Independent Electrocyclization and Oxidative Chain Cleavage along the Backbone of cis-Poly(phenylacetylene)

Abstract

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-C 6H3Ph3). 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 O 2. 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-C 6H3Ph3. Under these conditions, the most dramatic changes in molecular weight and molecular weight distribution are observed.

Synthesis and chiroptical properties of a helical poly(phenylacetylene) bearing optically active pyrene pendants

“…1). [41][42][43][44][45][46] This result demonstrates that poly-(R)-1 possesses a highly cis-transoidal stereoregular structure.…”

Characterization of substituted polyacetylene microstructure by pyrolysis gas chromatography

“…The second reaction path supposes just the opposite order of the two partial processes, i. e., intramolecular cyclization of a polymer chain yielding a cyclohexadiene sequence built into the polymer main chain followed by an extrusion of a cyclotrimer molecule and two fragments of the parent macromolecule undergoing further reaction [4,37,38]. It seems assured that both the above-outlined mechanisms participate in the overall decomposition of polyvinylene chains to cyclotrimers and other minority oligomers.…”

Molecular Shapes of Monosubstituted Polyacetylenes in their Liquid Crystalline Phases

“…[25] Particularly, the cis-rich polyacetylenes can degrade upon thermal treatment through a 6p-electroncyclization process of 1,3,5-hexatriene moieties along the polymer backbone. [22,26,27] Oxygen can further accelerate the degradation process by photo-oxidative cleavage of the polyene backbone. [26] Degradation is fatal to the LC properties of MSPAs, since it not only results in lowmolecular-weight fragments but also can completely destroy the initial molecular shape of the MSPAs.…”

“…[22,26,27] Oxygen can further accelerate the degradation process by photo-oxidative cleavage of the polyene backbone. [26] Degradation is fatal to the LC properties of MSPAs, since it not only results in lowmolecular-weight fragments but also can completely destroy the initial molecular shape of the MSPAs. Therefore, we shall avoid, as much as possible, the degradation when dealing with the genuine LC phase behavior of MSPAs.…”