Limiting conversion phenomenon in hybrid miniemulsion polymerization

A phenomenon seemingly unique to hybrid miniemulsion polymerization was observed where monomer conversion would either plateau at a limiting value or quickly switch to a dramatically lesser rate. This phenomenon has been attributed to a combination of three factors. The first factor is the degree to which the monomer and resinous component are compatible. Second is the resultant particle morphology after circa 80% monomer conversion, which roughly corresponds to the portion of reaction where this morphology is established. The third factor is the degree of interaction between the growing polymer and resin (grafting). Of these three, the first two factors were found much more significant in contributing to the limiting conversion. When particle morphology was found to be core/shell, a hard shell (high T-g polymer, PMMA) was found to form a barrier against newly formed initiator radicals derived in the aqueous-phase after appropriate conversion. Residual unreacted monomer solubilized in the resin-dominated particle core was thereafter unreachable by new radicals; hence a limited monomer conversion. In cases where the acrylic polymer (PBA) exhibited a glass transition significantly below the reaction temperature, instead of a core/shell morphology one where the acrylic polymer (and monomer) comprised the continuous particle-phase with small internal resinous island domains was observed. A portion of the monomer concentration was again found to be solubilized within the resin domains, yet in this case newly formed initiator radicals encountered a viscous environment instead of an effective barrier. Rate was found to be limited by the feed of monomer to local polymerization in the continuous particle phase from those resinous islands where residual monomer is solubilized. This is what led to continued polymerization, but at a considerably lesser rate.