Cross-Linking, Branching and Other Defects

Kovacic polyphenylene is brown with about 1 spin/chain detectable by ESR. Yamamoto polymer is yellow with a shorter chain length and fewer spins. In Kovacic polymer the spins and colour may both be due to polynuclear species. Polyphenylene produced from the poly(dihydrocatechol) precursor 249) is also yellow, but has a high molecular weight, of the order of 10s. It contains about 15% o-linkages, and the aromatization procedure may leave a high level of twists in the chain originating from the flexible precursor. This material dopes only to low levels of conductivity with sodium naphthalide (6x 10 3 Scm-1) and iron chloride (1.5 x 10-2 Scm-1) but reaches a level comparable to Kovacic and Yamamoto polyphenylenes with AsFs (102 S cm-1). 

Brown et al. 3S2) have recently emphasised the role of defect structures in heterocyclic polymers. They point out that the reported doped conductivities of these polymers may vary by as much as six orders of magnitude depending on the preparation procedure. They have applied the laser desorption method, discussed earlier for polyphenylene, to a range of polyheterocycles. Unlike polyphenylene, there was evidence for incomplete desorption and rearrangement of evaporated molecules. The results show that polymers prepared by Grignard coupling vary in their extent of bromination, the nature of the terminal species and the extent of formation of cyclic, polynuclear contaminants. 

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