An exploration of the effects of reversibility in chain transfer to metal in olefin polymerization

A kinetic model was derived to investigate the effects of reversibility in chain transfer to metal in olefin polymerization. The model predicts both number- and weight-average molecular weights, M-n and M-w, as a function of several reactor input variables including the constants for chain transfer and reversible transfer. A number of interesting and nonobvious insights into molecular weight distributions are gained from these simulations. The most revealing result is the variation of the molecular weight distribution (M-w/M-n) with conversion. In the absence of reversible transfer, the M-w/M-n is always greater than or equal to 2. Regardless of the magnitude of the reversible transfer constant, M-w/M-n approaches 2 as M-n approaches its maximum value. More dramatic deviations from M-w/M-n = 2 are observed for higher chain transfer constants. Polymerization data from two bis(phenoxyimine)zirconium catalyst systems are presented to demonstrate these effects. These results indicate that singular polymerizations should not be used to explore for reversible transfer characteristics, but rather a series of polymerizations should be conducted over a range of polymer conversions.