PURPOSE: To obtain a polyurethane having a narrow molecular weight distribu tion and a highly reactive bislactone skeleton and being useful as a biocompatible material or a degradable polyurethane material.

CONSTITUTION: This polyurethane is a polymer represented by the formula (wherein R is lower alkyl; (m) is 3 or 4; and (n) is an integer of 10-1500) or a derivative thereof obtained by opening part of the lactone rings thereof.