RAFT emulsion polymerization of styrene mediated by core cross-linked star (CCS) polymers

Core cross-linked star (CCS) polymers of poly(N,N-dimethylacrylamide) (PDMA) were prepared and explored as macromolecular chain transfer agents (Macro-CTAs) in the emulsion polymerization of styrene. The mass ratio of styrene : CCS and the solid content were systematically varied to elucidate their effect on the size and morphology of the particles. Control experiments using linear PDMA as a Macro-CTA were also performed in order to understand the mechanistic difference in CCS-mediated emulsion polymerization. While a two-phase process was typical of emulsion polymerization using linear Macro-CTAs, the CCS polymers functioned as "seeds" for the emulsion polymerization of styrene and exhibited a much higher polymerization rate. Significant clustering of spherical particles was observed when the mass ratio of styrene : CCS was increased while maintaining the solid content. On the other hand, linear and branched fibers were observed when the solid content was increased while maintaining the mass ratio of styrene : CCS. This morphology transformation is unique to CCS-mediated emulsion polymerization, and a protrusion mechanism caused by the cross-links in the core of the CCS was proposed to account for the aggregation of spherical particles into clusters or fibers.