The substantially linear polyether polyol component of the prepolymer is derived from a difunctional, active hydrogen-containing initiator, e.g., an aliphatic diol such as ethylene glycol, propylene glycol or 1,4-butanediol and one or more 1,2-epoxides which will impart hydrophilic properties to the resulting polyol, e.g., ethylene oxide or propylene oxide.