Ring-opening polymerization is a type of chemical reaction where a cyclic monomer, which is a molecule with a ring structure, opens up to form a long chain polymer. This process typically involves the use of a catalyst or initiator that helps break the ring, allowing the monomer to link with others and create a larger, more complex structure. This method is commonly used to produce various types of polymers, such as polylactic acid and polyethylene oxide. The resulting materials often have unique properties, making them useful in applications ranging from packaging to biomedical devices.