Synthesis of Stimuli-responsive, Water-soluble Poly[2-(dimethylamino)ethyl methacrylate/styrene] Statistical Copolymers by Nitroxide Mediated Polymerization

Abstract

2-(Dimethylamino)ethyl methacrylate/styrene statistical copolymers (poly(DMAEMA-stat-styrene)) with feed compositions f DMAEMA = 80-95 mol%, (number average molecular weights Mn = 9.5-11.2 kg mol -1) were synthesized using succinimidyl ester-functionalized BlocBuilder alkoxyamine initiator at 80 °C in bulk. Polymerization rate increased three-fold on increasing f DMAEMA = 80 to 95 mol%. Linear Mn increases with conversion were observed up to about 50% conversion and obtained copolymers possessed monomodal, relatively narrow molecular weight distributions (polydispersity = 1.32-1.59). Copolymers with f DMAEMA = 80 and 90 mol% were also cleanly chain-extended with DMAEMA/styrene mixtures of 95 and 90 mol% DMAEMA, respectively, confirming the livingness of the copolymers. Copolymer phase behavior in aqueous solutions was examined by dynamic light scattering and UV-Vis spectroscopy. All copolymers exhibited lower critical solution temperature (LCST)-type behavior. LCST decreased with increasing styrene content in the copolymer and with increasing solution concentration. All copolymers were completely water-soluble and temperature insensitive at pH 4 but were more hydrophobic at pH 10, particularly copolymers with f DMAEMA = 80 and 85 mol%, which were water-insoluble. At pH 10, LCST of copolymers with f DMAEMA = 90 and 95 mol% were more than 10 °C lower compared to their solutions in neutral, de-ionized water. Block copolymers with two statistical blocks with different DMAEMA compositions exhibited a single LCST, suggesting the block segments were not distinct enough to exhibit separate LCSTs in water.