The new C 1-symmetric complexes rac-[1-(9-η5-fluorenyl)-2-(2-methylbenz[e]-1-η5-indenyl)ethane]zirconium dichloride (14a), rac-[1-(9-η5-fluorenyl)-2-(4,5-cyclohexa-2-methyl-1-η5-indenyl)ethane]zirconium dichloride (14b), and rac-[1-(9-η5-fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-η5-indenyl)ethane]zirconium dichloride (15) were prepared in up to 93% yield. These compounds, activated with methyl aluminoxane, exhibit high active propene polymerization rates which remain constant over hours, even at elevated polymerization temperatures of 50 and 70 °C. The two different coordination sites of these “dual-side” catalysts lead to isotactic polypropenes with variable amounts of stereoerrors, depending on the monomer concentration. The 2-methyl substituent of the indenyl ligands results, at the same time, in significantly increased molecular weights of the polymer products (up to 2.3 × 105 g mol-1), the bulk properties of which can be adjusted from flexible, semicrystalline thermoplastic to excellent thermoplastic elastic.

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It is well known that the mechanical properties of highly isotactic polypropene depend on the crystalline structure of the material. Up to now, the polymer properties were modified by varying either the processing conditions—e.g., extrusion, annealing etc.—or the composition—e.g., blending, nucleation. Highly active ethylene bridged C1-symmetric zirconocene catalysts can be used to design the material properties by varying the crystallinity of thermoplastic (elastic) polypropenes. With the new unsymmetric catalyst rac−dichloro[1−(9−fluorenyl)−2−(R,S)−(5,6−cyclopenta−2−methylinden−1−η5−yl)ethane]zirconium (IV)/MAO (1) polypropenes of variable isotacticities are accessible. The variable and thus defined use of two different sides of the catalytically active species allows the placement of single, cumulated and consecutive [rr]-triads within an isotactic polymer chain. The formation of these stereoerrors is adjusted in an arbitrary manner by the polymerization conditions temperature and monomer concentration. The mechanical properties of polypropenes produced with 1/MAO vary from tough thermoplastic to excellent elastic. Stress—strain behavior of these physically crosslinked elastomers can be designed similar to the ones of polyisoprene or partly crosslinked natural rubber.

Synthesis, Characterization and Polymerization Behavior ofReceived November 13, 1996 Keywords: Epoxides / Fluorenyl ligands / unm-Zirconocenes, non-symmetrical / Polymers / Polymerizations Epoxystyrene (la) and oxirane (lb) were converted into their corresponding alcohols, 2a-c, by treatment with either fluorenyl-or indenyllithium. These alcohols were then further converted into their trifluoromethanesulfonate derivatives, 3a-c. Subsequent reaction of the triflates with fluorenyl-, indolyl-, or tetraphenylcyclopentadienyllithium resulted in formation of 2-(9-fluorenyl) and 6a were activated with methylalumoxane (MAO) and used for propene polymerization. The solid state structures of 4b, 4c, and 5a are reported.

On comparing this book with the previous edition which appeared 15 years earlier, it is seen that only one complete new chapter has been added, namely that on transition metal reagents. Also the selection of examples of natural products syntheses in the last chapter has been brought up to date. and the sections on silicon compounds, peptide syntheses, and the preparation of "preformed enolates" using lithiuin diisopropylamide have been considerably enlarged. By far the greater part of the material has simply been carried over from the previous edition. at least as regards information content, and sometimes even word-for-word. This hardly seems to d o justice to the rapid developments that have occurred in synthetic methods in the last two decades. Thus. in common with many other general textbooks. this book mainly provides a survey of classical preparative organic chetnist ry. structured according to reaction mechanisms. On the other hand, the princ,ipli~s of organic synthesis, announced in the title, arc scarcely covered. It is only in the accounts of nine examples of natural products syntheses that the redder learns something about the rational planning of a synthesis, at least in rudimentary form. The inadequacy of the treatment is illustrated by the fact that the concept of retrosynthetic analysis does not appear until Chapter 71 (pp. 679-681), where it receives a mere two pages.The book cannot be said to fully reflect the current state of research. This can be seen. for example. in the treatment of the synthesis of enantiomerically pure compounds. The Sharpless-Katsuki epoxidation merits less than twenty lines of text (pp. 589 --590). and the only method given for enantioselcctive reduction of "symmetrical" (i.e. achiral) ketones is the reaction with pinanyl-9-BBN (pp. 653-654). The diastereoselective (not "enantioselective") alkylation of Davies-Liebeskind enolates IS included (p. 577), but other methods such as the Evans aldol method and the asymmetric hydrogenation using Wilkinson catalysts are not mentioned. revealing some lack of balance in the choice of topics. Another very significant shortcoming i s that. apart from Chapter 72. there itre no references to original publications, and even the suggestions for further reading that were included in the second edition have now been dropped.The book is unlikely to offer new perspectives to synthetic chemists in universi-ties or industry, but seems more suitable for providing students starting a degree course in chemistry with a readily understandable survey of the repertoire of (mainly classical) synthetic methods.Munfred Bruun

