Mechanism of Stereoregulation

Several methods have been proposed to explain the stereoregulation mechanism in metallocene catalysis. Most now agree that the active spedes is cationic, the insertion of the monomer is between the transition metal and growing chains, and that the steric structure of the catalyst determines the orientation of the incoming monomer. Without giving detailed analyses of all the models, the essential features can be summarized as follows. 

The MAO methylates replace the halogens on the metallocene to form the active site  

This is a vacant coordination site that will bind the incoming monomer during the chain propagation. The polymerization process is thought to be essentially the same 

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We should like to consider a molecular mechanism of stereoregulation in the stereospecific polymerization of aliphatic monoaldehydes by carrying out a molecular model examination based on the structure, chemical behavior and catalytic behavior of the catalyst. 

Unfortunately, no catalyst is found which gives an isotactic polymer in quantitative yield. This fact obstructs the determination of the structure of the real active species for isotactic polymerization and the collection of unequivocal information about the mechanism of stereoregulation. Formation of the highly isotactic polymer which is cleanly separable from the atactic polymer indicates the existence of a highly stereospecific catalyst species in the polymerizing system. In order to answer to these unsolved problems, it will be necessary to do more experiments by utilizing new ideas or by more ingenious experimental techniques. 

Further investigations are necessary to clarify the mechanism of stereoregulation in the polymerization of olefins and diolefins probably the stereoelective polymerization of vinyl monomers and the asymmetric polymerization of diolefinic compounds will give further interesting contributions to the future progresses in this field. 

The mechanism of stereoregulation in the stereoselective polymerisation of propylene oxide with zinc dialkoxide and related zinc dialkoxide-ethylzinc alkoxide complexes has been satisfactorily explained by the enantiomorphic catalyst sites model prepared by Tsuruta et al. [52,75], According to this model, the presence of chiral sites with a central octahedral zinc atom, bearing the polymer chain and coordinating the monomer, was assumed to be the origin of the stereoregulation mechanism. 

Mechanism of stereoregulation on the basis of the data on polyolefin stereoregularity. The structure of a polymer chain is the recording of events proceeding in the insertion of olefin molecules into an active metal-carbon bond. To understand the stereochemistry of the propagation reaction, the data on the stereoregular structure of polymer chains are important. Recently, for this purpose, C-NMR spectroscopy has been extensively used... 

The sites for coordination of monomer (indicated by the open dotted lines in (I) to (III)) have been the subject of much speculation in regard to the mechanism of stereoregulation [23, 24]. It has also been suggested that the active site may contain two vacancies [25]. In the above, catalyst complexes have been represented as bridged structures but they have also been considered to be ionic complexes [26], e.g. 

The mechanism of stereoregulation, however, remained unclear for many years. Only more recently has an experimentally well-founded comprehensive reaction model been derived for the allylnickel complex-catalyzed 1,4-polymerization of butadiene, which convincingly explains the catalytic reaction mechanisms and the structure-reactivity relationships also involving the industrial nickel catalyst [26-28]. 

If the rate of anti-syn isomerization is relatively low, then the cis-trans selectivity can be determined by the formation of the anti- or the 5y/i-butenyl structure, for example from the t] -cis or the if-trans coordinated butadiene, in the catalyst complex. This is the mechanism of stereoregulation which was suggested in the mid-1960s by Cossee and Arlman [34, 35] for titanium-catalyzed butadiene polymerization, and which was reconsidered more recently for the allylne-odymium complex catalysts to explain their cis-trans selectivity [39], But it is also possible that the difference in reactivity between the anti and the syn structure of the catalytically active butenyl complex can determine the cis-trans selec-... 

The objective of this chapter is to examine the basic research on diene (butadiene, isoprene, and piperylene) polymerisation with the LnHalj-nL-AlRj (Ln = lanthanide, Hal = halogen, ligand (L) = tributyl phosphate (TBP), AlRj = triisobutylaluminum and diisobutylaluminum hydride) catalytic system. The chapter will analyse the role of such factors as the electronic and geometric structure of bimetallic active centres, anti-syn and 7t-o-transitions of the terminal units of the growing polymer chains and the nature of the lanthanide, diene, and organoaluminum component in the mechanism of stereoregulation. 

The Structure of Active Centres and the Mechanism of Stereoregulation in the Polymerisation of Butadiene... 

The mechanism of stereoregulation is still being debated. Some concepts are presented in the rest of this section. Natta believed that polymerizations initiated by oiganoaluminum compounds proceed by a coordinated anionic mechanism. The aluminum atoms were seen as forming complexes with the oxygen atoms on the penultimate units ... 

Heso dyad defects portrayed in Structure II have previously been associated with classical chain end controlled mechanisms of stereoregulation. However, the m errors are ambiguous. There are several other reaction mechanisms that can lead to them. The m placements can, for example, be due to a skipped insertion step in site controlled polymerizations. 

Metallocenes as homogeneous catalysts have been studied theoretically compared to heterogeneous catalysts. Most of the theoretical studies are related to the mechanism of stereoregulation in a-olefin polymer-... 

Since polyacetylene is insoluble in all solvents tested, identification of the isomers has been made by vibrational spectroscopic studies on thin films or by NMR spectra on the solid polymers (82). The structural aspects of polymers of acetylene or acetylene derivatives has recently been discussed by Simionescu et al. (83). The mechanism of stereoregulation in the stereospecific polymerization of acetylene or substituted acetylenes is still unclear. 

The mechanism of stereoregulation in the polymerization of 3-lactones and lactams is a subject in which very little definitive information is available and, consequently, many unanswered questions exist. Little is known about the molecular basis for the apparent stereoregulatlon in these ring-opening polymerization reactions, and unfortunately there is even very little quantitative information on tactlclty in these families of polymers. Because of this lack of reliable tacticlty information (as a result of the insensitivity of NMR spectra to differences in configurational sequences), stereoregularity is often inferred from crystallinity measurements. Unfortunately, however, there are many anomalies in the relationships between polymer crystallinity and apparent stereoregularity in the polyesters and polyamides obtained from 3-lactones and lactams. 

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