Poly(dimethyl biphenylene)

Two methods were investigated for the preparation of poly(dimethyl biphenylene) in a search for polymers combining good solubility with sufficiently high chain extension to produce a lyotropic nematic phase. The Ullmann reaction was used to condense 4,4′‐diiodo‐3,3′‐dimethyl biphenyl and the corresponding 2,2′‐dimethyl derivative with copper, and 4,4′‐dibromo‐2,2′‐dimethyl biphenyl was polymerized using the coupling reagent, bis(1,5‐cyclooctadiene)nickel(0), developed by Semmelhack. The Ullmann polymers were completely soluble in CHCl3 but only partially soluble in toluene, whereas earlier work had indicated similar polymers of higher molecular weight to be completely soluble in toluene. All the polymers produced were of low molecular weight and no evidence of the nematic phase was found by polarized light microscopy for CHCl3 solutions over the concentration range 6.8–25% by weight.