Mechanism of polymerization reactions forming the precursor polymers of poly(p-phenylenevinylene) (PPV) and its heteroaromatic derivatives are summarized. Reactions of a,a 0 -bis(tetrahydrothiophenio)-p-xylene dichloride with OH 2 in H 2 O proceed via 1,6-elimination-free radical polymerization mechanism. The rate of elimination reaction increased and that of subsequent polymerization decreased as the aromatic moiety was changed from phenyl to thienyl to furyl. Also, the elimination reaction mechanism changed from (E1cb) R to (E1cb) irr for the same variation of the aromatic moiety, whereas the polymerization mechanism remained the same. The polymerization mechanism for the Gilch route was proposed to be ionic. However, it might actually proceed by a free radical mechanism. The mechanism of the polymerization pathway of the sulfo(i)nyl route changed from a free radical to a competing ionic and free radical by the change in the solvent from protic to aprotic. The thermal decomposition of the bis-sulfonium salts proceeds by the S N 2 displacement of the tetrahydrothiophene by chloride followed by the thermal elimination of HCl. On the other hand, the elimination of the sulยฎ(o)nyl group is considered to proceed via a syn-elimination.