A process for the preparation of a conjugated poly(arylene vinylene) copolymer for use in a luminescent device, which comprises: (1) providing a precursor polymer comprising units of general formulain which Ar is substituted or unsubstituted arylene, L is a leaving group, R1 and R2 are each independently H, alkyl, alkoxy, aryl or an electron-withdrawing group, and n is an integer; (2) reacting the precursor polymer with a reactant comprising a carboxylate, an aldehyde, a ketone, a sulphonate, a thioate, a disulphide, a xanthate, an amine, a pyridine, a hydrazide, a phenoxide, an alcohol with a boiling point above 100° C., or a derivative thereof, under substitution conditions whereby a proportion of the leaving groups are substituted to form a substituted precursor copolymer comprising units of general formulain which Ar, R1, R2 and L are defined above, X is a substituent group from the reactant, l and m are independently integers; and (3) converting the substituted precursor copolymer to a conjungated poly(arylene vinylene) copolymer by elimination of the leaving groups from the substituted precursor copolymer.