No abstract

The relationship between the nature of catalyst systems and the resulting polymer has been well established in single-site olefin-polymerization systems and now provides the opportunity to tailor the polymers properties. [1][2][3][4] In particular, chiral ansa-metallocene catalysts, which follow an enantiomorphic site-control mechanism, have been intensively investigated to obtain stereochemical control of propylene polymerization by varying the ligand structure. [5][6][7] Unbridgedmetallocene-based systems, on the other hand, have received little attention owing to their aspecific nature [8,9] in spite of their ability to produce iso-rich [10,11] or isotactic-atactic block polypropylene [12,13] when containing rotationally hindered ligands. As unbridged metallocenes are far easier to synthesize than ansa-metallocenes, we have been investigating isospecific, unbridged-metallocene catalytic systems that can be generated in situ during the activation step. To this end, we have designed "class I" unbridged metallocenes, a new class analogous to the known aspecific, unbridged metallocenes of "class II". [8,9] The Lewis basic sites E in class I complexes are found to interact with [Me-MAO] À to generate rigid, rac-like cationic active species, thereby endowing aspecific, unbridged-metallocene precatalysts with isospecificity. Herein, we report a novel example of a sterically unhindered, unbridged zirconocene system that is able to produce highly isotactic polypropylene through the unprecedented role of methyl aluminum oxane (MAO). The amine-functionalized, unbridged zirconocenes [{1-(pMe 2 NC 6 H 4 )-3,4-Me 2 C 5 H 2 } 2 ZrX 2 ] [(AP) 2 ZrX 2 ; X = Cl (2), X = Me (3)] were obtained from newly synthesized ligand 1 as outlined in Scheme 1. The molecular structure of 2 has C 2 symmetry in the solid state (Figure 1). The polymerization of propylene with 2/MAO ([Al]/[Zr] = 1000) was performed at various temperatures (T p = 0, 25, 50, and 70 8C; Table 1, entries 1-4, respectively). The 2/MAO system shows lower catalytic activity but produces higher molecular weight polypropylenes than the well-known isospecific catalyst rac-[Et(Ind) 2 ZrCl 2 ] (EBIZr)/MAO (entry 9) under identical reaction conditions. The differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) traces indicate that all the crude polypropylenes from the 2/MAO system show multiple melting transitions (T m ) and broad molecular-weight distributions (M w /M n ) of between 4.5 and 11 (Figure 2).The crude polypropylenes were fractionated by stepwise solvent extraction [14] into three portions for further analysis, 1) diethyl ether soluble, 2) diethyl ether insoluble and nheptane soluble, and 3) diethyl ether insoluble and n-heptane insoluble. The mmmm methyl pentad values in Table 2 suggest that these portions correspond to atactic-like, mod- [*] Dr.

New unbridged zirconocenes functionalized with a Lewis base, [{1-(E-C(6)H(4))-3,4-Me(2)C(5)H(2)}(2)ZrCl(2)] (E = p-NMe(2) (3); p-OMe (4); p-SMe (5)) were prepared and their propylene polymerization behavior was examined. Under methylaluminoxane (MAO) activation at atmospheric monomer pressure, these complexes afford mixtures of polymers exhibiting multimelting transition temperatures and broad molecular weight distribution, whereas they produce completely atactic polypropylenes under [Ph(3)C][B(C(6)F(5))(4)] activation. Stepwise solvent extraction of the polymer mixtures reveals that the polymers consist of amorphous, moderately isotactic, as well as, highly isotactic portions and the weight ratio of each portion is dependent upon reaction temperature. The generation of rigid rac-like cationic active species in situ by the interaction between basic sites of catalysts and acidic sites of the [Me-MAO](-) counter anion is considered to be the origin of the observed isospecificity. Further investigation of bulk polymerization in liquid propylene shows not only a considerable increase of the isotactic portion of the obtained polypropylenes but also apparent isospecificity of 4 and 5/MAO systems even at high temperature. Variation of the Lewis basic center leads to a dramatic change in stereoselectivity of the catalyst in the decreasing order of 3>4>>5, in spite of their structural similarity.

