Triarylsulfonium hexafluorophosphate salts were shown to be effective catalysts for the ring-opening polymerisation of various cyclic monomers under UV irradiation. A dual basic/acidic catalytic system demonstrated the potential for UV-triggered formation of poly(delta-valerolactone)-b-poly(L-lactide)-b-poly(delta-valerolactone) in a 'one-pot' reaction.