The cover picture shows an autostereogram representation of the X-ray crystal structure of the smallest [2]catenane yet isolated. The catenane is formed in one step in an eight-molecule condensation from commercially available reagents and is isolated by simple washing and filtration of the reaction mixture. More about this catenane and analogously accessible [2]catenanes with differing dynamic properties is reported by D. A. Leigh et al. in two consecutive communications on p. 1209 ff. To see the 3-D image you must stare through the picture so that your focal point is behind the page. Placing the picture behind a piece of glass and focusing on your reflection helps if you cannot see the picture straight away. The autostereogram image is 2 x 108 times larger than the molecule itself. Did you see the molecule depicted here? REVIEWS Con tentsStereo-and regioregularity, molecular weight and molecular weight distribution of polyolefins, and incorporation of comonomers can be controlled efficiently with ansa-metallocene catalysts. Catalysts of this type can also be used for the polymerization of cyclic olefins and the production of functionalized polyolefins. In contrast to classical heterogeneous Ziegler-Natta catalysis, metallocene-catalyzed olefin polymerization occurs on uniform metal centers having a defined coordination environment. Recent research in this area has provided insights into fundamental mechanisms as well as advances in practical applications of this class of catalysts. ContentsTwo catena-like interpenetrating enantiomorphic frameworks of condensed tetragonal antiprisms. RhBi,,,, form the basis of the crystal structure of RhBi,. The electron localization function (ELF) shows high localization of the valence electrons only on the periphery of the frameworks (lone pairs of Bi atoms), whereas the electrons inside the nets are delocalized. The periodic nodal surface (PNS) I,-Y** describes perfectly the hyperbolic organization of the entire RhBi, structure. It separates the regions with low electron localization inside the polyhedral nets from the hyperbolic layer of the lone pairs.The optical rotation of poly(propy1ene imine) dendrimers with 64 identical enantiomerically pure amino acid end groups tends to zero. This remarkable behavior results from the packing of the end groups in the highly dense, multiply hydrogenbonded surface, which gives rise to different frozen-in conformations, leading to an internal compensation of optical activity.The condensation of eight molecules in one step provides 1, the smallest [2]catenane to date, in 20 YO yield. The cavity in one of the identical macrocycles of the catenane has dimensions of 4 x 6 A. Electrophilic a-alkylation and nucleophilic 1,2-addition to glycol aldehyde 1 (P = protecting group) affords syn-l,2-amino alcohols 2 in good yields and with high selectivities (89 to > 99% de, 94 to 2 9 9 % ee). This method, in which 1 is first converted into a chiral hydrazone, provides a flexible, direct route to y-amino-P-hydroxycarboxylic acids such as statin 3, an...

Untersuchungen an neuen Metallocenkatalysatoren zur Polymerisation von α‐Olefinen haben gegenwärtig weitreichende Auswirkungen auf die Entwicklung neuer Materialien und auch auf unser Verständnis der grundlegenden Reaktionsmechanismen, die für das Wachstum von Polymerketten an einem Katalysatorzentrum und für deren Stereoregularität verantwortlich sind. Im Gegensatz zu heterogenen Ziegler‐Natta‐Katalysatoren erfolgt die Polymerisation mit einem homogenen Metallocenkatalysator im Prinzip an einheitlichen Metallzentren mit definierter Koordinationssphäre. Dies macht es möglich, die Struktur der Metallocenkomplexe mit den Eigenschaften des Polymers, beispielsweise Molekulargewicht, stereochemischer Mikrostruktur, Kristallisationsverhalten und mechanischen Eigenschaften, zu korrelieren. Mit homogenen Katalysatorsystemen können Regio‐ und Stereoregularitäten, Molekulargewichte, Molekulargewichte, Molekulargewichtsverteilungen und Einbauverhältnisse von Comonomeren wirksam kontrolliert werden. Diese Katalysatoren eröffnen neue Zugänge zur Homo‐ und Copolymerisation cyclischer Olefine, zur Cyclopolymerisation von Dienen und sogar zu funktionalisierten Polyolefinen und erweitern somit das Spektrum und die Vielseitigkeit technisch verfügbarer Polyolefin‐Materialien.

