Catalysts multi-site

High activity Ti/Mg catalyst (multi-site) and heterogeneous branching distribution... 

Figure 1 Metallocene-single-site versus Ziegler-Natta multi-site catalysts. (From Ref. 34. Reprinted with permission from Chemical Engineering, McGraw-Hill, Inc., New York, 1993.)...

Homopolymers made with multi-site catalysts tend to have composite tacticities, with each site generating a distinctive set of intensities in accordance with certain reaction probabilities. Such polymers can be regarded as in-situ blends of several components, each one bearing a different tacticity. Fractionation would then separate the polymer into different fractions, and the tacticities of each fraction would reflect the different proportions of the various conqponents. The MMR data on the fractions potentially contain Information on the weight percent of each component as well as the reaction probabilities. 

The multi-functionality of metal oxides1,13 is one of the key aspects which allow realizing selectively on metal oxide catalysts complex multi-step transformations, such as w-butane or n-pentane selective oxidation.14,15 This multi-functionality of metal oxides is also the key aspect to implement a new sustainable industrial chemical production.16 The challenge to realize complex multi-step reactions over solid catalysts and ideally achieve 100% selectivity requires an understanding of the surface micro-kinetic and the relationship with the multi-functionality of the catalytic surface.17 However, the control of the catalyst multi-functionality requires the ability also to control their nano-architecture, e.g. the spatial arrangement of the active sites around the first centre of chemisorption of the incoming molecule.1... 

Several soluble multi-site ammonium catalysts have been synthesized, examples of which are shown in Scheme 1.1 [e.g. 41-43], Such catalysts have the advantage over single site catalysts in reactions involving divalent anions and, generally, in the lower amount of the salt required to obtain a catalytic effect. 

The syntheses of a variety of "multi-site" phase-transfer catalysts (PTCs) and the determination of their catalytic activity toward some simple Sn2 reactions and some weak nucleophile-weak electrophile SnAr reactions are described. In general, at the same molar ratio, the "multi-site" PTCs are as or more effective than similar "single-site" PTCs. Thus, the "multi-site" PTCs offer an economy of scale compared to "single-site" PTCs. 

A key aspect of metal oxides is that they possess multiple functional properties acid-base, electron transfer and transport, chemisorption by a and 7i-bonding of hydrocarbons, O-insertion and H-abstraction, etc. This multi-functionality allows them to catalyze complex selective multistep transformations of hydrocarbons, as well as other catalytic reactions (NO,c conversion, for example). The control of the catalyst multi-functionality requires the ability to control not only the nanostructure, e.g. the nano-scale environment around the active site, " but also the nano-architecture, e.g. the 3D spatial organization of nano-entities. The active site is not the only relevant aspect for catalysis. The local area around the active site orients or assists the coordination of the reactants, and may induce sterical constrains on the transition state, and influences short-range transport (nano-scale level). Therefore, it plays a critical role in determining the reactivity and selectivity in multiple pathways of transformation. In addition, there are indications pointing out that the dynamics of adsorbed species, e.g. their mobility during the catalytic processes which is also an important factor determining the catalytic performances in complex surface reaction, " is influenced by the nanoarchitecture. 

Polymer-supported multi-site phase-transfer catalysis seems to require the use of less material in order to provide activity comparable to others253 (Table 27). Quaternary phosphonium ions on polystyrene latices, the particles of which are two orders of magnitude smaller than usual, were shown to be capable of higher activity coagulation of the catalysts under reaction conditions was minimized by specific treatment904. The spacers may also contain ether linkages. 

For so-called multi-site catalysts, Ziegler catalysts for example, kinetic constants might vary with the type of active sites, see [10], In such a case, one should summarize Eqn.(5.4-1) over all contributions of different active sites, or kp is interpreted as an average value of all contributing sites. 

Under same reaction conditions, multi-site catalysts such as Ziegler catalysts provide a MWD that can be considered as a superposition of a number of Flory-Schulz distributions (see ref. 10). Current results show that even metallocene catalysts can be interpreted as "two-site" catalysts [13-15]. Often, it is not clear whether a certain shift of the molecular weight over the reaction time (at constant reaction conditions) can be attributed to small changes of the hydrogen concentration. 

Fig. 9.5-1, right, compares the co-monomer incorporated into the polymer, on a molecular basis. With the Ziegler-Natta, multi-site catalysts less monomer is incorporated in the high-molecular-weight fraction. The low-molecular-weight fraction is rich in co-monomer content as shown by the negative slope of curve a. In a metallocene-based polyethylene the comonomer is uniformly distributed (curve b). 

Due to their multi-sited nature, Ziegler-Natta and chromium catalysts produce structurally heterogeneous ethylene homo- and copolymers. This means that the polymers have broad MWD and broad composition (short-chain branching) distribution (Fig. 9). Catalyst active sites that produce lower molecular weights also have a tendency to incorporate more comonomer... 

In an effort to extend the use of the Pyrphos-derived dendrimers to asymmetric Pd-catalyzed coupling reactions, strongly positive selectivity effects were observed upon going to very large multi-site chiral dendrimer catalysts. This enhancement of the catalyst selectivity was observed in palladium-catalyzed allylic substitutions, such as that displayed in Scheme 3, which are known to be particularly sensitive to small changes in the chemical environment of the active catalyst sites [17]. 

Enzymes are capable of the kind of selectivity and rate enhancements discussed above because their active sites exhibit a number of distinctive features compared to the active sites employed by soluble transition metal complexes and solid state catalysts multi-point contact with the substrate, which is very hard to engineer in a synthetic catalyst the structural flexibility to undergo collective and rapid changes in structure to facilitate catalysis of a reaction and a unique ability to combine apparently incompatible features in catalysis, such as simultaneous acid and base catalysis and hydrophobic/hydrophilic interactions [62,63]. These points are discussed in more detail in the following sections. 

Balakrishnan, T., and J. P. Jayachandran, New Multi-Site Phase Transfer Catalyst for the Addition of Dichlorocarbene to Styrene, ... 

One key feature or limitation of Z-N catalyzed LLDPE and VLDPE resins is the non-uniform or broad composition distribution and broad molecular weight distribution (MWD or polydispersity index of 4 to 5) of the resins. This is due to multi-site nature of Z-N catalyst with differences in each site s ability to incorporate alpha-olefin comonomer. Catalyst sites in Z-N catalyst... 

Canada who developed Sclaire catalyst based on vanadium (V) instead of titanium (Ti). A majority of commercial polyolefins (POs) are still produced with heterogeneous, multi-sited, modified Z-N catalysts, e.g., TiCL) on MgCl2 support. 

Fig. 18.2 Melting point versus density dependencies for LLDPE prepared using Ziegler-Natta multi-site and a single-site metallocene catalyst...

In the case of PO blends, compatibilization most frequently aims for improved ductility and/or transparency. The Z-N-LLDPE obtained using multi-sited catalyst constitutes a specific case - the homopolymer may have phase-separated morphology that requires compatibilization. It has been known that addition of 5-20 wt% LDPE needs to be used for improved performance. However, explanation for this is rather recent (Robledo et al. 2009). The relaxation spectrum of the blend may be decomposed into three components (1) Z-N-LLDPE matrix, (2) LDPE dispersed drops, and (3) a thick interphase with its own viscoelastic properties, obtained by interaction between the high-MW linear fraction of the LLDPE and the low-MW linear LDPE macromolecules. 

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