Polymer precursors of polyacetylene. Thermal degradation of poly(vinyl esters). Part II—Effect of the n-acyl chain length on the autocatalytic thermal degradation of a homologous series of poly(vinyl n-alkyl esters) (PV- n-AEs)

Abstract

In order to determine the effect of the length of the n-acyl portion on the autocatalytic thermal elimination reaction, five of the shorter members of the homologous series of poly(vinyl n-alkyl esters) (PV- n-AEs) ▪ were studied under identical experimental conditions; namely, by a static procedure at several temperatures. The -R group was varied from one to five carbon atoms. All the PV- n-AEs were obtained by chemical modification of a unique sample of poly(vinyl alcohol) (PVA). Thus, by using a homologous series of PV- n-AEs with the same backbone molecular weight, any possible effect due to a change in molecular weight of the backbone and/or tacticity, structural chemical irregularities, etc., is eliminated. The shortest member of the series, i.e. poly(vinyl acetate) (PVAc) clearly degrades by an autocatalytic mechanism more readily than do longer ones. Thus, PVAc exhibits pure autocatalytic thermal degradation kinetics. On the contrary, the longest member of the series, poly(vinyl n-hexanoate ester) (PV- n-HE), degrades by very well defined first order kinetics. The other members of the series, poly(vinyl propionate) (PVPr), poly(vinyl butyrate) (PVBu) ≡ poly(vinyl n-butanoate ester) (PV- n-BE) and poly(vinyl valerate (PVVa) ≡ poly(vinyl n-pentanoate ester) (PV- n-PE) degrade by first order kinetics but with the participation, although to an insignificant extent, of the autocatalytic mechanism. For this and certain other reasons we have analyzed our experimental results by taking into account both types of mechanisms, separately. The three latter compounds followed first order rate kinetics quite well, the rate constants depending upon the n-alkyl chain length of the acyl portion. The energy of activation of the first order reaction decreases markedly as the length of the n-acyl portion increases. This decrease is neither constant nor regular as the acyl moeity increases in length but rather takes place in a quasi zig-zag fashion particularly with the longer members, suggesting an odd-even effect. On the basis of our experimental results we have arrived at the two following conclusions. Firstly, less stable poly(vinyl n-alkyl esters), i.e. poly(vinyl n-hexanoate ester) and, of course, higher members of the series which have not been investigated in the present work, would allow lower pyrolysis temperatures, thus increasing the synthetic applicability of this stripping reaction to the preparation of polyacetylene-like structures. Secondly, it was hoped that the kinetic study itself would shed more light on the mechanism of these decompositions in high molecular weight compounds.