Model compounds related to fluoroelastomers
The aims of the research described in this thesis were to explore the chemistry of some model compounds related to fluorohydrocarbon polymers, in particular the copolymer of (I), especially to obtain definitive information about mechanisms of cross-linking and polymer degradation and to explore new approaches to cross-linking. A range of model compounds of the form (III) and (IV) were synthesized and the results obtained demonstrate that aryl-ether cross-links of the form (II) are stable in the presence of various bases at elevated temperatures. This indicates that these links are not responsible for the instability of copolymers cured in this way. Models of the form (III) and (IV) show surprisingly low reactivity under typical curing conditions, indicating that either the polymers themselves are more reactive or reaction occurs by an unexpected mechanism. In pursuit of the likely cross-linking mechanism it was shown that polymers of the forms (I) and (CF(_2CH(_x)F(_2-X))(_n), x=1,2,are highly susceptible to elimination of HF in the presence of Lewis acids, e.g. SbF(_5) to yield highly unsaturated materials. Model chemistry involving SbF(_5) resulted in novel eliminations of HF, e.g. (quant.) (V) (VI) the formation of some stable long-lived carbocations, e.g. and other novel chemistry, e.g. a 1,5 elimination of HF new approach to cross-linking was also developed involving the incorporation into the copolymer (I) of crowded comonomer units, e.g. when copolymers containing the structures (XI) where heated radicals were found via hemolytic cleavage to promote cross-linking: examination of the model compounds yielded new re-actions including a route to seven-membered cyclic ethers, and an unexpected ring closure reaction catalysted by NBU(_3),(HF)(_X), The reactivity of the models was also examined in the areas of nucleophilic attack, base-catalysed elimination of HF, addition of 1,3-dipoles, the formation of long-lived carbanions and one-electron transfer chemistry. The results obtained considerably broaden the chemistry of this class of compound.