There are provided according to the invention, novel compounds of formula wherein R1, R2, R3, R4, R5, R6, X a, b, and Z are as defined in the specification, processes for preparing them, formulations containing them and their use in therapy for the treatment of inflammatory diseases. The polymerizable composition has a high refractive index and excellent light resistance and is thus useful for an optical sheet assembly for a backlight unit. comprising about 60 to 70 volume % of finely powdered filler material in a polymer matrix, are made by dissolving the polymer in a volatilisable solvent and forming a homogeneous mixture of the components by high shear mixing.