Polyphenylene

This polymer is completely aromatic in character [182]. Polymerization of benzene to polyphenylene was, therefore, investigated quite thoroughly [184, 185]. Benzene [186] and other aromatic structures [184, 185] polymerize by what is believed to be a radical-cationic mechanism. In this type of polymerization, benzene polymerizes under mild conditions in the presence of certain Lewis acids combined with oxidizing agents [186–188]:

The yield of polymers reaches a maximum value (close to quantitative) at an aluminum chloride to cupric chloride molar ratio of 2:1 [180]. Solvents, concentrations, and temperatures affect the molecular weights of the products [189]. Other Lewis acids that are effective in benzene polymerizations are MoCl₅, FeCl₃, and MoOCl₄. The products, however, possess greater degrees of structural irregularity [183]. Theoretical considerations indicate [183] that during the polymerization the benzene rings become associated in a stacked end to end arrangement. As a result, the radical-cation becomes delocalized over the entire chain:

The σ bond formation shown above can also be accompanied by simultaneous depropagation and loss of benzene molecules. Chain buildup stops when the radical-cation on the terminal phenyl group becomes too small to promote further association. The polymer is very stable up to 500–600C and oxidizes very slowly. It is, however, quite insoluble with a very high melting point that makes is difficult to process and even to determine its molecular weight. Introduction of irregularity into the polymeric structure by copolymerizing terphenyl, biphenyl, or triphenylbenzene with benzene results in formation of soluble products. Their molecular weights, however, are low up to 3,000 [192]. They melt between 300 and 400C and contain phenyl branches and some fused rings. The copolymers can be cross-linked with xylylene glycol or with benzene-1,3,5-trisulfonyl chloride. Soluble alkyl substituted poly(p-phenylene) s were prepared [193] by a coupling process, using palladium catalysts:

Syntheses also include formations of copolymers with other aromatics compounds that lack substituents. The polymers with hexyl or longer side chains are soluble in toluene [193]. Goodson et al. [246] reported formation of soluble derivatives of poly(p-phenylene) of high molecular weight via Suzuki coupling reactions catalyzed by palladium (0) precursors in the presence of either triphenylphosphine or tri(o-tolyl) phosphine. Use of triphenylphosphine, however, apparently resulted in incorporation of the phosphine [246]. Formation of poly(benzoyl-1,4-phenylene) with the aid of a nickel-catalyzed reaction was reported [247]:

When sodium iodide is replaced with 2,20-bipyridine, the reaction proceeds faster. The polymers prepared with bipyridine exhibit a glass transition of 217C and loose only 3% of their weight at 500C either in air or in nitrogen [247]. Carter et al. [247] used organolithium-activated nickel catalysts to synthesize polyarylates

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مواضيع ذات صلة

Cardo Polymers

Oligomers with Terminal Functional Groups

Diels–Alder Polymers

Fluorine Containing Aromatic Polymers

High-Performance Polymers

Phosphonitrile Polymers

Polysilanes

Polyarylsiloxanes

Fluorosilicone Elastomers

Polysiloxane Coating Resins

Silicone Elastomers

Silicone Elastomers