The present invention provides polythiol oligomer formed by the reaction of at least two or more different dienes and at least one or more dithiol wherein stoichiometric ratio of the sum of the number of equivalents of all polythiols to the sum of the number of equivalents of all dienes used to form said polythiol oligomer is greater than 1.0 : 1.0; and wherein said two or more different dienes comprise (a) at least one non-cyclic diene and at least one cyclic diene; or (b) at least one aromatic ring-containing diene and at least one non-aromatic cyclic diene; or (c) at least one non-aromatic monocyclic diene and at least one non-aromatic polycyclic diene. Sulfur-containing polyurethane of the present invention can be prepared by combining polyisocyanate, polyisothiocyanate, or mixture thereof; the polythiol oligomer described above; and active hydrogen-containing material.