A cyclic compound comprising m units of the formula Ia. 1 and n units of the formula (Ib) 2 joined together to form a ring, wherein Y and Y2 are divalent bridging groups which may be the same or different in each unit; R0 is H or (C1-C6) alkyl or is a metal or ammonium cation; R5 is H or (C1-C60) alkyl or OH group; j is 1 or 2; R3 is hydrogen, a hydrocarbyl or a hetero-substituted hydrocarbyl group; each of R1, R2 and R4, which may be the same or different, is hydroxyl, hydrogen, hydrocarbyl or hetero-substituted hydrocarbyl, with the provisos that i) at least one of R1, R2, R4 is hydroxyl, and mnulln is 4 to 20, m is 1-8 and n is at least 3, and ii) in formula Ia at least one OH group is in the meta or para position to the COOR0 group and/or at least one R5 group is OH.