A method for the synthesis of soluble polyphenylenes that involves the polymerization of l-halo-4-lithiobenzenes, l-halo-3-lithiobenzenes, and 1,3-and 1,4-dilithiobenzenes in ethereal solvents is described. The lithiobenzenes are prepared from the corresponding dihalogenated benzenes using tertbutyllithium . By varying the monomers and conditions used, a variety of soluble halogenated polyphenylenes can be obtained with weight average molecular weights (Afw) as high as 6000 with polydispersities (Mw/M") equal to ~2.8. The prequenched polymers can exist as multilithiated polymers if 2.0 equiv of tert-butyllithium is used to form the aryllithium intermediates. The polymers are usually amorphous and contain significant amounts of phenylated polyphenylene units capped with halogens. The polymers are soluble in organic solvents such as THF, chloroform, and methylene chloride. Several of the polymers exhibited low melting and glass transition temperatures. The brominated polymers are easily dehalogenated upon treatment with tert-butyllithium in THF at -78 °C followed by quenching with water. The molecular weights of several of the brominated polymers increased upon debromination with tert-butyllithium. When the polymerizations were carried out in the presence of 1,3-diphenylisobenzofuran, o-benzyne intermediates were trapped as their Diels-Alder adducts. LiH appears to be present in significant amounts upon completion of the polymerization.
The functionalization of brominated polyphenylenes with several alkynes is described. The bromide groups on the polymer were replaced with terminal alkynes using a Pd/Cu catalyst system.Substitution of nearly all the original bromide locations was achieved. The functionalized polymers, freely soluble in THF, exhibited high char yields of up to 89 % on thermolysis to 900 °C under a nitrogen atmosphere. Several of the functionalized polymers, derived from brominated polyphenylenes with approximately equal amounts of meta and para linkages, flowed on heating. The polymer that was functionalized with phenylacetylene exhibited a well-defined melting point at approximately 200 °C and flowed to produce a monolithic disk of glassy carbon. The amount of acetylenic cross-linking units in the polymers functionalized with phenylacetylene appears to be directly related to the char yields of the functionalized polymers. Predominantly meta-linked brominated polyphenylenes were also functionalized with a number of terminal alkynes, and though high char yields of up to 86% were observed, these materials offered no improvements in flow properties. The structure/property relationships and material applications are discussed.
Polyphenylene exhibits a number of diverse properties that have prompted its use in numerous composite materials including insulating layers for semiconductors, doped electrical conductors, metal catalyst supports, and lubricant additives. While poly(p-phenylene) (PPP) is intractable even with degrees of polymerization of 10-15, the presence of ortho or meta linkages destroys the crystallinity to such an extent that soluble material can be formed.2We recently described a method for the formation of l-bromo-4-lithiobenzene (1) by the treatment of p-dibromobenzene with ferf-butyllithium in 1,4-dioxane. Compound 1 could be instantaneously polymerized, even at -78 °C, by the addition of hexamethylphosphoramide (HMPA) to afford brominated polyphenylene.3 The FTIR absorption intensity for the para linkage was 3-4 times more intense than the non-para-linked intensities. It was the presence of the non para linkages or phenylated units that caused the crystallinity to be destroyed, rendering polymers that were soluble even with degrees of polymerization over 40 (SEC relative to polystyrene). Here we describe the polymerization of 1 in THF without the need for the highly toxic cancer suspect agent HMPA.The polymerization procedure is as follows. To a solution of p-dibromobenzene (16 mmol) in THF (16 mL) at -78 °C was slowly added ferf-butyllithium (24 mmol, 2.3 M in pentane). Note that 12 mmol of ferf-butyllithium were utilized for the lithium-halogen exchange and 12 mmol were consumed for the elimination of the tertbutyl bromide to afford lithium bromide, isobutylene, and isobutane. Thus the use of ferf-butyllithium allowed all the byproducts to be innocuous, i.e., no alkyl halides remained in solution. Accordingly, 4 mmol of p-dibromobenzene remained unreacted. If we used 2.0 equiv of ferf-butyllithium to convert all of the p-dibromobenzene to 1, significantly lower yields (26%) of the final etherinsoluble polymer were obtained. This may be due to the formation of small amounts of p-dilithiobenzene, a possible chain-termination agent.
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