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Ethene/propene copolymerizations were performed with catalytic systems composed of a stereorigid racemic isospecific metallocene and polymethylalumoxane. A number of complexes were used as representatives of three different classes of metallocenes. r-Me2C(3-t-Bu-Cp)2ZrCl2, r-Me2C(3-t-Bu-Ind)2ZrCl2, r-EBDMIZrCl2, r-EBDMTHIZrCl2, and r-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 were chosen as sterically hindered complexes. The parent metallocenes, r-Me2C(Ind)2ZrCl2, r-EBIZrCl2, and r-Me2Si(Ind)2ZrCl2 were also employed as well as other compounds bearing different substituents on the indenyl ligands such as r-Me2C(3-Me3Si-Ind)2ZrCl2, r/m-Me2C(3-Me-Ind)2ZrCl2, and r-Me2Si(2-Me-Ind)2ZrCl2. The reactivity ratios of copolymerization (rE and rP) were determined and examined in the light of the metallocene structure. The complexes that were particularly demanding from the steric point of view were identified as suitable candidates for promoting copolymerizations with a high product of reactivity ratios, giving rErP in a range from 1.7 to 3.0. The other two classes of metallocenes gave rise to rErP clearly lower than 1. A more complex situation was observed as far as the reactivity ratios rE and rP are concerned. The enhancement of the steric hindrance brought about in most cases an increase of rE but not a substantial decrease of rP. In some cases, rE was found to remain almost unchanged and rP dramatically increased. A correlation between the reactivity ratios and the metallocene structure is attempted.

The influence of molecular hydrogen on propene polymerization promoted by the regio- and isospecific rac-Me2C(3-t-Bu-1-Ind)2ZrCl2/MAO catalytic system has been investigated both by experiment (liquid propene, Tp = 50 °C) and molecular modeling (quantum mechanics/molecular mechanics). A competitive secondary (2,1) propene insertion into the Zr−H bond, originated after chain transfer to hydrogen, is detected by ¹³C NMR analysis of these polymers:  increasing amounts of the new CH3−CH(CH3)−CH(CH3)−CH2− chain-end group are observed when the hydrogen partial pressure is increased from 1 to 4 bar. The ratio between primary (1,2) and secondary initiations is found to be constant and nearly equal to 5. On the other hand, the complete regiospecificity of propene insertion during chain propagation is preserved. A combined quantum mechanics/molecular mechanics study on the model cation [rac-H2C(3-t-Bu-1-Ind)2Zr−H(propene)]⁺ shows that secondary propene coordination, followed by secondary insertion into the Zr−H bond which generates the observed structure of the saturated end group, is even favored over primary coordination/insertion, thanks to a stabilizing β-agostic bond between the metal center and one methyl hydrogen of the secondary coordinated propene. Two different mechanisms have been identified for primary propene insertion into the Zr(n-propyl) and Zr(isopropyl) species, with energy barriers of nearly 4 and 9 kcal/mol, respectively. The experimentally observed ratio of [1,2]/[2,1] ≈ 5 for initiations is explained in terms of side reactions that can transform the “slow” propagating Zr(isopropyl) site into the “fast” Zr(n-propyl) one. The model [rac-H2C(3-t-Bu-1-Ind)2Zr(isopropyl)]⁺ cation shows indeed comparable energy barriers for monomer insertion and β-H transfer followed by associative displacement with a primary propene to generate the “fast” propagating species.

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.

Ethene/propene copolymerizations were performed with a catalytic system composed of a metallocene and an alumoxane. The role played by the alumoxane was investigated by comparing the effects of three different compounds:  poly(methylalumoxane) (MAO), tetraisobutylalumoxane (TIBAO), and tetraisooctylalumoxane (TIOAO). A thorough comparison was performed using rac-ethylenebis(tetrahydroindenyl)zirconium dichloride (r-EBTHIZrCl2) as the metallocene. Further data were obtained with isopropylidenebis(3-tert-butyl-cyclopentadienyl)zirconium dichloride (r-Me2C(3-t-BuCp)2ZrCl2) and dimethylsilylenebisfluorenylzirconium dichloride (Me2SiFlu2ZrCl2). Main parameters of copolymerization and copolymers were determined. MAO gives rise to higher polymerization activities and lower molecular masses of the copolymers. A higher productivity is obtained with TIOAO than with TIBAO, whereas these two alumoxanes behave very similarly in determining the copolymer properties. A statistical elaboration allowed to derive the reactivity ratios of the comonomers. The copolymer microstructure is essentially due to the metallocene, even though a MAO-based catalytic system shows a slightly higher reactivity for propene.

Ethene and propene were copolymerized using a metallocene-based catalytic system, composed of rac-isopropylidenebis(3-tert-butylcyclopentadienyl) zirconium dichloride, and poly(methylalumoxane) (MAO). Ethene/propene copolymers were prepared in a broad range of chemical composition, from 17% to 92% as ethene molar content. The use of an isospecific metallocene allowed the preparation of bipolymers composed of ethene and propene with regioregular propene units inserted, in a single macromolecular chain, with the same enantioface, whereas the careful control of the polymerization conditions led to the preparation of copolymers with a very narrow distribution of the chemical composition. A careful analysis of the copolymers microstructure was performed, essentially aimed at the investigation of the comonomers distribution. A statistical treatment of the ¹³C NMR data brought to the determination of the reactivity ratios (r1 and r2) of the comonomers. Data are reported and discussed to demonstrate that a single center homogeneous catalyst is able to prepare ethene/propene copolymers with long sequences of both comonomers.