hich is the residue of a polyol R5 (OH)n wherein n is an integer equal to two or more, 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 groups or may form parts of a cyclic group; and wherein such linear chains may be crosslinked to