W. BURCKHABDT et al. : Structure and Molar Refraction of Halides 97 phys. stat. sol. (a) 85, 97 (1984) Subject classification: 4 and 20.1; 22.5; 22.5.2 Sektion Cisernie (a) and Hektwn Physa'k (b) der Friedrich-&hiller-Universitit Jenul)The wavelength dependence of the molar refraction of AB-compounds, crystallizing in the NaCl-or CsC1-structure type, is analysed by use of a two-term Sellmeier dispersion formula. I n the range of the validity of a single-oscillator model a clear distinction between series with Na Clor Cs Clstructure is possible on the base of the influence of the parameters I,, the resonance wavelength of the electronic absorption, and r a e , the effective number of electrons involved in the electron transition, on the molar refraction and its slope. The Sellmeier model is compared with the dispersion model of Wemple. Die Wellenliingenabhiingigkeit der Molrefraktion von AB-Verbindungen, die im NaCl-oder im CsC1-Strukturtyp kristallisieren, wird unter Verwendung einer zwei-gliedrigen Sellmeier-Dispers ionsformel analysiert. Im Giiltigkeitsbereich eines Ein-Oszillator-Modellsist auf der Grundlage des Einflusses der Parameter A,, Resonanzwellenliinge der Elektronenabsorption, und I Z~, effektive Zahl der am Elektroneniibergang beteiligten Elektronen, auf die Molrefraktion und deren Anstieg eine eindeutige Unterscheidung zwischen Reihen mit NaC1-bzw. CsC1-Struktur moglich. Dem Sellmeier-Model1 wird das Dispersionsmodell von Wemple gegenubergestellt.

According to Wemple the "static" electronic constant E(0,z) of a binary mixture (z the composition) is interpreted as the ratio of two characteristic energy functions Ed(z) and E,(z) described by a fractional linear interpolation, connecting them with the chemical structure of the original substances. Simple algebraic operations yield the existence conditions for relatively low (E < el, EJ or high (E > E,, 8,) values of E. These inequalities connect optical with stoichiometric coefficients. Despite their restrictive nature it should be possible to find out suitable material combinations. Corresponding hypothetical combinations are indicated for the four characteristic cases. Die ,,statische" elektronische dielektrische Konstante F(0, z) einer biniiren Mischsubstanz (z Mischungsverhlltnis) wird nach Wemple als der Quotient zweier charakteristischer Energiefunktionen Ed(Z) und E,(r) aufgefalt, fur die gebrochen h e a r e Interpolationsformeln angenommen werden, welche diese GroDen mit der chemischen Struktur der Ausgangsstoffe verknupfen. Mittels einfacher algebraischer Operationen lassen sich die Bedingungen dafiir herleiten, daO die Mischung ein tieferes oder hoheres i aufweist als beide Ausgangsstoffe. Diese Ungleichungen verbinden optische Daten mit stochiometrischen Koeffizienten. Trotz deren restriktiver Natur sollten sich dennoch geeignete Substanzkombinationen finden lassen, fur die hypothetische Beispiele in den vier moglichen Fallen angegeben werden.

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