Mechanisms, coordinated anionic

On the basis of the X-ray structural data as well as the mode of polymerization, Yasuda et al. [3a] proposed a coordination anionic mechanism involving an eight membered transition state for the organolanthanide-initiated polymerization of MM A (Fig. 6). The steric control of the polymerization reaction may be ascribed to the intermolecular repulsion between C(7) and C(9) (or the polymer chain), since completely atactic polymerization occurred when the monomer was methyl or ethyl acrylate. 

However, the practical, direct synthesis of functionalized linear polyolefins via coordination copolymerization olefins with polar monomers (CH2 = CHX) remains a challenging and industrially important goal. In the mid-1990s Brookhart et al. [25, 27] reported that cationic (a-diimine)palladium complexes with weakly coordinating anions catalyze the copolymerization of ethylene with alkylacrylates to afford hyperbranched copolymers with the acrylate functions located almost exclusively at the chain ends, via a chain-walking mechanism that has been meticulously studied and elucidated by Brookhart and his collaborators at DuPont [25, 27], Indeed, this seminal work demonstrated for the first time that the insertion of acrylate monomers into certain late transition metal alkyl species is a surprisingly facile process. It spawned almost a decade of intense research by several groups to understand and advance this new science and to attempt to exploit it commercially [30-33, 61]. 

A remarkable feature of iridium enantioselective hydrogenation is the promotion of the reaction by large non-coordinating anions [73]. This has been the subject of considerable activity (anticipated in an earlier study by Osborn and coworkers) on the effects of the counterion in Rh enantioselective hydrogenation [74]. The iridium chemistry was motivated by initial synthetic limitations. With PFg as counterion to the ligated Ir cation, the reaction ceases after a limited number of turnovers because of catalyst deactivation. The mechanism of... 

In contrast to the above polymerizations via anionic and/or coordination anionic mechanisms, radical polymerization initiated with metalloporphyrins remains to be studied. The only example of controlled radical polymerization by metalloporphyrins has been reported by Wayland et al. where the living radical polymerization of acrylic esters initiated with cobalt porphyrins was demonstrated. In this section the radical polymerization of MMA initiated with tin porphyrin is discussed. 

The discussion of Mnetic work will be here preceded by a summarized description of the chemical nature of the polymerization, to which we have attributed a mechanism of anionic coordinated type. Such a definition of the reaction mechanism depends upon the fact that the catalyst is a complex in which, generally, a transition metal acts as a coordinating agent and that a carbon atom, which belongs to the extremity of a growing polymeric chain, is coordinated to such a complex and, in the activated state, it possesses a negative charge. 

Polymerization of isobutylene, in contrast, is the most characteristic example of all acid-catalyzed hydrocarbon polymerizations. Despite its hindered double bond, isobutylene is extremely reactive under any acidic conditions, which makes it an ideal monomer for cationic polymerization. While other alkenes usually can polymerize by several different propagation mechanisms (cationic, anionic, free radical, coordination), polyisobutylene can be prepared only via cationic polymerization. Acid-catalyzed polymerization of isobutylene is, therefore, the most thoroughly studied case. Other suitable monomers undergoing cationic polymerization are substituted styrene derivatives and conjugated dienes. Superacid-catalyzed alkane selfcondensation (see Section 5.1.2) and polymerization of strained cycloalkanes are also possible.118... 

Stereospecific action of the alkoxyaluminum compound was interpreted by Natta independently (24). They proposed the coordinate anionic mechanism, and suggested that an aluminum atom forms a complex with an oxygen atom immediately preceding the terminal oxygen atom of the growing chain. An activated complex thus formed has a steric configuration which permits the minimum nonbonded... 

These cocatalyst effects observed in the stereospecific polymerization of aliphatic monoaldehyde by the organoaluminum catalyst are similar to those reported by Letort for free cationic polymerization. We prefer the coordinated cationic mechanism to the coordinated anionic one proposed by several workers. 

In the past few years the use of aluminum alkyls as catalysts for cyclic ether polymerizations has received much attention. Two different mechanisms have been proposed to explain the catalytic activity of the aluminum alkyl catalysts. Saegusa, Imai, and Furukawa (75) suggest that a cationic mechanism is produced. They feel it is not related to the coordinate anionic mechanism presumed to take place with related catalyst systems used for aldehydes and epoxides. They propose that the Lewis acid first reacts with adventitious water to form a Bronsted acid. ... 

