT dispersion polymerization undergoes an initial slow homogeneous polymerization and a subsequent fast heterogeneous one as reported previously.75−80 The turning point in the two-stage plot represents onset of block copolymer micellization. The onset of micellization in the presence of PDMA582 occurs 4 h later than those in the absence of PDMA582, and it becomes later with the increasing concentration of the introduced PDMA582 until to almost a constant at cases of 10 wt % PDMA582 (Figure S11). As shown in Figure S8, PDMA is somewhat compatible with P4VP-b-PS, and therefore it acts as a plasticizer to delay block copolymer nucleation and to decelerate RAFT dispersion polymerization. After the onset time of micellization, the heterogeneous RAFT polymerization decelerates with the PDMA582 concentration increasing as indicted by the decreasing apparent polymerization rate constant (Kpapp),81 which is calculated by the slope of the ln([M]0/[M]) vs the polymerization time curve in the linear part, from 0.54 to 0.29 h−1. Clearly, the viscosity of the polymerization medium increases with the amount of introduced PDMA582 (Figure S12). Herein, it is thought, with the increasing introduced PDMA582, the polymerization medium becomes more viscous, and this makes more PDMA to be included in the P4VP-b-PS nanoassemblies. At this condition, the accessibility of the styrene monomer decreases, since PDMA is solvophilic and the St monomer is hydrophobic. This decreased monomer accessibility leads to decreasing

CTA and a general CTA in RAFT dispersion polymerization, the synthesized AB/B nanoassemblies were found to be much different from the AB nanoassemblies.65,73 In the present study, the presynthesized solvophilic homopolymer of PDMA is added in RAFT dispersion polymerization, and the included solvophilic PDMA changes the solvophilic/solvophobic balance in P4VP-b-PS block and therefore changes the block copolymer morphology. To further verify this, 20 wt % PDMA582 was added into the presynthesized P4VP24-b-PS182 nanoassemblies of 42 ± 4 nm nanospheres (seeing TEM images in Figure 2A), and then the mixture was heated at 55 °C for 24 h. It was observed that the 42 ± 4 nm nanospheres converted into 85 ± 13 nm vesicle-like aggregates (Figure S9), confirming that the introduced PDMA582 really changed the P4VP24 -b-PS182 nanoassemblies. Note: in TEM sampling, the in situ synthesized nanoassemblies were dispersed in the 80/20 methanol/water mixture with much diluted concentration, and therefore most of the included PDMA was diffused into the solvent in a given time of about 30 min, and just BCP nanoassemblies were observed. 3.4. Hydrophilic Homopolymers Affecting RAFT Dispersion Polymerization and BCP Nanoassemblies. In this section, the introduced hydrophilic homopolymers affecting RAFT dispersion polymerization and BCP nanoassemblies are investigated in detail. Initially, the concentration of the introduced PDMA582 affecting P4VP-b-PS nanoassemblies is explored. To fulfill this, RAFT dispersion polymerizations in the presence of PDMA582 with different PDMA582 concentrations were conducted, and then they were stopped at similar monomer conversion of 91−94.5% to ensure P4VP-b-PS having similar chemical composition (Figure S5 and Table S2), and finally the BCP nanoassemblies were checked. Figure 4 indicates that the P4VP-b-PS nanoassemblies change

Figure 4. TEM images of P4VP24-b-PS nanoassemblies prepared by RAFT dispersion polymerization with the introduced PDMA582 concentration at 0 (A), 5 (B), 10 (C), and 20 wt % (D).

from 42 ± 4 nm nanospheres (Figure 4A) to the mixture of 55 ± 9 nm nanospheres and nanowires (Figure 4B), then to the mixture of 62 ± 16 nm nanospheres, nanowires, and vesicles (Figure 4C), and finally to 159 ± 59 nm vesicles (Figure 4D) with PDMA582 concentration increasing from 0 to 20 wt %. RAFT dispersion polymerization with PDMA582 concentration E

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Figure 5. TEM images of BCP nanoassemblies of P4VP15-b-PS185/184 (A), P4VP24-b-PS182/186 (B), P4VP47-b-PS186/187 (C), and P4VP160-b-PS185/187 (D) prepared by RAFT dispersion polymerization in the absence of PDMA582 and in the presence of 20 wt % PDMA582.

