Particle formation in thermally initiated radical-mediated thiol-ene emulsion polymerizations

Radical-mediated thiol-ene polymerizations (RMTEPs) conducted under typical emulsion polymerization conditions were studied with the aim of determining how various experimental parameters impact particle nucleation and growth. These colloids are unusual since they are made by step-growth polymerizations, which can be thermally initiated in an aqueous emulsion system. Four pairs of thiol-ene monomers were used to probe how composition impacts the nucleation loci. Kinetic studies were carried out to understand how monomer conversion, molecular weight, and particle size change throughout the reaction. Additionally, the number of particles formed as a function of surfactant and initiator concentrations were studied and this data, along with information from dye partition experiments, were used to determine polymer colloid nucleation mechanisms and how monomer/polymer structure impacts nucleation processes. Within the four pairs of monomers studied, three proceeded via homogeneous nucleation (via chain collapse in the aqueous phase), while one pair, which gave the highest degree of crosslinking, underwent heterogeneous nucleation (nucleation occurs in the micellar phase). Possible factors that impact which nucleation mechanism dominates a given set of monomers, such as crosslink density, water solubility, and thiol/ene reactivity, is discussed, although no clear set of delineating guidelines can be offered with this current set of data.