RAFT-mediated synthesis of graft copolymers via a thiol-ene addition mechanism

[摘要] ENGLISH ABSTRACT: The main objective of this project was the controlled synthesis of graft copolymers via athiol-ene addition mechanism. The Reversible Addition-Fragmentation chain Transfer(RAFT) process was used in all polymerization reactions with the aim to achieve acertain degree of control over the molecular weight. Several synthetic steps wererequired in order to obtain the final graft copolymer and each step was investigated indetail.Firstly, two RAFT agents (cyanovaleric acid dithiobenzoate and dodecyl isobutyric acidtrithiocarbonate) were synthesized to be used in the various polymerization reactions ofstyrene and butyl acrylate. This was done successfully and the RAFT agents were usedto synthesize low molecular weight polystyrene branches of the graft copolymer.Different molecular weights were targeted. It was found that some retardationphenomena were present especially at high RAFT agent concentrations.The polystyrene branches that were synthesized contained RAFT end-groups. Variouspathways were explored to modify these RAFT end-groups to form thiol end-groups tobe used in the thiol-ene addition reaction during the grafting process. The use of sodiummethoxide for this purpose proved most successful and no evidence of the formation ofdisulfide bridges due to the initially formed thiols was detected.Allyl methacrylate (AMA) was chosen as monomer to be used for the synthesis of thepolymer backbone because it has two double bonds with different reactivities. For thefirst time, RAFT was used to polymerize AMA via the more reactive double bond toobtain linear poly(allyl methacrylate) (PAMA) chains with pendant double bonds.However, at higher conversions, gelation occurred and the molecular weight distributionswere uncontrolled. NMR was successfully used to study the tacticity parameters of thefinal polymer.Finally, the synthesis of the graft copolymer, PAMA-g-polystyrene, was carried out bymeans of the 'grafting onto approach. The thiol-functionalized polystyrene brancheswere covalently attached to the pendant double bonds of the PAMA polymer backbonevia a thiol-ene addition mechanism in the presence of a free radical initiator. A Multi-Angle Laser Light Scattering (MALLS) detector was utilized in conjunction with Size-Exclusion Chromatography (SEC) to obtain molecular weight data of the graftcopolymer. The percentage grafting, as determined by 1H-NMR, was low.

[发布机构] Stellenbosch University