Metallocene rigidity

In the polymer filed, new-generation metallocenes, which are currently used in many polyethylene and polypropylene processes, can polymerize proplylene in two different modes alternating blocks of rigid isotactic and flexible atactic. These new developments and other changes and approaches related to polymerization are noted in Chapters 11 and 12. 

Finally, Brintzinger s synthesis of stereo-rigid ansa-metallocenes [18,19] provided the basis for modern high-activity metallocene catalysts and greatly increased their economic potential. However, although MAO activation and stereo-rigid structures of the new metallocene systems were extremely successful, the complexity of these catalytic systems was such that precise information about the structure of the active species remained elusive. 

Hiickel band calculations, rigid-rod transition metal-acetylide polymers, 12, 371-372 Human health, tin toxic effects, 12, 637 Hybrid magnets, metallocene-containing bimetallic M(II)—Cr(II) oxalates, 12, 427 bimetallic M(II)-Fe(III) oxalates, 12, 432 bimetallic M(III)-Ru(III) oxalates, 12, 435 materials, 12, 437 properties, 12, 425 trimetallic oxalates, 12, 436 Hydantoins, with lead reagents, 3, 888 Hydration... 

Depending on the structure of the metallocene, different polymer microstructures are formed. Generally, among the rigid metallocenes, different structures may be distinguished 152,153), and there are also metallocenes that have fluctuating structures. 

Introduction of another ruthenium bipyridine moiety or bridging metallocene (ferrocene, cobaltocenium) results in sensors that specifically bind chloride anions (Figure 16.15). The structural modification of the amide receptor results in a decease in cavity dimensions and significant rigidity of the macrocycle. Therefore it cannot accommodate hydrogen phosphate anions, but only much smaller CF [45, 46]. 

Figure 21 Changes in component ratio for metallocene (filled symbols) and Ziegler films (open symbols) (A) Mobile, (B) intermediate and (C) rigid amorphous components. The shaded area indicates the plateau stress region.

A notable substitution product of ruthenocene is ethynyl-ruthenocene, which can be bound to estradiol, a form of estrogen studied for breast cancer treatment. With the rigid alkyne spacer, the steroid s affinity to the estrogen receptor was surprisingly unchanged upon addition of the metallocene. The addition of organometallic complexes to biomolecules may allow for the selective tuning of properties of the molecule. ... 

Many (i7 -diene)metallocenes (5) of conjugated dienes, fixed in their 5-cis conformation by a rigid carbon framework, have been obtained in high yields by most of the synthetic routes used for the preparation of 3... 

For the production of ethylene/l-octene copolymers, metallocenes in combination with oligomeric methylalumoxanes or other compounds are now used [31, 63]. Half-sandwich transition metal complexes such as [(tetramethyl- / -cyclopentadienyl) (A-/-butylamido)dimethylsilyl]titanium dichloride are applied to synthesize linear low-density copolymers and plastomers, called constrained geometry catalysts [31]. Ethylene and styrene can be copolymerized to products ranging from semicrystalline mbber-like elastomers to highly amorphous rigid materials at room temperature [64]. 

This type of selectivity originates solely from steric interactions between the auxiliary ligands, polymer chain, and the incoming propene. It was first explained qualitatively by means of visualization of the structure of the catalyst precursors. A more quantitative approach led naturally to molecular mechanics models in order to explain and even predict the stereospecificity of catalysts with different ligand environments. Due to the limitations of MM models to describe metallocene complexes as well as bond breaking and bond formation processes (see Section 3.1.2.1), the models were initially based on some rigid core structures derived from the measured structures, e. g., of the dichloride precursors [25, 26]. In order to achieve more accurate results, core structures, calculated by ab initio methods, were employed later. A further step in this direction is the joint description of the... 

One successful approach for the selective formation of conformationally well-defined -symmetric -metallocenes has been the use of bis-aryl or binaphthyl bridges to link Gp or Ind ligands. Another approach has been to use doubly bridged bis-Cp ligands. A third methodology implies the application of the Nazarov cyclization for the preparation of rigid doubly bridged bis-indene compounds which lead to -symmetric -metallocenes. 

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