One-pot synthesis of double and triple polybetaine block copolymers and their temperature-responsive solution behavior

Di- and triblock betaine copolymers composed of a sulfobetaine with a carboxy/phosphobetaine were synthesized by one-pot aqueous reversible addition-fragmentation chain transfer (RAFT) polymerization. The betaine methacrylate monomers were fully converted at each step by tuning the concentrations of the monomers, the chain transfer agent, and the initiator. AB-type diblocks poly(carboxybetaine methacrylate)-b-poly(sulfobetaine methacrylate) (PGLBT-b-PSPE) and poly(2-(methacryloyloxy)ethyl phosphorylcholine)-b-poly(sulfobetaine methacrylate) (PMPC-b-PSPE) were prepared to have identical unit numbers with different total molecular weights. Unlike homo-polysulfobetaine solutions or PGLBT-b-PSPEs in our previous report, the aqueous solutions of those diblock copolymers showed weak upper critical solution temperature (UCST) behavior, monitored by transmittance/dynamic light scattering. Even at the lowest temperature, the diblock chains remained in the unimer/aggregate polydisperse state, which is the intermediate stage prior to the emergence of monodisperse micelles. It is thought that the UCST behavior that originated from electrostatic attraction among PSPE motifs was interfered by temperature-inert motifs (PGLBT/PMPC) on the opposite side, having nearly the same unit number. The BAB-type triblock copolymer PSPE-b-PGLBT-b-PSPE with a 1:1:1 stoichiometry showed a clearer transmittance shift and size variation due to the increased ratio of PSPE at both chain ends. While the transmittance gradually decreased by cooling, larger aggregates (~ 700 nm) than those of diblocks (~ 180 nm) emerged with unimers. The small-large diffusive modes transformed into a monodisperse mode at much lower temperatures, which is assumed to be flower-like micelles consisting of PGLBT-loop coronas and PSPE core. However, another sample of 1:2:1 stoichiometry showed incomplete transition as diblocks having a similar AB ratio.