. "diene which is substituted with one or more hydrocarbyl groups, silyl groups, hydrocarbylsilyl groups, silylhydrocarbyl groups, or mixtures thereof, at least one of said substituents being located at the 1- or 4-position, and said D' having from 5 up to 40 atoms other than hydrogen, M' is a Group 1 metal cation, a Group 2 metal or zinc dication, a magnesium or zinc monohalide cation, a mono(C1-20" . . .