PURPOSE: To obtain a ring-opened monomer useful as an intermediate capable of efficiently providing a monodisperse polymer or a raw material of an anti- foaming agent and so on in a high yield under mild conditions by reacting a heterocyclic compound with a specified reaction initiator in the presence of a specified catalyst to cleave the ring.

CONSTITUTION: A reaction initiator of the formula X4-nMeRn [X is halogen, alkoxy, etc.; Me is Si, Sn, etc.; R is (oxygen or nitrogen substituted)hydrocarbon residue; n is 0-3] is reacted with a heterocyclic compound (pref. -caprolactone, etc.) in the presence of a catalyst selected from a Lewis acid catalyst, a cation catalyst, an anion catalyst, a Lewis acid/anion cocatalyst and a cation/anion cocatalyst to cleave the ring of the above mentioned heterocyclic compound. The reaction initiator is preferably (CH3)3SiOCH3, etc.