alkyl group of 1 to 4 carbon atoms, preferably methyl or ethyl; R is the same as R', or is a long chain alkyl or alkenyl group of at least 12 carbon atoms, preferably at least 16 carbon atoms, more preferably at least 18 carbon atoms, and most preferably at least 20 carbon atoms, optionally substituted in available positions by N, O, or S atoms, or R is an aralkyl or aralkenyl group of at leas