Abstract: Compounds of general formula (I), wherein R.sup.1 is hydrogen, C.sub.1-4 alkyl or a group of formula (II): --O(CH(R.sup.10)CH.sub.2 O).sub.n R.sup.20, R.sup.2 is hydrogen or a group of formula (III): --O(CH(R.sup.11)CH.sub.2 O).sub.m R.sup.21, R.sup.3 is a group of formula (IV): --(CH(R.sup.12)CH.sub.2 O).sub.p R.sup.22, and R.sup.4 is a group of formula (V): --(CH(R.sup.13)CH.sub.2 O).sub.r R.sup.23, where R.sup.10, R.sup.11, R.sup.12 and R.sup.13 each is hydrogen or CH.sub.3, R.sup.20 R.sup.21, R.sup.22 and R.sup.23 each is an acyl moiety of a polycarboxylic acid, and n, m, p and r each is an integer from 1 to 15; q is 1 or 2; with the provisos that if q is 2, then the 2 groups R.sup.1 may be the same or different and the 2 groups R.sup.2 may be the same or different; and, if one or more of the groups R.sup.20, R.sup. 21, R.sup.22, and R.sup.23 is an acyl moiety of a polycarboxylic acid group, then 0-3 of the remaining carboxy groups of the polycarboxylic acid form the acyl function(s) of a moiety R.sup.