| 초록 |
The insertion mechanism of the styrene polymerization was elucidated by the density functional theory (DFT). We propose a detailed mechanism of styrene insertion into a Cp-based ansa-metallocene catalyst (Cp = η5-C5H5) resulting in chain propagation or Hβ-transfer via frontside or backside attack. The Cp-based ansa-metallocene catalyst adopted in this study, SiH2Cp2Ti+-CH3, models the active site of Si(CH3)2Cp2Ti(CH3)2 which is experimentally known as a highly syndiospecific catalyst for styrene polymerization. In the first styrene insertion, the secondary insertion produces thermodynamically more favorable structure, while the primary insertion results in the structure blocking the additional monomer insertion. For the second insertion of styrene, frontside attack of the styrene results in kinetically more faster Hβ-transfer reaction to the approaching monomer, while backside insertion yields the monomer-inserted product without Hβ--transfer reaction. This means that the syndiotactic insertion via backside insertion is kinetically more favorable process than any other insertion processes. The preference of the syndiotactic insertion to the isotactic insertion is explained in terms of the chain-end-control mechanism. |
