Morphological Stabilization of Block Copolymer Worms Using

7 days ago - Block copolymer worm stabilization via cross-linking during polymerization-induced self-assembly (PISA) is challenging. This is because b...
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Morphological Stabilization of Block Copolymer Worms Using Asymmetric Cross-Linkers during Polymerization-Induced SelfAssembly Baohua Zhang, Xiaoqing Lv, Anqi Zhu, Jinwen Zheng, Yongqi Yang, and Zesheng An* Institute of Nanochemistry and Nanobiology, College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China S Supporting Information *

ABSTRACT: Block copolymer worm stabilization via crosslinking during polymerization-induced self-assembly (PISA) is challenging. This is because block copolymer worms typically occupy a narrow regime in the phase diagram, and in situ crosslinking may hinder a morphological transition from sphere to worm. In this work, in situ cross-linking of block copolymer worms during PISA was studied using three different asymmetric cross-linkers, each bearing a pair of double bonds with different reactivities. Specifically, ethanolic PISA syntheses targeting poly(2-(dimethylamino)ethyl methacrylate)-bpoly(benzyl methacrylate) diblock copolymer worms were investigated in the presence of vinyl methacrylate, allyl methacrylate, or 4-allyloxybenzyl methacrylate. The copolymerizations of benzyl methacrylate with the asymmetric cross-linkers underwent progressive branching to finally cross-linking of the block copolymer worms. While all the three asymmetric cross-linkers were able to cross-link worms, 4-allyloxybenzyl methacrylate with a structure mimicking benzyl methacrylate showed the best results with minimal perturbation to the worm morphology. sterilization,49 cell culture,50 stem cell storage,51 and blood cell cryopreservation.52 Worms have also been shown to be effective Pickering emulsifiers,53,54 superflocculants for micrometer-sized silica particles,55 and rheology modifiers56−58 due to their highly anisotropic character. Stimuli-responsive PISA-generated nano-objects offer intriguing opportunities for applications that require a postpolymerization morphological transition, for instance, in sterilizable worm hydrogels.49,51,59,60 Several research groups have studied the stimulus-responsive properties of PISA-generated worms and vesicles upon changes in temperature, pH, or redox potential.25,36,61−70 On the other hand, many applications demand strict preservation of nano-object morphology in order to maintain its morphology-dependent performance. Undesired morphology degradation may occur when nano-objects are used in the presence of organic solvents, surfactants, colloidal particles, and proteins or are subject to high shearing force or high dilution.55,71−74 Under such circumstances, effective stabilization of nano-object morphology is crucial for the intended application, e.g., worms as emulsifiers.53 Morphological stabilization of block copolymer nano-objects via covalent cross-linking is well-established in the literature of traditional block copolymer self-assembly.75−77 Current strategies for morphological stabilization of PISA-generated nanoobjects include either postpolymerization cross-linking that

1. INTRODUCTION Polymerization-induced self-assembly (PISA) is a highly efficient and robust method for the synthesis of block copolymer nano-objects via either dispersion or emulsion polymerization.1−4 High solids content and reliable morphology control are the two most notable advantages of PISA in comparison with conventional block copolymer self-assembly.5−7 Simultaneous chain extension and in situ self-assembly of block copolymers can lead to a continuous alteration of the packing parameter of block copolymer assemblies, producing nano-objects that typically undergo sphere-to-worm-to-bilayer transitions.8−10 In many formulations, block ratio and solids content are the two main synthetic parameters that have been employed to tune PISA morphologies. Recently, significant advances have been made that show PISA morphologies can be affected by a number of other factors, including Flory−Huggins parameter (χ),11 polymer topology and architecture,12,13 polymerization-induced cooperative assembly (PICA),14 polymerization-induced electrostatic self-assembly (PIESA),15−17 degree of solvophobicity of the core-forming block,18−20 and others.21−32 PISA allows reproducible access to higher-order morphologies such as worms and vesicles,15,24,33−48 once a morphology diagram is constructed, a protocol first advocated by Armes and co-workers.10 This has enabled new applications being actively explored with the robust and high-efficiency synthesis of block copolymer worms in particular. Worm synthesis has been shown in several studies to directly result in formation of hydrogels, which have been exploited as biomaterials for © XXXX American Chemical Society

