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

Stegmann, Jacobus Christiaan (2007-12)

Thesis (MSc)--University of Stellenbosch, 2007.

Thesis

ENGLISH ABSTRACT: The main objective of this project was the controlled synthesis of graft copolymers via a thiol-ene addition mechanism. The Reversible Addition-Fragmentation chain Transfer (RAFT) process was used in all polymerization reactions with the aim to achieve a certain degree of control over the molecular weight. Several synthetic steps were required in order to obtain the final graft copolymer and each step was investigated in detail. Firstly, two RAFT agents (cyanovaleric acid dithiobenzoate and dodecyl isobutyric acid trithiocarbonate) were synthesized to be used in the various polymerization reactions of styrene and butyl acrylate. This was done successfully and the RAFT agents were used to synthesize low molecular weight polystyrene branches of the graft copolymer. Different molecular weights were targeted. It was found that some retardation phenomena were present especially at high RAFT agent concentrations. The polystyrene branches that were synthesized contained RAFT end-groups. Various pathways were explored to modify these RAFT end-groups to form thiol end-groups to be used in the thiol-ene addition reaction during the grafting process. The use of sodium methoxide for this purpose proved most successful and no evidence of the formation of disulfide bridges due to the initially formed thiols was detected. Allyl methacrylate (AMA) was chosen as monomer to be used for the synthesis of the polymer backbone because it has two double bonds with different reactivities. For the first time, RAFT was used to polymerize AMA via the more reactive double bond to obtain linear poly(allyl methacrylate) (PAMA) chains with pendant double bonds. However, at higher conversions, gelation occurred and the molecular weight distributions were uncontrolled. NMR was successfully used to study the tacticity parameters of the final polymer. Finally, the synthesis of the graft copolymer, PAMA-g-polystyrene, was carried out by means of the “grafting onto” approach. The thiol-functionalized polystyrene branches were covalently attached to the pendant double bonds of the PAMA polymer backbone via 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 graft copolymer. The percentage grafting, as determined by 1H-NMR, was low.

AFRIKAANSE OPSOMMING: Die hoofdoel van hierdie projek is die beheerde sintese van ‘n entkopolimeer via ‘n merkaptaan-een addisiereaksie. Die sogenaamde “Reversible Addition-Fragmentation chain Transfer” (RAFT) proses is in al die polimerisasiereaksies gebruik met die doel om ‘n mate van beheer oor die molekulêre massa van die polimere te verkry. Verskeie stappe (waarvan elkeen ten volle ondersoek is) was nodig om die finale entkopolimeer te verkry. Eerstens is twee RAFT-agente (sianovaleriaansuur ditiobensoaat en dodekielisobottersuur tritiokarbonaat) gesintetiseer vir gebruik in verskeie polimerisasiereaksies van stireen en butielakrilaat. Hierdie stap was suksesvol en die RAFT-agente is toe gebruik vir die sintese van lae molekulêre massa polistireensytakke vir die entkopolimeer. Die molekulêre massas van die sytakke is gevarieer en daar is gevind dat vertragings in die polimerisasiereaksies voorgekom het, veral by hoë konsentrasies van die RAFT-agente. Die polistireensytakke wat gemaak is, besit almal ‘n RAFT-eindgroep. Verskeie roetes is bestudeer ten einde die RAFT-eindgroepe tot merkaptaan-eindgroepe te modifiseer om sodoende tydens ‘n merkaptaan-een addisiereaksie gebruik te word. Die gebruik van natriummetoksied was hier die suksesvolste en daar was geen teken van die vorming van disulfiedbrûe as gevolg van die oorspronklik gevormde merkaptane nie. Allielmetakrilaat (AMA) is gekies as die monomeer wat gebruik sou word vir die sintese van die polimeerruggraat omdat die monomeer twee dubbelbindings met verskillende reaktiwiteite besit het. Vir die eerste keer is RAFT gebruik vir die polimerisasie van AMA via die meer reaktiewe dubbelbinding om lineêre poli(allielmetakrilaat) (PAMA) kettings met dubbelbindings in die sygroepe te verkry. Gelvorming en onbeheerde molekulêre massaverspreiding het egter by hoër monomeeromsettings voorgekom. KMR is susksekvol gebruik om die taktisiteitsparameters van die finale polimeer te bestudeer. Ten slotte is die sintese van die entkopolimeer, PAMA-g-polistireen, uitgevoer deur die aanhegting van voorafgevormde sytakke. Die polistireensytakke met die merkaptaaneindgroepe is kovalent geheg aan die dubbelbindings in die sygroepe van die PAMA-polimeerruggraat via ‘n merkaptaan-een addisiemeganisme in die teenwoordigheid van ‘n vrye radikaalinisieerder. ‘n Kombinasie van gelpermeasiechromatografie en multi-hoeklaserligverstrooiing is gebruik om die molekulêre massa van die entkopolimeer te bepaal. Die persentasie sytakke soos bepaal deur 1H-KMR was laag.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/18700
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