AbstractRecent investigations indicate that the base‐promoted polymerization reaction of a 1,4‐phenylenebis(methylene)disulfonium salt in a aqueous solution involves the formation of a highly reactivep‐xylylene intermediate, which polymerizes through formation of a sulfur ylide active center by an anionic mechanism. If so, it would be expected that the concentration of this intermediate should be very important in controlling both polymer yield and molecular weight, so it should be possible to increase the concentration of that intermediate by removing the organic sulfide released in the initial elimination reaction by extraction into an organic solvent. This expected effect was verified by the very large increases in both reaction conversion and polymer intrinsic viscosity obtained when the polymerization reaction was carried out in the presence of pentane. The effect was greater with the cyclic sulfide monomer than with the dimethyl sulfide monomer, presumably because of the higher water solubility of the l