Figure 6. Monomer conversion (A) and ln([M]0/[M])−time plot (B) in RAFT dispersion polymerization in the absence of PDMA and in the presence of PDMA582 with PDMA582 concentration ranging from 5 to 20 wt %, the GPC traces (C), and the evolution of Mn,th, Mn,NMR, Mn,GPC, and Đ of P4VP24-b-PS prepared through RAFT dispersion polymerization in the presence of 20 wt % PDMA582 (D).

weight with monomer conversion, and the low Đ below 1.3 demonstrate a controlled RAFT dispersion polymerization in the presence of PDMA582. The P4VP24-b-PS nanoassemblies undergo a nanospheres-to-nanowires-to-vesicles transition with increasing DP of the PS block (Figure 7). In comparison, just P4VP24-b-PS nanospheres, the size of which increases with the DP of the PS block, are formed via RAFT dispersion polymerization in the absence of PDMA582 (Figure S14).

polymerization rate. However, the exact reason that dispersion RAFT polymerization decelerates with PDMA needs further study. The P4VP-b-PS BCPs prepared employing the typical P4VP24-TTC macro-CTA were analyzed by GPC (Figure 6C) and 1H NMR (Figure S13), and the results are shown in Figure 6D. The well-consistent molecular weight of P4VP-b-PS, Mn,GPC, Mn,NMR, and Mn,th, the linearly increasing molecular F

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Figure 7. TEM images of P4VP-b-PS nanoassemblies prepared by RAFT dispersion polymerization in the presence of 20 wt % PDMA582 at 4 (A), 5 (B), 6 (C), 7 (D), 9 (E), and 14 h (F).

Figure 8. TEM images of P4VP24-b-PS182/185/186 nanoassemblies prepared by RAFT dispersion polymerization in the absence of homopolymers (A) and in the presence of 20 wt % P4VP369 (B) or 20 wt % PEG6000 (C).

polymerization kinetics and the size/morphology of P4VP-b-PS nanoassemblies. RAFT dispersion polymerization decelerates with increasing amount of the introduced PDMA, and introducing PDMA into RAFT dispersion polymerization favors synthesis of complex BCP nanoassemblies. The introduced solvophilic homopolymers changing polymerization kinetics and P4VP-b-PS nanoassemblies are ascribed to three reasons. First, the introduced solvophilic homopolymers increase the viscosity of the polymerization medium, and with increasing viscosity of the polymerization medium more solvophilic PDMA is included in P4VP-b-PS nanoassemblies, and accessibility of monomer to BCP nanoassemblies decreases and therefore RAFT dispersion polymerization decelerates. Second, the introduced PDMA can act as plasticizer for P4VPb-PS nanoassemblies and therefore delays nucleation of P4VPb-PS in the polymerization medium. Third, the included PDMA in P4VP-b-PS nanoassemblies change the solvophilic/solvophobic balance of P4VP-b-PS and therefore change the size/ morphology of BCP nanoassemblies. We believed that introducing solvophilic homopolymers is a promising alternative to modulate RAFT dispersion polymerization.

This confirms that introducing PDMA into the RAFT dispersion polymerization favors synthesis of complex morphologies of the P4VP-b-PS nanoassemblies, e.g., nanowires and vesicles. Lastly, introducing other two solvophilic homopolymers of 20 wt % P4VP369 and 20 wt % PEG6000 into RAFT dispersion polymerization is also explored. Three cases of RAFT dispersion polymerization in the absence of homopolymers, in the presence of 20 wt % P4VP369, and in the presence of 20 wt % PEG6000 led to synthesis of 42 ± 4 nm nanospheres, 106 ± 22 nm vesicles, and 367 ± 38 nm vesicles, respectively, even though all the P4VP-b-PS BCPs almost have the same chemical composition (Figure 8). These results show that these two homopolymers of P4VP and PEG can also lead to a great change in the P4VP-b-PS nanoassemblies as similarly as PDMA discussed above.

4. CONCLUSIONS RAFT dispersion polymerization is an efficient method to synthesize BCP nanoassemblies. Herein, we propose a new method to modulate RAFT dispersion polymerization by introducing solvophilic homopolymers into polymerization medium. It is discovered that in RAFT dispersion polymerization of styrene employing P4VP-TTC macro-CTA, introducing solvophilic homopolymers of PDMA, P4VP, and PEG into the polymerization medium can greatly change both G

