Although unstrained five- or six-membered carbocyclic systems undergo free radical ring-closing polymerization, the introduction of an oxygen into the ring will promote free radical ring-opening polymerization, since the resulting carbon–oxygen double bond is approximately 50 kcal more stable than a carbon-carbon double bond. Thus, cyclic ketene acetals, cyclic ketene aminals, cyclic vinyl ethers, unsaturated spiro ortho carbonates, and unsaturated spiro ortho esters will all undergo free radical ring-opening polymerization. Copolymerization of these monomers with a wide variety of vinyl monomers will introduce an ester, amide, keto or carbonate group into the backbone of an addition polymer and can make possible the synthesis of functionally terminated oligomers, biodegradable addition polymers, thermally stable polymers, and monomers that expand on polymerization.