R5 is an essentially hydrocarbon group which is the residue of a polyol R5 (OH)a wherein a is an integer equal to two or more, preferably 2 to 4, and R5 is as defined previously, such polyol being a single molecular species or a mixture of molecular species; R6 is a valence bond or an essentially hydrocarbon group; R7 and R8 are hydrogen or essentially hydrocarbon groups which may be separate grou