Ring-opening polymerization is defined as a polymerization in which a cyclic monomer yields a monomeric unit which is acyclic or contains fewer cycles than the monomer. Generally, the ring-opening polymerization is carried out under mild conditions, and the by-product is less than in the polycondensation reaction. A high molecular weight polymer is easily obtained. 
Common ring-opening polymerization products includes polypropylene oxide, polytetrahydrofuran, , polyoxymethylene, polycaprolactam and polysiloxane.