(6ag) Reversible Addition-Fragmentation Chain Transfer in Microemulsion Polymerizations

Over 20 million tons per year of polymer latex is produced worldwide by free radical emulsion polymerization for applications such as coatings, adhesives and rheological modifiers. Unfortunately, these polymerizations often suffer from poor control of molecular weight and yield high polydispersity polymers. Therefore, there is strong interest in incorporating the controlled polymerization mechanism of reversible addition-fragmentation chain transfer (RAFT) into heterogeneous polymerizations. RAFT controls the bulk free radical polymerization of many monomers over a wide range of reaction conditions and allows for the production of polymers with predetermined molecular weight, but frequently results in high polydispersity polymers and loss of colloidal stability in heterogeneous polymerizations. Additionally, rate retardation is often observed in RAFT polymerizations, and the source of this rate retardation is not fully understood.

The goals of this work were twofold: (1) to incorporate RAFT into microemulsion polymerizations to produce stable latex nanoparticles composed of low polydispersity polymers of predetermined molecular weight and (2) to identify the key parameters for achieving control in RAFT microemulsion polymerizations. Microemulsion polymerization is an attractive alternative to emulsion polymerization because of the absence of large monomer droplets and the elimination of biradical termination reactions. These characteristics were expected to increase the colloidal stability of the resulting latex and enhance the control of the polymerization. In fact, RAFT microemulsion polymerizations of acrylates with xanthate chain transfer agents produced stable latex nanoparticles (20-30 nm in diameter) composed of low polydispersity polymers of predetermined molecular weight. The RAFT microemulsion polymerization kinetics, polymer molecular weights and polydispersities, and latex particle sizes identified the key parameters for achieving control as the chain transfer agent per micelle ratio, and the water solubilities of the monomer and the chain transfer agent. With this information, a simple kinetic model was developed to further examine the RAFT microemulsion polymerization mechanism. The model demonstrates the significance of the rate of fragmentation of the intermediate radical formed during the equilibrium transfer reaction and the rate of diffusion of the chain transfer agent to the locus of polymerization. The model is also useful for the selection of appropriate chain transfer agents for different microemulsion polymerizations.