Heteroarm core cross-linked star (CCS) polymers consist of two chemically different arms emerging from a densely cross-linked core. If the two arms are immiscible, the intramolecular segregation can occur to produce two different “faces” within the CCS polymers. Such nanoobjects are expected to behave as Janus nanoparticles, useful as interfacial stabilizers and emulsifiers. Realizing intramolecularly segregation has been considered challenging in heteroarm CCS polymers, however, since the covalently connection of the arms to the core prevents rearrangement. For the synthesis of intramolecularly segregated heteromarm CCS polymers, we explored an “in-out” route by reversible addition-fragmentation chain transfer (RAFT) copolymerization of 1,2-bis(maleimidoethane) (BMI) with styrene (S) in the presence of a macro-chain transfer agent (CTA). Because of the strong alternating tendency of maleimide and styrenic double bonds, the cross-linked core effectively forms and ties up a number of macro-CTAs. Subsequent chain extension from the core grows another polymer chain, completing heteroarm CCS polymer. We demonstrate that such a heteroarm CCS polymer consisting of polylactide (PLA) and polystyrene (PS) arms (PLAnPSn) can be synthesized from PLA-CTA. We also show that use of an excess S in the alternating copolymerization directly results in PLAnPSn in one-shot polymerization. Transmission electron microscopy (TEM) analysis of PLAnPSn films cast from neutral and selective solvents showed lamellar and spherical morphologies, indicating that PLA and PS arms can be segregated within the CCS architecture and further self-assemble. To shed light on the segregation mechanism, we investigated dependence of the synthetic pathways to the degree of intramolecular segregation by analyzing their microphase-separated morphologies. Assuming that a higher degree of intramolecular segregation would result in more well-defined structures with a narrower interface, we find that the incompatibility of the growing PS chains to the local environment with high PLA concentration can be further amplified during the “out” step by using a more selective solvent. This promotes intramolecular segregation in the heteroarm CCS polymer, to produce a narrower interface between the compartments, leading to more well-defined self-assembled structures than those synthesized via “multi macro-initiator” approach.

Synthesis of heteroarm core cross-linked star polymers as Janus nanoparticles

ACS Fall 2020 Virtual Meeting

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