Structure of the Active Centers

As previously mentioned, Mg/Ti catalysts appear to lead to a single type, no matter what the preparation method, with MgCl2 being the activating principle. It is 

Formation of active centers proceeds by alkylation and reduction of the transition metal complex during interaction with the organoaluminum cocatalyst. 

For the supported catalysts for propylene polymerization, the basic question has been to establish whether or not the donor participates in the formation of active centers. The role of the aluminum alkyl, on the other hand, is still subject to debate even with regard to non-supported Ziegler-Natta catalysts. 

It is known that isotactic polypropylene is also formed over catalysts which do not contain a Lewis base. According to Keii n9), the addition of the donor does not modify the MWD of the isotactic fraction of polypropylene obtained with the MgCl2/TiCl4/EB—AlEtj system. This would suggest that the Lewis base does not participate in forming isospecific active centers having different Mn or MWD. 

The same model has been used lt6) to explain the copolymerization of ethylene and propylene with TiCl+/EB/MgCl2—AlEt3, with various amounts of EB added to the cocatalyst. The triad sequence distribution calculated for the copolymer obtained without EB was in disagreement with reactivity ratios, while the values obtained with high concentrations of EB did agree. Thus, the two active species mentioned, having two and one vacancies respectively, would be characterized by 

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These kinetic and stereochemical results give us a direct evidence for the bimetallic structure of the active center in which alkylaluminum components are involved as important ligands. 

The value of KM decreases with increasing electronwithdrawing capability of the aluminum component, i.e. with decreasing electron density at the vanadium induced by the aluminum component bonded to the vanadium in the bimetallic structure of the active center. This result seems to suggest that electron back-donation from a filled vanadium d orbital to the empty propylene jc obital (it-bonding) is the main factor in determining the vanadium-propylene interaction. 

While this review discloses the kinetic and stereochemical features of soluble Ziegler-Natta catalysts, we have little information on the structure of the active center. The steric environments of active centers must be very important in determining the monomer reactivity, regiospecificity and stereospecificity of soluble catalyst. The influence of ligands such as the aluminum components on the rates of chain propagation and chain-terminating steps should be correlated to the electronic structure of... 

Figure 3 Proposed structure of the active center of poly(l-vinylimidazole)-Cu complex.

The structure of the active center in tyrosinase has been proposed as shown in Figure 4 [39,40],... 

Scheme 33. Structures of the active centers in [NiFe] and [FeFe] hydrogenases.

Several fundamental problems are still to be solved, such as the exact knowledge of the structure of the active centers and the mechanism involving the single elementary polymerization stages. Only an in-depth knowledge of these phenomena may bring about further scientific progress. 

In this section a brief review of quantum-chemical studies on the electron structure of the active center and the nature of the elementary steps of the chain propagation and transfer reactions for olefin polymerization is given. 

The catalytic oligomerization of olefins in the presence of OAC and the olefin polymerization in the presence of transition metals are based on similar olefin insertions into the metal-carbon and metal-hydrogen bonds (see Section 3.2). However, in organoaluminium compounds, the structure of the active center is defined more simply and more reliably. Data on its coordination state, thermodynamic and kinetic parameters have been reported (e.g. Table 13). 

Although Friedel-Crafts or Lewis acid catalysts are often used to initiate carbocationic polymerizations and are very important from an industrial viewpoint, very little is known about the active intermediates involved. Such information is important because, in general for ionic polymerization reactions, small changes in the structure of the active center can result in large changes in molecular weight, molecular weight distribution (MWD),... 

Many papers have been published concerning the structure of the active centers in anionic and cationic ring-opening polymerization reactions of oxacyclic monomers. Recently, attention has been paid in our laboratory to the influence of the structure of complex carbonium salt initiators, especially of the dioxolanyllum salts used for initiating the cationic polymerization reactions of trioxane, tetrahydrofuran and dioxolane, on the course of the polymerization ( ). 

