Thesis (MSc (Chemistry and Polymer Science))--Stellenbosch University, 2008. / The Reversible addition fragmentation chain transfer (RAFT) technique is a robust and
versatile technique that enables the synthesis of polymers of controlled molecular
weight and polydispersity. The application of the RAFT technique in heterogeneous
aqueous media has attracted great interest in academics and industry due to it being
more environmentally friendly, besides its other advantages. To date, the synthesis of
well-defined high molecular weight polymers via the RAFT process under industrially
relevant conditions still remains a challenge for polymer chemists.
The study addresses the application of the RAFT process in heterogeneous media,
namely in miniemulsion polymerization, for the synthesis of AB diblock copolymers of
n-butyl methacrylate and styrene.
AB diblock copolymers of high molecular weight were successfully prepared via a twostep
method. In the first step, a dithiobenzoate monofunctional RAFT agent was used in
bulk polymerization with the first monomer, n-butyl methacrylate. After the
polymerization, the majority of the polymer chains contained the thiocarbonyl-thio
RAFT agent functionality, which makes the chains potentially active for chain
extension. The polymeric RAFT agent (also referred to as the starting block) obtained in
the first step was chain extended in the second step, in miniemulsion, upon further
addition of fresh initiator and the second monomer, styrene.
The effects of the initiator/RAFT agent concentration ratio on the miniemulsion systems
were investigated. The miniemulsion systems used for the high molecular weight AB
diblock copolymers exhibited living features despite the high polydispersity indices.
Kinetic results showed an increase in the rate of polymerization throughout the
polymerization. Size exclusion chromatography (SEC) results indicated significant
broadening in the molecular weight distributions and a steep increase in the
polydispersity during the polymerization. It was concluded that the broad molecular
weight distributions and steep increase in the polydispersity was not only related to the
initiator concentration but possibly due to other factors such as inhomogeneity in the miniemulsion system and a transition in the kinetic behavior during the polymerization.
Secondary particle formation emerged from kinetic data and transmission electron
microscopy (TEM) results, but this were not supported by the SEC results.
The effect of the use of a water-soluble initiator on the miniemulsion system was also
investigated. Results indicated a similar behavioral pattern as observed in the AIBNinitiated
systems, and not much improvement in terms of the molecular weight
distributions and polydispersity was seen.
The effect of the molecular weight of the diblock copolymers on the miniemulsion
system was investigated. Poly(n-butyl methacrylate)-b-poly(styrene) diblock
copolymers of lower molecular weight were synthesized via the two-step process.
Kinetic results indicated a similar behavioral trend as to that of the high molecular
weight diblock copolymers synthesized, however SEC chromatograms showed
narrower molecular weight distributions and low polydispersity indices.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/2707 |
| Date | 12 1900 |
| Creators | Bailly, Nathalie |
| Contributors | Sanderson, R. D., Tonge, M. P., Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | Stellenbosch University |
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