Chromocene supported by silica is a traditional industrial ethylene polymerization catalyst that does not require aluminum alkyl cocatalyst

however

its initiating mechanism for ethylene polymerization remains unclear up to now. The feasibility of metallacycle and oxachromacycle mechanisms suggested in literatures

as well as the chroma-Cpcycle mechanism which was firstly proposed in this work were systematically investigated by DFT method. It was found that chain initiation could be achieved with a relatively low barrier (

ca

. 24.9kcal/mol) in the metallacycle mechanism

which was consistent with the experiment results. However

because of the large steric effect of the metallacycle

the subsequent propagation would be very slow due to the high insertion barrier (

ca

. 31kcal/mol). For the oxachromacycle mechanism

both the initiation and insertion barriers were very high

and the bonding between Cr and silica support would be impaired by C

2

H

4

insertion

which made this mechanism nearly impossible. With respect to the chroma-Cp-cycle mechanism

the energy for cyclopentadienyl slippage to weaken its coordination with Cr center is very high

making the initiation barrier as high as 37.3kcal/mol

which contradicted with the fast initiation nature of S-9 catalyst. Nevertheless

the low barrier (

ca

. 20kcal/mol) for insertion ensured a fast propagation

which was in agreement with experimental observations. In this work

molecular simulation methods are used to study three possible initiation mechanisms including metallacycle mechanism

oxachromacycle mechanism and chroma-Cp-cycle mechanism for S-9 catalyst

which gave a relatively deep understanding on this catalyst under a molecular level.