The structural details of isotactic polypropene (iPP) produced with the moderately isospecific racemic-ethylenebis( 1-indeny1)zirconium dichloriddmethylaluminoxane (rac-(EBI)ZrClzflMAO) catalyst are strongly dependent on monomer concentration. Polymerizing propene at 50 "C in toluene, at propene concentrations in the experimentally measurable range 0.4-11 mol&, rF-(EBI)ZrC12/MAO shows activities in the range 2-350 kgpp/(mmolzr * h) and yields polypropenes with M. , 's from 8800 to 36 600, percent mmmm pentads ranging from 54 to 86%, and corresponding melting temperatures from 86 to 136 "C. Two types of regioirregular (secondary) placements in an isotactic sequence of primary propene insertions are observed in iPP synthesized in liquid monomer, erythro (E, ca. 0.4%) and threo (T, ca. 0.2%). These secondary units are gradually converted into 1,3 propene units as the monomer concentration is lowered.In "starved-catalyst" conditions, that is, as [MI -0, ruc-(EBI)ZrClz/MAO produces atactic propene oligomers (M, = 1080, mmmm = 9.1%). These effects are the most probable cause for the discordance of literature data on metallocene-catalyzed propene polymerization. Three chain transfer mechanisms have been detected: P-hydrogen transfer to the metal and /3-hydrogen transfer to the monomer, both occurring after a primary insertion, and P-hydrogen transfer to the monomer after a secondary insertion with the exclusive formation of a cis-2-butenyl end group.

Heterocycle-fused titanium indenyl silylamido dimethyl complexes produce very high molecular weight polypropylene having a prevailingly syndiotactic microstructure with syndiotactic pentad contents rrrr up to 40-55% (sam-PP). The samples are basically amorphous and may slowly develop a low level of crystallinity (16-20%) at room temperature. A structural characterization has shown that sam-PP samples crystallize in disordered modifications of the helical form I of syndiotactic polypropylene (s-PP). The stretching of compression-molded films of sam-PP samples produce oriented crystalline fibers in the trans-planar mesomorphic form of s-PP. The low stereoregularity prevents the formation of the ordered trans-planar form III of s-PP, which instead is obtained in stretched fibers of the highly stereoregular and crystalline s-PP. The trans-planar mesomorphic form, obtained in stretched fibers, in turn transforms into the helical form I upon releasing the tension. The analysis of the mechanical properties has shown that sam-PP samples show good elastic behavior in a large range of deformation with remarkable strength, due to the presence of crystallinity. A comparison with the mechanical properties of less syndiotactic and fully amorphous samples is reported. These fully amorphous samples present lower strength and experience rapid viscous flow of the chains at high deformations and/or by application of stresses for long times. The higher strength in the semicrystalline sam-PP samples makes these materials interesting thermoplastic elastomers showing high toughness and ductility.

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A kinetic model for chain propagation that can account for reaction orders higher than one on monomer concentration, observed for several insertion polymerizations, is discussed. This model is based on the presence of a “single-center, two-state catalyst” system, where the slower state is of lower energy and the interconversion rate between the two states is intermediate between the fast and slow propagation rates. This kinetic model fits available experimental data on polymerization rates, as a function of propene concentration, in the presence of zirconocene catalysts. Different possibilities for the chemical nature of the two catalyst states are discussed.

The crystal structures of three MgCl(2)·nEtOH complexes with n=1.5, 2.8, and 3.3 have been fully determined. Such complexes are the fundamental precursors for Ziegler-Natta polymerization catalysts used to produce polyolefins on a multimillion-ton scale worldwide. The ab initio structure solution showed that the structure of MgCl(2)·nEtOH complexes with n=1.5 and 2.8 are based on ribbons of metal-centered octahedra, whereas for n=3.3 this chainlike arrangement breaks into a threadlike structure of isolated octahedra linked by hydrogen bonds. A clear correlation between catalyst performance and the crystal structure of precursors has been found, and reveals the fundamental role of the latter in determining catalyst properties. The direct knowledge of building blocks in the precursor structures will help to develop more accurate models for activated catalysts. These models will not require the arbitrary and oversimplified assumption of locating the catalyst active sites on selected cut surfaces of the α-MgCl(2) crystal lattice.

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penheimer approximation). Rather, as Jameson has noted,21 the chief contribution to arises from the anharmonic segment of the expression for chemical shielding. The effect of a shorter C-D bond is to deshield C5, C6, and C7 because these atomic centers are particularly responsive to the charge redistribution that accompanies the difference in zero-point vibrational energy.Acknowledgment. We are indebted to the National Institutes of Health for generous financial support of our research (Grant CA-12115).

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