Studies on the Thermal Degradation of Several Polymer-Additive and Copolymer Systems

Transition metal chelates of acetylacetone generally decompose in the first instance by a ligand scission mechanism with the release of acac' radicals. The ligand scission can be promoted by complexation with electron-donating species, and as described in Chapter 1 , this has led to an interest in the use of the chelates as polymerisation initiators Additional interest in the chelates comes from their use in modifying the degradation behaviour of polymers. The ability of the transition metal acetylacetonate chelates to interact with electron-donating sites within the polymers renders them particularly worthy of investigation. Chapter 2 presents an introduction to the thermal analysis techniques employed in this research, with emphasis on the comparative merits of each system. The preparation of the chelates is described in Chapter 3 and the nature of their interaction with electron-donating compounds considered on the basis of spectroscopic evidence. The thermal decomposition of the chelates is also discussed and a mechanism is proposed for the fragmentation of the chelates at high temperature. The thermal degradation of polymers (particularly those employed in this research) is the subject of Chapter 4. In Chapters 5, 6 and 7, the thermal degradation of blends of Co3+ , Co2+ and Mn3+ chelates with poly(methyl methacrylate) and a methyl methacrylate-methacrylic acid copolymer is described. Both polymers contain electron-donating structures (ester and acid side groups, unsaturated linkages) and these are found to promote the chelate ligand scission reaction in the manner of the low molecular analogues discussed in Chapters 1 and 3. The acac radicals produced in the initial decomposition attack the polymer backbone and initiate depolymerisation, whilst small radicals produced in the advanced stages of chelate decomposition attack the substituent groups, inducing the formation of cyclic anhydride structures. The interaction of the chelates with the polymer side groups can promote scission of these groups from the polymer chain with the associated formation of unsaturated sites on the polymer backbone. Such modifications of the polymer block the usual depolymerisation reaction and leads to chain fragmentation. The influence of Cu(acac)2 on the degradation behaviour of poly(methyl methacrylate) and the copolymer is discussed in Chapter 8. Although the Cu(acac)2-PMMA blend behaves similarly to the blends of PMMA with the chelates considered earlier, the Cu(acac)2 copolymer blend behaves in a markedly different fashion, with two new major unzipping processes and little fragmentation. Evidence is provided in this chapter which indicates that the different behaviour stems from a copper catalysed decarboxylation process In Chapter 9, the influence of the chelates on the thermal degradation of poly(vinyl acetate) is investigated. Interactions with the ester substituents are observed, similar to the case of the poly(methyl methacrylate) blends. Attack of acac' radicals on the poly(vinyl acetate) will not initiate depolymerisation as the polymer does not degrade by this mechanism but instead causes structural changes along the backbone. Structural changes are also expected to occur when acac' radicals attack polystyrene and poly(vinyl chloride) in the blends of these polymers with the chelates. The TVA behaviour of these blends is described in Chapter 10. In the final chapter, Chapter 11, the thermal degradation of a series of styrene-methacrylic acid copolymers is described. It is found that anhydride ring structures, which block the unzipping of the styrene sequences, form between neighbouring methacrylic acid units. A slight overall stabilisation of the copolymer relative to polystyrene results