Compromise between dominant polymerization mechanisms in preparation of polymer microspheres

A system consisting of uniform-sized styrene monomer droplets dispersed in an aqueous phase was employed to study the compromise between dominant polymerization mechanisms in the preparation of polymer microspheres. Monomer droplets were prepared by a microporous membrane emulsification technique, with negligible size distribution. Two polymerization mechanisms were identified-polymerization in micro-sized monomer droplets and nano-sized secondary nuclei, even in the presence of a hydrophobic initiator. Factors affecting the compromise between these two dominant mechanisms were investigated. Increasing the amount of hydrophobic additive (hexadecane) and/or adding water-soluble inhibitor in the aqueous phase suppressed the polymerization in the secondary nuclei, while adding a relatively hydrophilic monomer in the styrene feed and/or increasing the amount of the initiator and the size of the monomer droplets promoted the polymerization in the secondary nuclei. Under the condition where a hydrophilic monomer was added, hollow microspheres were formed when polymerization in the secondary nuclei was dominant, while one-hole microspheres resulted when polymerization occurred only inside the larger monomer droplets, indicating that the compromise between dominant mechanisms is critical to the polymerization process. (C) 2004 Published by Elsevier Ltd.