Independent electrocyclization and oxidative chain cleavage along the backbone of cis-poly(phenylacetylene)

Article abstract of DOI:10.1021/ma051060y

cis-Poly(phenylacetylene) (PPA) and cis-poly(pentadeuteriophenylacetylene) (PPA-d₅) undergo similar thermally induced transformations in bulk and solution. The structures identified in thermally treated samples include cis - trans isomerization of the backbone, cyclohexadiene repeat units in the main chain, and extruded 1,3,5-triphenylbenzene (1,3,5-C₆H ₃Ph₃). Among these structures, cyclohexadiene sequences along the backbone formed by intramolecular cyclization are the principal products in all solvents regardless of temperature and the presence or absence of ambient light or O₂. We propose that cyclohexadiene structures form via 6π electrocyclization of triene sequences and, therefore, are an unavoidable consequence of cis-alkene units of the polyene backbone. Concurrent with cyclization, decrease of molecular weight and increase of molecular weight distribution are observed. Traces of acid lead to a dramatic increase in cyclization. Extrusion of 1,3,5-C₆H₃Ph₃ results from cyclohexadiene repeat units and is slower than electrocyclization, even in the presence of acid, light, or O₂. In the presence of ambient light, O₂ accelerates the rate of cyclization and extrusion of 1,3,5-C₆H₃Ph₃, the decrease of molecular weight, and the formation of carbonyl-containing products detected in a 5:1 molar ratio to extruded 1,3,5-C₆H₃Ph₃. Under these conditions, the most dramatic changes in molecular weight and molecular weight distribution are observed.