Aminopyridinato-ligand-stabilized organoyttrium cations are accessible in very good yield through alkane elimination from trialkyl yttrium complexes with sterically demanding aminopyridines, followed by abstraction of one of the two alkyl functions using ammonium borates. At 80 degrees C and in the presence of small amounts of aluminum alkyl compounds, very high ethylene polymerization activities are observed if very bulky aminopyridinato ligands are used. During these polymerizations a reversible polyethylene chain transfer is observed between the organoyttrium cations and aluminum alkyls. The chain-transfer catalyst system described here is able to produce relatively long-chain (up to 4000 g mol-1) Al-terminated polyethylene with a molecular-weight distribution<1.1. In the synthesis of higher molecular PE a slight increase in polydispersity with increasing chain length (15,600 g mol-1, approximately 1.4) is observed owing to reduced reversibility caused by higher viscosity and precipitation of polymer chains (temperature of 80-100 degrees C).

The ™metallocene breakthrough∫ [1,2] has made the dream of tailoring an olefin polymerization catalyst for a desired polymer architecture move closer to reality. In less than one decade since the initial discoveries in the mid-1980s, classes of metallocene catalysts were developed with almost any conceivable kind of stereocontrol (isotactic, syndiotactic, atactic, hemiisotactic), and-most importantly-the relationship was clarified between selectivity on the one hand and symmetry and structure of the catalytic species on the other. [1,2] Therefore, when in 1995 a rational route to isotactic/atactic stereoblock polypropylene (a material of high potential interest for applications as a thermoplastic elastomer [3] ) was announced, [4] the scientific community was-in a way-prepared and there was little room for skepticism.The idea behind the catalyst design was elegant and conceptually simple (Scheme 1). As is well-known, stereorigid ansa-metallocene catalysts in which two indenyl ligands are locked by a bridge in a rac-C 2 -symmetric (Scheme 1 a) or a meso-C s symmetric (Scheme 1 b) configuration afford isotactic and atactic polypropylene, respectively. [1,2] The former is a semicrystalline thermoplastic material, with a melting temperature of up to 165 8C; the latter, instead, is uncrystallizable and moderately elastomeric. [3] Thus, it was considered worthwhile to try to prepare unbridged bis(indenyl) catalysts with substituents of tunable size on the rings, in such a way that hindered ligand rotation is allowed, and a rac/meso conformational rearrangement occurs at a rate intermediate between those of monomer insertion and chain growth (transfer). Support for this strategy was gained from results with [(2-Ar-indenyl) 2 ZrCl 2 ] complexes, where Ar is an aryl group which can range from a simple phenyl (1; Scheme 1 c) [4] to much more complicated and bulky moieties, such as 3,5-ditert-butyl-4-methoxyphenyl (2). [5] Single-crystal X-ray diffrac-Scheme 1. Mechanistic basis for isotactic/atactic stereoblock propene polymerization with [(2-Ar-indenyl) 2 ZrCl 2 ]-based ™oscillating∫ metallocene catalysts (c), according to Coates and Waymouth.[4] P Polymer chain; only CÀC bonds traced in the saw-horse chain representations. Stereorigid ansa-metallocene catalysts in which two indenyl ligands are locked by a bridge in a rac-C 2 -symmetric (a) or a meso-C s symmetric (b) configuration afford isotactic and atactic polypropylene, respectively. tion had proved that complex 1 crystallizes in mixed ™rac-like∫ and ™meso-like∫ conformations. [4] Once combined with suitable cocatalysts, [1,2] it gives rise to a moderately active propene polymerization catalyst. [4] Remarkably, the polypropylene produced is largely stereoirregular, but also contains a highly isotactic part; [4, 5] the fact that it does perform as a thermoplastic elastomer [4±6] was taken as an indication that at least part of the isotactic and stereoirregular sequences are chemically bound, and that therefore crystalline domains act as physical crosslinks between...

The titanium complex Cp[1,3-(2',6'-Me2C6H3)2(CH2N)2C=N]Ti(CH2Ph)2, with a monoanionic eta 1-iminoimidazolidide ancillary ligand, is shown to be a highly efficient catalyst for olefin polymerisation when activated with the Lewis acid B(C6F5)3.