. "A method according to claim 19 wherein R4 is a group of the sub-formula (a), (b) or (c): --CO2 H (a) --CO2 1 /q Aq+ (b) --CO2 R8 (c) where Aq+ is an ion wherein q is 1 or 2; and R8 is a group such that CO2 R8 is an ester group. 24." . . .