m 0.1 to 2.7; and x, y and z satisfy 2???x+y+z???4; with the provisos that:R4 and R5 are other than both H; when R1 is H or C1-4 alkyl then R4 is other than C1-4 alkyleneSO3 H; when R1 is H or C1-4 alkyl and R6 is H, C1-4 alkyl or hydroxyC1-4 alkyl; then R7 is other than C1-4 alkyl or hydroxyC1-4 alkyl; and when R1 is H or C1-4 alkyl then R6 and R7 are other than both H. 2.