PROBLEM TO BE SOLVED: To solve such a problem that, in the method for ring-opening polymerizing a ring-opening polymerizable monomer by using a compressible fluid such as supercritical carbon dioxide, when using as the catalyst an organic catalyst not containing metal atoms, high molecular weight polymer products are not obtained even prolonging the reaction over long hours, which results in the degradation of durability and softening point of the polymer product under the influence of low molecular weight components.SOLUTION: This polymer product does not substantially contain an organic solvent and a metal catalyst, the amount of residual ring-opening polymerizable monomer is 1,000 ppm or less, and the number-average molecular weight is 15,000 or more. Thereby an advantage is achieved that lowering of durability or softening point under the influence of low molecular weight components can be controlled.