The relationship between the nature of catalyst systems and the resulting polymer has been well established in single-site olefin-polymerization systems and now provides the opportunity to tailor the polymers properties. [1][2][3][4] In particular, chiral ansa-metallocene catalysts, which follow an enantiomorphic site-control mechanism, have been intensively investigated to obtain stereochemical control of propylene polymerization by varying the ligand structure. [5][6][7] Unbridgedmetallocene-based systems, on the other hand, have received little attention owing to their aspecific nature [8,9] in spite of their ability to produce iso-rich [10,11] or isotactic-atactic block polypropylene [12,13] when containing rotationally hindered ligands. As unbridged metallocenes are far easier to synthesize than ansa-metallocenes, we have been investigating isospecific, unbridged-metallocene catalytic systems that can be generated in situ during the activation step. To this end, we have designed "class I" unbridged metallocenes, a new class analogous to the known aspecific, unbridged metallocenes of "class II". [8,9] The Lewis basic sites E in class I complexes are found to interact with [Me-MAO] À to generate rigid, rac-like cationic active species, thereby endowing aspecific, unbridged-metallocene precatalysts with isospecificity. Herein, we report a novel example of a sterically unhindered, unbridged zirconocene system that is able to produce highly isotactic polypropylene through the unprecedented role of methyl aluminum oxane (MAO). The amine-functionalized, unbridged zirconocenes [{1-(pMe 2 NC 6 H 4 )-3,4-Me 2 C 5 H 2 } 2 ZrX 2 ] [(AP) 2 ZrX 2 ; X = Cl (2), X = Me (3)] were obtained from newly synthesized ligand 1 as outlined in Scheme 1. The molecular structure of 2 has C 2 symmetry in the solid state (Figure 1). The polymerization of propylene with 2/MAO ([Al]/[Zr] = 1000) was performed at various temperatures (T p = 0, 25, 50, and 70 8C; Table 1, entries 1-4, respectively). The 2/MAO system shows lower catalytic activity but produces higher molecular weight polypropylenes than the well-known isospecific catalyst rac-[Et(Ind) 2 ZrCl 2 ] (EBIZr)/MAO (entry 9) under identical reaction conditions. The differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) traces indicate that all the crude polypropylenes from the 2/MAO system show multiple melting transitions (T m ) and broad molecular-weight distributions (M w /M n ) of between 4.5 and 11 (Figure 2).The crude polypropylenes were fractionated by stepwise solvent extraction [14] into three portions for further analysis, 1) diethyl ether soluble, 2) diethyl ether insoluble and nheptane soluble, and 3) diethyl ether insoluble and n-heptane insoluble. The mmmm methyl pentad values in Table 2 suggest that these portions correspond to atactic-like, mod- [*] Dr.

Dinuclear group 4 metallocene catalysts linked by a biphenylene or 1,2‐diphenylethylene bridge, namely [4,4′‐(C5Me4)2(C6H4)2][CpZrCl2]2 (2a), [p‐(C5Me4)C6H4CH2]2[CpZrCl2]2 (2b), [p‐(3,4‐Me2C5H2)C6H4CH2]2[CpZrCl2]2 (2c), [(C5Me4)2(C6H4)2][TiCl3]2 (3a), [p‐(C5Me4)C6H4CH2]2[TiCl3]2 (3b), and [p‐(3,4‐Me2C5H2)C6H4CH2]2[TiCl3]2 (3c), have been prepared and the crystal structures of 2b and 3b determined by X‐ray diffraction methods. The crystal structures reveal that these complexes consist of two equivalent metal units inverted with respect to the center of the bridge. All the complexes were tested for the polymerization of ethylene and styrene in the presence of methylaluminoxane (MAO), and direct comparisons of their catalytic properties with those of the corresponding mononuclear analogues [(PhC5Me4)CpZrCl2] (4a), [(p‐TolC5Me4)CpZrCl2] (4b), [(1‐p‐Tol‐3,4‐Me2C5H2)CpZrCl2] (4c), [(PhC5Me4)TiCl3] (5a), [(p‐TolC5Me4)TiCl3] (5b), and [(1‐p‐Tol‐3,4‐Me2C5H2)TiCl3] (5c) were performed. The dinuclear zirconocenes show a high activity in ethylene polymerization comparable with those of the corresponding mononuclear catalysts and give an increased molecular weight of polyethylene. The dinuclear half‐sandwich titanocenes exhibit similar or slightly lower activity and molecular weight in styrene polymerization compared with their mononuclear analogues. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)