Studies on hexolic anhydride based polyesters, III. mechanistic and comparative aspects of the product distributions in the thermal degradation

Abstract

Two systems of unsaturated polyesters based on maleic anhydride, phthalic anhydride, hexolic anhydride (5,6,7,8,9,9-hexachloro-1,2,3,4,4a,5,8,8a-octahydro-5,8-methanonaphthalene-2,3-dicarboxylic anhydride) and 2-butene-1,4-diol or 2,3-dichloro-2-butene-1,4-diol were synthesized. The polyesters were pyrolysed under programmed heating from 40° to 400° C at a rate of 10 K/min. The volatile pyrolysates were separated and identified by combined gas chromatography/mass spectrometry. The formation of anhydrides, diols, hydrocarbons, chloro compounds and polychlorinated cyclopentadiene compounds etc., is explained in the light of single and double β-scission, acyl-oxygen and alkyl-oxygen bond fission and carbon-carbon bond homolysis mechanisms. It is observed that β-scission plays a vital role in the primary degradation process of polyesters. The decreasing number of β-hydrogens in the diol part of the polyester is reflected in the quantity of anhydrides ejected from the polymer backbone. Further it is observed that hexachlorocyclopentadiene and related compounds are ejected from the hexolic anhydride set free during pyrolysis and not from the hexolic moiety incorporated in the polymer backbone. An approximate calculation shows that about 75% of the total amount of maleic anhydride, formed in the pyrolysis of saturated diol based polyesters having fullest number of β-hydrogens in the diol part, is derived from β-scission and the rest via alkyl-oxygen bond fission mechanism. A similar observation is also noted-indirectly for the formation of hexolic anhydride.