Configurational properties of polyphenylene precursor polymers

Abstract

The configurationally changes of a soluble precursor polymer. poly(5,6- dlmethylcarboxycyclohexadi-l.S-ene) have been studied during it’s conversion to poly(phenylene) by thermal aromatisation. This was performed principally in solution in N-methylpyirolidinone and chloroform, by size exclusion chromatography, viscometry and scattering techniques. The results show the polymer is initially a random coil, and during aromatisation stiffens to a wormlike chain, the intermediate to the rodlike molecule. This is evidenced by increased persistence length and increased dependency of size on molecular weight. The conversion to a stiffer molecule is accompanied by chain scission, more pronounced for high molecular weight polymers, and agglomeration, with consequential increased polydispersity and scatter in results. At 30% aromatisation aggregates dominate the behaviour of the solution, and at 40% the polymer becomes insoluble. Results from each technique differ according to the relative sensitivity to the two species present Aggregates exist as low as 10% aromatisation, suggesting the formation of contiguous phenylene nuclei, dispersed phenylene would not cause aggregation. When separated from the solution the aggregated species was found to be only slightly more aromatised than the free chain equivalent, and chemically very similar, suggesting aggregation arises from the arrangement rather than the quantity of phenylene monomers. The scattering behaviour of lightly aromatised polymer conformed to a wormlike chain model, while the aggregated polymer's scattering is close to that of a star model, suggesting a fringed micelle structure with a core of closely packed phenylene blocks, and arms preferentially composed of precursor polymer, with randomly dispersed phenylene. The insolubility of partially aromatised poly(DHCD-DMC) is common to other studies of polymers with conjugated backbones in which change from a good to a poor solvent causes a change from random coil to aggregated stiff chains.