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, such polyol being a single molecular species or a mixture of molecular species; and wherein such linear chain may be crosslinked with other similar chains, and wherein R5 may be a single quadrivalent radical attached to all the interim acetal forming oxy