Some representative examples of ethylenically unsaturated monomers that can potentially be synthesized into rubbery polymers which can be asymmetrically tin-coupled in accordance with this invention include alkyl acrylates, such as methyl acrylate, ethyl acrylate, butyl acrylate, methyl methacrylate and the like; vinylidene monomers having one or more terminal CH2???CH-- groups; vinyl aromatics su