Received: February 1, 2018 Revised: March 14, 2018

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Scheme 1. Structures of Cross-Linkers (A), Synthesis of PDMAEMA Macro-CTA via RAFT Solution Polymerization (B), and Synthesis of in Situ Cross-Linked PDMAEMA-b-P(BnMA-co-X) Worm via RAFT Dispersion Polymerization (C)

Although Cai and co-workers have elegantly demonstrated that nanowires with extremely high aspect ratios can be prepared in methanol/water mixtures via PIESA,16,17 the worm morphology, typically spanning a very narrow regime in the morphology diagram, is known to be extremely sensitive to synthetic conditions and block compositions.8,66 In situ crosslinking of worms during PISA synthesis is therefore particularly challenging for at least two reasons: (1) cross-linking lowers chain mobility and may impede a morphological transition from sphere to worm and (2) the presence of cross-linkers may change the block copolymer composition away from the narrow worm regime. To the best of our knowledge, in situ crosslinking during PISA synthesis to afford stabilized worms with a uniform cross-link density has not been realized. In this article, we examine the ability and scope for in situ cross-linking of poly(2-(dimethylamino)ethyl methacrylate)-b-poly(benzyl methacrylate) (PDMAEMA-b-PBnMA) diblock copolymer worms in reversible addition−fragmentation chain transfer (RAFT) ethanolic dispersion polymerization using three types of asymmetric cross-linkers, including vinyl methacrylate (VMA), allyl methacrylate (AMA), and 4-allyloxybenzyl methacrylate (ABMA) (Scheme 1).

relies on efficient organic chemistries or in situ cross-linking in the presence of divinyl comonomers during PISA. Postpolymerization cross-linking is a highly reliable approach to afford covalently stabilized spheres, worms, and vesicles with crosslinks being installed either in the shell-forming block or more commonly in the core-forming block.20,35,55,66,71,78−82 In principle, in situ cross-linking using divinyl comonomers during PISA is a more straightforward approach, eliminating an additional second step for postpolymerization reaction. However, sufficient cross-linking lowers chain mobility of the in situ generated block copolymers and can therefore inhibit morphological transitions during PISA when symmetric divinyl comonomers (e.g., ethylene glycol di(meth)acrylate) are added prior to polymerization.83 For this reason, higher-order morphologies such as worms and vesicles that require smooth chain reorganization and curvature alteration during PISA have not been successfully cross-linked with prior added symmetric divinyl comonomers.84−87 In several studies block copolymer vesicles or worms were stabilized by adding symmetric divinyl comonomers at the late stage of polymerization to form a third, short cross-linker block.53,55,72,88 Although this two-stage method is effective, the cross-links are only confined to the third block, and their distribution across the core of the nanoobjects is not uniform. Recently, an in situ cross-linking strategy using a prior added asymmetric cross-linker (e.g., allyl acrylamide) was reported to effectively stabilize vesicles during PISA.89 Because this asymmetric cross-linker has a highreactivity acrylamido group and a low-reactivity allyl group, cross-linking is delayed to the late stage of dispersion polymerization when vesicles have already formed.89,90 However, it remains elusive if block copolymer worms could be in situ cross-linked using this asymmetric cross-linker strategy.

2. RESULTS AND DISCUSSION 2.1. Synthesis of PDMAEMA Macro-CTAs. The synthesis of PDMAEMA-b-PBnMA worms involves two sequential polymerization steps. The first step is RAFT solution polymerization of 2-(dimethylamino)ethyl methacrylate (DMAEMA) in DMF at 70 °C using 2-cyano-2-propyl ethyl trithiocarbonate as the chain transfer agent (CTA) and 2,2′azobis(2-methylpropionitrile) (AIBN, [AIBN]/[CTA] = 0.2) as the initiator (Scheme 1B). PDMAEMAs with two different number-average degree of polymerizations (DP = 26 and 36) B

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Figure 1. Kinetic plots of ln([M]0/[M]) vs time (A) and evolution of weight-average molar mass (Mw) with conversion (B) during dispersion (co)polymerizations for the syntheses of linear PDMAEMA26-b-PBnMA80 and cross-linked PDMAEMA26-b-P(BnMA0.97-co-X0.03)80 worms at 20 wt % solids, 70 °C.

