A kinetic study of ethylene and 1-hexene homo- and copolymerization catalyzed by a silica-supported Cr(IV) complex: Evidence for propagation by a migratory insertion mechanism

The surface organometallic fragment (equivalent to SiO)(2)Cr=CHCMe3 initiat es the polymerization of ethylene and 1-hexene at room temperature in the a bsence of organoaluminum or other activators. The kinetics at the gas-solid interface were studied by in situ IR spectroscopy in a constant volume, va riable pressure reactor. At low pressures, the reaction is first order in P (C2H4), first order in the number of moles of chromium, and linearly depend ent on the Cr loading on silica. The second-order rate constant for polymer ization is (177 +/- 3) s(-1) (mol Cr)(-1) at 21 degrees C, and is virtually unchanged upon subsequent additions of ethylene to the catalyst. The kinet ic isotope effect, k(C2H4)/k(C2D4), is a meagre 1.29. The temperature depen dence of the second-order rate constants yields Delta H double dagger(obs) = (30.2 +/- 0.9) kJ/mol and Delta S double dagger(obs) = (-99 +/- 3) J/(K.m ol). These values are consistent with a propagation mechanism of agosticall y assisted migratory insertion at an alkylchromium(IV) active site. They ar e inconsistent with an alternating alkylidene/metallacycle mechanism requir ing rate-determining C-H cleavage. The rate of polymerization is accelerate d by a factor of 3.2 in the presence of 18 mol % H-2 Homopolymerization of 1-hexene is only 4.7 times slower than that of ethylene, an effect attribut ed to weaker preequilibrium binding of the substituted olefin which is part ially compensated by faster migratory insertion. Copolymerization of ethyle ne and 1-hexene is random. It results in slower incorporation of ethylene b ut faster incorporation of 1-hexene relative to their respective homopolyme rization rates.