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Three different external donors, ethyl benzoate (EB), 2,2,4,4-tetramethylpiperidine (TMPip), and phenyltriethoxysilane (PTES), have been used with catalysts MgCh/EB/TiCLt and MgCh/BEHP/TiClj (bis(2-ethylhexyl) phthalate (BEHP)), using selectively 13C enriched AlEts as cocatalyst. The heptaneinsoluble-octane-soluble and octane-insoluble fractions of all the samples have been characterized by the gel permeation chromatography (GPC) and 13C 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.

A polynuclear Nd-AI bimetallic complex in single crystalline form, which is active for diene polymerization, was isolated and its detailed structure was determine9 by X-ray diffraction technique. The results indicate that the crystal is triclinic, space group P1; the lattice parameters are: a = 15,195 (6), b = 15,263 (9, c = 13,749 (4) A; a = 90,Ol (2), / 3 = 95,12 (3), y = 82,65 (3)"; V = 3 149,5 A'; Z = 1 . The coordination number of the Nd atom is seven and the configuration is monocap-prism. Nd atoms can be divided into three groups according to their coordination environment.

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The 13C NMR analysis of the initiation step is applied to the study of the effect of the Lewis bases on high-yield supported Ziegler-Natta catalysts for isotactic propene polymerization. Catalytic systems containing ethyl benzoate (EB) and bis(2-ethylhexyl) phthalate (BEHP) as internal bases are studied in the absence of external base and in the presence of 2,2,6,6-tetramethylpiperidine (TMPip) and phenyltriethoxysilane (PTS), as external bases. The fact that the increase of isotactic productivity here observed, due to the presence of either internal or the external base, is accompanied by a change of the first step stereoregularity gives evidence of the direct participation of the Lewis base on the formation of the isospecific centers.