Oxidative coupling of the phenylethynyl group in .mu.-(1-3.eta.:2-4.eta.-trans,trans-1,4-diphenylbutadiene)-bis(bis(.eta.5-methylcyclopentadienyl)titanium) and the reaction of 1,4-diphenyl-1,3-butadiene with bis(cyclopentadienyl)titanium(II)