Polymerization Catalysts DOI: 10.1002/ange.201007678 Salalen Titanium Complexes in the Highly Isospecific Polymerization of 1-Hexene and Propylene** Konstantin Press, Ad Cohen, Israel Goldberg, Vincenzo Venditto, Mina Mazzeo, and Moshe Kol* Dedicated to Professor Adolfo Zambelli on the occasion of his 77th birthday Isotactic polypropylene (iPP) is a thermoplastic material of vast importance. The ever-increasing demand for it is derived from its useful physical properties and the availability of its feedstock, propylene. The most important microstructural property of polypropylene is the degree of isotacticity, which, combined with sufficiently high molecular weight, determines its melting point (T m ) and thereby its possible applications. [1] iPP produced by heterogeneous Ziegler–Natta catalysts has typical T m values not exceeding 165 8C. [2–5] Homogeneous catalysts of the metallocene, [6] and, more recently, nonmetal- locene [7] families lead to polymers possessing narrower molecular weight distributions. However, in spite of consid- erable research efforts in the last 25 years, only a few such systems were found to lead to iPP having T m values approaching those obtained by the heterogeneous cata- lysts. [8, 9] Herein, we describe a family of nonmetallocene catalysts for olefin polymerization based on a new design concept. Certain members of this family led to polypropylene of exceptionally high isotacticities and T m values. Isospecific catalysts are capable of discriminating between the two enantiotopic faces of an incoming olefin. This differentiation is achieved through the different interactions of these faces with the preferred conformation of the bound polymeryl chain, which is oriented by its interactions with substituents in the vicinity of the chiral metal environment. C 2 -symmetric catalysts are relatively accessible, and their two coordination sites are homotopic, so their induction of isospecificity is independent of possible epimerization events of the polymeryl chain. C 1 -symmetric complexes are structurally more diverse, but the directing abilities of their two diastereotopic sites are usually different. So, a successful design of highly isospecific C 1 -symmetric catalysts should include a directional polymeryl chain migration to the more selective site. [10] This approach was previously developed for C 1 -symmetric metallocenes bearing an overly crowded site from which the polymeryl chain skips into the less crowded and more directing site, [11, 8d] as visualized in the appropriate quadrant representation (Figure 1, left). [12] In designing the current catalysts, we envisioned that a directional site epimerization might also be promoted electronically, namely, by placing donors of different trans influence trans to the two coordination sites in an octahedral environment (Figure 1, right). The tetradentate {ONNO}-type salan ligands tend to wrap around Group 4 metals in the symmetric fac–fac mode, giving octahedral complexes of the type [{ONNO}MX 2 ] (X = O-iPr, benzyl (Bn), etc.). [13] The ability of the dibenzyl complexes to promote isospecific polymerization in the presence of coca- talysts such as B(C 6 F 5 ) 3 or methylalumoxane (MAO) depends on the steric bulk of the phenolate substituents and on the metal. [14] Recently, we showed that C 1 -symmetric zirconium complexes derived from nonsymmetrically substituted salan ligands led to averaging of tacticities, thus implying that a random polymeryl site epimerization was taking place. [15] To promote a directional site epimerization, a tetradentate ligand wrapping in a nonsymmetric manner would be required. Thus, we turned to the salalens. Salalens are half- salan/half-salen hybrid ligands, found to preferably wrap around octahedral Group 4 metal centers so that the half- salan O,N,N donors bind in a fac mode and the half-salen O,N,N donors bind in a mer mode. [16] This fac–mer wrapping places one coordination site trans to the neutral imine N donor and the other site trans to the anionic phenoxy O donor, thus satisfying the above requirement. [17] Complexes Figure 1. Induction of directional polymeryl site epimerization by steric pressure in C 1 -symmetric metallocenes (left) and electronic trans influence in octahedral fac–mer complexes of tetradentate {OD 1 D 2 O}- type ligands (right; D = donor). The bottom drawings are the corre- sponding quadrant representations. [*] K. Press, A. Cohen, Prof. I. Goldberg, Prof. M. Kol School of Chemistry, Tel Aviv University Ramat Aviv, Tel Aviv 69978 (Israel) Fax: (+ 972) 3-640-7392 E-mail: moshekol@tau.ac.il Prof. V. Venditto, Dr. M. Mazzeo Dipartimento di Chimica, Università di Salerno Via Ponte don Melillo, 84084 Fisciano (Italy) [**] We thank the Israel Science Foundation for financial support, Ms Dvora Reshef (Tel Aviv) for technical assistance, Dr. Marina Lamberti (Salerno) for valuable discussions, and Dr. Maria Grazia Napoli (Salerno) for GPC analysis. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201007678. Angewandte Chemie 3591 Angew. Chem. 2011, 123, 3591 –3594  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim