| Two reversible addition-fragmentation chain transfer (RAFT) agents, cumyl dithiobenzoate (CDB) and benzyl dithiobenzoate (BDB) were prepared in the lab and were found suitable for the RAFT miniemulsion polymerization due to their low water solubility and the strong phase partitioning towards the monomer oil phase.; The RAFT process in the CDB-mediated conventional emulsion polymerization was found to be unsuccessful, showing loss of molecular weight control and latex stability as a result of insufficient diffusion of the RAFT agent through the aqueous phase from the monomer droplets to the micellar nucleated particles.; The RAFT process in the CDB- or BDB-mediated miniemulsion polymerizations exhibited living characteristics similar to the RAFT bulk polymerizations. However, limited coagulum, bimodal molecular weight distributions, and relatively broad particle size distributions were observed in the RAFT miniemulsion latexes with low surface coverage of the surfactant on the initial droplets. The minor (low molecular weight) species may originate from some large, RAFT agent-concentrated droplets, some of which lose stability during the polymerization and became the pink solid coagulum found at the end of the reaction. The system stability and the related molecular weight control can be improved by adding an extra 30 mM SLS post-homogenization or reducing the sonification time from 10 minutes to 1 minute.; Further kinetics studies of the RAFT miniemulsion polymerization suggests the effect of radical segregation: faster polymerization rate (R p), higher radical concentration ([P*]). However, compared to the nonliving miniemulsion without any RAFT agents, the average number of radicals per particle n¯ and [P*] in the RAFT miniemulsion polymerizations were found to be much lower (often 0.01∼0.12, and 10−8 mol/L oil, respectively), yet still higher than the values in nitroxide-mediated stable free-radical polymerization (SFRP) miniemulsion polymerizations.; Before an attempt was made to prepare block copolymers in miniemulsion latexes, four types of RAFT agents (CDB, BDB, ECX (o-ethyl s-cyanomethyl xanthate) and EPX (o-ethyl s-(1-phenylethyl) xanthate)) were screened in the bulk homopolymerizations of styrene, vinyl acetate (VAc) and vinyl 2-ethylhexanoate (V2EH). The ECX-mediated block copolymer PVAc-b-PV2EH and the chain extension of PVAc were successfully carried out. The attempt to prepare PS-b-PV2EH proved unsuccessful. |
