Expanding the scope of RAFT polymerization for (multi)block copolymer synthesis by exploitation of the nanoreactor concept in emulsion polymerization

RAFT polymerization is a versatile technique for synthesis of polymers of a range of complex architectures. Multiblock copolymers composed of polymer segments of sufficiently high molecular weight such that microphase separation and self-assembly can occur offer a pathway to a myriad of nano-engineered nanoparticles and materials (Fig. 1). In the present work, we demonstrate how aqueous RAFT emulsion polymerization can be used to significantly expand the synthetic capabilities of RAFT polymerization by understanding and exploiting kinetic and mechanistic features of polymerization within nanoreactors (i.e. polymeric nanoparticles).
The degree of livingness (chain-end fidelity) in RAFT polymerization for a given degree of polymerization can be markedly increased in a compartmentalized system such as (mini)emulsion polymerization. This is a consequence of a reduced rate of bimolecular termination caused by physical segregation of propagating radicals in submicron size particles (nanoreactors). Exploitation of this feature makes it possible to prepare multiblock copolymers comprising slowly propagating monomers such as styrenes and methacrylates, which is not possible in the corresponding homogeneous (solution/bulk) system. Another important intrinsic feature of emulsion polymerization is that the polymerization occurs in submicron size polymer particles with monomer being supplied from micron size monomer droplets via diffusion through the aqueous phase. Consequently, the ratio of monomer to RAFT end groups within the particles is lower than in the corresponding homogeneous system. This enables (macro)RAFT agent / monomer pairs with chain transfer coefficients that are too low for efficient RAFT control to be employed successfully in emulsion polymerization, contrary to in the corresponding homogeneous systems. This has important implications in regard to polymer synthesis; for example, it significantly expands the range of synthetically accessible multiblock copolymers as the restriction of “monomer order” in block synthesis is markedly relaxed.

Fig 1. Graphic illustration of multilayered nanoparticle obtained via sequential RAFT emulsion polymerization.