r methyl, R1 is the residue of an aliphatic polyol group having m+1 primary hydroxyl groups and 1 to 10 carbon atoms, said residue being formed by the removal of hydroxyl groups from the polyol, and having a valence of m+1, R3 is selected from hydrogen, ##STR20## wherein R4 is an alkenyl or alkyl group, and R5 is an aliphatic or aromatic group of up to 8 carbon atoms, m is an integer of 2 to 5, an