A polymerization in which a cyclic monomer yields a monomeric unit which is acyclic or contains fewer cycles than the monomer. Note: If monomer is polycyclic, the opening of a single ring is sufficient to classify the reaction as ring-opening polymerization.

Modified from the earlier definition.

In polymer chemistry, ring-opening polymerization (ROP) is a form of chain-growth polymerization in which the terminus of a polymer chain attacks cyclic monomers to form a longer polymer (see figure). The reactive center can be radical, anionic or cationic.

Ring-opening of cyclic monomers is often driven by the relief of bond-angle strain. Thus, as is the case for other types of polymerization, the enthalpy change in ring-opening is negative. Many rings undergo ROP.

Many cyclic monomers are amenable to ROP. These include epoxides, cyclic trisiloxanes,^{[citation needed]} some lactones and lactides, cyclic anhydrides, cyclic carbonates, and amino acid N-carboxyanhydrides. Many strained cycloalkenes, e.g norbornene, are suitable monomers via ring-opening metathesis polymerization. Even highly strained cycloalkane rings, such as cyclopropane and cyclobutane derivatives, can undergo ROP.

Ring-opening polymerization has been used since the beginning of the 1900s to produce polymers. Synthesis of polypeptides which has the oldest history of ROP, dates back to the work in 1906 by Leuchs. Subsequently, the ROP of anhydro sugars provided polysaccharides, including synthetic dextran, xanthan gum, welan gum, gellan gum, diutan gum, and pullulan. Mechanisms and thermodynamics of ring-opening polymerization were established in the 1950s. The first high-molecular weight polymers (M_n up to 10⁵) with a repeating unit were prepared by ROP as early as in 1976.

New research shows that ROP can be completed with cyclic esters with minimal to no use of solvents by using resonant acoustic mixing.

An industrial application is the production of nylon-6 from caprolactam.