Raft Polymerization of N,N-Dimethylacrylamide Utilizing Novel Chain Transfer Agents Tailored for High Reinitiation Efficiency and Structural Control

Abstract

The syntheses of the novel acrylamido-based RAFT chain transfer agents (CTAs) N,N-dimethyl-s-thiobenzoylthiopropionamide (TBP, 1c) and N,N-dimethyl-s-thiobenzoylthioacetamide (TBA, 1d) for the controlled polymerization of N,N-dimethylacrylamide (DMA) are described. The results from a comparative study of 1c and 1d with two CTAs previously reported for the RAFT polymerization of DMA (benzyl dithiobenzoate (BDB, 1a) and cumyl dithiobenzoate (CDB, 1b)) are disclosed and demonstrate the effectiveness of the newly reported CTAs. Polymerizations at CTA/I ratios of 5/1 yield poly(N,N-dimethylacrylamide) (PDMA) homopolymers with high molecular weight impurity (1b−1d) or bimodal molecular weight distributions (1a). The high molecular weight impurity observed in polymerizations mediated by 1b−1d has been attributed to species arising from bimolecular termination reactions, as determined by multiple detector size exclusion chromatography (SEC). The bimodality observed for 1a-mediated polymerizations is due to the presence of two distinct “active” species, as determined by end group analysis using SEC. We show that it is possible to reduce/eliminate the high molecular weight impurity for 1b−1d-mediated polymerizations simply by increasing the CTA/I ratio to 80/1. Under these conditions, 1a-mediated polymerization still yields a bimodal molecular weight distribution. The differences in the efficiency of the CTAs 1a−1d are rationalized in terms of their structural and electronic characteristics and their relative fragmentation/reinitiation efficiencies.