POLYSTYRENE CYCLIZATION UNDER HIGH HYDROSTATIC-PRESSURE

Hydrostatic pressure effects on the cyclization of polystyrene chains (M(n) = 2600, M(w)/M(n) = 1.07; M(n) = 4500, M(w)/M(n) = 1.08), both e nds labeled with 1-pyrenyl groups,were studied by measuring fluorescen ce spectra and pyrene monomer and excimer decay curves. The experiment s were performed at room temperature in toluene (good solvent), cyclop entane (Theta solvent), and methylcyclohexane (poor solvent), covering a pressure range of 1-3500 bar. The cyclization rate constant decreas es monotonically with pressure, as a result of the increase of solvent viscosity. However, changes in the quality of solvent for the polymer induced by pressure also influence chain cyclization. Activation volu mes for chain cyclization compare well with solvent viscosity activati on volumes when solvent quality to the polymer is pressure independent . The rate of excimer dissociation and ring-opening increases with pre ssure, especially in Theta and poor solvents. This effect is ascribed to the lowering of the excimer binding energy, induced by changes on t he excimer conformation.