Transition metal ions polymerization processes

The specific feature of polymerization as a catalytic reaction is that the composition and structure of the polymer molecule formed show traces of the mechanism of the processes proceeding in the coordination sphere of the transition metal ion to which a growing polymer chain is bound. It offers additional possibilities for studying the intimate mechanism of this heterogeneous catalytic reaction. 

Alkenes are known to form d - complexes with low valent transition metal ions (or atoms), thus stabilizing their low valent complexes (152). Complexes of this type are key intermediates in a variety of catalytic processes, e.g., hydrogenations, polymerizations,... 

N2 ligand is not able to induce appreciable surface mobility or relaxation. The tendency toward strong relaxation in the presence of adsorbates differentiates the chemistry of transition metal ions on silica from the chemistry of the same ions on crystalline oxides (on which relaxation and mobility are definitely smaller). This property is likely to play a fundamental role in determining the properties of Cr2+ (Ni2+) on silica in catalytic processes (e.g., ethene polymerization) for which a large number of coordination vacancies are needed. 

The oligomerization/polymerization of alkenes on catalysts containing transition metal ions can be discussed on the basis of a scheme very similar to that reported in Fig. 2, where the active center is represented by an isolated transition metal on the catalyst surface (F ). The only difference with respect to the previous case concerns the height of the energy barriers associated with the insertion process, which is definitely lower (A isteric constraints, the scheme of Fig. 2a can be modified to give that as shown in Fig. 3. 

Among the most significant developments in the field of catalysis in recent years have been the discovery and elucidation of various new, and often novel, catalytic reactions of transition metal ions and coordination compounds 13, 34). Examples of such reactions are the hydrogenation of olefins catalyzed by complexes of ruthenium (36), rhodium (61), cobalt (52), platinum (3, 26, 81), and other metals the hydroformylation of olefins catalyzed by complexes of cobalt or rhodium (Oxo process) (6, 46, 62) the dimerization of ethylene (i, 23) and polymerization of dienes (15, 64, 65) catalyzed by complexes of rhodium double-bond migration in olefins catalyzed by complexes of rhodium (24,42), palladium (42), cobalt (67), platinum (3, 5, 26, 81), and other metals (27) the oxidation of olefins to aldehydes, ketones, and vinyl esters, catalyzed by palladium chloride (Wacker process) (47, 48, 49,... 

Catalysts used in the polymerization process. For example, commercial isotactic polypropylene is polymerized from a hetereogenous organo-aluminium-titanium complex (Ziegler-Natta process) [110,1260], or less frequently from metallic oxides of chromium, vanadium or molybdenum bonded to an inert support (e.g. the Philips process) [447]. Transition metal ion contents vary in different commercial samples (Table 2.4). 

It is well known (11) that transition metal ions act as senatizers promoting the photooxidation of polyolefins. Japanese authors (38) have recently found that the degradation of polypropylene induced by U V light (3650, 2537, 1800 A) depends on the oxygen concentration and on the residues of the polymerization catalyst. They concluded that degradation in an oxygen atmosphere is a photooxidative process sensitized by metal residues. 

Other macrocyelic eompoimds sueh as cyelodextrins (CD) can be also polymerized and then used as ion carriers for transport of metal ions from aqueous solutions. The hydrophobie CD polymers have been used as ion earriers for separation of transition metal eations from dilute aqueous solutions by transport across polymer inelusion membranes. Recent developments of the CD polymers applieation in sorption processes of heavy metal ion from aqueous solution will be also presented in this ehapter. It is also shown that hydrophilic, water soluble CD polymers obtained by cross-linking of CD with anhydrides are useful in the removal of transition metal ions from diluted aqueous solutions by ion flotation process. 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

The formation of the active metal-carbon bond as a result of the interaction of low-valent ions of the transition metal with olefin is the most intriguing step of the polymerization process by one-component catalysts. The possibility of the formation of the transition metal-carbon bond resulting from the reaction of titanium low-valent ions with ethylene is shown in Dzsabiev et al. (182) ... 

CT excitation is more common in the photochemistry of coordination compounds. Besides the d — d transition (ligand field band), the CT band is often observed and the redox reaction between central metal ion and ligand is frequently induced by photoirradiation. This process has been used to initiate vinyl polymerization. 

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