Reversible Addition-fragmentation Chain Transfer Dispersity Regulation by Chain Transfer Agent Micelles
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.
DispersityMolecular weight distributionReversible addition-fragmentation chain transfer polymerizationChain transfer agentMicelle
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