Figure 2. Evolution of GPC traces during dispersion (co)polymerizations for the syntheses of linear PDMAEMA26-b-PBnMA80 (A) and cross-linked PDMAEMA26-b-P(BnMA0.97-co-X0.03)80 worms in the presence of VMA (B), AMA (C), and ABMA (D) at 20 wt % solids, 70 °C.

perturbation to the worm morphology. Each of these asymmetric cross-linkers has a pair of double bonds with different reactivities. Their reactivity ratio (r1/r2) was estimated according to the Alfrey−Price equation on the basis of (Q, e) values for model monomers, 91 and allyl acetate and allyloxybenzene were assumed to have similar (Q, e) values. VMA has a higher reactivity ratio (r1/r2 = 1700) than both AMA (r1/r2 = 28) and ABMA (r1/r2 = 37) while the latter two have similar values. Polymerizations of VMA and AMA have been previously studied by RAFT or atom transfer radical polymerization (ATRP) with aims to synthesize either alkenefunctionalized polymers (with low dispersities) or branched polymers (with high dispersities).92−94 On the basis of these studies, we reason that the high-reactivity methacrylic units should statistically copolymerize with BnMA at the early stage of dispersion polymerization and the pendant, low-reactivity vinyl or allyl units on the generated copolymers should gradually contribute to branching as the overall monomer conversion increases. Thus, it is possible, under suitable conditions, to produce higher-order morphologies such as worms when the polymer chains within the nanoparticles are still mobile before significant cross-linking reactions finally occur upon monomer depletion.

were prepared, purified via dialysis against ethanol, and isolated by evaporation of solvent. A gel permeation chromatograph (GPC) equipped with both refractive index and light scattering detectors was used to provide absolute number-average molar masses (Mn) and dispersities (Đ): Mn = 4.87 kg/mol, Đ = 1.16 for PDMAEMA26 and Mn = 6.15 kg/mol, Đ = 1.19 for PDMAEMA36. Similar molar masses were also determined by comparing the proton resonances of the ester methylene group (−COOCH2−) at 4.3−3.9 ppm and the CTA methylene group (−CH2−SC(S)S−) at 3.3−3.1 ppm from the 1H NMR spectra (Figure S3). These PDMAEMAs were subsequently used as the macromolecular chain transfer agents (macroCTAs) for RAFT dispersion polymerization of BnMA in the absence or presence of cross-linkers to provide either linear PDMAEMA-b-PBnMA or cross-linked PDMAEMA-b-P(BnMA-co-X) (X denotes cross-linker) worms (Scheme 1C). 2.2. Dispersion Polymerization Kinetics. Dispersion copolymerizations of BnMA with three asymmetric crosslinkers (VMA, AMA, and ABMA) were investigated to in situ cross-link block copolymer worms. While both VMA and AMA are commercially available, ABMA was synthesized via three steps (see Supporting Information) and was used as a crosslinker structurally mimicking BnMA in order to minimize C

