摘要
聚合物的分子量和分子量分布(分散度)是决定聚合物性能的重要参数.目前通过聚合调控分散度的方法大都针对有机溶液聚合体系,缺乏有特色的在水相聚合体系中调控分散度的方法.本文中提出一种链转移剂胶束调控聚合物分散度的新方法.选用葡萄糖氧化酶(GOx)除氧-氧化还原引发可逆失活-断裂链转移(RAFT)聚合方法,利用双亲性链转移剂在水溶液中存在的游离分子与胶束的热力学平衡,实现对聚合物分散度的调控.通过监测聚合过程中链转移剂胶束的变化,证实了链转移剂胶束对分散度调控的重要作用.研究了溶液的pH、添加剂,包括电解质、表面活性剂和助表面活性剂对聚合物分散度的影响.通过"一锅法"成功合成嵌段共聚物,验证了宽分散度的聚合物同样具有高的链端保真度.
Abstract
Molecular weight and molecular weight distribution(dispersity,D)of polymers have an important effect on polymer properties.In recent years,most methods for regulating dispersity are focused on polymerization in organic solution,but innovative strategies for regulating dispersity in aqueous solution are still scarce.Herein,we report a novel chain transfer agent(CTA)micellar strategy for dispersity control in reversible addition-fragmentation chain transfer(RAFT)polymerization.This is realized by choosing an amphiphilic CTA which is able to form micelle and polymerization was carried out via cascade reactions involving enzymatic deoxygenation and redox initiation.During polymerization,CTA in aqueous solution is first consumed,biasing the micellar equilibrium towards dissolution and consequently broadening of dispersity due to the difference in polymerization time experienced by the polymer chains.Solution pH,sodium chloride,1-octadecanol,but not sodium dodecanesulfate,all have effect on dispersity control.We show that by varying the solution pH(5.8-7.1)for polymerizations at a monomer concentration of 1 mol/L,a range of dispersities can be obtained at different degrees of polymerization:for DP 1018,D is in the range of 1.33-1.18;for DP 504,D is in the range of 1.39-1.26;for DP 339,Ð is in the range of 1.51-1.32.Besides,the dispersity can be increased to 1.55 or reduced to 1.33 after additives were added in aqueous solution(pH=5.8).Generally,higher pH resulted in lower Ð values due to increased degree of ionization of the carboxylic acid group of the CTA,which lowered the CMC and thus the size and stability of the micelles formed by the CTA.The importance of micelles played in dispersity tuning was further confirmed by(1)polymerization at a monomer concentration of 2 mol/L and(2)the use of a totally hydrophilic CTA.In both cases,polymers with narrow dispersities(D≤1.17)were obtained regardless of the pH of the solution,which were attributed to no micelle formation in solution at a high organic(monomer)content or the use of a totally hydrophilic CTA.Further kinetic studies were conducted following the changes of the fluorescence of nile red and dynamic light scattering(DLS)during the polymerization.As polymerization proceeded,the fluorescence of nile red experienced a blue shift,indicating the microenvironment of nile red was gradually changed from being inside of the micelle to being surrounded by aqueous solution.In addition,DLS showed a reduction in the size of micelles during the polymerization.The fluorescence and DLS studies suggest disintegration of the micelles during polymerization,and it is this gradually disintegration of CTA micelles that contributes to the broadening of dispersity.However,polymers with a high dispersity still possessed high end-group fidelity,which was confirmed by one-pot synthesis of block copolymers.
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