PHOTOASSOCIATION OF WATER-SOLUBLE COPOLYMERS CONTAINING PHOTOCHROMIC SPIROBENZOPYRAN MOIETIES

Photoinduced random association of HPMA copolymers containing photochr omic spirobenzopyran (SBP) moieties in side-chain termini have been st udied by static and dynamic light-scattering and spectroscopic methods . Experimental results gave evidence of photoinduced association in so lvents with different polarity (water, dioxane). The driving force of this association is electrostatic dipole-dipole interactions between z witter-ionic merocyanine forms of SEP molecules formed upon UV irradia tion. The intermolecular association is only possible in solvents with a low thermodynamic quality far the copolymer close to its solubility : limit. An increase in solvent quality or screening by co-ions suppre sses the photoassociation. The kinetics of cluster formation and disso lution were also investigated by monitoring time dependences of the in tensity of scattered light, hydrodynamic size and merocyanine absorban ce. A common feature of all association processes is the initial growt h of cluster sizes at short irradiation times followed by a plateau af ter the photostationary state of SEP moieties is achieved. lit contras t to hydrodynamic diameter, the static light-scattering intensity incr eases monotonously, reflecting increasing mass and/or concentration of clusters. The dissolution was very slow in aqueous solutions. Both th e formation and dissolution of clusters ave faster in dioxane solution s. The kinetics of cluster dissolution was found to be twice as slow a s the relaxation time of the spontaneous photochromic conversion of SE P moieties. (C) 1998 John Wiley & Sons, Ltd.