A Density Functional Study on the Stereoselectivity of Styrene Polymerization with ansa-Metallocene Catalyst Sung Hoon Yang, June Huh, Jae Shick Yang, and Won Ho Jo* Hyperstructured Organic Materials Research Center and School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea Received February 2, 2004; Revised Manuscript Received April 19, 2004 ABSTRACT: The insertion mechanism of styrene polymerization with a Cp-based ansa-matallocene catalyst (Cp ) η 5 -C5H5) was investigated by the density functional theory (DFT). The metallocene catalyst adopted in this study, Si(CH3)2Cp2Ti(CH3)2, is experimentally known as a highly syndiospecific catalyst for styrene polymerization. A cationic species, SiH2Cp2Ti + -CH3, is used as an activated form of the catalyst for modeling the initiation of styrene polymerization when one calculates the first insertion of a styrene monomer into the cationic species. Calculations for the second insertion of styrene monomer into the product of the first insertion were also performed. The results of the second insertion of styrene monomer suggest that the frontside insertion of the second styrene leads to a H -transfer reaction, while the backside insertion, which is favored over the frontside insertion, produces a highly syndiotactic product without the H -transfer reaction. This is in agreement with the experimental result. In the backside styrene insertion, the transition state is characterized by a -agostic interaction than an R-agostic interaction because the R-agostic interaction has the repulsive interactions between catalytic ligands and substituents bonded to CR of growing polymer chain. The stereoselectivity of the backside insertion is rationalized by the chain-end-control mechanism, and this mechanism also rationalizes higher insertion steps of styrene monomers. I. Introduction There have been substantial interests in homoge- neous metallocene catalysts as an alternative to clas- sical Ziegler-Natta heterogeneous catalysts. 1 These new compounds, metallocene catalysts, have several desirable properties such as high activity and an ability to produce high-molecular-weight polymers with a nar- row molecular-weight distribution. Moreover, it is pos- sible to control rigorously stereoselectivity by tailoring metallocene ligands. 2-5 Many experimental investigations have been under- taken to understand the mechanism of metallocene- based polymerization. 6-9 Experiments have focused mainly on the effect of metallocene structure on the stereospecificity of polymer. However, the experimental works have been unsatisfactory to fully elucidate the insertion mechanism of metallocene-based polymeriza- tion due to limitation of instrumental facility in moni- toring the reaction in a molecular level. As supplementary methods, theoretical studies such as ab initio calculation, 10-20 semiempirical approachs, 21-27 and the density functional theory (DFT) 28-35 have been performed with much attention on the subject of met- allocene-based polymerization. The methods have given us a basic insight into the real mechanism, which controls stereo- or regioselectivity of polymerization, and have also provided a guideline to design new catalyst for polymers having desired properties. Up to date, however, most of these theoretical studies have concentrated on olefin polymerization. Conse- quently, very few of the theoretical studies have been carried out for elucidating the reaction mechanism of metallocene-catalyzed styrene polymerization, although the styrene polymerization has attracted much interest from industry in the past decade 36 since syndiotactic polystyrene was first synthesized in 1985. 37 The only theoretical study on the metallocene-based styrene polymerization, to our knowledge, is a density functional study by Cavallo and co-workers, 38,39 who investigated the mechanism of chain-end stereocontrol in syndiospe- cific polymerization of styrene with the catalytic model based on CpTi + CH(Ph)CH 3 species (Cp ) η 5 -C 5 H 5 , Ph ) phenyl ring). Although their work provides some meaningful information on the coordination behavior of styrene monomer to the catalytic site, the explanation for the insertion mechanism is still unsatisfactory due to simplification of their model used for the catalytic ligand and a growing polymer chain. Moreover, possible side reactions were not fully investigated since the directionality of styrene approach was not taken into account. In summary, the complete mechanism of syn- diospecific styrene polymerization with metallocene catalyst has been still unraveled. In this paper, we investigate the insertion mechanism for styrene monomer into a Cp-based ansa-metallocene catalyst in order to elucidate the origin of the stereo- selectivity of metallocene-based styrene polymerization. Especially, we focus on how the chirality of growing polymer chain affects the directionality of styrene insertion in relation with the stereoselectivity. This paper is organized as follows. The details of the computational method are presented in the next section. In section III, the results of DFT calculations are presented for the first insertion into the cationic cata- lyst, i.e., SiH 2 Cp 2 Ti + -CH 3 , which is defined as an initiation in this study. In section IV, the results for the second monomer insertion are presented, and the di- rectionality of the styrene monomer insertion is dis- cussed in detail by comparing the energy profile of the * To whom correspondence should be addressed: e-mail whjpoly@plaza.snu.ac.kr; Tel +82-2-880-7192; Fax +82-2-885- 1748. 5741 Macromolecules 2004, 37, 5741-5751 10.1021/ma0497815 CCC: $27.50 © 2004 American Chemical Society Published on Web 06/29/2004