4 diol 2 3 dibromo but 2 ene 1 4 diols 3 4 dibromo but 2 ene 1 2 diol 2 2 bis bromomethyl propane 1 3 diol and 1 2 5 6 tetrabromo hexane 3 4 diol the oligomerization of epichlorohydrin can also be initiated by a mixture of brominated and or unsaturated diols the molar ratio of epichlorohydrin to polyol initiator is not critical and can vary within wide proportions this ratio controls the hydroxyl number of the resulting polyether polyol the oligomerization catalyst can be any one of the acid catalysts known for this type of reaction boron tri fluoride in the free or complexed state is nevertheless preferably used diglycidyl and polyglycidyl ethers of brominated epichlorohydrin oligomers can also be prepared by partial or total molecular bromination of the diglycidyl or polyglycidyl ethers of unsaturated epichlorohydrin oligomers prepared by dehydrochlorination in an alkaline medium of the unsaturated chlorinated polyether polyols resulting from the catalytic oligmerization of epichlorohydrin initiated by an unsaturated dihydroxylic or polyhydroxylic aliphatic compound the halogen content of the polyether polyols according to the invention which still possess unsaturated bonds can be increased further and consequently the flame resistance of the polyurethanes which are derived therefrom by partial or total bromination of these unsaturated bonds according to this technique unsaturated polyether polyols prepared by alcoholysis of diglycidyl or polyglycidyl ethers of saturated or unsaturated epichlorohydrin oligomers employing a c 1.