Metal-containing catalysts, polymerization

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

C. Dynamic Aspects of Unsaturated Hydrocarbons Oligomerization and Polymerization on Metal-Containing Catalysts... 

In conclusion, also in the case of alkene oligomerization/polymerization on metal-containing catalysts, the rate of surface reactions and hence, the concentrations of the surface species can be modified by tuning temperature and pressure... 

Styrene-divinyl benzene resins have been used as typical support for the metal containing catalysts bound to them. The synthesis of these polymeric reagents has involved several approaches depending on the application which they are intended. The immobilization of transition metals on polymer supports involves using polymers containing ligands which can complex with the metal such that the coordination sphere of the metal remains essentially the same as in... 

Catalysts prepared from these polymeric ligands were said to be useful for the production of methacrylic esters as well as acetates by the transesterification process. The reader is referred to European Patent EP 05557131A2 for details of the preparation of polymeric metal containing catalysts of the latter type. 

Polymeric metal-containing catalysts Various applications 1... 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases, it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips catalysts) for ethylene polymerization ... 

This homopol)TTierization is carried out in the presence of a catalytically effective amount of a suitable metal-containing catalyst such as stannous octoate (stannous-2-ethylhexoate — Sn(CgHi502)2) or stannous oxalate. Typically the catalyst levels are reported as monomerxatalyst molar ratios. Level vary widely depending on the catalyst but stannous octoate is preferably used from 15,000 1 to 40,000 1. The polymerization is carried out in the presence of an initiator such as an alkanol, a... 

A new class of substituted ionic polyacetylenes was synthesized by metathesis polymerization with transition metal-containing catalysts [77]. The monomers used were dihexyldipropargylammonium salts with bromide and tosylate as anions. It was found that MoClj-based catalytic systems were very effective for the cyclopolymerization of ionic monomers. The resulting dark red polymers exhibited good solubility in polar organic solvents such as methanol, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), THE, and CHCI3, which indicates their ionic nature. The structure of the polymers was confirmed by IR, UV-visible, and H and NMR spectroscopy. Doping the polymers with I, resulted in a substantial increase of conductivity (10 S/cm) compared with the undoped state (10" S/cm). 

Recently some information became available on a new type of highly active one-component ethylene polymerization catalyst. This catalyst is prepared by supporting organometallic compounds of transition metals containing different types of organic ligands [e.g. benzyl compounds of titanium and zirconium 9a, 132), 7r-allyl compounds of various transition metals 8, 9a, 133), 7r-arene 134, 185) and 71-cyclopentadienyl 9, 136) complexes of chromium]. 

Organometallic usage is shown in the preparation of titanium- or vanadium-containing catalysts for the polymerization of styrene or butadiene by the reaction of dimethyl sulfate with the metal chloride (145). Free-radical activity is proposed for the quaternary product from dimethylaniline and dimethyl sulfate and for the product from l,l,4,4-tetramethyl-2-tetrazene and dimethyl sulfate (146,147). 

Organic compounds having labile hydrogen atoms, such as phenols, anilines, and acetylenes, are also oxidatively polymerized by metal-complex catalysts (Eqs. 1-3). The oxidative coupling is a dehydrogenation reaction the polymer chain produced contains the dehydrogenated monomer structure as a repeating unit. As a remarkable example, poly(phenylene ether), one of the... 

Overall, ADMET polymerization is an advantageous strategy for synthesizing transition metal-containing polymers because of the functional group tolerance of the metathesis catalysts employed, the mild conditions under which it operates, the wide range of accessible architectures, and the precise structural control afforded by the method. 

Metal ions and particularly heavy metal ions tend to sensitize peroxy radical formation, and hence, the presence of metal scavenging or chelating species can offset this effect. This form of stabilization is particularly important for polymers in which metal-containing polymerization catalyst residues are present, such as polyolefins. While simple additives like calcium stearate may be used, more sophisticated ones based on bifunctional chelating species also are available commercially. 

In coordinated polymerizations with alkyl metal and Ziegler-type catalysts, vacant p- or d-orbitals of a metal component coordinate with the jr-electrons of olefins, diolefins and non-polar monomers. When the polymer chain end is fixed in position and partially stabilized by its metal-containing gegen-ion, repetitive insertion of the polarized and oriented monomer between the chain end and gegen-ion yields stereoregular polymers. Of the various factors which affect polymer stereoregularity, the most important appears to be the gegen-ion structure and its ability to coordinate and orient the monomer. 

We wish also to point out that the fact that in the above schemes only the transition metal, the last polymerized unit, and the coordinated monomer are represented does not mean that the catalysts prepared from A1(C2H5)3 and a titanium alkoxyde or from A1(C2H5)2C1 and a Co compound are pure organo-metallic compounds of Ti or Co, respectively. These catalysts are presumably complexes containing A1 and the transition metal, the latter probably being part of a cation, the A1 of an anion. In the above schemes we have represented only the transition metal for the sake of simplicity. 

His proposal involved a metal carbene and a metallocyclobutane intermediate and was the first proposed mechanism consistent with all experimental observations to date. Later, Grubbs and coworkers performed spectroscopic studies on reaction intermediates and confirmed the presence of the proposed metal carbene. These results, along with the isolation of various metal alkyli-dene complexes from reaction mixtures eventually led to the development of well-defined metal carbene-containing catalysts of tungsten and molybdenum [23-25] (Fig. 2). After decades of research on olefin metathesis polymerization, polymer chemists started to use these well-defined catalysts to create novel polymer structures, while the application of metathesis in small molecule chemistry was just beginning. These advances in the understanding of metathesis continued, but low catalyst stability greatly hindered extensive use of the reaction. 

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