Polymerisable resins which comprise a porphyrinogenic ring system obtained by the reaction of: (a) one or more compounds selected from the group consisting of pyrrole and N-(lower)alkyl pyrroles, any of which may be optionally substituted, and (b) a C4-C6 saturated alicyclic ketone which is capable of reacting with the 2 or 5 position of the pyrrole ring. Resin coating systems comprising the said resins.