SYNOPSISPropene was polymerized a t 40°C and 2-bar propene in toluene using methylalumoxane (MAO) activated ra~-Me~Si(Benz[e]Indenyl)~Zr C12 (BI) a n d rac-Me2Si(2-Me-Ben~[e]Indenyl)~ZrCl~ (MBI). Catalyst BI/MAO polymerizes propene with high activity to afford low molecular weight polypropylene, whereas MBI/MAO is less active and produces high molecular weight polypropylene. Variation of reaction conditions such as propene concentration, temperature, concentration of catalyst components, and addition of hydrogen reveals that the lower molecular weight polypropylene produced with BI/MAO results from chain transfer to propene monomer following a 2,l-insertion. A large fraction of both metallocene catalyst systems is deactivated upon 2,l-insertion. Such dormant sites can be reactivated by H2-addition, which affords active metallocene hydrides. This effect of H2addition is reflected by a decreasing content of head-to-head enchainment and the formation of polypropylene with n-butyl end groups. Both catalysts show a strong dependence of activity on propene concentration that indicates a formal reaction order of 1.7 with respect to propene. MBI/MAO shows a much higher dependence of the activity on temperature than BI/MAO. At elevated temperatures, MBI/MAO polymerizes propene faster than BI/ MAO. 0 1995 John Wiley & Sons, Inc.

Poly(propy1ene)s with narrow molecular weight distributions were prepared with various methylaluminoxane-activated metallocene-based Ziegler-Natta catalysts to study the influence of randomly incorporated regio-and stereoirregularities on the crystallization behavior. As a function of the metallocene type and the polymerization temperature, the molecular weights varied between 11500 < M,, < 63 OOO, melting temperatures of annealed samples between 125 to 158 "C, and the corresponding degrees of crystallinity, as measured by wide-angle X-ray scattering, between 49 and 67%. While the virgin poly(propy1ene)s exhibited exclusively the amodification, annealing and melt crystallization favored the development of the y-modification. The microstructure analysis by I3C NMR spectroscopy revealed a linear correlation between the content of the y-modification and the average length of the isotactic segments.

Metallocene/borate catalysts, generated from zirconocene dimethyl compounds, L n ZrMe2, and anilinium borate, [HNMe2Ph]+[B(C6F5)4]-, were used to copolymerize 5-N,N-diisopropylamino-1-pentene with 1-hexene and 4-methyl-1-pentene. The selected zirconocenes, bis(pentamethylcyclopentadienyl)zirconium(IV) dimethyl (Cp*2ZrMe2) and rac-ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium(IV) dimethyl (rac-EB(THI)ZrMe2), provided atactic and isotactic materials, respectively. The isotactic polymers produced were amorphous or crystalline depending of the monomer. The calculated reactivity ratios for the copolymerization of 5-N,N-diisopropylamino-1-pentene with 1-hexene and metallocene rac-EB(THI)ZrMe2 indicate that this system approximates an ideal azeotropic copolymerization with r 1 = 1.11 and r 2 = 0.87. Estimates for the reactivity ratios for the copolymerization of 5-N,N-diisopropylamino-1-pentene with 4-methyl-1-pentene were r 1 = 3 and r 2 = 1 for rac-EB(THI)ZrMe2 and r 1 = 5 and r 2 = 0.5 for Cp*2ZrMe2. The polymerization of 1-hexene in the presence of the saturated 1-N,N-diisopropylaminopentane with rac-EB(THI)ZrMe2/borate was compared with analogous copolymerizations of 1-hexene/5-N,N-diisopropylamino-1-pentene. The aminopentene was more effective than the aminopentane in inhibiting the rate of total monomer conversion, implicating both intra- and intermolecular mechanisms for inhibition by the amine. Copolymers of 4-methyl-1-pentene/5-N,N-diisopropylamino-1-pentene produced with rac-ethylenebisindenyl zirconium(IV) dimethyl (rac-EBIZrMe2)/borate have higher decomposition temperatures with increasing amounts of aminopentene. This copolymer can be protonated with HCl to yield a methanol-soluble material.

