Abstract: Block copolyethers having general formula (I):RX--(CH.sub.2 --CHR.sub.1 --O--).sub.x --(--CH.sub.2 CHR.sub.2 --O--).sub.y --(--CH.sub.2 --CHR.sub.3 --O--).sub.z --R.sub.4wherein:R is selected from H, alkyl, alkylaryl;X is selected from O, S and N;R.sub.1, R.sub.2 and R.sub.3 are selected from H, CH.sub.3, and C.sub.2 -C.sub.10 alkyl radicals;R.sub.4 is selected from H, alkyl, acyl;x,y,z represent the number of ether units and are integers which have such values that the molecular weight of the compound having general formula (I) is between 600 and 4000, and in addition (x+z)/y is between 1.5 and 5.0; with the proviso that:a) either R.sub.1 and R.sub.3 are selected from H and CH.sub.3, in which case R.sub.2 is selected from C.sub.2 to C.sub.10 alkyl radicals,b) or R.sub.1 and R.sub.3 are selected from C.sub.2 -C.sub.10 alkyl radicals, in which case R.sub.2 is selected from H and CH.sub.3.