Density Functional Study on the Regioselectivity of Styrene Polymerization with an ansa-Metallocene Catalyst

Abstract

The mechanism of regioselectivity of styrene polymerization with a Cp-based ansa-matallocene catalyst (Cp = η5-C5H5) was investigated using density functional theory (DFT). The metallocene catalyst studied in this work, (CH3)2SiCp2Ti(CH3)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.

On the Initiation Mechanism of Syndiospecific Styrene Polymerization Catalyzed by Single‐Component ansa‐Lanthanidocenes

“…[15] This is consistent with DFT studies for titanocene catalysts which showed that primary insertion is favored in the initiation step but leads to a stable product that blocks additional primary insertions, thus favoring secondary insertion in the propagation step. [16,17] In order to get a better insight in these reactivity and mechanistic issues, we have undertaken a DFT study of the initiation process of styrene polymerization promoted by our neodymocene complexes. The influence of the nature of the bridge in the ancillary Cp'XMe 2 Flu' ligand (CMe 2 vs. SiMe 2 ) and of the "active" (allyl vs. alkyl) group has been investigated.…”

DFT mechanistic study of the H2-assisted chain transfer copolymerization of propylene andp-methylstyrene catalyzed by zirconocene complex

“…In the past decades, many computational studies have been reported to understand the mechanisms of transition metal‐catalyzed olefin polymeriza‐tions . Karttunen et al studied the influences of the ligands on hafnocene‐catalyzed polymerization computationally, and found that steric effects of the ligand affect the activation energy in chain propagation.…”

“…Karttunen et al studied the influences of the ligands on hafnocene‐catalyzed polymerization computationally, and found that steric effects of the ligand affect the activation energy in chain propagation. Yang et al's DFT study of Ti‐catalyzed copolymerization of ethylene and styrene indicated that styrene prefers 1,2‐insertion to 2,1‐insertion initially, but the subsequent styrene insertion is dominated by 2,1‐insertion mode. The QM/MM study of Kang et al on the 1‐hexene polymerization catalyzed by a dicationic rare earth metal catalyst found that 1‐hexene polymerization prefers 1,2‐insertion to 2,1‐insertion.…”

Theoretical investigation of ethylene/1-butene copolymerization process using constrained geometry catalyst (CpSiH2NH)-Ti-Cl2

“…Although the Ziegler-Natta catalysts have been extensively studied both experimentally [3] and theoretically [4], only few studies have been dealt with copolymerization of CGCs [5,6]. Theoretical studies on copolymerization can give an insight into the mechanism of monomer insertion, including the structure of transition state, activation energy and origin of regioselectivity.…”