The present invention relates to a compound having the structure of Formula I: A-N=N-Z Iwherein:A is an optionally substituted aryl radicalZ is the residue of a moiety selected from dimedone (5,5-dimethy-1,3-cyclohexanedione), acetoacetamides, malonamides, barbituric acid and 1,3-propanedioneswherein the compound comprises at least one ethylenically-unsaturated polymerizable group (Q).