Stereoblock Copolymers and Tacticity Control in Controlled/Living Radical Polymerization

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Abstract
Three controlled/living radical polymerization processes, atom transfer radical polymerization (ATRP), reversible addition-fragmentation transfer (RAFT) polymerization, and nitroxide-mediated polymerization (NMP), were investigated for the polymerization of N,N-dimethylacrylamide in the presence of Lewis acids known to enhance isotacticity, such as yttrium trifluoromethanesulfonate (Y(OTf)3) and ytterbium trifluoromethanesulfonate (Yb(OTf)3). Poly(N,N-dimethylacrylamide) with controlled molecular weight, low polydispersity (Mw/Mn < 1.2), and a high proportion of meso dyads (∼85%) was prepared by ATRP (with initiating system methyl 2-chloropropionate/CuCl/Me6TREN) and RAFT (with cumyl dithiobenzoate transfer agent) in the presence of Y(OTf)3. The combination of NMP (using N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide, SG1) and a Lewis acid complexation technique led to less precise control over chain architecture and microstructure (∼65% meso dyads), as compared to RAFT/Y(OTf)3 or ATRP/Y(OTf)3. The latter two systems were used for the first one-pot synthesis of stereoblock copolymers by radical polymerization. Well-defined stereoblock copolymers, atactic-b-isotactic poly(N,N-dimethylacrylamides), were obtained by adding Y(OTf)3 at a given time to either RAFT or ATRP polymerizations, initially started without the presence of the Lewis acid.