Polymerization, alkene, catalysis

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

The first example of homogeneous transition metal catalysis in an ionic liquid was the platinum-catalyzed hydroformylation of ethene in tetraethylammonium trichlorostannate (mp. 78 °C), described by Parshall in 1972 (Scheme 5.2-1, a)) [1]. In 1987, Knifton reported the ruthenium- and cobalt-catalyzed hydroformylation of internal and terminal alkenes in molten [Bu4P]Br, a salt that falls under the now accepted definition for an ionic liquid (see Scheme 5.2-1, b)) [2]. The first applications of room-temperature ionic liquids in homogeneous transition metal catalysis were described in 1990 by Chauvin et al. and by Wilkes et ak. Wilkes et al. used weekly acidic chloroaluminate melts and studied ethylene polymerization in them with Ziegler-Natta catalysts (Scheme 5.2-1, c)) [3]. Chauvin s group dissolved nickel catalysts in weakly acidic chloroaluminate melts and investigated the resulting ionic catalyst solutions for the dimerization of propene (Scheme 5.2-1, d)) [4]. 

This is clo.sely related to the Tertiary radical synthesis" scheme for the preparation of organocobalt porphyrins, in which alkenes insert into the Co—H bond of Co(Por)H instead of creating a new radical as in Eq. (13). If the alkene would form a tertiary cobalt alkyl then polymerization rather than cobalt-alkyl formation is observed. " " " The kinetics for this process have been investigated in detail, in part by competition studies involving two different alkenes. This mimics the chain transfer catalysis process, where two alkenes (monomer and oligomers or... 

The range of reactions which have been examined is wide (248) and includes hydrogenations (256), ammonia synthesis (257), polymerizations (257), and oxidations (258). Little activity has occurred in this area during the past few years. Recent reports of the effects of sonication on heterogeneous catalysis include the liquefaction of coal by hydrogenation with Cu/Zn (259), the hydrogenation of olefins by formic acid with Pd on carbon (260), and the hydrosilation of 1-alkenes by Pt on carbon (261). 

As a simple computational model for the catalysis of alkene polymerization, let us consider some aspects of the general chain-propagation reaction... 

Cationic zirconocenes serve as useful reagents in such diverse fields as alkene polymerization, carbohydrate chemistry, asymmetric catalysis, and so on. Reagents that were originally developed for polymerization reactions (MAO, ansa-metallocenes, non-nucleophi-lic borate counterions) have now found use in organic synthesis and are being employed for carbometalation reactions, hydrogenation, and Diels—Alder catalysis. 

The properties of siloxide as ancillary ligand in the system TM-O-SiRs can be effectively utilized in molecular catalysis, but predominantly by early transition metal complexes. Mono- and di-substituted branched siloxy ligands (e.g., incompletely condensed silsesquioxanes) have been employed as more advanced models of the silanol sites on silica surface for catalytically active centers of early TM (Ti, W, V) that could be effectively used in polymerization [5], metathesis [6] and epoxidation [7] of alkenes as well as dehydrogenative coupling of silanes [8]. 

In the field of alkene metathesis ruthenium-allenylidene precursors have made, since 1998, an important contribution to catalysis [12, 31, 32], for the formation of cycles and macrocycles via RCM, ROMP and acyclic diene metathesis (ADMET) polymerization. 

In a number of classes of systems, the catalytic and other chemical effects of metal ions on reactions of organic and inorganic molecules are generally recognized the catalysis of nucleophilic reactions such as ester hydrolysis the reactions of alkenes and alkynes in the presence of metal carbonyls (8, 9, 69) stereospecific polymerization in the presence of Ziegler catalysts (20, 55, 56) the activation of such small molecules as H2 (37), 02 (13), H202 (13), and possibly N2 (58) and aromatic substitution reactions of metal-cyclopentadienyl compounds (59, 63). 

The mechanism of catalytic polymerization of alkenes to motor fuel recently has been abiv discussed by Schmerling and Ipatieff in one chapter of a current book on catalysis (5). 

Several patents of BP Chemicals relate to the use of ionic liquids in catalysis. The polymerization of alkenes in ionic liquids was claimed (128), as was the alkylation of aromatic hydrocarbons with alkenes in the presence of an ionic liquid (129). 

Silver ions form similar alkene complexes which are soluble in aqueous solution and may be used to effect the separation of unsaturated hydrocarbons from alkanes. Catalysis for the polymerization of alkenes also form metal-alkene complexes which lead to polymerized product. 

One of the great discoveries of organomctallic chemistry was the catalyzed polymerization of alkenes at atmospheric pressure and ambient temperature. Vast quantities of polyethylene and polypropylene (over 15 million tons annually) are made by Ziegler-Natta catalysis. Ziegler and Natta received the Nobel prize in chemistry in 1963, and the importance of their work w stimulating interest in organometallic chemistry should not be underestimated. 

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