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

In theory, a miniemulsion should be an ideal environment for "living" radic al polymerization via the reversible addition-fragmentation chain transfer process (RAFT). Compartmentalization minimizes radical-radical termination events, and droplet nucleation eliminates the mass transfer limitation foun d 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 p olydispersity over the reaction, and the appearance of a separate organic p hase after initiation were all indications of particle instability. A disti nct difference between standard polymerizations and those that involve high ly active RAFT agents is the fact that in RAFT polymerization there is a ti me interval early in the reaction where oligomers dominate the molecular we ight distribution. The presence of large quantities of oligomers is postula ted to be the culprit behind the destabilization observed through a detrime ntal interaction with the ionic surfactant of the miniemulsion. Conductivit y measurements verified the increase of free surfactant in the aqueous phas e over the course of reaction. Despite this, results showed clear indicatio n of "living" character with a linear evolution of molecular weight until r oughly 40% monomer conversion, after which the molecular weight showed cont ributions from initiator-derived chains.