No abstract

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.

The presence of "free" trimethylaluminum (TMA) in methylalumoxane (MAO) solutions can be highly detrimental to the performance of metallocene and "post-metallocene" olefin polymerization catalysts. The most used strategy to remove "free" TMA is to evaporate MAO solutions to dryness, until a free-flowing white powder ("solid MAO") is left. This procedure is tedious and potentially hazardous, because in some cases the distillate is a concentrated hydrocarbon solution of TMA. Moreover, "solid MAO" is poorly soluble in common polymerization media, and once in solution it can regenerate TMA to some extent. This communication reports on a facile alternative, which consists in the controlled addition of a sterically hindered phenol, such as 2,6-di-tert-butylphenol, effectively trapping "free" TMA. We show here that 2,6-di-tert-butylphenol/MAO solutions activate equally well the dichloro-precursors of well-known zirconocene and bis(phenoxyimine)Ti catalysts, and that their use in propene polymerization results in a substantially higher productivity, polymer stereoregularity, and/or average molecular mass compared with activation by MAO alone.

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.

Classical" MgCl 2 -supported Ziegler-Natta catalysts (ZNCs) continue to dominate the industrial production of isotactic polypropylene. There is a growing awareness of the inherent competitive edge of these low-cost systems over single-center (primarily metallocene) catalysts and of the potential for further improvement, particularly if deeper insight into the structure of the catalytic surfaces and the mechanisms of their modification by means of electron donors can be achieved. In the framework of a project ultimately aiming at the implementation of ZNCs with known and controlled surface structures, we are revisiting this whole area by using a combination of advanced computational (periodic DFT) and spectroscopic (high-resolution magicangle-spinning 1 H NMR spectroscopy) tools. In this article, we report on the neat MgCl 2 matrix and on model MgCl 2 /electron-donor adducts. The results indicate that the (104) surface, with five-coordinate Mg cations, is the dominant lateral termination in well-formed large crystals, as well as in highly activated MgCl 2 samples prepared by ball-milling. In the latter case, a minor fraction of surface Mg sites with a higher extent of coordinative unsaturation [e.g., four-coordinate Mg cations on (110) edges and/or at crystal corners or other defective locations] also appear to be present. RMe 2 Si(OMe) (R ) octadecyl) binds to both types of Mg sites, albeit with different strengths resulting in different mobilities. The less-electron-donating RMeSi-(OMe) 2 , in contrast, binds to the more unsaturated Mg sites only. The approach described herein is currently being extended to MgCl 2 /TiCl n systems, as well as to their adducts with internal and external donors of different natures, strengths, and steric demands.

Pyridyl-amido catalysts have emerged recently with great promise for olefin polymerization. Insights into the activation chemistry are presented in an initial attempt to understand the polymerization mechanisms of these important catalysts. The activation of C1-symmetric arylcyclometallated hafnium pyridyl-amido precatalysts, denoted Me2Hf{N(-),N,C(-)} (1, aryl = naphthyl; 2, aryl = phenyl), with both Lewis (B(C6F5)3 and [CPh3][B(C6F5)4]) and Brønsted ([HNR3][B(C6F5)4]) acids is investigated. Reactions of 1 with B(C6F5)3 lead to abstraction of a methyl group and formation of a single inner-sphere diastereoisomeric ion pair [MeHf{N(-),N,C(-)}][MeB(C6F5)3] (3). A 1:1 mixture of the two possible outer-sphere diastereoisomeric ion pairs [MeHf{N(-),N,C(-)}][B(C6F5)4] (4) is obtained when [CPh3][B(C6F5)4] is used. [HNR3][B(C6F5)4] selectively protonates the aryl arm of the tridentate ligand in both precatalysts 1 and 2. A remarkably stable [Me2Hf{N(-),N,C2}][B(C6F5)4] (5) outer-sphere ion pair is formed when the naphthyl substituent is present. The stability is attributed to a hafnium/eta(2)-naphthyl interaction and the release of an eclipsing H-H interaction between naphthyl and pyridine moieties, as evidenced through extensive NMR studies, X-ray single crystal investigation and DFT calculations. When the aryl substituent is phenyl, [Me2Hf{N(-),N,C2}][B(C6F5)4] (10) is originally obtained from protonation of 2, but this species rapidly undergoes remetalation, methane evolution, and amine coordination, giving a diastereomeric mixture of [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (11). This species transforms over time into the trianionic-ligated [Hf{N(-),C(-),N,C(-)}NR3][B(C6F5)4] (12) through activation of a C-H bond of an amido-isopropyl group. In contrast, ion pair 5 does not spontaneously undergo remetalation of the naphthyl moiety; it reacts with NMe2Ph leading to [MeHf{N(-),N}NMe2C6H4][B(C6F5)4] (7) through ortho-metalation of the aniline. Ion pair 7 successively undergoes a complex transformation ultimately leading to [Hf{N(-),C(-),N,C(-)}NMe2Ph][B(C6F5)4] (8), strictly analogous to 12. The reaction of 5 with aliphatic amines leads to the formation of a single diastereomeric ion pair [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (9). These differences in activation chemistry are manifested in the polymerization characteristics of these different precatalyst/cocatalyst combinations. Relatively long induction times are observed for propene polymerizations with the naphthyl precatalyst 1 activated with [HNMe3Ph][B(C6F5)4]. However, no induction time is present when 1 is activated with Lewis acids. Similarly, precatalyst 2 shows no induction period with either Lewis or Brønsted acids. Correlation of the solution behavior of these ion pairs and the polymerization characteristics of these various species provides a basis for an initial picture of the polymerization mechanism of these important catalyst systems.

13C NMR spectroscopy is the main source of information on the stereochemistry of Ziegler−Natta and related transition metal catalyzed propene polymerizations. In simple cases, like those of polypropylenes formed under pure enantiomorphic-site or chain-end control, the origin of the stereoselectivity can be easily recognized from the steric pentad distribution obtained from routine 13C NMR spectra. On the other hand, the variety of innovative polymers that can now be prepared with “high-yield” heterogeneous and metallocene-based homogeneous catalysts under hybrid, multiple, or oscillating stereocontrol represent very complex systems, which are beyond the possibilities of configurational analysis by routine 13C NMR. In such cases, high-field 13C NMR can be highly advantageous. Indeed, in this paper we show that from the methyl and methylene regions of 150 MHz 13C NMR spectra of polypropylenes of various tacticities, the stereosequence distribution can be determined at a much finer level of detail, so as to obtain an adequate experimental basis for the investigation of the many complicated mechanisms of stereocontrol presently encountered in Ziegler−Natta catalysis.

An influence of monomer concentration on the stereospecificity of group 4 ansa-metallocene catalysts for the polymerization of 1-alkenes1 2has been pointed out for Cs-symmetric syndiotacticspecific catalysts. [1][2][3]