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all the three cross-linking copolymerizations remained monomodal and the molar mass distributions had low dispersities (Đ ∼ 1.2 or smaller) for conversions lower than 40%, but obvious shoulders were present for those of around 60% conversions. However, the molar masses obtained for the cross-linking copolymerization using VMA were only slightly higher than those obtained for the corresponding linear block copolymers in the range 60−85% conversions. For higher conversions, GPC measurements were not conducted because the polymer samples became insoluble in DMF. Although the GPC trace for 85% conversion appeared to be multimodal, the dispersity (Đ = 1.41) was only modestly larger than that achieved for the corresponding linear block copolymer (Đ = 1.09 for 86% conversion) synthesized in the absence of cross-linker. However, the cross-linking copolymerizations with both AMA and ABMA showed a significant departure from linearity in Mw at high conversions, reminiscent of the behavior observed in the synthesis of branched methacrylic copolymers using a substoichiometric ratio of a symmetric dimethacrylate brancher relative to RAFT agent in statistical solution copolymerization. 96 Indeed, the GPC traces for the cross-linking copolymerizations using AMA and ABMA exhibited multimodal and broad molar mass distributions with high Đ values being obtained at high conversions, Đ = 4.42 at 86% conversion and Đ = 3.21 at 85% conversion using AMA and ABMA, respectively. The lower degree of branching using VMA in comparison with that using AMA or ABMA is ascribed to the higher reactivity ratio of the methacrylic/vinyl units of VMA, which leads to a more selective polymerization to its methacrylic unit and a lower possibility for reactions to the incorporated vinyl units on the copolymers. It is also worth noting that for a given polymer chain length the degree of branching is dependent on the polymer concentration. Previous studies on solution branching copolymerizations have shown that branching is more effective when the polymer concentration is much higher than the critical overlap concentration (c*). In dispersion copolymerizations, the generated block copolymers self-assemble into nanoparticles once a critical chain length is reached. Subsequently, the copolymerization occurs in the monomer-swollen nanoparticles where the local copolymer chain concentration is high such that branching at high conversions and cross-linking at the final stage is expected to be highly effective. 2.3. Morphological Stabilization of Block Copolymer Worms. Next, the ability and scope of different cross-linkers to in situ stabilize worm morphology were investigated. Linear or cross-linked PDMAEMA36-b-P(BnMA(1‑x)-co-Xx)110 block copolymer worms were synthesized in the absence or presence of varying amounts of different cross-linkers at 30 wt % solids and 70 °C. The dispersion (co)polymerizations were conducted for a sufficiently long time (24 h) to achieve near-quantitative conversions (>99%) and to ensure complete cross-linking of the final morphologies. This precaution is necessary in order to make a valid comparison of the effect of cross-linker structures on worm stabilization. The linear PDMAEMA36-b-PBnMA110 worms had a diameter of 30 nm and a polydisperse length in the range 0.2−3 μm (Figure 3). The TEM micrographs for the cross-linked PDMAEMA36-b-P(BnMA(1−x)-co-Xx)110 worms are shown in Figure 4. In comparison with the linear worms, the VMA-crosslinked worms showed both a reduction in worm length and an increase in sphere population. This morphology degradation was more severe for the sample synthesized using 6% VMA

Dispersion polymerization of BnMA mediated by PDMAEMA macro-CTAs has been previously studied95 and is chosen here as the model system for the synthesis of block copolymer worms. For dispersion polymerization kinetic studies, (co)polymerizations of BnMA in the absence or presence of different asymmetric cross-linkers were conducted at 20 wt % solids and 70 °C using PDMAEMA26 as a macro-CTA to target either linear PDMAEMA26 -b-PBnMA 80 or cross-linked PDMAEMA26-b-P(BnMA0.97-co-X0.03)80 worms. The dispersion (co)polymerizations were followed by periodically extracting aliquots from the polymerization solutions which were subject to characterization by 1H NMR spectroscopy for monomer conversion, GPC for molar mass and dispersity of the generated soluble polymers (before cross-linking was reached in the presence of a cross-linker), and dynamic light scattering (DLS) for hydrodynamic diameter (Dh) and polydispersity index (PDI) of the generated nanoparticles. Dispersion polymerization of BnMA targeting PDMAEMA26b-PBnMA80 proceeded to a high conversion (94%) within 10 h as determined by 1H NMR analysis (Figure S5). TEM analysis indicated the final morphology was a worm phase (Figure S6). The polymerization kinetic plot shown in Figure 1 suggested that the growing PDMAEMA-b-PBnMA self-assembled into particles (particle nucleation) at around 2.2 h when the polymerization rate was observed to increase, which has been previously explained by polymerization in monomer-swollen nanoparticles.3 The kinetic plots were linear both before and after this nucleation time point, and a linear evolution in molar mass vs conversion was also observed with monomodal and narrow molar mass distributions (Figure 2A). These features, as expected for a controlled RAFT dispersion polymerization, were consistent with previously reported studies.95 However, dispersion copolymerizations of BnMA with the asymmetric cross-linkers to target cross-linked PDMAEMA 26 -b-P(BnMA0.97-co-X0.03)80 worms showed some notable changes in the polymerization kinetics. In comparison with the dispersion polymerization of BnMA only, the cross-linking copolymerizations showed similar polymerization rates before particle nucleation, but their nucleation points were delayed to longer times (higher conversions) and higher polymerization rates were observed after nucleation. The delayed nucleation may be attributable to the more soluble nature of the asymmetric crosslinkers in comparison with BnMA. Although clear explanations for the observed higher polymerization rates after nucleation in the presence of cross-linkers and for the differences among the cross-linkers are still lacking, we surmised that the subtle balance between the degree of branching (or viscosity) and the solubilization/plasticization by monomer/solvent resulted in such behavior. Although all the cross-linking copolymerizations finally produced insoluble worms (Figures S7, S9, and S11), the soluble copolymers generated during the polymerization prior to complete cross-linking were characterized by GPC equipped with both refractive index and light scattering detectors. As shown in Figure 1B, the molar masses were linearly dependent on the total monomer conversions at low conversions but started to deviate from linearity at approximately 60% conversion, suggesting that at early copolymerization stage the generated copolymers were primarily linear chains. Both copolymerizations through the vinyl or allyl groups of the asymmetric cross-linkers and side reactions taking place to the incorporated vinyl or allyl units on the copolymers are negligible. Consistent with these results, the GPC traces for D

