The process of claim 25 wherein R3 and R300 are identical or R3 is a methyl group or a linear, cyclic or branched hydrocarbon group of from 7 to about 10 carbon atoms which may optionally be halogenated, an alkylaryl group of from 7 to about 10 carbon atoms or an alkylalkenyl group of from 3 to about 10 carbon atoms, with the proviso that R3 is not cyclic or branched in ??-position.