• niobium-based catalysts
• olefins polymerization
• x-ray strucutre

This thesis describes the synthesis and characterization of niobium-based metal complexes and their usefulness as catalysts for the polymerization of ethylene, norbornene,pinene and vinyl monomers.
The polymerization of above described monomers was studied using niobium complexes based on N,N-dialkylcarbamates, mixed chloro alkoxides, simple oxygen-containing ligands and the starting material for the preparation of such derivatives, i.e. NbCl5.
Niobium N,N-dialkylcarbamates Nb(O2CNR2)n [n = 5, R = Me, 1, Et, 2; n = 4, R = Et, 3; n = 3, R = Et, 4], exhibited high activity for polymerization of ethylene when activated with TMA-depleted MAO and 1,2-dichloroethane as activator working at 1 bar ethylene pressure. Ta(O2CNEt2)5, 5, and NbCl5 were tested by comparison and low activity for ethylene polymerization was ascertained. The catalytic activities of the different precursors increase along the series 2 > 1 > 3 > 4 > NbCl5 > 5. Poly(ethylene)s produced by niobium N,N-dialkylcarbamates complexes were linear and exhibited high molecular weights and broad molecular weight distributions. The 1:1 reaction of 2 with Al2Et3Cl3 proceeds via ligand exchange giving the X-Ray characterized mixed alkyl-carbamato derivative [Al3(Et)2(O2CNEt2)6][Cl] (6) and, presumably, niobium species in oxidation state < 5.
Ring-opening metathesis polymerization (ROMP) of norbornene (NBE) catalyzed by 1-4 was studied in the presence of methylaluminoxane (MAO) as a co-catalyst. These novel catalytic systems resulted very active in chlorobenzene: 2 in the presence of MAO catalyzes the ROMP of NBE with the highest activity never reported up to now for niobium catalysts. Analogous to what found by ethylene polymerization, 5 and NbCl5 was also tested for NBE polymerization and trace amount of polynorbornene (PNBE) was obtained. A new FT-IR method for the swift determination of the cis/trans content of the polymer is presented.
This high productivity on ethylene and NBE polymerization appears particularly attractive considering that these precursors(1-4) are rather cheap and easy to synthesize and to handle.
In this context, it was found that these niobium N,N dialkylcarbamates (1-2) were almost inactive in β-pinene, styrene, butylvinylethers, cyclopentene and cyclooctene polymerization.
On the other hand easily accessible NbCl5 works as an excellent catalyst for the polymerization of β-pinene in the absence of co-catalyst. β-pinene was polymerized with NbCl5 in toluene at room temperature. High conversion and low molecular weights, Mn = 1200-1600, were obtained under these conditions. The effect of solvent polarity, temperature, catalyst concentration, time and co-catalysts on conversion and molecular weights was investigated. 1H NMR analyses have showed that β-pinene polymer is structurally identical to that formed by conventional cationic Lewis acid initiators.
The dinuclear niobium compounds [NbCln(OR)(5n)]2 and the mononuclear NbCl4[OCH2CH(R′)OR] were tested in ethylene polymerization. Variable polymerization conditions were considered, and the use of aluminium co-catalysts was required in every case. Complex 13, whose X-Ray structure is described here for the first time, exhibited the highest activity [151 kgpolymer×molNb–1×h–1×bar–1] ever reported for a niobium catalyst in ethylene polymerization.The obtained polymers resulted linear and were characterized by FT-IR, NMR, GPC and DSC analyses.
Simple oxygen-containing derivatives of niobium pentachloride were tested as catalytic precursors in ethylene polymerization reaction by using DMAO as co-catalyst. Highly linear polyethylene was obtained in every cases. Mostly niobium pentachloride compounds showed catalytic activities in the range 89÷116 kgpolymer×molNb–1×h–1×bar–1. The dinuclear compounds provided lower performance, whereas the tantalum compounds resulted not active. Relatively high molecular weight polymer was achieved by means of the ester complex, the alkoxyether, the oxo-bridged and the phosphine, i.e. specific combinations of structure and ligand. Fluorine-containing derivatives are less reactive with respect to those obtained for analogous chlorine-containing species.
The polymerization of styrene and iso (i) and normal (n) butylvinyl ethers (BVEs) by means of NbCl5 or Nb(V) N,N-dialkylcarbamato (2) as catalytic precursors, in the presence of methylaluminoxane as co-catalyst, was investigated. The results concerning styrene and BVEs are in sharp contrast with those regarding norbornene and ethylene polymerization. In the former cases, (2) is not effective provide while NbCl5 provides high activity. The influence of the reaction parameters on the catalytic activity and the characteristics of the resulting polymers was discussed.