A new type of ring-opening monomer has been investigated and found to undergo facile and efficient free-radical polymerization. The monomers are cyclic allylic sulfides with 6-methylene-1,4-dithiepane (3) and 3-methylene-1,5-dithiacyclooctane (4) synthesized and investigated as two examples. 6-Methylene-1,4-dithiepane (3) is a clear oil and 3-methylene-1,5-dithiacyclooctane (4) has a low, near ambient temperature melting point, and in contrast to many ring-opening monomers, they readily survive exposure to moisture and aqueous acids and bases. These compounds were polymerized in bulk with complete ring opening to give highly insoluble crystalline homopolymers of high molecular weight (M(w) ca. 500 000-700 000) with glass transition temperatures of ca. -30 to -50 degrees C and melting points of ca. 100-130 degrees C. The homopolymers and their 1:1 copolymer were characterized by FT-IR and high-temperature H-1 and C-13 NMR spectroscopies, high temperature GPC, and DSC. They displayed volume shrinkages in the range expected of ring-opening monomers (4.5-8.5%) of their molecular weights in spite of the crystallinity present in the final polymers.