INTERMOLECULAR EXCIMER FORMATION FOR PYRENE-END-CAPPED POLYSTYRENE - A MODEL FOR THE TERMINATION PROCESS IN FREE-RADICAL POLYMERIZATION

The kinetics of intermolecular excimer formation are reported for a polystyrene sample containing one pyrene end group (PS-Py). This reaction serves as a model for the termination process in free-radical polymerization. Fluorescence decay profiles were measured for both monomer and excimer emissions, and from these, the transient contribution to the diffusion-controlled reaction was evaluated. In this way, the mutual diffusion coefficients characteristic of the reaction could be determined. These values (D(o) = 1.4 x 10(-6) cm2 s-1, toluene 20-degrees-C; 1.2 x 10(-6) cm2 s-1, cyclohexane, 34.5-degrees-C) for the polymer of M(w) = 10 400 (M(w)/M(n) = 1.05) are remarkably close to values reported for polystyrene itself (M(w) = 10 700) as determined by quasielastic light scattering (D(o) = 1.57 x 10(-6), toluene 20-degrees-C; 1.23 x 10(-6), cyclohexane, 34.5-degrees-C). This close correspondence between the two sets of values implies that, in this concentration region, excimer formation (and the termination reaction for polymers of this length) is controlled by center-of-mass diffusion of the polymer.