Determination of the Number of Active Sites for Olefin Polymerization Catalyzed over Metallocene/MAO Using the CO Inhibition Method

Abstract

For a determination of the active site concentrations, [C*], the simultaneous kinetics and inhibition method using CO as a catalyst inhibitor were examined. It is noticed that this evaluation method is not quantitatively exact because MAO-activated alkyl Zr sites exist in dynamic equilibrium between dormant states and a chain propagating site and because some CO molecules coordinate even to inactive species. The number of active sites was evaluated on the assumption that two molecules of CO coordinate to each active center. The increase in Al/Zr ratio and temperature increased both the number of active sites and the value of k p for ethylene polymerization over Cp2ZrCl2/MAO. Especially, a drastic increase of activity is mainly due to a much higher value of k p above a Al/Zr ratio of 2000. A comonomer enhanced drastically the catalytic activity to more than 10 times for ethylene−propylene copolymerization over the rac-Me2Si(Ind)2ZrCl2/MAO system. This is due to the drastic increase in the propagating rate constant, k p, rather than the increase in the formation of new active sites by the addition of comonomer.

Poisoning effect of CO on ethylene polymerization with Ni(II)–diimine/MAO

“…This decreased polymerization rate was not recovered during polymerization. This phenomenon was also observed in the early transition metal olefin catalyst system [9]. Our group reported that CO affected steep decrease of ethylene polymerization rates on Cp 2 ZrCl 2 /MAO system and ethylene/ propylene copolymerization rates on rac-Me 2 Si(Ind) 2 ZrCl 2 / MAO system [9].…”

“…Simultaneous kinetic and inhibition method was developed by Caunt, Tait and coworkers [6,7]. A small amount of suitable inhibitor was injected into polymerization run and decrease in the overall rate of polymerization were measured simultaneously as well as the amount of inhibitor adsorbed [8][9][10][11][12].…”

Influence of support friability and concentration of α‐olefins on gas‐phase ethylene polymerization over polymer‐supported metallocene/methylaluminoxane catalysts

“…16 Chemically, a-olefins are thought to participate in the formation of new catalyst sites and/or the activation of dormant sites 17,18 and in the increase of the propagation rate constant. 19,20 Ethylene/a-olefin synergism seems to occur only in nonhomogeneous polymerization systems, that is, polymerization over supported catalysts 21 or initially homogeneous systems in which the polymer formed is insoluble in the solvent and precipitates out of the solution. 11,22 When the polymer precipitates from an initially homogeneous system, the active center becomes encapsulated in the precipitated polymer.…”

Metallocenes

“…Metallocene cations are subject to equilibria between catalytically active and inactive forms, complicating the quantification of active-site concentrations (190). For example, a kinetic model for ethylene polymerization by constrained geometry catalysts requires activation and deactivation rates, and a trigger mechanism (191).…”