An insulated glass sealant having the best characteristics of polysulfides and polyurethanes rather than the worst is made from a polysulfide-based polyurethane prepared by a one-step procedure wherein a randomly copolymeric hydroxyl-terminated polysulfide polyacetal is reacted with a polyisocyanate at a ratio of from about 1:1 to about 1:1.2 on an equivalents basis. The copolymeric hydroxyl-terminated polysulfide polyacetal is made by the condensation of formaldehyde with a mixture of a dithiodialkylene glycol and an aliphatic or cycloaliphatic diol.