Luciano Noristi’s scientific contributions

Cross polarization/magic angle spinning 13C NMR has been applied to the investigation of some solid organometallic compounds of the general formulae TiCl4 · n RCOOR′, MgCl2 · n RCOOR′ (RCOOR′ = p-CH3C6H4COOCH3 and C6H5COOC2H5, n = 1; 2; and 0,15). Compounds with n = 1 and 2 show narrow resonance lines indicative of crystalline regular structures while for MgCl2 · 0,15 RCOOR′ a disordered structure is found with the organic ligands strongly bonded to the surface of MgCl2 crystallites. For n = 2 splitting into a doublet is found for some resonances due to the spatial non-equivalence of the two ligands induced by crystalline packing. A shift has been found for some resonances in going from Mg to Ti complexes which reflects the different Lewis acidity of the two metal atoms.

A study on a new generation of MgCl2-supported Ziegler-Natta catalyst, which contain dialkylpropane diether as internal donors and have the property of producing highly active and stereospecific catalyst without any external donor, has been performed. The behavior of three different diethers (2,2-diisobutyl-1,3-dimethoxypropane, 2,2-dicyclopentyl-1,3-dimethoxypropane, and 2-ethyl-2-butyl-1,3-dimethoxypropane) as internal donors is compared with the behavior of the same diethers when they are uaed as external donors with a catalyst containing diisobutyl phthalate as internal donor. The results obtained show that the same isospecific centers are produced when the same diether donor is present as an internal donor or replace diisobutyl phthalate during polymerization when it is added as an external donor. The sum of the present results and of the previously obtained findings on traditional supported Ziegler-Natta catalysts has enabled us to reach some conclusions on the role and reciprocal behavior of internal and external donors.

Three pairs of alkoxysilanes, (a) Me(EtO)3Si, Ph(EtO)3Si, (b) Ph2(MeO)2Si, Ph2(EtO)2Si, (c) Ph2(EtO)2Si, Ph(EtO)3Si, differing only in one parameter, have been used as external donors with the catalysts MgCl2/TiCl4 and MgCl2/DIBP/TlCl4 (DIBP = diisobutyl phthalate), using selectively C-13-enriched AlEt3 as cocatalyst. The heptane-insoluble fractions of all the samples have been characterized by C-13 NMR analysis and gel permeation chromatography. The catalyst chemical modifications due to the external donors have been studied by GC analysis of the base content of the solid catalyst under the polymerization conditions. On the basis of all these data we can conclude that (i) all the alkoxy silane donors used, even those that do not produce any evident increase of isotactic productivity, interact with the isospecific centers making them more stereospecific and more active and (ii) it is possible to modulate the activation of the isospecific centers and the poisoning of the nonstereospecific ones by varying the number and the relative bulkiness of both hydrocarbon and alkoxy substituents. The mechanism of the activation effect is discussed.

Three different external donors, ethyl benzoate (EB), 2,2,4,4-tetramethylpiperidine (TMPip), and phenyltriethoxysilane (PTES), have been used with catalysts MgCl2/EB/TiCl4 and MgCl2/BEHP/TiCl4 (bis(2-ethylhexyl) phthalate (BEHP)), using selectively C-13 enriched AlEt3 as cocatalyst. The heptane-insoluble-octane-soluble and octane-insoluble fractions of all the samples have been characterized by the gel permeation chromatography (GPC) and C-13 NMR analysis. The stereochemical and GPC data have been compared with the results of the study of the exchange between internal and external donors by GC analysis of the solid catalyst. On the basis of all these data, we can conclude that (i) the effectiveness of a catalytic system depends on the choice of the pair of internal and external donors rather than on the single internal and external donor, and (ii) while the amount of activation clearly depends on the external donor, the internal donor affects the stereoregularity.

In this paper we will approach the study of the machanism of the Lewis base activation in high yield supported Ziegler-Natta catalysts by three different experimental routes: i) stereochemical study, by 13C NMR analysis, of samples of polypropene prepared using selectively 13C enriched AlEt3 as cocatalyst; ii) GPC characterization of the most isotactic fractions; iii) study of the exchange between internal and external bases by GC analysis of the base content of the solid catalyst. Despite the well known complexity of the problem, on the basis of all these data it is possible to single out some general trends of the Lewis base activation and these trends depend more on the characteristics of the specific pair of internal and external bases than on those of the single internal or external base.