Initiators organometallics

Here M is the transition metal and L are other ligands of the initial organometallic compounds. In this case individual organometallic compounds are considered to be true catalysts, and the question of the dependence of the polymerization rate on the character of metal-ligand bonds in the initial organometallic compounds is discussed (123). 

However, initial organometallic compounds may be reagents in a more... 

Some electrochemically initiated organometallic reactions of rhodium and iridium porphyrins have been explored and reviewed by Kadish et al. [178,306], For more recent papers concerning this matter, the reader is referred to Sect. 5. 

Ulman, M., Grubbs, R. H. Relative Reaction Rates of Olefin Substrates with Ruthenium(ll) Carbene Metathesis Initiators. Organometallics 1998, 17, 2484-2489. 

Addition of compounds RMI (R = alkyl, aryl M = Ca, Ba) to terminal vinylacety-lenes occurs exothermically in Et20 at < RT jjjg initial organometallics are not... 

Trost, B.M., and T. Sato Dimethyl(methylthio)sulfonium Tetrafluoroborate Initiated Organometallic Additions to and Macrocyclizations of Thioketals. J. Am. Chem. Soc., 107, 719 (1985). 

The above model for monometallic mechanism, though now widely accepted, is still occasionally questioned. Some evidence, for instance, has been presented over the years to support a bimetallic mechanism [242]. It was shown that elimination of the organometallic portion of the complex catalyst during polymerization of propylene results in deactivation of the catalyst. By contrast, replacement of the initial organometallic compotmd with another one results in a change in the polymerization rate, but not in deactivation of the catalyst. 

To achieve this goal, a number of different types of initiators and catalysts were evaluated for the polymerization of both the racemic and optically active monomers, including macrozwitter ions, nucleophilic initiators, cationic catalysts and initiators, organometallic catalysts, and nucleophilic initiator-crown ether complexes. The results obtained in these polymerizatio reactions are collect d in Tables 1 and 2 for the optically active and racemic monomers, respectively. 

Wang, Q. Quyoum, R. Gillis, D. J. Tudoret, M.-J. Jeremic, D. Hunter, B. K. Baird, M. C. Ethylene, styrene, and a-methylstyrene polymerization by mono(pentamethylcyclopentadienyl) (Cp ) complexes of titanium, zirconium, and hafnium Roles of cationic complexes of the type [Cp MR2] (R = alkyl) as both coordination polymerization catalysts and carbocationic polymerization initiators. Organometallics 1996,15, 693-703. 

Sanchez-Barba LF, Garces A, Femandez-Baeza J, et al. Stereoselective production of poly(rac-kctide) by ROP with highly efficient bulky heteroscorpionate alkyknagnesium initiators. Organometallics. 2011 30 2775-2789. 

Mehrkhodavandi, P, Schrock, R.R., and Pryor, L.L. (2003) Living polymerization of 1-hexene by cationic zirconium and hafnium complexes that contain a diamido/donor ligand of the type H3CC(2-C5H4N)(CH2Nmesityl)2. A comparison of methyl and isobutyl initiators. Organometallics, 22,4569 583. 

Zhang, Y.H., Reeder, E.K., Keaton, R.J., and Sita, L.R. (2004) Goldilocks effect of a distal substituent on living Ziegler-Natta polymerization activity and stereoselectivity within a class of zirconium amidinate-based initiates. Organometallics, 23,3512-3520. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Today the term anionic polymerisation is used to embrace a variety of mechanisms initiated by anionic catalysts and it is now common to use it for all polymerisations initiated by organometallic compounds (other than those that also involve transition metal compounds). Anionic polymerisation does not necessarily imply the presence of a free anion on the growing polymer chain. 

Low surface energy substrates, such as polyethylene or polypropylene, are generally difficult to bond with adhesives. However, cyanoacrylate-based adhesives can be effectively utilized to bond polyolefins with the use of the proper primer/activa-tor on the surface. Primer materials include tertiary aliphatic and aromatic amines, trialkyl ammonium carboxylate salts, tetraalkyl ammonium salts, phosphines, and organometallic compounds, which are initiators for alkyl cyanoacrylate polymerization [33-36]. The primer is applied as a dilute solution to the polyolefin surface, solvent is allowed to evaporate, and the specimens are assembled with a small amount of the adhesive. With the use of primers, adhesive strength can be so strong that substrate failure occurs during the course of the shear tests, as shown in Fig. 11. 

After a vigorous initial activity, whereby new fluorinated organometallic compounds were first synthesized, much of the research effort has decreased Some activity has been concerned with the reactions of perfluoroaromatic magnesium compounds... 

Mechanistically the reaction can be divided into two steps. Initially the alkyl halide 1 reacts with sodium to give an organometallic species 3, that can be isolated in many cases. In a second step the carbanionic R of the organometallic compound 3 acts as nucleophile in a substitution reaction with alkyl halide 1 to replace the halide ... 

The most important practical application of the organometallic complex photoinitiators is the possibility of using these types of initiators in modifying the pre-existing polymer chain, e.g., block, graft, and crosslinked copolymers preparation. 

The initial formation of an ate complex by attack of the nucleophile on the aluminum reagent, followed by reaction with the ketone, is unlikely since treatment of the ketone with a mixture of MAD and the organometallic reagent gave results comparable to those obtained with the organometallic reagent in the absence of MAD. 

The availability of organic ligands in the surface complexes obtained by the reaction of organometallic compounds with supports allows us to consider two possibilities of the initiation process (1) the monomer insertion into the organic ligand-metal bond (bensyl, allyl, etc.) (9a) ... 

The catalysts formed by the support of organometallic compounds of transition elements are also of great interest for nonpolymerization reactions. Generally speaking, these catalysts can be used in three various states (a) in the initial state, (b) after reduction, and (c) after oxidation... 

The application of these catalysts in the initial state (without any special treatment of the surface organometallic complexes of such cata-lysts) for ethylene polymerization has been described above. The catalysts formed by the reaction of 7r-allyl compounds with Si02 and AUOj were found to be active in the polymerization of butadiene as well (8, 142). The stereospecificity of the supported catalyst differed from that of the initial ir-allyl compounds. n-Allyl complexes of Mo and W supported on silica were found to be active in olefin disproportionation (142a). 

Unfortunately, at present the information characterizing the properties of the active bond in polymerization catalysts is very scant. The analogy between the features of the active bonds in the propagation centers and those of the transition metal-carbon bond in individual organometallic compounds is sure to exist, but as in the initial form the latter do not show catalytic activity in olefin polymerization this analogy is restricted to its limits. 

Richards et at. carried out extensive studies on the use of mercury,2 6 277 lead278 279 and silver compounds to terminate anionic polymerization and form polymeric organometallic species which can be used to initiate polymerization. 

---