Controlled free radical polymerization in miniemulsion using Reversible Addition-Fragmentation Chain Transfer (RAFT)

[摘要] ENGLISH ABSTRACT:A novel approach to conducting controlled free radical polymerization in aqueoussystems using Reversible Addition-Fragmentation Chain Transfer (RAFT) has beenstudied. When conducting RAFT in aqueous systems, reaction conditions must bechosen such that monomer transport across the aqueous-phase is either eliminated orfacilitated. This is to prevent the formation of the red layer associated with RAFT inemulsions. The formation of the red layer is ascribed to the inability of waterinsoluble,dithiobenzoate-endcapped oligomers to be sufficiently transported acrossthe aqueous phase. The novel approach in this study focussed on eliminatingmonomer transport and comprises two fundamental steps: the synthesis ofdithiobenzoate-encapped oligomers in bulk followed by miniemulsification of theseoligomers to yield a polymerizable miniemulsion. Dithioesters that act as chaintransfer agents in the RAFT -process were synthesized in situ, thereby eliminatinglaborious and time-consuming organic purification procedures of dithioesters.In situ formation of the RAFT-agents involved conducting the reaction betweendi(thiobenzoyl) disulfide and conventional azo-initiators of differing structures in thepresence of monomer. The structure of the chosen azo-initiator played a role in theefficiency of the RAFT process when the reaction was conducted in the presence ofmonomer to control the free radical polymerization process.Synthesis of the oligomers was performed by heating di(thiobenzoyl) disulfide and aselected azo-initiator, in the presence of monomer for a specific reaction duration inbulk. After the reaction was stopped, these oligomers were then miniemulsified byadding water, surfactant and cosurfactant, followed by the application of shear to formthe resulting mini emulsion.The free radical polymerization of the dithiobenzoate-endcapped oligomers in theminiemulsion proceeded in a controlled manner with molecular weight increasing in alinear fashion with increasing conversion, while polydispersities remained low. Thefamiliar red layer formation associated with RAFT polymerization in conventionalemulsions was not observed under these conditions.The effects of changing the cosurfactant (hydrophobe) as well as changing the degreeof polymerization of the emulsified oligomers were also investigated and described.

[发布机构] Stellenbosch University