Living radical polymerization by reversible addition-fragmentation chain transfer in ionically stabilized miniemulsions

In theory, a miniemulsion should be an ideal environment for "living" radical polymerization via the reversible addition-fragmentation chain transfer process (RAFT). Compartmentalization minimizes radical-radical termination events, and droplet nucleation eliminates the mass transfer limitation found in conventional "living" emulsion polymerizations. In practice, however, several phenomena were observed when using the RAFT technique, indicating a deviation from this idealized theory when the miniemulsion was stabilized by an ionic surfactant. Inefficient droplet nucleation, a steadily rising polydispersity over the reaction, and the appearance of a separate organic phase after initiation were all indications of particle instability. A distinct difference between standard polymerizations and those that involve highly active RAFT agents is the fact that in RAFT polymerization there is a time interval early in the reaction where oligomers dominate the molecular weight distribution. The presence of large quantities of oligomers is postulated to be the culprit behind the destabilization observed through a detrimental interaction with the ionic surfactant of the miniemulsion. Conductivity measurements verified the increase of free surfactant in the aqueous phase over the course of reaction. Despite this, results showed clear indication of "living" character with a linear evolution of molecular weight until roughly 40% monomer conversion, after which the molecular weight showed contributions from initiator-derived chains.