Thesis (MSc)--Stellenbosch University, 2001. / ENGLISH ABSTRACT: A novel approach to conducting controlled free radical polymerization in aqueous
systems using Reversible Addition-Fragmentation Chain Transfer (RAFT) has been
studied. When conducting RAFT in aqueous systems, reaction conditions must be
chosen such that monomer transport across the aqueous-phase is either eliminated or
facilitated. This is to prevent the formation of the red layer associated with RAFT in
emulsions. The formation of the red layer is ascribed to the inability of waterinsoluble,
dithiobenzoate-endcapped oligomers to be sufficiently transported across
the aqueous phase. The novel approach in this study focussed on eliminating
monomer transport and comprises two fundamental steps: the synthesis of
dithiobenzoate-encapped oligomers in bulk followed by miniemulsification of these
oligomers to yield a polymerizable miniemulsion. Dithioesters that act as chain
transfer agents in the RAFT -process were synthesized in situ, thereby eliminating
laborious and time-consuming organic purification procedures of dithioesters.
In situ formation of the RAFT-agents involved conducting the reaction between
di(thiobenzoyl) disulfide and conventional azo-initiators of differing structures in the
presence of monomer. The structure of the chosen azo-initiator played a role in the
efficiency of the RAFT process when the reaction was conducted in the presence of
monomer to control the free radical polymerization process.
Synthesis of the oligomers was performed by heating di(thiobenzoyl) disulfide and a
selected azo-initiator, in the presence of monomer for a specific reaction duration in
bulk. After the reaction was stopped, these oligomers were then miniemulsified by
adding water, surfactant and cosurfactant, followed by the application of shear to form
the resulting mini emulsion.
The free radical polymerization of the dithiobenzoate-endcapped oligomers in the
miniemulsion proceeded in a controlled manner with molecular weight increasing in a
linear fashion with increasing conversion, while polydispersities remained low. The
familiar red layer formation associated with RAFT polymerization in conventional
emulsions was not observed under these conditions. The effects of changing the cosurfactant (hydrophobe) as well as changing the degree
of polymerization of the emulsified oligomers were also investigated and described. / AFRIKAANSE OPSOMMING: Hierdie studie is geloods om 'n nuwe benadering tot die beheerde vry-radikaal
polimerisasie in water gebaseerde sisteme te ondersoek. Daar is spesifiek gekyk na die
uitvoer van die RAFT (Reversible Addition-Fragmentation Chain Transfer) proses in
emulsies. Wanneer RAFT in emulsies toegepas word, moet die toestande waaronder die
reaksie uitgevoer word, versigtig opgestel word. Die toestande moet so gekies word dat
die vervoer van monomere deur die waterfase óf geëlimineer word óf gefasiliteer word.
Dit word gedoen om die faseskeiding in die vorm van 'n rooi laag, wat so kenmerkend
van RAFT -polimerisasie in emulsies is, te voorkom. Hierdie faseskeiding vind plaas
omdat die vervoer van ditiobensoaat endgroep-bevattende oligomere deur die waterfase
tydens interval II, moeilik is a.g.v. hulle oplosbaarheid in water.
Die nuwe benadering wat hier bestudeer is, het twee basiese stappe. Eerstens word die
ditiobensoaat endgroep-bevattende oligomere in bulk gesintetiseer. Dit word gevolg deur
die emulsifisering van die oligomere. Hierna vind verdere polimerisasie van die
oligomere plaas deur die dormante oligomere te heraktiveer.
Die ditio-esters wat as kettingoordrag agente optree in die RAFT proses, word in situ
gesintetiseer. Hierdie modifikasie sny tydrowende organiese suiweringsmetodes uit.
Die in situ RAFT agente word gesintetiseer deur di(tiobensoïel) disulfied met verskillende
konvensionele azo-inisieerders te laat reageer. Die struktuur van die spesifieke azoinisieerder
het wel 'n rol gespeel in die effektiwiteit van die RAFT proses om molekulêre
massa te beheer as bg. reaksie in die teenwoordigheid van monomere uitgevoer is.
Die sintese van die oligomere is gedoen deur di(tiobensoïel) en 'n azo-inisieerder te verhit
in die teenwoordigheid van monomere. Die reaksie is gedoen in bulk en die graad van
polimerisasie van die oligomere is beheer deur die reaksie te stop by verskillende
tydstippe. Nadat die bulk reaksie gestop is, is hierdie oligomere ge-emulsifiseer deur die
oligomere te meng met 'n seep, hidrofoob en water. Hierdie mengsel word dan
onderwerp aan 'n vermengingskrag om 'n polimeriseerbare mini-emulsie te vorm. Die voortsetting van die polimerisasie van die oligomere in die mini-emulsie het op 'n
beheerde wyse verloop, m.a.w. molekulêre massa wat linieêr toeneem met stygende
omsetting. Polidispersiteit indekse van die polimere het deurentyd laag gebly in die
stabielste sisteme. Onder hierdie toestande was daar geen kenmerkende rooi laagvorming
te bespeur nie.
Die effekte wat die verandering van die hidrofoob, asook die verandering van die graad
van polimerisasie van die oligomere op die sisteem gehad het, is onder andere ook
ondersoek en beskryf.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/52174 |
| Date | 12 1900 |
| Creators | Vosloo, Johannes Jacobus |
| Contributors | Sanderson, R. D., De-Wet Roos, D., Stellenbosch University. Faculty of Science. Dept. of Chemistry & Polymer Science . |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 114 p. : ill. |
| Rights | Stellenbosch University |
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