Abstract: Compounds of formulae I and II ##STR1## wherein D is H or --OR wherein R is phenyl and R.sub.1 to R.sub.4 are H or a substituent such as phenylthio, and one of the substituents R.sub.1, R.sub.2, R.sub.3 and R.sub.4 is a group of formula III--E-CR.sub.5 =C(R.sub.6).sub.2 (III)or R is substituted by a group of formula III, wherein E is a direct bond or --X-C(O)-- wherein X is a linking group, and R.sub.5 and R.sub.6 are each independently of the other H, halogen, C.sub.1 -C.sub.12 alkyl or C.sub.6 -C.sub.10 aryl, can be polymerised to homopolymers or copolymers. The monomers and polymers are reversible photochromic systems which can be used as color indicators, as photochemically modifiable color filters, or for the optical storage of information.