ng of alkyl radicals having 1 to 4 carbon atoms, a phenyl radical and a hydroxyl radical; each A is independently selected from the group consisting of R2 and a diol group represented by the general formula --R3 OCH2 CH(OH)CH2 OH where R3 is an alkylene radical having from 1 to 6 carbon atoms, with the provision that at least one A is a diol group; x has a value of at least 1; and y has a value of