Expanding the Scope of RAFT Multiblock Copolymer Synthesis Using the Nanoreactor Concept: The Critical Importance of Initiator Hydrophobicity

Precise multiblock copolymer synthesis coupled with self-assembly offers morphology control on length scales ranging from a few nanometers to micrometer scale, providing enormous opportunities for future development of advanced materials and applications. The scope of multiblock copolymer synthesis via RAFT polymerization has recently been expanded by application of the nanoreactor concept for emulsion polymerization. This enabled use of slow propagating monomers, such as styrenes and methacrylates, in multiblock synthesis. However, severe limitations attributed to the high polymer glass transition temperature (Tg) of some polymers have hitherto remained. The use of monomers that give such high-Tg polymers effectively prevented penetration of aqueous-phase-generated radicals into the polymer particles wherein the RAFT functionality is located. We here demonstrate that these constraints can be relieved by judicious choice of the radical initiator. Multiblock homopolymers were synthesized by seeded RAFT emulsion polymerization using initiators that differ substantially in hydrophobicity. Ten sequential chain extensions using tert-butyl methacrylate (Tg of PtBMA = 118 °C) with targeted block DP = 100 were conducted at 80 °C for each initiator. Markedly narrower molecular weight distributions were obtained when more hydrophobic initiators were used. The same polymerizations targeting low -Tg polymers (PnBMA; Tg = 20 °C) resulted in only minor differences in control when the different initiators were used, supporting our hypothesis on the role of radical penetration. The present results are anticipated to significantly expand the scope of RAFT polymerization in aqueous emulsion by allowing access to a wider range of low-dispersity multiblock copolymers.