Fabrizio Forlini

Vinylic Polymerization of Norbornene by Late Transition Metal‐Based Catalysis

The polymerization of norbornene with two new families of late transition metal-based catalysts derived from: (i) Ni and Pd complexes bearing bulky diammine ligands, with the general formula [ArN=C(R)-C(R)=NAr]MeX 2 and (ii) Fe and Co complexes bearing bulky arylimine ligands with the general formula [(2,6-ArN=C(Me)) 2 C 5 H 3 N]MX 2 is reported. New Co-based complexes have been tested as well. A prevailingly vinylic, amorphous polymer was obtained with all the catalysts tested, whose solubility in organic solvents is substantially dependent on the molucular weight. Polymerization activity greatly varies from one catalyst to another and depends rather on the metal than on the ligand. Solvent polarity and temperature greatly affect the polymerization yields. Besides a dramatic reduction of molecular weights, the addition of small amounts of 1-hexene produces a noticeable increase in the catalytic activity. NMR analysis shows that in all cases a certain percentage of ROMP polymer is present and that, in general, the variations in polymerization conditions, which produce an increase in activity, simultaneously affect a reduction of the ROMP percentage.

13C NMR studies of zirconocene-catalyzed propylene/1-hexene copolymers: in-depth investigation of the effect of solvent polarity

Full Paper: Propylene/1-hexene copolymers were prepared with the isospecific Et(Ind) 2 ZrCl 2 /MAO catalyst by using a wide range of solvents. In the order of increasing dielectric constant (e) they are: heptane (1.92), methylcyclohexane (2.02), toulene (2.38), chlorobenzene (CB) (5.68), methylene chloride (8.93) and o-dichlorobenzene (o-DCB) (10.12). The effect of solvent structure and polarity on yield, composition, and microstructure of copolymers was investigated. The data show that solvent polarity, which has an enormous effect on the activity, has a negligible effect on the copolymer composition. Very close 1-hexene contents are obtained with all the solvents used except for toluene, in which the comonomer content is remarkably lower than that in the other solvents for all compositions. Detailed 13 C NMR analysis of the copolymers obtained was performed: the reactivity ratios obtained by statistical elaboration of the experimental values of triad and tetrad comonomer compositions confirm the actual lower accessibility of the active site to 1-hexene in toluene than in the other solvents. This fact can be explained by the competition of the nucleophilic toluene with 1-hexene in the coordination to the active site.

13C NMR Study of the Effect of Coordinating Solvents on Zirconocene-Catalyzed Propene/1-Hexene Copolymerization

The solvent effect observed in propene/1- hexene copolymerizations performed with the isospecific catalyst rac-Et(Ind) 2 ZrCl 2 /MAO is studied, A range of solvents with increasing donor character and steric hindrance has been tested, and their effect on copolymer yield, composition, and microstructure has been thoroughly analyzed. Our results demonstrate that the solvent can have a significant influence on the comonomer reactivities, even though the solvent polarity is not the relevant factor. At the same comonomer compositions in solution, polymerizations carried out in coordinating solvents (e.g., aromatic solvents), lead to the formation of products with considerably decreased content of 1-hexene. The reduced incorporation of the higher a-olefin is explained in terms of competition between the nucleophilic medium and the olefin monomer for coordination to the active polymerization site. These results give us valuable information regarding the mechanism of polymerization at the active centers.

Microwave-assisted chemical-free hydrolysis of wool keratin

Wool fibers were submitted to “green hydrolysis” with superheated water in a microwave reactor, in view of the potential exploitation of keratin-based industrial and stock-farming wastes. The liquid fraction was separated by filtration from the solid fraction, which consists mainly of small fragments of wool fibers and other insoluble protein aggregates. The liquid fraction contains free amino acids, peptides and low molecular weight proteins, with a small amount of cystine and lanthionine, and has a different secondary structure when compared with keratins extracted from wool via reductive or oxidative methods. Cleavage of the cystine disulfide bonds without the use of harmful, often toxic, reductive or oxidative agents allows the extraction of protein material from keratin wastes, offering the possibility of larger exploitation and valorization.

Unbridged bicyclic cyclopentadienyl zirconocene complexes: Their possible application as fluxional catalysts in propene polymerization

Three series of novel unbridged bicyclic cyclopentadienyl zirconium complexes, containing six-, seven- and eight-membered saturated rings fused to the cyclopentadienyl unit and bearing different substituents (-H, -CH 3 , -Ph) in position 2, were used in combination with methylalyminoxane (MAO) for the polymerization of propene at different temperatures. All the catalysts have been shown to be active, the lowest activity being observed with the eight-membered ring systems. With most catalysts the polymers obtained are fully atactic when the polymerizations are conducted at 30°C, and become partially isotactic at lower temperatures. The 13 CNMR analysis shows that in one case, that is, when the seven-membered ring is associated with the phenyl substituent in position 2, a preponderance of the isotactic heptad in the methyl region of the 13 C NMR spectrum is observed, which allows us to hypothesize the formation of stereoblocks.

Mechanistic Aspects of Olefin Polymerization with Metallocene Catalysts: Evidence from NMR Investigations

Isotopically 13C enriched Cp2M13CH3X metallocenes (M = Ti, Zr; X = 13CH3 , Cl) have been used as a probe for the reactivity of metallocenemethylaluminoxane catalysts for olefin polymerization. 1H and 13C NMR studies of the reactions between metallocenes and Lewis acids such as methylaluminoxane (MAO), AlMe3 , and the well-defined B(C6F5)3 have been performed in order to clarify the role of MAO in the activation of metallocenes. The isotopic enrichment has permitted us to study these Systems in conditions as close as possible to usual polymerization conditions. Evidence of the formation in Solution of “cationic” metallocene species having [X MAO] as counterion has been provided. In situ-polymerization of 13C enriched ethylene permitted us to gain an insight into the role of the observed products in the catalytic activity and to make the first direct Observation of methylenes of polymeryls of Cp2Zr-polymeryl species. A survey of these results is presented. Recent results of the study of the reactivity of two s-alkyl substituted aluminoxanes with two metallocenes Cp2ZrCl2 and Cp2 *ZrCl2 are also reported. It was found that the Cp2 *Zr-i- BuCl and Cp2 *Zr-i-OctCl stability is greater than that of Cp2Zr-i-BuCl and Cp2Zr-i-OctCl. The polymerization activities exhibited by different cocatalysts could be explained.

In-depth Investigation of Unsaturated Chain-end Groups: a Tool for Understanding Hydrogen Activation Mechanism in Zirconocene Catalysed Propene Polymerization

Detailed NMR analysis of the microstructure of polypropenes prepared using a range of methylaluminoxane-activated zirconocene catalysts, with and without hydrogen present, have shown that chain transfer to hydrogen by the hindered metal polymeryls which result from secondary monomer insertion cannot be the only mechanism for the well known hydrogen activation effect. It is proposed that another mechanism which may give rise to the activation is that hydrogen should renew catalytic sites which have reacted with unsaturated chain terminal groups in the polymerization mixture to form inert species (such as metal allylics or other sterically hindered sites). Propene was polymerized in solution at partial pressures from 0.4 to 1.1 bar and temperatures ranging from 30°C to 100°C, using a range of three methylaluminoxane-activated zirconocene catalysts. It was found that of the several types of unsaturated terminal group observed using NMR spectroscopy, only vinylidene, 2-butenyl and 4-butenyl are actually formed during polymerization reactions conducted at less than 60°C. Preliminary investigations have shown that the activity of the zirconocene based catalysts is lessened on addition of low molecular weight olefins as models for unsaturated chain-end groups.