Ring-opening polymerization

Ring-opening polymerization is a chain-growth process in which cyclic monomers open and join into polymer chains. It can be driven by ring strain or by the formation of favorable bonds in the polymer.

Initiation (polymer chemistry): The step that creates an active species capable of adding monomer during a polymerization. An initiator or catalyst generates the active chain end that opens the first ring.
Cyclic monomer: A monomer whose atoms are joined in a ring before polymerization. Its ring bonds are broken as the monomer becomes part of the growing chain.
Polylactic acid: A biodegradable aliphatic polyester made from lactic acid or its cyclic dimer, lactide. Industrial polylactic acid is commonly produced by opening lactide rings.
Step-growth polymerization: A polymerization in which molecules of many sizes react stepwise through functional groups. Unlike ring-opening chain growth, high molecular weight generally develops only at high conversion.
Chain propagation: The repeated addition of monomer units to an active polymer chain end. Propagation extends the chain as each added cyclic monomer opens.
Lactone: A cyclic ester formed by intramolecular esterification of a hydroxy acid. Lactones open to form polyester chains in widely used ring-opening reactions.
Polycaprolactone: A biodegradable polyester made by polymerizing the cyclic ester ε-caprolactone. Its synthesis is a standard example of lactone ring-opening polymerization.
Free-radical polymerization: A chain-growth polymerization in which radicals add to unsaturated monomers. Its radical addition mechanism differs from the ionic or coordination pathways common in ring opening.
Ring strain: The excess energy of a cyclic molecule caused by geometric or electronic constraints on its bonds. Releasing ring strain can make opening and incorporating a cyclic monomer favorable.
Lactam: A cyclic amide whose nitrogen is part of a ring. Lactams can open to produce polyamides such as nylon-6.

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