Elastin-Based Side-Chain Polymers:  Improved Synthesis via RAFT and Stimulus Responsive Behavior

Abstract

Elastin-based side-chain polymers (EBPs) were prepared by the polymerization of a methacrylate derivative of the pentapeptide valine−proline−glycine−valine−glycine (VPGVG) using reversible addition−fragmentation chain transfer (RAFT) polymerization. The polymerizations proceeded in a controlled manner, yielding polymers with a narrow molecular weight distribution (polydispersity indices 1.03−1.23) and molecular weights in good agreement with those predicted from the initial monomer:initiator ratio for the conversion obtained. The dithioester end groups of the resulting polymers were removed by reaction with azo initiator-derived radicals. The lower critical solution temperature (LCST) behavior of the series of EBPs so obtained was investigated in solutions of varying pH (1.5−5.1) and polymer concentration (0.11−0.97 mg/mL) and for polymers of different degrees of polymerization (29−88 repeating units). These EBPs behaved similarly to linear polypeptides, known as elastin-like peptides (ELPs); the transition temperature decreased with increasing polymer concentration and molecular weight. Unlike ELPs, but in common with previously reported EBPs, a strong dependence of transition temperature on pH was observed due to the presence of the carboxylic acid from the C-terminal residue in the peptide side chains. Significant differences between the EBPs described here and those reported earlier were found, however, regarding the transition temperature at a given pH and its variation with molecular weight. These variations are attributed to differences in architecture between the polymers described here (higher molecular weight homopolymers) and those reported earlier (A−B−A triblock copolymers with short EBP A blocks and a PEG B block).