INFLUENCE OF PRESSURE ON CYCLIZATION OF A POLYSTYRENE CHAIN - TIME-RESOLVED FLUORESCENCE MEASUREMENTS

The cyclization of a polystyrene chain (M(n) = 4500, M(w)/M(n) = 1.08) , both ends labelled with 1-pyrenyl groups, was studied in toluene and methylcyclohexane from 1 to 3500 bar. The major effect of pressure on the cyclization rate constant comes from its effect upon the viscosit y. Nevertheless, for a full understanding of the results, changes in t he quality of the solvent for the polymer should occur in methylcycloh exane (a poor solvent), but not in toluene (a good solvent). The rate of the excimer dissociation process increases with pressure, this bein g attributed to a decrease of the excimer binding energy, owing to the diminution of the distance between pyrenes in an excimer sandwich con formation.