Synthesis of soluble halogenated polyphenylenes. Mechanism for the coupling of halogenated lithiobenzenes

A method for the synthesis of soluble polyphenylenes that involves the polymerization of 1-halo-4-lithiobenzenes, 1-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 tert- butyllithium. By varying the monomers and conditions used, a variety of soluble halogenated polyphenylenes can be obtained with weight average molecular weights (Mw) as high as 6,000 with polydispersities (Mw/Mn) equal to approx. 2.8. The pre-quenched polymers can exist as multi-lithiated polymers if 2.0 equivalents of tert-butyllithium are 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 deg 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, ortho-benzyne intermediates were trapped as their Diels- Alder adducts. LiH appears to be present in significant amounts upon completion of the polymerization.