A series of MAO-activated C(1)-symmetric indenyl-ansa-dithienocyclopentadienyl-based zirconocenes have been used to produce isotactic polypropylenes of medium to high molecular weights, of different degrees of stereoregularity, and free from regioerrors. The effect of the presence of rr defects on the polymorphic behavior and mechanical properties of polypropylene is analyzed. The presence of rr defects induces crystallization of gamma form and of disordered modifications intermediate between alpha and gamma forms. A linear relationship between the amount of gamma form and the average length of isotactic sequences has been found. Samples with low concentration of rr defects, up to 3-4%, present high melting temperatures, in the range 160-130 degrees C, and behave as stiff-plastic materials; sample with higher rr content, in the range 4-6% and melting temperatures around 115-120 degrees C are highly flexible thermoplastic materials, and, finally, samples with concentration of rr defects in the range 7-11% and melting temperatures in the range 80-110 degrees C are thermoplastic elastomers with high strength. The fine-tuning of the chain microstructure, achieved by a tailored design of new metallocene catalysts, has allowed production of new polypropylenes having desired properties, intermediate between those of stiff plastic and elastomeric materials.

Isotactic propylene−ethylene (iPPEt) and propylene−butene (iPPBu) copolymers have been prepared with different metallocene catalysts. The different influences of stereodefects (isolated rr triads), ethylene and butene comonomeric units on the crystallization of the α and γ forms of isotactic polypropylene (iPP) have been discriminated. Both iPPEt and iPPBu copolymers crystallize from the melt as mixtures of the α and γ forms. The amount of the γ form increases with increasing crystallization temperature, comonomer concentration, and content of rr stereodefects. In iPPBu copolymers, the amount of the γ form decreases for concentration of butene units higher than 10−14 mol % and is always lower than that crystallized in iPPEt copolymers. Butene units, therefore, favor crystallization of the γ and α forms at low and high concentrations, respectively. These data have indicated that the crystallization of the γ form of iPP is not only related to the value of the average length of the regular propylene sequences 〈L iPP〉, but is also related to the inclusion of stereodefects and constitutional defects in the crystals of iPP. Very different proportions of ethylene and butene units are included in crystals of the α and γ forms of iPP. Butene units are included indifferently in crystals of the α and γ forms, but probably more easily in the α form at high concentrations. Therefore, at low butene concentration, up to nearly 10 mol %, the effect of shortening of the length of regular isotactic propylene sequences prevails and induces crystallization of the γ form. For butene concentrations higher than 10 mol %, the effect of inclusion of butene units in crystals of the α form prevails, producing a decrease of the amount of the γ form and crystallization of the pure α form for butene contents higher than 30 mol %.

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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.

Random copolymers of propylene with ethylene, butene, and hexene comonomers have been prepared with different metallocene catalysts. The used catalysts have allowed a precise control of concentration of stereodefects and high degree of comonomer incorporation while maintaining high molecular mass and random distribution of comonomers. Incorporation of ethylene, butene, and hexene produces great enhancement of ductility, flexibility, and toughness, as compared to highly isotactic polypropylene, but with important differences in the values of elastic modulus and strength depending on the comonomer. In propylene−ethylene copolymers, the values of elastic modulus, ductility, toughness, and strength can be tuned by changing the concentrations of rr stereodefects and ethylene units, so that stiff and fragile plastics, highly flexible materials, and elastomers can be obtained. Propylene−butene copolymers behave as highly flexible materials with remarkable ductility and toughness and contemporary high strength and stiffness. Propylene−hexene copolymers are highly flexible materials with plastic resistance and strength that decrease with increasing hexene content. These outstanding properties are not easily accessible with propylene-based copolymers produced with traditional Ziegler−Natta catalysts because of nonrandom distribution of comonomers and non-homogeneous composition.

Highly isotactic polypropylene samples, containing a very low amount of rr stereodefects (0.1−0.2%) and slightly higher concentration of defects of regioregularity (0.8−0.9% of 2,1 erythro units), with different molecular masses have been prepared with an isospecific but not fully regioselective metallocene catalyst. The effects of the presence of rr defects and of 2,1 regiodefects and the effect of the molecular mass on the mechanical properties and crystallization behavior of polypropylene have been analyzed. Samples containing 2,1 regiodefects are very stiff and much more fragile than the samples containing only rr stereodefects. The presence of rr defects, even for low concentrations, produces instead increase of ductility and improvement of drawability at room temperature. The different effect of stereo- and regiodefects is probably related to their different levels of inclusion inside the crystalline phase. The uniform inclusion of rr defects in the crystals makes the stereodefective and regioregular samples more homogeneous, where crystalline and amorphous phases have the same composition. The study of the crystallization behavior has shown that the molecular mass strongly influences the amount of α and γ forms that crystallize from the melt. Samples containing regiodefects and with high molecular masses (higher than 200 000) show the same amount of γ form and the same crystallization rate. A lower amount of γ form and higher crystallization rates are instead observed for the sample having similar microstructure but lower molecular mass. This indicates that the crystallization of the γ form is favored over the α form when the crystallization is slow, that is, for samples with high molecular mass. When the molecular mass is lower than 100 000, the crystallization is faster and the formation of the α form is kinetically favored. The comparison with regioregular samples containing only rr stereodefects has shown that the effects of rr stereodefects and 2,1 regiodefects on the crystallization properties of polypropylene are very similar, at least when the concentration of defects is small (1% of rr stereodefects or 2,1 erythro units). Both defects produce a shortening of the regular crystallizable isotactic and regioregular sequences, inducing crystallization of the γ form.

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