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 --(C2 H4 O)p --(C3 H6 O)s -- of between about 1,000 to 3,000.