Active site measurements in the coordinated anionic polymerization of propylene

The active-site concentration was determined by using tritium labeling on three coordinate anionic catalysts containing violet titanium(III) chloride hereafter referred to as titanium(III) chloride or TiCl₃]. These catalysts were used in the polymerization of propylene. Three-component catalysts, as well as two-component catalysts, were investigated. Previous estimates of active-site concentrations for (C₂H₅)₂AlCl—TiCl₃ catalysts appear to be too high by at least a factor of 10. The reason for the previous high estimates can be attributed to the marked reduction in chain-transfer rate which occurs when the amount of polymer formed exceeds about 2 g./g. TiCl₃. The chain-transfer process appears to involve both alkylaluminum and monomer, but the effective monomer concentration is apparently not reduced until a later stage in the polymerization. The propagation rate is, therefore, unaffected; but the transfer rate is reduced, leading to the formation of polymer with much higher molecular weight. Extension of this process could lead to the observed broad distribution of molecular weights in polypropylene. As a result of the low value of active-site concentrations about 10^?3 mole/mole titanium(III) chloride], the absolute value calculated for the propagation rate constant for propylene polymerization is in the range of 50 l./mole sec., and the average lifetime of a growing polymer molecule during the early stages of the polymerization is about 1 min.