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have similar characteristics to the linear analogues even when 6% ABMA was used. In addition to asymmetric cross-linkers, two symmetric crosslinkers ethylene glycol dimethacrylate (EGDMA) and 1,4phenylenebis(methylene) bis(2-methyl acrylate) (PBMBMA) were also tested in control experiments under dispersion copolymerization conditions. As shown in Table S5 and Figure S13, macroscopic gelation was observed when 3% and 2% EGDMA were used. Although reducing EGDMA to 1% generated block copolymer worms, they were not completely cross-linked as the worms could be destructed by a good solvent such as DMF. PBMBMA was synthesized as a symmetric cross-linker that has the basic motif of BnMA. Cross-linked worms could be produced with PBMBMA up to 3% (Figure S14). However, further increasing PBMBMA up to 6% resulted in macroscopic gelation. These results suggest that the ability to cross-link worms is highly dependent on the structure of symmetric cross-linkers. In comparison with EGDMA, the structural similarity of PBMBMA to BnMA and the longer spacer of PBMBMA relative to EGDMA may together contribute to the higher ability of PBMBMA to in situ cross-link worms. However, the use of the asymmetric crosslinker ABMA is still favorable because ABMA can be used up to 6% without macrogelation. DMF dissolution tests were also conducted to evaluate if the worms were completely cross-linked using the asymmetric cross-linkers. For these experiments, the worms were dispersed into DMF and were incubated in DMF for several hours. DMF was then removed via dialysis against ethanol to produce ethanolic dispersions again. We found this redispersion into ethanol could assist in acquiring high quality TEM images as DMF was difficult to remove from the nanoparticles. In comparison with the images shown in Figure 4 for the assynthesized samples, the cross-linked worms using different amounts of asymmetric cross-linkers retained their worm characteristics after DMF dissolution tests (Figure 5), suggesting that 3% or 6% asymmetric cross-linker is effective at realizing complete cross-linking of the block copolymer worms.

Figure 3. TEM micrograph of linear PDMAEMA36-b-PBnMA110 worms synthesized at 30 wt % solids, 70 °C.

Figure 4. TEM micrographs of cross-linked PDMAEMA36-bP(BnMA(1−x)-co-Xx)110 worms synthesized at 30 wt % solids, 70 °C.

than using 3% VMA; the maximum length was 0.8 vs 1.7 μm, and the diameter was 23 vs 34 nm. While the worms crosslinked using 3% AMA seemed to better retain the worm morphology (0.5−2.2 μm in length, 35 nm in diameter), an obvious reduction in both worm length (maximum length 0.7 μm) and diameter (30 nm) as well as an increase in sphere population was again observed for the 6% AMA-cross-linked sample. However, the worms cross-linked using ABMA, which has a structure mimicking that of BnMA, appeared to be more resistant to morphology degradation and were less sensitive to the amount of cross-linker used. The worm length was 0.3−3 and 0.2−2.8 μm, and the worm diameter was 35 and 36 nm, for the worms cross-linked using 3% and 6% ABMA, respectively. The effect of the cross-linkers on the characteristics of the worm morphology can be explained from a structural point of view. Although the presence of only 3% cross-linker may seem to a marginal compositional alteration to the dispersion polymerization formulation, it should be emphasized that the worm phase is narrow and thus is highly sensitive to any compositional perturbation. Both VMA and AMA are more solvophilic and have a smaller volume than BnMA, which is expected to reduce both the phase segregation strength and the packing parameter of the block copolymer worms. On the other hand, ABMA has a structure similar to that of BnMA and thus causes little perturbation to the final packing parameter. In addition, it should also be emphasized that the difference in the spacer length in different cross-linkers can lead to different degrees of entropic loss during the cross-linking process. The longest spacer in ABMA may also positively contribute to the success of worm cross-linking. As such, the cross-linked worms

Figure 5. TEM micrographs of cross-linked PDMAEMA36-bP(BnMA(1−x)-co-Xx)110 worms after DMF dissolution tests.

