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We present a theoretical study of the basic interactions occurring at supported hetereogenous Ziegler-Natta catalytic systems. We first investigated the interaction between prototypes of each class of industrially relevant internal donors (1,3-diethers, alkoxysilanes, phthalates, succinates) and the MgCl 2 support. Our analysis indicates that donors can be separated into two classes. 1,3-Diethers and alkoxysilanes belong to the former because they have a short spacer between the coordinating O atoms and coordinate preferentially to the same Mg atom of the (110) lateral cut. Conversely, phthalates and succinates belong to the latter class because they have a longer spacer between the coordinating O atoms and thus can adopt a variety of coordination modes. Indeed, they can coordinate to both the ( 100) and ( 110) lateral cuts. In the last part of this manuscript we report on the stereo-and regioselective behavior of possible active Ti species with and without two succinate molecules coordinated in the proximity of the Ti atom. We show that the two succinate molecules confer a remarkable stereoselectivity in both primary and secondary propene insertions. This model very simply rationalizes the effect of the donors, and it is consistent with the models so far developed to rationalize the stereoselectivity of metallocene and octahedral nonmetallocene catalysts.

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.

A new class of isospecific and highly regiospecific C 2 - symmetric ansa-zirconocenes, characterized by a bisindenyl ansa ligand with bulky substituents in the 3 position of indene and a single carbon bridge is disclosed:  variation of the size of the substituent in C(3) has a strong effect on the extent of chain transfer and isospecificity in propene polymerization. In fact, while rac-[Me2C(1-indenyl)2]ZrCl2 produces low molecular weight and moderately isotactic polypropene (iPP) also containing some regioirregularities (M̄ n = 6500, mmmm ca. 81% and 2,1tot = 0.4% at 50 °C in liquid monomer), rac-[Me2C(3-tert-butyl-1-indenyl)2]ZrCl2 produces iPP with molecular weights between 25 000 (T p = 70 °C) and 410 000 (T p = 20 °C) and a fairly high isotacticity (mmmm ca. 95% at 50 °C), with no detectable 2,1 units. The influence of polymerization temperature on the catalyst performance has been investigated by polymerizing liquid propene in the temperature range of 20−70 °C:  the experimental ΔΔE ⧧ values for enantioface selectivity have been estimated for two members of the new class (rac-[Me2C(3-tert-butyl-1-indenyl)2]ZrCl2 ΔΔE ⧧ enant = 4.6 kcal/mol; rac-[Me2C(3-(trimethylsilyl)-1-indenyl)2]ZrCl2 ΔΔE ⧧ enant = 2.6 kcal/mol). For comparison, Brintzinger's moderately isospecific, benchmark catalyst rac-[ethylene(1-indenyl)2]ZrCl2 (ΔΔE ⧧ enant = 3.3 kcal/mol), the single carbon bridged, unsubstituted rac-[Me2C(1-indenyl)2]ZrCl2 (ΔΔE ⧧ enant = 2.8 kcal/mol), and the C 2-symmetric, practically aspecific, rac-[ethylene(3-methyl-1-indenyl)2]ZrCl2 (ΔΔE ⧧ enant = 1.9 kcal/mol) are also reported. The molecular structures of rac-[Me2C(3-tert-butyl-1-indenyl)2]ZrCl2 and rac-[Me2C(3-(trimethylsilyl)-1-indenyl)2]ZrCl2 have been determined.

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Ziegler‐Natta catalysts, discovered in the years 1953–1954, account today for a production volume of ∼65 million tons of polyolefins, including mainly polyethylene and polypropylene products. Since their first discovery, the development of Ziegler‐Natta catalysts has been relentless, and their evolution is the result of the exploitation, starting from the mid‐1960s, of four major breakthroughs: the active form of MgCl 2 ; the stereoregulating effect of electron donors (isotactic polypropylene); the chemical route to the active form of MgCl 2 ; and, finally, the control of the morphology of the catalyst/polymer particles. Thus, the most advanced Ziegler‐Natta catalysts are today prepared starting from controlled morphology MgCl 2 , or its precursors, TiCl 4 , or other titanium derivatives and, especially for isotactic polypropylene, electron donors. With respect to the first‐generation of Ziegler‐Natta catalysts based on TiCl 3 , MgCl 2 ‐based catalysts are not only much more efficient in terms of productivity ( both increased number of active centers and value of their propagation constant), but they also offer an unprecedented level of customization to meet any process (slurry, bulk, or gas‐phase) or product requirements (polyethylene, polypropylene, and their copolymers). Actually, MgCl 2 ‐supported catalysts are complex but also versatile systems whose architecture (particle shape, size, size distribution, and porosity) and performances (activity, hydrogen response, stereocontrol, and capability to tune polymer MW and MWD) can be directed acting on both the morphology of the MgCl 2 precursors and the nature of the electron donors. Many mechanistic aspects of the Ziegler‐Natta catalysis have been elucidated, eg, the mechanism of polymer particle growth, the role of prepolymerization, the activating effect of hydrogen in propylene polymerization, and the stereoblock nature of isotactic polypropylene. On the other hand, many other aspects are still open to debate or, at least, still need clarification; eg, the exact nature and number of active centers and their mechanism of deactivation, the intimate mechanism of action of electron donors, the difference between the species that are active in ethylene or propylene polymerization, and the comonomer effect in ethylene copolymerization. Thus, there is still a lot of room for further investigation and improvements in Ziegler‐Natta catalysts. A big step forward in this field would be the generation of catalysts combining the economics and morphological features of MgCl 2 ‐based systems with the peculiarities of single‐site systems.

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The known rac-[ethylenebis(4,7-dimethyl-η5-1-indenyl)]ZrCl2 (2r) and its meso isomer (2m) have been compared with the prototypal chiral isospecific rac-[ethylenebis(η5-1-indenyl)]ZrCl2 (1r) and its aspecific meso isomer 1m in terms of molecular structures, solution dynamics, and ligand substitution effect on polymerization performance. In liquid propene at 50 °C, 2r/MAO produces iPP with appreciably higher isotacticity but lower molecular weight and regiospecificity than 1r/MAO. The lower molecular weight obtained with 2r in liquid monomer is due to predominant chain transfer to the monomer after a secondary propene insertion, producing >90% cis-2-butenyl− end groups. At lower propene concentration, 2r/MAO allows both β-hydrogen transfer after a primary insertion and β-methyl transfer. The low-activity 2m/MAO catalyst produces low molecular weight aPP. The diastereoselective synthesis of 2r,m via the corresponding rac- and meso-bis(4,7-dimethyl-1-(trimethylsilyl)-3-indenyl)ethane is reported. The crystal and molecular structures of meso-bis(4,7-dimethyl-1-(trimethylsilyl)-3-indenyl)ethane, 2r,m, have been determined.