Abstract: The invention provides a polymer derivative comprising the reaction product of a metal compound and a polymer having functional groups. The metal compound may be an acetate, carbonate, chloride, hydroxide, oxalate, oxide or sulphate of Al, Ba, Ca, Ce, Cd, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, Na, Pb, Sn, Ti, V, Zn and Zr. The functional group may be a carboxylic acid group, a thiol group or an olefinic group.The invention includes a process for efficiently reacting a functional group on a polymer with at least one co-reagent wherein the co-reagents are rendered compatible with the polymer, for example by being dissolved in the polymer or in a solution thereof whereby the reaction is effected in a single phase. The process may be a two stage process which comprises forming a metal derivative of a polymer of relatively low molecular weight and then incorporating the metal derivative into a polymer of higher molecular weight.