RAFT Polymerization of Acrylamides Containing Proline and Hydroxyproline Moiety: Controlled Synthesis of Water-Soluble and Thermoresponsive Polymers

Two acrylamides containing proline and hydroxyproline moiety, N -acryloyl- l -proline (A-Pro-OH) and N -acryloyl-4- trans - l -proline (A-Hyp-OH), were polymerized by reversible addition-fragmentation chain transfer (RAFT) process to afford well-defined amino acid based polymers. Two chain transfer agents (CTAs), benzyl dithiobenzoate (CTA 1) and benzyl 1-pyrrolecarbodithioate (CTA 2), were compared for the direct polymerization of these monomers without protecting chemistry. With 2,2′-azobis(isobutyronitrile) as an initiator, the dithiocarbamate-type RAFT agent (CTA 2) was efficient for the controlled synthesis of poly(A-Pro-OH)s, which can be regarded as a weak polyelectrolyte. Controlled character of the polymerization of A-Hyp-OH, which has a carboxylic acid and a hydroxyl group in the monomer unit, in the presence of CTA 1 was confirmed by the formation of narrow polydispersity products and the linear relationship between the molecular weight and conversion. Water-soluble poly(A-Hyp-OH)s with number-average molecular weights between 8.2 × 10 3 and 2.21 × 10 4 and relatively low polydispersities were obtained, depending on the monomer/CTA ratio. Their methylated samples, poly(A-Pro-OMe) and poly(A-Hyp-OMe), and random copolymers showed characteristic thermal phase transitions in aqueous solutions.