Preferably each of R10, R11, R12, and R13 is a methyl group; Rc is H; m is preferably three or four whereby the group Rb is most preferably the radical ???(CH2)3???; and the values of p and s are such as to provide a molecular weight of the oxyalkylene segment ???(C2H4O)p???(C3H6O)s??? of between about 1,000 to 3,000.