Structure of the active center. The active centers of this dimeric enzyme are so well embedded into its protein structure that they are inaccessible to the solvent. The two centers are situated approximately 30 A apart from each other but connected by /3-strands. The active center consists of a type 2 copper center and a cofactor. Sequence comparisons have established that the residues His 8, His 246, and His 357 coordinate the copper ions in both yeast and plants (e.g., lentil seeds) [120,122]. The participating cofactor is typical for amine oxidases, diamine oxidases, and lysyl oxidases but has not yet been found in any other protein - 2,4,5-trihydroxy-phenylalanine quinone [123, 124] (also known as TOPA-quinone, TPQ or 6-hydroxy-DOPA quinone), an internal cofactor which is created by post-translational modification of the tyrosine in position 387 [120]. The consensus sequence of the amino acids neighboring the TOPA cofactor are conserved in all known amine oxidases - Asn-TOPA-Asp/Glu [113,120, 123,125-127]. The positions of the histidine ligands relative to TOPA quinone are conserved in all known amine oxidases as well. The chain lengths of the amine oxidase monomers vary according to the organism of origin 692 residues in yeast [128], 762 in bovine serum amine oxidase [128,129] and 569 in the enzyme from lentil seeds [120,130]. 

Structure of the active center. The active center of Cu,Zn-SOD is situated at the end of a channel whose walls consist of conserved, charged residues [229], The superoxide radical does not reach the active center by diffusion, it is directed... 

Up till now, the predominant and, it should be mentioned, successfully solved problems have been related to the determination of the nature (cationic, free-radical or anionic) and the structure of the active center of the growing polymer chain represented by an asterisk in Scheme 1. However, the investigation of the process of the direct insertion of the monomer in the polymer chain, i.e. everything represented in Scheme 1 by an arrow - was considered to be of secondary importance, with the exception of anionic coordination polymerization. It is usually a priori assumed that this is an elementary single-stage activation transition in the literal sense without any peculiar features, and if these features even exist, they are completely predetermined by (Fig. 1). 

In contrast to free-radical polymerization, in anionic polymerization it is possible to change gradually the structure of the active center passing from the ion-covalent carbon-metal bond to the ion pair and, in the limit, to a free anion by increasing the polarity and solvating ability of the solvent. For the simplification of the interaction picture without reducing the consideration area five basic states of the carbon-metal bond in an anionic active center will be considered 1) the slightly polar carbon-metal bond (R-X), 2) the polarised carbon-metal bond (Re X ), 3) the contact ion pair (Re, X ), 4) the solvent-seperated ion pair (Re X ) and 5) the free anion (Re + X ). 

The cationic polymerization of lactones is not yet sufficiently well understood. The structure of the active centers is not known with certainty and the direction of the opening of the lactone ring (O-alkyl (a) vs. O-acyl (b)) has been established only recently for a few representative monomers 16,17) ... 

Specific inhibitors for Zn hydrolases have been very actively developed, mostly on the basis of the acquired information on the structures of the active centers. They are extremely helpful in... 

Comparison of the chiral bimetallic catalysts, Cu-Pd-TA and Cu-Ru-TA, showed significant differences. In the case of Cu-Ru-TA catalyst, introducing 0.1-0.5% Ru into Cu-TA leads to almost complete loss of enantioselectivify, while in the cases of Cu-Ru and Cu-Pd catalysts such chiral deactivation proceeds only after introduction of more than 5% Pd. The general catalytic activity of the Cu-Ru-TA catalysts increased with increasing Ru content, while the Cu-Pd catalysts exhibited a synergism of catalytic activity, which was explained by a peculiar structure of the active center and by invoking a ligand effect A similar effect for skeletal Cu-Ru-TA catalysts was... 

Tyrosinase is an oxygenase with a molecular weight of 33,000. In contrast to laccase, there are only one or two copper atoms per molecule. Tyrosinase catalyzes the oxidation of catechins to the respective quinones with the reduction of O2 to H2O. Since one of the cooper ions is presnet in the form not detected by EPR, it is assumed that it is a diamagnetic Cu " ion. The essentially different structure of the active center of tyrosinase and of laccase points also to a different mechanism of their operation. Apparently, in the case of tyrosinase a progressive transport of electrons takes place from the substrate... 

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