Adolfo Zambelli

Syndiotactic Polymerization of Propylene: Single‐Site Vanadium Catalysts in Comparison with Zirconium and Nickel

The syndiotactic-specific polymerization of propylene can be promoted by single-site catalysts based on vanadium, nickel and group IV metals. The polymerization mechanisms are reasonably well established for nickel and group IV metals. For vanadium catalysts the polymerization mechanisms reported in the literature are still debatable, since the stereochemistry of the monomer insertion is well known, but the active species have never been isolated. In this paper the mechanisms concerning Ni and group IV metal catalysts are briefly summarized and proposed mechanisms concerning vanadium are compared within one another.

Group 4 Cs Symmetric Catalysts and 1-Olefin Polymerization

Abstract Polymerization of propene and 1-butene promoted by group 4 Cs symmetric metallocene precursors has been tested in different reaction conditions. The stereochemical structure of the polymer is affected by the radius of the transition metal, the substituent of the olefin and in some cases the reaction temperature and the monomer concentration. The results are tentatively rationalized considering the possible catalytic cycles that may produce either syndiotactic or isotactic polymers. A sample of syndiotactic polypropylene melting at 170°C is also reported.

Syndiotactic-specific polymerization of propene with Nickel-based catalysts. 3. Polymer end-groups and regiochemistry of propagation

Abstract Low molecular weight prevailingly syndiotactic polypropylene samples have been synthesized under proper conditions with homogeneous catalytic systems based on Ni(II) diimine derivatives and methylaluminoxane. 13 C NMR analysis of the natural abundance end groups formed in both the initiation and termination steps suggests that the regiospecificity of propene insertion during the propagation is prevailingly 1,2. The effects of ligand variation at the Ni catalytic center on the regiospecificity of the polymerization is in agreement with 1,2 insertion as the main mode of propagation.

Stereospecific polymerization of α-olefins: End groups, polymer structure and reaction mechanism☆

Abstract In this paper are reviewed some topics concerning polymerization of α-olefins to either isotactic or syndiotactic polymers in the presence of both heterogeneous and homogeneous initiator systems. Isotactic polymerization of chiral α-olefins and isotactic and syndiotactic specific polymerization of styrene are also discussed. The evidence relating to reaction mechanisms which arises from the stereochemical structure of the polymers and especially of the end groups is considered particularly.

Non-metallocene group 4 organometallic complexes as catalysts for olefin polymerization : synthesis and catalytic activity of the cationic complex [Zr(CH2Ph)3]+ [B(CH2Ph)(C6F5)3]-

Abstract The title cationic Cp-free zirconium hydrocarbyl has been synthesized by ligand abstraction from tetrabenzylzirconium with the strong Lewis acid tris (pentafluorophenyl)boron. NMR analysis indicates that the [Zr(CH 2 Ph) 3 ] + cation is stabilized by coordination of the phenyl ring of the anion [B(CH 2 Ph) (C 6 F 5 ) 3 ] − , and that this structure is maintained in aromatic solvents up to 100°C. The highly electrophilic zirconium cation reacts rapidly with Lewis bases such as THF, affording the 6-coordinate [Zr(CH 2 Ph) 3 (THF) 3 ] + adduct. The title compound is the first non-metallocene group 4 cationic complex which functions as a single-component olefin polymerization catalyst.

Evaluation of the dimethylsilyl-bis(2-methyl-4-phenyl-1-indenyl) ligand with group 4 triad metals in propene polymerizations with methylaluminoxane

Dimethylsilyl-bis(2-methyl-4-phenyl-1-indenyl)MCl2 (M = Ti, Zr, Hf) complexes in combination with methylaluminoxanes produce isotactic polypropene. The Ti catalysts produce isotactic polypropene which is more stereoregular than that obtained with traditional, heterogeneous catalysts. Zr produces low molecular weight, weakly isotactic waxes at phenomenal rates greater than 32 · 106 gPP/[(mol catalyst) · h · (mol/l monomer)]. Hf produces long, softer isotactic chains with good activities.

Group 4 Metallocene Catalysts with Hapto-Flexible Cyclopentadienyl-Aryl Ligand

Catalytic precursors Ti(IV) and Zr(IV) complexes bearing cyclopentadienyl and substituted cyclopentadienyl anionic ligands, bonded to phenyl or substituted phenyl through an isopropylidene bridge have been utilized in the polymerization of propene and styrene and in ethylene-styrene copolymerization. In the presence of trichloro[(1,2,3,4,5-η)-1-(1-methyl-1-phenylethyl)-2,4-cyclopentadien-1-yl]titanium (LTiCl 3 ) we have obtained either partially isotactic (chain-end type) or atactic poly-(propylene), and either atactic or syndiotactic polystyrene depending on the reaction temperature. [1-Methyl-1-naphthylethyl-2-inden-1-yl]titanium(IV) behves like (LTiCl 3 ) in styrene polymerization, while it affords metal-controlled partially isotactic poly(propylene), as well as the corresponding zirconium compounds. The experimental data are tentatively explained by the temperature dependence of coordination of the bridged aryl group of the ligand.

Single insertion of propene into a cationic zirconium(IV) complex: Isolation and X-ray crystal structure of [(C5Me5)Zr(CH2CHMeCH 2Ph)(CH2Ph)]-[B(CH2Ph)(C6F5)3]

Abstract The reaction of propene with the half-sandwich salt complex [Cp′Zr(CH 2 Ph) 2 ][B(CH 2 Ph)(C 6 F 5 ) 3 ] ( 1 ) (Cp′ = C 5 Me 5 ) affords cleanly the single-insertion adduct [Cp′Zr(CH 2 CHMeCH 2 Ph)(CH 2 Ph)][B(CH 2 Ph)(C 6 F 5 ) 3 ] ( 2 ). The crystal structure of 2 reveals an unusual ‘back-biting’ η 6 -arene coordination to the d 0 metal, accounting for the unexpected stability of this compound either towards further propene insertion or towards β-hydrogen elimination.

A Density Functional Theory Study of the Syndiotactic-Specific Polymerization of Styrene

Preliminary theoretical results relative to a possible mechanism of syndiospecific polymerization of styrene with models based on the CpTiP* (P = Polymeryl) species are presented. The styrene-free CpTiCH2Ph* species is characterized by a η7 coordination of the benzyl-type chain end, which is preserved also in the most stable species upon coordination of a styrene molecule. The transition State for the insertion step is characterized by the classical four centers geometry, usually proposed for the polymerization of olefins. The insertion can occur with a energy barrier of 47 kJ/mol. As for the mechanism of stereocontrol, the transition State which leads to formation of a syndiotactic diad is favored over the transition State which leads to formation of an isotactic diad by 5 kJ/mol. The former is favored because in the transition State leading to formation of an isotactic diad the growing chain is placed within the coordination sphere of the titanium atom, and interacts repulsively with the other ligands.

Secondary syndiotactic-specific propene insertion in the presence of homogeneous V-based catalysts

Abstract This paper shows, by terminating the polymerization through deuterolysis with CF 3 CH 2 OD, that the propyl end groups of syndiotactic polypropylene obtained in the presence of homogeneous catalysts, based on vanadium salts and alkylaluminium halides, such as VCl 4 –Al(C 2 H 5 ) 2 Cl–Anysole, arise, at least mostly, from termination, thus confirming the Coleman–Fox type of the statistical model of the propagation already proposed in previous papers. Direct evidence is also provided that secondary insertion of propene is the only syndiotactic specific polymerization step. In view of this, the early hypotheses concerning the structure of the active species promoting syndiotactic specific polymerization are critically reconsidered.