Rod-sphere transition in polybutadiene-poly(L-lysine) block copolymer assemblies

This paper describes the synthesis and characterization of poly(butadiene)m-poly(L-lysine)n (m-n = 107-200, 107-100, and 60-50) block copolymers. The polymers are prepared in a two-step process whereby amine-terminated polybutadiene is used to initiate the ring-opening polymerization of the epsilon-benzyloxycarbonyl L-lysine N-carboxyanhydride. After deprotection, the self-assembly of the block copolymers in aqueous media were studied using dynamic light scattering, transmission electron microscopy, and circular dichroism spectroscopy. These block copolymers were found to form either spherical micelles or rod-like micelles at high pH depending on the composition of the block copolymer. As the pH is decreased, the micelles swell due to charge-charge repulsions between corona chains and from the helix-coil transition of the polypeptide block. The two systems that form rod-like micelles at high pH also exhibit a pH-induced rod-sphere transition at low pH. This transition was verified from Kratky analysis of the static light scattering data and via CONTIN analysis of the dynamic light scattering data, which shows a bimodal distribution in particle sizes.