Heteroarm core cross-linked star polymers via RAFT copolymerization of styrene and bismaleimide

We explored reversible addition-fragmentation chain transfer (RAFT) copolymerization of 1,2-bis(maleimidoethane) (BMI) with styrene (S) in the presence of polylactide macro-chain transfer agent (PLA-CTA) as a means to synthesize heteroarm core cross-linked star (CCS) polymers consisting of PLA and PS arms (PLA(n)PS(n)). Because of the strong alternating tendency of maleimide and styrenic double bonds, copolymerization of BMI with an excess of S depleted BMI in the early stage of polymerization forming a cross-linked core. The remaining S was successively polymerized to grow PS arms from the core, completing PLA(n)PSn via "in-out" mechanism. Use of a stoichiometric amount of S produced PLA(n), which could be used as a macro-CTA for the synthesis of more well-defined PLA(n)PSn. Compared with divinylbenzene, copolymerization of BMI with S was much more effective for core formation suggesting the importance of the alternating character of the copolymerization. While PLA(n)PSn existed as stable nanoparticles in a neutral solvent in contrast to linear PLA-b-PS, it also self-assembled to form microphase-separated structures in a selective solvent and in bulk indicating that PLA and PS arms can be intramolecularly segregated.