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Synthesis of multiblock copolymers using seeded reversible addition−fragmentation chain transfer (RAFT) emulsion polymerization has been explored with a view to elucidate how certain experimental conditions influence the control over molecular weight distribution (MWD). Two separate parameters have been explored in detail: (i) the ratio of monomer concentration to the RAFT end group concentration within particles and (ii) the glass transition temperature (T g ) of the particles. The parameters (i) and (ii) are interrelated as an increase in the ratio [monomer]:[RAFT] leads to a lower T g because of the increased plasticization of the polymer particle by the monomer. Three different monomers were employed, each giving a polymer of different T g values: n-butyl methacrylate (T g = 20 °C), iso-butyl methacrylate (T g = 57 °C), and tert-butyl methacrylate (T g = 118 °C). The results show that the level of control over the MWDs via the RAFT mechanism is markedly reduced under conditions where T g of the polymer particles is high. This is attributed to the high T g value, leading to low radical penetration rates (low diffusion rates) of radicals generated via initiation in the aqueous phase, preventing propagating radicals from reaching the core region of the particles before bimolecular termination occurs. In the present system, the RAFT end groups are predominantly (but not at all exclusively) located in the core region of the particles.

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 (T g ) of some polymers have hitherto remained. The use of monomers that give such high-T g 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 (T g 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 -T g polymers (PnBMA; T g = 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.