Abstract: The invention relates to a process for polymerising a monomer, polymerizing and/or crosslinking an oligomer, or crosslinking a polymer or copolymer, said monomer, oligomer, polymer and copolymer being selected among precursor monomers, urethane, thiourethane and episulfide oligomers, and urethane, thiourethane and episulfide polymers and copolymers, which comprises: (a) adding to precursor monomers or oligomers or polymers or copolymers an effective amount of at least one complex of Co, Mo or W which under irradiation conditions photochemically reacts to release at least one anionically charged nucleophile or uncharged Lewis base; and (b) irradiating the mixture of (a) to release the at least one anionically charged nucleophile or uncharged Lewis base to initiate polymerisation and/or crosslinking of the monomer, oligomer, polymer or copolymer.