Aminosilylene-bridged ansa-zirconocenes for branched polyethylenes with bimodal molecular weight distributions Yonggyu Han a , Hyoseok Kim a , Min Hyung Lee b , Youngjo Kim c , Junseong Lee a , Yoon Sup Lee a , Youngkyu Do a, * a Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea b Department of Chemistry, University of Ulsan, 680-749, Republic of Korea c Department of Chemistry and Basic Science Research Institute, Chungbuk National University, Cheongju, Chungbuk 361-763, Republic of Korea article info Article history: Received 26 June 2009 Received in revised form 1 September 2009 Accepted 3 September 2009 Available online 6 September 2009 Keywords: Bimodal molecular weight distribution Branched polyethylene Ansa-metallocene Aminosilylene-bridge abstract Reactions of the dilithium salt of aminosilylene-bridged ligands with (Me 2 N) 2 ZrCl 2 (THF) 2 followed by the treatment of Me 3 SiCl are found to be an efficient synthetic route to aminosilylene-bridged ansa-zirconoc- enes, R 2 N(Me)Si(g 5 –C 5 H 4 ) 2 ZrCl 2 (R = Me (1), Et (2)) and Me 2 N(Me)Si(g 5 –C 5 H 4 )(g 5 –C 5 Me 4 )ZrCl 2 (3). Crys- tal structure of 3 determined by X-ray diffraction study reveals the presence of p-bonding interaction between N and Si atoms, which is further supported by DFT calculation results. These complexes are very active (>1  10 3 Kg/(mol Cat.atmh)) for homopolymerization of ethylene in the presence of methylalu- moxane (MAO) cocatalyst, generating polyethylenes that contain branches as well as bimodal molecular weight distribution (MWD). Methyl, ethyl, butyl, and other longer branches (n P 6) are observed in the resulting polyethylenes. The polyethylenes from 1, 2 and 3/MAO show a broad MWD range (6.3–42.2, 3.5–4.0 and 2.6–3.4, respectively). Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Metallocene-based polyethylene (PE) possesses excellent properties such as high clarity and high impact strength [1,2], but its bulk production has been hampered by significant processing problems due to a lack of branching and narrow molecular weight distribution (MWD). Thus, the enhancement of the processibility of metallocene-based PE has become an important research subject in recent years, with particular focus on introducing branches and modulating the MWD. The former objective has been approached via copolymerization with higher a-olefins [3], the use of mono- [4] and bi-nuclear [5] ‘‘constrained geometry catalysts”, the use of meso ansa-metallo- cene-based catalysts [6], and homopolymerization with ‘‘tandem catalytic systems” [7]. The latter goal has been tackled by using mixed metallocene systems [8], and by designing metallocene precatalysts that produce multi catalytically active species dur- ing the polymerization process [9–11]. In order to develop metallocene catalytic system that can simultaneously carry out both functions of the formation of branches and the modulation of the MWD, we have designed functionalized silylene-bridged ansa-zirconocenes 1–3. Si N Zr Cl Cl R' 4 2: R = Et; R' = H 1: R = Me; R' = H 3: R = Me; R' = Me R R The purpose of introducing NR 2 group to the bridging Si atom is three-fold. Firstly, the amine group is expected to cause the alter- ation of the electron donating ability of the ansa-ligands in point via direct p-bonding interaction between the lone pair electrons of N atom and the Si orbital. The foregoing electronic effect may lead to a change in polymerization process. Secondly, in the ethyl- ene polymerization with methylalumoxane (MAO), the amine group is also expected to interact with the cocatalyst without caus- ing steric encumbrance to a metal center, generating dual active species and thereby resulting in bimodal MWD of PE. Thirdly, even though group 4 metallocenes bearing amine groups have attracted interest from strategic viewpoint in developing new catalytic sys- tems, the amine groups were only introduced to the cyclopentadi- enyl ring fragments as demonstrated in the following examples of rac-Me 2 Si(2-(N,N-dimethylamino)indenyl) 2 ZrCl 2 (I) [12], (9-(N,N- dimethylamino)fluorenyl) 2 ZrCl 2 (II) [13] and Me 2 Si(cyclopentadi- enyl)(3-(N,N-dimethylaminoethyl)cyclopentadienyl)ZrCl 2 (III) [11] 0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2009.09.003 * Corresponding author. E-mail address: ykdo@kaist.ac.kr (Y. Do). Journal of Organometallic Chemistry 694 (2009) 4216–4222 Contents lists available at ScienceDirect Journal of Organometallic Chemistry journal homepage: www.elsevier.com/locate/jorganchem