Droplet nucleation in miniemulsion thiol-ene step photopolymerization

Complex and unique polymer colloids have been prepared by miniemulsion polymerization through particle nucleation confined to monomer droplets. Unlike radical chain polymerization, the reaction conditions for a predominant droplet nucleation have not been investigated in step polymerizations such as thiol-ene polymerization. To clarify this issue, an aqueous thiol-ene miniemulsion based on diallyl adipate and 2,2-(ethylenedioxy)diethanedithiol is prepared. Droplet stability is affected by Ostwald ripening due to the limited solubility of hexadecane (costabilizer) in the monomer phase. The control of chemical stability is also difficult, since a self-initiated polymerization is caused by adventitious radicals generated in the emulsification stage (ultrasonication). However, the addition of an appropriate concentration of a radical scavenger (quinone) can halt polymerization for several hours. Under these conditions, batch photopolymerization kinetics, molecular weight progress and particle size distribution have been determined with reproducible results. Emphasis is placed on understanding how initiator solubility, droplet size and monomer solubility affect droplet nucleation. For this purpose, a reliable measurement of droplet (particle) size distributions is achieved by combining size data from dynamic light scattering and transmission electron microscopy. When using a water-soluble photoinitiator, a substantial homogeneous nucleation is reported even for highly water-insoluble monomers. Only when the droplet diameter is low enough (about 100 nm), then droplet nucleation prevails. Conversely, a water-insoluble initiator drives a robust and complete droplet nucleation irrespective of the reaction conditions. Reaction parameters, such as droplet size, initiator solubility and monomer solubility, which are important in favouring droplet nucleation in a miniemulsion thiol-ene step polymerization are reviewed.