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The polymerization kinetics of propene polymerization using metallocene/methylaluminoxane (MAO) homogeneous catalysts have been investigated to explore the role of donor/acceptor interactions and to enhance the catalyst productivities. In the case of the non‐stereospecific Cp2ZrCl2/MAO model system it has been demonstrated that, in addition to the well known irreversible deactivation, reversible deactivations, which are second order relative to the zirconium active site concentration, account for the decay of the polymerization rate. While MAO injection during polymerization enhances the polymerization rate, zirconocene addition deactivates the catalyst which can be reactivated by injecting additional MAO. A sequence of dynamic equilibria involving the formation of active cationic metallocene intermediates as well as inactive zirconocene species, e.g., zirconocene dimers, is proposed. Lewis base and Lewis acid additives have been added as probes to examine the role of such equilibria in the case of metallocene‐based catalyst systems such as MAO‐activated Cp2ZrCl2, racemic ethylenebisindenyl zirconium dichloride (EBIZrCl2), and racemic ethylenebis (4,5,6,7‐tetrahydroindenyl) zirconium dichloride (EBTHIZrCl2). While the conventional donors such as 2,6‐ditert.butyl‐4‐methylphenol (BHT) and 2,2,6,6‐tetramethylpiperidine (TMP) reduce catalyst productivities, even at very low donor/Al molar ratios, increasing propene concentration and addition of trimethylboroxine (TMB) substantially enhance catalyst productivities and affect molecular weights of the polypropylene produced with metallocene/MAO catalysts.

Propene and 1 -octene were copolymerized with the syndiospecific homogeneous metallocene catalyst Me,C(Cp)(Flu)ZrCl,/MAO. Large amounts of octene were incorporated randomly. While catalyst activity was not affected markedly by low octene content, molecular weight, crystallinity, Young's modulus, and glass transition temperature were reduced with increasing octene content. Blends of atactic oligopropene with syndiotactic polypropene and poly(propene-co-octene) were prepared from toluene solution and compared with a reactor blend prepared with a hybrid catalyst containing a mixture of syndiospecific Me,C(Cp)(Flu)ZrCl,/MAO and non-specific Cp,ZrCl,/MAO. Atactic oligopropene acted as plasticizer reducing Young's modulus and glass transition temperature of the blends. ZUSAMMENFASSUNG: Propen und 1 -0cten wurden mit dem syndiospezifischen Metallocen-Katalysator Me,C(Cp)(Flu)ZrCl,/MAO copolymerisiert. Es wurde ein hoher, statistischer Octeneinbau beobachtet . Niedrige Octenkonzentrationen beeinflufiten die Aktivitat des Katalysators nur wenig; Molekulargewicht, Kristallinitlt, E-Modul und Glastemperatur wurden dagegen mit ansteigendem Octengehalt erniedrigt. Blends aus ataktischem Oligopropen und syndiotaktischem Polypropen bzw. Poly(propen-co-octen) wurden aus einer Toluol-Liisung hergestellt. Diese Liisungsblends wurden mit einem Reaktorblend verglichen, der mit einem Hybrid-Katalysator bestehend aus einer Mischung von syndiospezifischem Me,C(Cp)(Flu)ZrCl,/MAO und unspezifischem Cp,ZrCl,/ MA0 hergestellt wurde. Das ataktische Oligopropen wirkte als Weichmacher, der E-Modul und Glastemperatur der Blends erniedrigte. aspific cp2Zrc1, syndiospecific M~$(Cp)(Flu)zrCl, ExperimentalThe polymerization procedure has been described previou~ly'~~ 20. All catalyst components, toluene and monomers were handled and stored under argon. The metallocene was dissolved in a dilute MA0 solution (50 mmol/L A1 in toluene, prepared from a 10 wt.-Vo solution of MA0 in toluene from Witco GmbH with a, = 800 g/mol) and injected into a thermostated mixture of toluene/MAO/octene/propene