2.4. Synthesis of Cross-Linked Triblock Copolymer Worms. This in situ cross-linking during PISA is a straightforward and feasible approach to provide stabilized block copolymer worms that are amenable to further functionalization/modification without changing the worm morphology. To illustrate this possibility, ABMA-cross-linked block copolymer worms were used as seeds from which a third PDMADEM block was initiated to result in cross-linked E

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Macromolecules triblock copolymer worms. The TEM images of PDMAEMA26b-P(BnMA0.97-co-ABMA0.03)80 diblock copolymer and PDMAEMA26-b-P(BnMA0.97-co-ABMA0.03)80-b-PDMAEMA72 triblock copolymer worms are shown in Figure 6 as representative

Figure 7. pH-dependent zeta-potential of cross-linked PDMAEMA36b-P(BnMA0.97-co-ABMA0.03)110 diblock copolymer and PDMAEMA36b-P(BnMA0.97-co-ABMA0.03)110-PDMAEMA94 triblock copolymer worms.

consumed, and the extent of branching correlates with the estimated reactivity ratio of the two double bonds on each of the cross-linker. Block copolymer worms can be finally crosslinked because the cross-linking reactions become more effective in the polymer-rich nanoparticles at high monomer conversions. Although all three asymmetric cross-linkers lead to cross-linked worms, morphology degradation can occur when using VMA or AMA. However, cross-linking with ABMA that has a similar structure to BnMA proves to be more robust, with worm characteristics similar to those of the corresponding linear block copolymer worms being obtained. The diblock copolymer worms can be completely cross-linked with 3% or 6% asymmetric cross-linkers as confirmed by DMF dissolution tests. Cross-linked triblock copolymer worms can be prepared using the cross-linked diblock copolymer worms as the precursor without morphology degradation. This study provides an effective approach to synthesize block copolymer worms with robust morphological stabilization, and further functionalization or modification is possible without changing the worm morphology.

Figure 6. TEM micrographs of PDMAEMA26-b-P(BnMA0.97-coABMA 0.03 ) 80 diblock copolymer (A) and PDMAEMA 26 -b-P(BnMA0.97-co-ABMA0.03)80-b-PDMAEMA72 triblock copolymer (B) worms.

examples. It is clear that the worm morphology was retained for the triblock block copolymer that had an additional solvophilic PDMAEMA block. In contrast, the morphology underwent a dramatic change from worm to mixed morphologies (sphere, worm, and bilayer) when initiating a third PDMAEMA block from the corresponding linear diblock copolymer counterpart (Figure S15). The presence of a third block was supported by an increase in worm diameter from 38 nm for the cross-linked diblock copolymer to 47 nm for the cross-linked triblock copolymer. Zeta-potential measurements on cross-linked worms dispersed in water provided further evidence for the successful generation of a third PDMAEMA block on the worm surface. Figure 7 compares the pH-dependent zeta-potential of cross-linked diblock and triblock worms. At pH < 6, the zetapotential for the triblock copolymer worms was more positive than the diblock copolymer worms due to the presence of more PDMAEMA on the worm surface that could be protonated.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b00246. Experimental details and additional data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.A.). ORCID

Zesheng An: 0000-0002-2064-4132 Notes

The authors declare no competing financial interest.

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3. CONCLUSION Dispersion copolymerizations of BnMA with three different asymmetric cross-linkers (VMA, AMA, and ABMA) have been investigated aiming to produce in situ cross-linked block copolymer worms. Kinetic studies indicate that at the early stage linear copolymers are produced by statistical copolymerization of BnMA with the methacrylic units of the asymmetric cross-linkers. Branching due to reactions to the incorporated vinyl or allyl units on the copolymers becomes progressively more significant as the monomers are gradually

ACKNOWLEDGMENTS We thank financial support by National Natural Science Foundation of China (21674059) and assistance of Instrumental Analysis and Research Center, Shanghai University.

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REFERENCES

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