These fundamental steps of the catalytic cycle have been confirmed by stoichiometric reactions starting from isolated stable complexes, and by DFT calculations [11], Although many aspects of the Heck olefination can be rationalized by this textbook mechanism , it provides no explanation of the pronounced influence that counter-ions of Pd(II) pre-catalysts or added salts have on catalytic activity [12], This led Amatore and Jutand to propose a slightly different reaction mechanism [13]. They revealed that the preformation of the catalytically active species from Pd(II) salts does not lead to neutral Pd(0)L2 species a instead, three-coordinate anionic Pd(0)-complexes g are formed (Scheme 3, top). They also observed that on the addition of aryl iodides la to such an intermediate g, a new species forms quantitatively within seconds and the solution remains free of iodide and acetate anions. It may then take several minutes before the expected stable, four-... 

Natta, G. and Porri, L., Elastomers by Coordinated Anionic Mechanism , in Polymer Chemistry of Synthetic Elastomers Diene Elastomers, Wiley-Interscience, John Wiley Sons, New York, 1969, Part II, pp. 597-678. 

Ring-opening polymerization is an important field of research in the chemistry of polymer synthesis. Usually, it proceeds by ionic mechanisms, i.e. cationic, anionic and coordinate anionic mechanisms. Research on ring-opening polymerization proceeding via free-radical propagating species in which the so-called molecular design of monomer plays an important role has recently been reported. 

The importance of the electrophilic character of the cation in organo-alkali compounds has been discussed by Morton (793,194) for a variety of reactions. Roha (195) reviewed the polymerization of diolefins with emphasis on the electrophilic metal component of the catalyst. In essence, this review willattempt to treat coordination polymerization with a wide variety of organometallic catalysts in a similar manner irrespective of the initiation and propagation mechanisms. The discussion will be restricted to the polymerization of olefins, vinyl monomers and diolefins, although it is evident that coordinated anionic and cationic mechanisms apply equally well to alkyl metal catalyzed polymerizations of polar monomers such as aldehydes and ketones. 

It is possible for some catalysts to operate by either anionic or coordinated anionic mechanism depending upon the nature of the monomer, the solvent, and the polymerization conditions. This is more easily seen by considering that the ionization of a metal-carbon bond 192) proceeds through stages analogous to those proposed for carbonium ion formation 214) ... 

Zinc and cadmium alkyls have not been successful as stereospecific catalysts in the absence of co-catalysts, presumably because they do not complex strongly enough with the monomer and the metal-carbon bonds are too covalent. Cadmium alkyls were first reported by Furukawa and coworkers (260) to induce vinyl polymerization, but it was shown later (267, 262) that oxygen was a co-catalyst and the reactions were free radical in nature. Similar free radical results were obtained with zinc alkyls (261—263) and vinyl monomers. However, with more basic and more easily polarized monomers, such as olefin oxides and aldehydes, the zinc catalysts operate by a coordinated anionic mechanism (250). 

Boron alkyls are expected to be inactive for coordinated anionic polymerization of olefins because the boron-carbon bond is not sufficiently ionic. The diazomethane polymerization with boron alkyl catalyst reported by Bawn, Ledwith and Matthies (275) is a special case of the growth reaction. A coordination mechanism seems most probable, but it has not been ascertained whether the polymer chain migrates as a car-banion or as a radical. If the complex between diazomethane and boron decomposes into a boron-carbene complex, then the polymer chain could migrate as a carbanion with the driving force provided by the electrophilic carbonium ion ... 

Natta et al. (167,188,287,298,312) have built a strong case in favor of a coordinated anionic mechanism in which an electropositive metal complexes and polarizes the monomer and a polymer anion adds to the positively polarized carbon of the monomer. One of the points which was used to support the anionic mechanism was that the order of reactivity for ethylene, propylene and butene is opposite to that of cationic catalysts. The lower reactivity of propylene and butene versus ethylene was attributed to the electron releasing alkyl groups (287), but steric hindrance is believed to be a better explanation. Support for the steric effect is indicated by the influence of bulk placed at some distance from the double bond (116). For example, reactivity decreases sharply in the order pentene-1 > 4-methylpentene-l > 4,4-dimethylpentene-l, although basicity of the double bonds must change only very slightly. 

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