Abstract: A method of effecting cross-linking of a resin comprises generating vinyl sulfonyl moieties in situ with the resin, said sulfonyl moieties then undergoing a reaction which effects cross-linking of the resin. The vinyl sulfonyl moieties may be generated as a result of a loss of a liquid carrier for the resin to be cross-linked. The cross-linking reaction may result from reaction of the vinyl sulfonyl moieties with nucleophilic groups in the resin composition. The resin may be a co-polymer of a compound of formula (IV) with other olefinically unsaturated monomers.