The surface organometallic fragment (tSiO) 2 CrdCHCMe 3 initiates the polymerization of ethylene and 1-hexene at room temperature in the absence of organoaluminum or other activators. The kinetics at the gas-solid interface were studied by in situ IR spectroscopy in a constant volume, variable pressure reactor. At low pressures, the reaction is first order in P(C 2 H 4 ), first order in the number of moles of chromium, and linearly dependent on the Cr loading on silica. The second-order rate constant for polymerization is (177 ( 3) s -1 (mol Cr) -1 at 21 °C, and is virtually unchanged upon subsequent additions of ethylene to the catalyst. The kinetic isotope effect, k(C 2 H 4 )/k(C 2 D 4 ), is a meagre 1.29. The temperature dependence of the second-order rate constants yields ∆H ‡ obs ) (30.2 ( 0.9) kJ/mol and ∆S ‡ obs ) (-99 ( 3) J/(K‚mol). These values are consistent with a propagation mechanism of agostically assisted migratory insertion at an alkylchromium(IV) active site. They are inconsistent with an alternating alkylidene/metallacycle mechanism requiring ratedetermining C-H cleavage. The rate of polymerization is accelerated 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 attributed to weaker preequilibrium binding of the substituted olefin which is partially compensated by faster migratory insertion. Copolymerization of ethylene and 1-hexene is random. It results in slower incorporation of ethylene but faster incorporation of 1-hexene relative to their respective homopolymerization rates.