Cationic Polymerization of Vinyl Monomers in Aqueous Media: From Monofunctional Oligomers to Long-Lived Polymer Chains

Abstract

Polymer latexes are easily prepared on a multimillion ton scale in industry using free radical initiated emulsion and suspension polymerizations in water, a cheap, nonviscous, heat-controlling, and environmentally benign solvent. Until recently, researchers had done little investigation into ionic polymerization because even a small amount of water would easily deactivate the conventional catalysts used in these processes. In the last decade, however, cationic polymerization in aqueous media has emerged as a new and attractive method for controlling the polymerization reactions using mild experimental conditions. This Account reviews the current science of and future outlook for cationic polymerization of vinyl monomers in aqueous media. We particularly emphasize the design and evolution of catalytic systems and the precision synthesis of functional polymers. Early work to tailor the suspension and emulsion cationic polymerizations of reactive monomers such as p-methoxystyrene and vinyl ethers used long-chain strong acids, called INISURF for their dual roles as initiators and surfactants, and lanthanide triflates. These polymerization processes shared two main features: (i) all reactions (initiation, propagation, and termination) occurred at the particle interface; (ii) synthesized polymers have limits on their molecular weight, attributed to the "critical DP" effect, related to the entry of oligomers inside the particles as they become increasingly hydrophobic. The next generation of catalysts, named "Lewis acid-surfactant combined catalysts" (LASC), shifted the polymerization locus from the interface to the inside of the monomer droplets, allowing for the production of long polymer chains. Recently, catalytic systems based on boranes, (BF(3)OEt(2), B(C(6)F(5))(3), (C(6)F(4)-1,2-[B(C(6)F(5))(2)]), and (C(6)F(4)-1,2-[B(C(12)F(8))](2))), have shown great potential in controlling the cationic polymerization in "wet" solution, containing an excess of water relative to Lewis acid, or aqueous media of such industrially important monomers as styrene, cyclopentadiene, and even isobutylene.