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(13) Hanisch, A.; Yang, P.; Kulak, A. N.; Fielding, L. A.; Meldrum, F. C.; Armes, S. P. Phosphonic Acid-Functionalized Diblock Copolymer Nano-Objects via Polymerization-Induced Self-Assembly: Synthesis, Characterization, and Occlusion into Calcite Crystals. Macromolecules 2016, 49, 192−204. (14) Xu, S.; Ng, G.; Xu, J.; Kuchel, R. P.; Yeow, J.; Boyer, C. 2(Methylthio)ethyl Methacrylate: A Versatile Monomer for Stimuli Responsiveness and Polymerization-Induced Self-Assembly in the Presence of Air. ACS Macro Lett. 2017, 6, 1237−1244. (15) Zhao, W.; Gody, G.; Dong, S.; Zetterlund, P. B.; Perrier, S. Optimization of the RAFT polymerization conditions for the in situ formation of nano-objects via dispersion polymerization in alcoholic medium. Polym. Chem. 2014, 5, 6990−7003. (16) Huang, J.; Zhu, H.; Liang, H.; Lu, J. Salicylaldehydefunctionalized block copolymer nano-objects: one-pot synthesis via polymerization-induced self-assembly and their simultaneous crosslinking and fluorescence modification. Polym. Chem. 2016, 7, 4761− 4770. (17) Xiao, X.; He, S.; Dan, M.; Su, Y.; Huo, F.; Zhang, W. Brush Macro-RAFT Agent Mediated Dispersion Polymerization of Styrene in the Alcohol/Water Mixture. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 3177−3190. (18) Fielding, L. A.; Derry, M. J.; Ladmiral, V.; Rosselgong, J.; Rodrigues, A. M.; Ratcliffe, L. P. D.; Sugihara, S.; Armes, S. P. RAFT dispersion polymerization in non-polar solvents: facile production of block copolymer spheres, worms and vesicles in n-alkanes. Chem. Sci. 2013, 4, 2081−2087. (19) Huo, M.; Ye, Q.; Che, H.; Wang, X.; Wei, Y.; Yuan, J. Polymer Assemblies with Nanostructure-Correlated Aggregation Induced Emission. Macromolecules 2017, 50, 1126−1133. (20) Wang, X.; Zhou, J.; Lv, X.; Zhang, B.; An, Z. TemperatureInduced Morphological Transitions of Poly(dimethylacrylamide)Poly(diacetone acrylamide) Block Copolymer Lamellae Synthesized via Aqueous Polymerization Induced Self-Assembly. Macromolecules 2017, 50, 7222−7232. (21) Tan, J.; Liu, D.; Bai, Y.; Huang, C.; Li, X.; He, J.; Xu, Q.; Zhang, L. Enzyme-Assisted Photoinitiated Polymerization-Induced SelfAssembly: An Oxygen-Tolerant Method for Preparing Block Copolymer Nano-Objects in Open Vessels and Multiwell Plates. Macromolecules 2017, 50, 5798−5806. (22) Chen, X.; Liu, L.; Huo, M.; Zeng, M.; Peng, L.; Feng, A.; Wang, X.; Yuan, J. Direct Synthesis of Polymer Nanotubes via Aqueous Dispersion Polymerization of Cyclodextrin/Styrene Complex. Angew. Chem. 2017, 129, 16768−16772. (23) Karagoz, B.; Esser, L.; Duong, H. T.; Basuki, J. S.; Boyer, C.; Davis, T. P. Polymerization-Induced Self-Assembly (PISA)-control over the morphology of nanoparticles for drug delivery applications. Polym. Chem. 2014, 5, 350−355. (24) Zhou, W.; Qu, Q.; Yu, W.; An, Z. Single Monomer for Multiple Tasks: Polymerization Induced Self-Assembly, Functionalization and Cross-Linking, and Nanoparticle Loading. ACS Macro Lett. 2014, 3, 1220−1224. (25) Huo, M.; Zhang, Y.; Zeng, M.; Liu, L.; Wei, Y.; Yuan, J. Morphology Evolution of Polymeric Assemblies Regulated with Fluoro-Containing Mesogen in Polymerization-Induced Self-Assembly. Macromolecules 2017, 50, 8192−8201. (26) Zhang, W.-J.; Hong, C.-Y.; Pan, C.-Y. Efficient Fabrication of Photosensitive Polymeric Nano-objects via an Ingenious Formulation of RAFT Dispersion Polymerization and Their Application for Drug Delivery. Biomacromolecules 2017, 18, 1210−1217. (27) Wang, X.; Figg, C. A.; Lv, X.; Yang, Y.; Sumerlin, B. S.; An, Z. Star Architecture Promoting Morphological Transitions during Polymerization-Induced Self-Assembly. ACS Macro Lett. 2017, 6, 337−342. (28) Pei, Y.; Noy, J.-M.; Roth, P. J.; Lowe, A. B. Thiol-reactive Passerini-methacrylates and polymorphic surface functional soft matter nanoparticles via ethanolic RAFT dispersion polymerization and postsynthesis modification. Polym. Chem. 2015, 6, 1928−1931.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00690. Experimental details and Figures S1−S14 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (T.G.). *E-mail: [email protected] (W.Z.). ORCID

Tianying Guo: 0000-0001-6587-6466 Wangqing Zhang: 0000-0003-2005-6856 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support by the National Science Foundation for Distinguished Young Scholars (no. 21525419), the National Science Foundation of China (no. 21474054), the National Key Research and Development Program of China (2017YFC1103501) is gratefully acknowledged.

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