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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 study of the structure of isotactic propylene-hexene random copolymers, prepared with singlecenter metallocene catalysts, is reported. For low concentrations of hexene comonomeric units, up to nearly 10 mol %, copolymers crystallize in the R form of isotactic polypropylene. Copolymers with hexene contents higher than 10 mol % crystallize in a new polymorphic form that melts at nearly 50 °C. The hexene units are partially included in both crystals of R form and of the new form. The crystallization of the new crystalline form allows incorporation of a high amount of hexene units. The melting temperature and the crystallinity decrease rapidly with increasing hexene content. A slower decrease of melting temperature and crystallinity is observed at high hexene concentrations because of the crystallization of the new form. The crystal structure of the new form has been studied by analysis of X-ray powder and fiber diffraction patterns of samples containing hexene concentration higher than 10 mol %. Chains in 3-fold helical conformation of propylene-hexene copolymers are packed in a trigonal unit cell according to the space group R3c or R3 hc. The values of a and b axes of the unit cell depend on hexene concentration, and values of a ) b ) 17.5 Å and c ) 6.5 Å have been found for the sample with 26 mol % of hexene. The structure contains high degree of disorder due to the constitutional disorder of the random copolymer chains that produces disorder in the positioning of the lateral groups in the unit cell. Moreover, statistical disorder in the up-down positioning of the helical chains and slight disorder in the orientation of chains around the 3-fold axes are present. The inclusion of hexene units in the crystals induces a suitable increase of density that allows crystallization of 3-fold helical chains in the trigonal form, where the helical symmetry of the chains is maintained in the crystal lattice. The structure is similar to that of form I of isotactic polybutene. This form does not crystallize, and has never been observed so far for the polypropylene homopolymer because, in the absence of bulky side groups, it would have a too low density. This structure represents an example of entropydriven phase formation.

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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.

Propene-hexene copolymers crystallize in a new polymorphic form of isotactic polypropylene when the concentration of hexene is higher than nearly 10-15 mol %. The hexene units are included in the crystals, inducing an increase of density that allows crystallization of 3-fold helical chains in a trigonal unit cell according to the space group R3c or Rc, where the helical symmetry of the chains is maintained in the crystal lattice. The structure of this new form is similar to those of isotactic polybutene and polystyrene and does not crystallize in polypropylene homopolymer because it would have too low density. The crystal structure of isotactic polypropylene is therefore no longer an exception to the principles of polymer crystallography, but the new structure represents the fulfillment of these principles and indicates that the packing of polymer molecules is mainly driven by density.