An interpretation of ethylene polymerization with alkyl‐promoted transition‐metal catalysts

A concept has been developed to explain the kinetic behavior of six transition‐metal catalyst systems for ethylene polymerization: CrO3 · SiO2 and V2O5 · SiO2 promoted by (i‐C4H10)3Al; VOCl3 promoted by (C2H5)2AlCl; TiCl4 promoted by (C2H5)3Al or (i‐C4H10)3Al; and (C5H5)2TiCl2 promoted by (CH3)2AlCl. Despite obvious differences in the chemical nature of their active species and known behavioral differences, all of these systems show a second‐order deactivation of active species with time. Production of polymer in batch polymerizations has been shown to obey an equation relating propagation and deactivation constants, monomer concentration, reaction time, and initial number of active sites. This equation has been extended to fit batch runs with continuous catalyst addition. One possible reaction mechanism has been proposed. Qualitative steps are: (1) complex formation between the transition‐metal complex and alkyl, (2) alkylation of the complex, (3) polymerization by the alkylated species, and (4) second‐order decay of active species.