The stereosequence distribution of the “atactic” and “isotactic” fractions of a polypropylene sample made with a MgCl2-supported catalyst was determined by means of high-resolution 13C NMR and analyzed in terms of statistical models of increasing sophistication. Two-site models, including the one normally used for the interpretation of “routine” 13C NMR data at pentad level, were shown to be inconsistent with the much finer high-resolution data. A good agreement between experimental and calculated distributions could be obtained only in terms of a three-site model, describing each fraction as a mixture of highly isotactic, weakly isotactic (“isotactoid”) and syndiotactic sequences. According to such model, the two fractions comprise the same three building blocks (the configuration of the three different types of stereosequences being almost invariant) and differ merely in their relative amounts (which indicates a stereoblock nature). The correlations with the physical properties of the materials and the implications on the nature of the catalytic species are also briefly discussed.

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A new class of isospecific and highly regiospecific C 2 - symmetric ansa-zirconocenes, characterized by a bisindenyl ansa ligand with bulky substituents in the 3 position of indene and a single carbon bridge is disclosed:  variation of the size of the substituent in C(3) has a strong effect on the extent of chain transfer and isospecificity in propene polymerization. In fact, while rac-[Me2C(1-indenyl)2]ZrCl2 produces low molecular weight and moderately isotactic polypropene (iPP) also containing some regioirregularities (M̄ n = 6500, mmmm ca. 81% and 2,1tot = 0.4% at 50 °C in liquid monomer), rac-[Me2C(3-tert-butyl-1-indenyl)2]ZrCl2 produces iPP with molecular weights between 25 000 (T p = 70 °C) and 410 000 (T p = 20 °C) and a fairly high isotacticity (mmmm ca. 95% at 50 °C), with no detectable 2,1 units. The influence of polymerization temperature on the catalyst performance has been investigated by polymerizing liquid propene in the temperature range of 20−70 °C:  the experimental ΔΔE ⧧ values for enantioface selectivity have been estimated for two members of the new class (rac-[Me2C(3-tert-butyl-1-indenyl)2]ZrCl2 ΔΔE ⧧ enant = 4.6 kcal/mol; rac-[Me2C(3-(trimethylsilyl)-1-indenyl)2]ZrCl2 ΔΔE ⧧ enant = 2.6 kcal/mol). For comparison, Brintzinger's moderately isospecific, benchmark catalyst rac-[ethylene(1-indenyl)2]ZrCl2 (ΔΔE ⧧ enant = 3.3 kcal/mol), the single carbon bridged, unsubstituted rac-[Me2C(1-indenyl)2]ZrCl2 (ΔΔE ⧧ enant = 2.8 kcal/mol), and the C 2-symmetric, practically aspecific, rac-[ethylene(3-methyl-1-indenyl)2]ZrCl2 (ΔΔE ⧧ enant = 1.9 kcal/mol) are also reported. The molecular structures of rac-[Me2C(3-tert-butyl-1-indenyl)2]ZrCl2 and rac-[Me2C(3-(trimethylsilyl)-1-indenyl)2]ZrCl2 have been determined.

The effects of regiochemical and stereochemical errors on the kinetic course of isotactic propene polyinsertion promoted by two typical homogeneous metallocene catalysts are analyzed in detail. It is shown, in particular, that occasional regioirregular 2,1 insertions not only slow down chain propagation to a substantial extent-as already reported in a preliminary communication-but also practically inhibit chain transfer to the monomer and to the aluminum alkyl cocatalyst. Active centers bearing a growing chain with a 2,1 last-inserted propene unit are therefore trapped in a "dormant" state, in which the only feasible alternative to the formation of a sterically demanding head-to-head enchainment, at least at sufficiently high reaction temperatures, is 2,1 to 1,3 isomerization. Experimental data suggesting that chain propagation is hindered after a stereoirregular monomer insertion are also discussed.

Investigation of the effects of hydrogen and different external donors on the stereoregularity and chain-end distribution of polypropylene prepared using MgCl,/TiCl,/phthalate ester-AIEt,-alkoxysilane catalyst systems has not only confirmed the importance of regiospecificity in relation to hydrogen activation but has also indicated a significant effect of stereospecificity. Alkoxysilanes giving high isospecificity also give high molecular weight polypropylene. Polymer stereoregularity also increases with increasing hydrogen concentration in polymerization. These effects indicate that not only regioirregular but also stereoirregular insertion slows down chain propagation. In each case the probability of chain transfer with hydrogen is higher than that following a stereoregular insertion, and in each case chain transfer with hydrogen leads to the regeneration of isospecific propagation.

I3C NMR analysis of the chain-end distribution of poly(propy1enes) prepared using the highly active catalyst system MgCI,/TiCl,/diether -AlEt, has revealed particularly high proportions of butyl chain-ends in polymers prepared at relatively low hydrogen pressures. This indicates that the high sensitivity of this type of catalyst to hydrogen, both with respect to catalyst activity and polymer molecular weight, can largely be ascribed to chain transfer following regioirregular (2,l-) insertion, such an insertion leading to a species having low activity in chain propagation. Isotactic stereoregularity increases with increasing hydrogen pressure, indicating that a stereoirregular insertion may also slow down the chain propagation, again leading to chain transfer and resulting in the conversion of a potential chain defect into an isobutyl chain-end. Analysis of highly isotactic polymer fractions isolated via temperature rising elution fractionation revealed the presence of both butyl and isobutyl chain-ends, indicating that even the most highly stereospecific sites in MgC1,-supported catalysts are not totally regiospecific.

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I3C NMR analysis of relatively low-molecular-weight polypropylenes prepared using the catalyst system MgCI,/TiCl,/phthalate ester-AIEt,-external Lewis base has revealed the presence of up to around 20% butyl (n-Bu) chain ends, indicative of regioirregular (2,l-) monomer insertion followed by chain transfer with hydrogen. The effect of hydrogen on catalyst activity depends on the nature of the external donor, and the results indicate that at least part of the observed increase in catalyst activity in the presence of hydrogen can be ascribed to the regeneration of active species via chain transfer at dormant (2,l -inserted) sites.