Photochromic behaviour of polymeric systems and related phenomena
ABSTRACT The different photochromic systems which have been used in polymer chemistry are mainly based on cis-trans isomerization of azomethines and azocompounds which were built into the polymeric backbone, or attached as sidegroups. These systems are reviewed and briefly commented upon. Special attention is paid to spirobenzopyran derivatives of which the photochromism is very important and characterized by a strong negative solvatochromism. Different spirobenzopyran photochrome-containing polymers are considered: (1) copolymers with vinyl monomers, e.g. methyl methacrylate-styrene. (2) polypeptides such as polytyrosine and polylysine. (3) polycondensation products. Their behaviour in solution is discussed and the existence of different isomeric mesoxy amines demonstrated. In the solid state when these photochromes are used in a polymeric matrix, they permit detection of secondary glass transition phenomena; by contrast, when attached on a copolymer, they permit accurate determination of T g . In crosslinked systems the copolymers show a reversible photochromic behaviour. Finally a comparison of these photochromic phenomena with thermal racemization of some new optically active polymers is presented.