ARTICLE pubs.acs.org/Organometallics
QM/MM Studies on Scandium-Catalyzed Syndiospecific Copolymerization of Styrene and Ethylene Yi Luo,*,† Yunjie Luo,‡ Jingping Qu,† and Zhaomin Hou*,†,§ †
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, People's Republic of China School of Biological and Chemical Engineering, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, People's Republic of China § Organometallic Chemistry Laboratory, RIKEN Advanced Science Institute, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan ‡
bS Supporting Information ABSTRACT: The copolymerization of styrene and ethylene by the cationic half-sandwich scandium alkyl species (η5-C5Me5)Sc(CH2SiMe3) has been computationally investigated by using the quantum mechanics/molecular mechanics (QM/MM) method. It has been found that the initiation of styrene polymerization both kinetically and energetically prefers 2,1-insertion (secondary insertion, free-energy barrier of 12.6 kcal/mol, and exergonic by 19.1 kcal/mol) to 1,2insertion (primary insertion, free-energy barrier of 19.0 kcal/mol, and exergonic by 8.9 kcal/mol). This is in contrast to a titanocene-based catalyst system, in which the initiation of styrene polymerization was computationally found to prefer 1,2-insertion, while the subsequent styrene insertion (polymerization) proceeds in a 2,1-insertion pattern. In the current Sc-based catalyst system, although the insertion of styrene into the metalalkyl bond of the active species is kinetically slower than that of ethylene, the formation of a styrene π-complex is more favorable than that of an ethylene complex. Also, the insertion of styrene into an ethylene-preinserted species is more energetically favorable than continuous ethylene insertion into the ethylene-preinserted species. These thermodynamic factors could add to a better understanding of styreneethylene copolymerization. The thermodynamic preference for the insertion of styrene rather than that of ethylene into the active species with an ethylene end group was not reported for group 4 catalyst systems. It is also found that the syndiospecific selectivity is inherently determined by the substituent of the ancillary ligand η5-C5Me5.
’ INTRODUCTION Syndiotactic polystyrene (sPS) is a promising engineering plastic, which has many useful properties such as high melting point (ca. 270 °C), high crystallinity, high modulus of elasticity, low dielectric constant, and excellent resistance to heat and chemicals.1,2 However, its brittleness has remained a drawback, limiting the application scope of sPS. For improvement of the toughness of sPS, the incorporation of a comonomer such as ethylene is expected to be a useful method, and toward this end, extensive studies on the copolymerization of styrene with ethylene have been carried out by both academic and industrial researchers.24 Several types of group 4 catalysts were applied for styreneethylene (SE) copolymerization. For Cp0 TiXYZtype catalysts (Cp0 = substituted or unsubstituted cyclopentadienyl; X, Y, Z = Cl, alkyl, alkoxyl, etc. ligand),3f,jm substantial quantities of homopolymer contaminants are coproduced in addition to SE copolymers, likely due to multiple active species. The ansa-zirconocene-based catalyst was used for stereospecific SE block copolymerization with isotactic homosequences of styrene, but the content of styrene in the polymer chain was rather low and uncontrollable.3e ansa-Amido-monocyclopentadienyl Ti and Zr constrained-geometry catalysts (CGCs) could produce SE copolymers with no more than two consecutive styrene units r 2011 American Chemical Society
along the copolymer chain (styrene incorporation
