Etude cinetique de la polymerisation cationique du cyclopentadiene amorcee par l'hexachloroantimoniate de triphenylmethyle

A study of cyclopentadiene polymerization, initiated by φ₃C⁺SbCl₆^? in methylene chloride solution, has been carried out at temperatures between ?70 and +20° using a dilatometric method. An overall external second order with respect to monomer has been found. At very low temperature (?70°), the concentration of active centres remains low and roughly constant, in agreement with a quasi-stationary state assumption. Between ?50 and + 10°, experimental determination of (k_p. M^*), obtained from variation of v_p and M] with time, shows that the concentration of centres goes through a maximum, sharper and more rapidly reached as the temperature is raised. Initiation is slower than propagation and active centres are rapidly destroyed when termination becomes faster than initiation. This explains the partial conversions and the observed maximum for concentration of active centres. Propagation and unimolecular termination rate constants have been determined at each temperature: activation energies are E_p = ?8 ± 0·5 kcal mole^?1 and E_p = ?0·3 ± 0·1 kcal mole^?1. These negative values can be explained by an exothermic process of solvation of active centres, leading to more reactive propagating species.