Temperature- and pH-Responsive Dense Copolymer Brushes Prepared by ATRP

Abstract

Dual responsive molecular brushes consisting of statistical copolymers of di(ethylene glycol) methyl ether methacrylate (MEO2MA) and either methacrylic acid (MAA) or N,N-dimethylaminoethyl methacrylate (DMAEMA) were synthesized by grafting from poly(2-(2-bromoisobutyroyloxy)ethyl methacrylate (PBIEM) macroinitiators using atom transfer radical polymerization (ATRP). Copolymer brushes with controlled composition and molecular weights ranging from Mn = 600 000 to 1 400 000 with polydispersity indexes (Mw/Mn) between 1.18 and 1.45 were obtained. The lower critical solution temperature (LCST) of MEO2MA-stat-MAA decreased with the increasing molar fraction of MAA in the copolymer in deionized water but increased in buffer solution at pH 7. At pH 9, the hydrophilicity of polymer increased with ionization of carboxylic acid to further raise the LCST. On the other hand, the LCST of MEO2MA-stat-DMAEMA copolymers increased with increasing DMAEMA content at pH 4, 7, and 9. A bottle-brush terpolymer prepared from all three comonomers exhibited LCST at pH 4 and 7 but not at pH 9, which can be attributed to the stronger ionization of MAA. The responsive nature of the copolymer is enhanced by the densely graft structure of a brush copolymer.