The polymerization can be carried out in the presence of an inert normally-liquid organic vehicle such as, for example, aromatic hydrocarbons such as benzene, toluene, xylene, ethylbenzene and the like; oxygenated organic compounds such as anisole, dimethyl and diethyl ethers of ethylene glycol; normally-liquid saturated hydrocarbons including open chain, cyclic and alkyl-substituted cyclic satura