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Controlled Radical Polymerization in Surfactant-Free Emulsion Polymerization

Gray, Dominic M (2019) Controlled Radical Polymerization in Surfactant-Free Emulsion Polymerization. PhD thesis, University of Sheffield.

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Abstract

This Thesis describes the polymerization of methyl methacrylate (MMA) in the presence or absence of 1,1-diphenylethylene (DPE) in surfactant-free emulsion polymerization (SFEP), with subsequent polymerizations of either styrene (St) or benzyl methacrylate (BzMA) in the presence of the latex formed. The effect of varying amphiphilicity of several RAFT agents in reversible addition-fragmentation chain-transfer (RAFT) SFEP was also investigated. Firstly, the precursor polymer is synthesised by SFEP of methyl methacrylate in the presence of varying concentrations of DPE. The molecular weight and (dispersity) Ð of the resulting PMMA can be reduced by increasing the DPE concentration in the polymerization, with varying initiator concentration having little effect. Increasing the DPE content also reduced the rate of polymerization. Mass spectroscopy (MS) indicated the presence of only 1 DPE unit per polymer chain, contrary to other emulsion polymerization studies, with diffusion-ordered nuclear magnetic resonance (DOSY NMR) spectroscopy showing the presence of free DPE and DPE bound to the polymer chains. Utilizing DPE in SFEP yielded a bimodal particle distribution as judged by transmission electron microscopy (TEM), with a secondary particle distribution forming over the first hour of the polymerization. Secondly, St and BzMA are polymerized in the presence of PMMA latex synthesised in the presence or absence of DPE. The rate of polymerization of each of these two monomers is increased by the presence of DPE in the precursor, indicating that the precursor is a source of radicals by either thermal cleavage of the polymer chains or the uncapping of the polymer by DPE. Only very low BzMA conversion is achieved when the precursor did not contain DPE. Polymerizations of St contained a considerable amount of precursor after the attempted extension, whereas the polymerization of BzMA contained very little precursor, as judged by SEC. PSDs analysed by TEM show an increase in particle diameter, broadening of the size distribution, and the formation of a secondary particle size distribution during the polymerization of St or BzMA. Finally, the polymerization of methyl methacrylate is conducted in the presence of several RAFT agents. Solution polymerizations confirmed that these RAFT agents enabled good control over the molecular weight and Ð, but their success in SFEP depended on the amphiphilicity of the RAFT agent. For two RAFT agents, this could be altered by changing the pH, with the anionic or cationic derivatives giving considerably better control. Size exclusion chromatography (SEC), incorporating a UV detector at λ = 260 nm, showed the presence of the amphiphilic RAFT agents throughout the MWD and the absence of RAFT end-groups when using hydrophobic RAFT agents. However, control over molecular weight and Ð is achieved at the cost of reduced control over the particle size distribution (PSD) of the latex. Overall, the information given in this Thesis brings us closer to the controlled SFEP synthesis of diblock copolymers. With the DPE method being used to form diblock copolymers of PMMA-b-PBzMA, despite the hydrophobicity of DPE. While the success of the use of RAFT agents to form precursor polymers for later extension is dependent on the amphiphilicity of the RAFT agent. These methods can be used in the SFEP synthesis of diblock copolymers for coatings where the use of diblock copolymers helps to prevent polymer phase separation and the presence of a surfactant can have negative effects on the quality of the final product.

Item Type: Thesis (PhD)
Academic Units: The University of Sheffield > Faculty of Science (Sheffield) > Chemistry (Sheffield)
Depositing User: Mr Dominic M Gray
Date Deposited: 18 Dec 2019 13:20
Last Modified: 18 Dec 2019 13:20
URI: http://etheses.whiterose.ac.uk/id/eprint/25556

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