Density functional study on the regioselectivity of styrene polymerization with an awsa-metallocene catalyst

The mechanism of regioselectivity of styrene polymerization with a Cp-based ansa-matallocene catalyst (Cp = η⁵C₅H₅) was investigated using density functional theory (DFT). The metallocene catalyst studied in this work, (CH₃)₂SiCp₂Ti(CH ₃)₂, is known to produce polystyrene via the secondary insertion of styrene. A cationic species, SiH₂Cp₂Ti ⁺-CH₃, is used as an activated form of the catalyst for modeling the initiation of styrene polymerization. The results show that primary insertion is more favorable than secondary insertion in the initiation of styrene polymerization, and the most stable product obtained from primary insertion blocks an additional primary insertion of styrene monomer. For propagation of styrene polymerization, secondary insertion is more favorable than primary insertion. If primary insertion occurs during the propagation, the most stable product of the primary insertion blocks again the additional primary insertion of styrene and thus enhances the secondary insertion. The main driving force blocking the primary insertion is the repulsive interaction between two phenyl rings of the preinserted styrene and the next approaching styrene.