Site epimerization mechanism

Scheme 9. Rzindom Site Epimerization Mechanism for Sjmthesizing Stereoblock Polymers...

The stereochemical mechanism responsible for the isoselectivity of Ci-symmetric metallocene catalysts has been a topic of considerable debate. There are two limiting mechanisms possible for the formation of isotactic polypropylene with such Ci-symmetric catalysts having diastereotopic coordination sites. These are the site epimerization mechanism and the alternating mechanism, as shown in Scheme 2.3. 

The vast majority of published reports for isotactic polypropylene formation with metallocenes based on i-1 invoke the site epimerization mechanism to account for the observed isoselectivity.When the growing polymer chain occupies the coordination site distal to the terf-butyl group (Scheme 2.3, site a), it is directed away from the benzo substituent of the fluorenyl ligand in the transition state for monomer insertion. The methyl group of the incoming monomer is... 

SCHEME 2.3 There are two limiting mechanisms for isoselective polymerization with Cj-symmetric catalysts the site epimerization mechanism and the alternating mechanism. The bridge substituents have been omitted for clarity P represents the growing polymer chain and M = cationic Zr" " or Hf ", generally. (Reprinted with permission from Miller, S. A. Bercaw, J. E. Organometallics 2006, 25, 3576-3592. Copyright 2006 American Chemical Society.)... 

A more recent set of experiments favors the alternating mechanism. If polymerizations are conducted in dilute monomer (10% by volume in toluene), the melting temperatures and [mmmm] fractions exceed those obtained from bulk polymerizations. For example, Tm = 135 °C and [mmmm] = 88.8% are obtained in 10% monomer solution as compared to Tm = 129 °C and [mmmm] = 78.0% in neat monomer at the same Tp(40 °C). In addition, under these dilute conditions, [mmmm] is found to increase from 82.2% to 89.4% as the polymerization temperature increases over the range of 0-60 °C. A similar effect was reported with 2 bar of propylene monomer in toluene the [mmmm] fraction increased from 83.5% to 87.7% as Tp increased from 10 to 50 These results are consistent with an alternating mechanism that increasingly yields to a site epimerization mechanism as the monomer concentration decreases or the polymerization temperature increases. This conclusion is... 

Cl-symmetric catalysts such as 9 and 10 bear a Cp ring substituent pointing in the same direction as one of the chlorine atoms on the Zr center. In the active catalyst species, the chlorine is replaced by the growing polymer chain or a n-complex-bonded olefin. The coordination site on the same side as the Cp substituent (site B) is more sterically hindered than the other coordination site (site A). After insertion of a monomer coordinating at site B, the polymer chain is walked back to less sterically hindered site A by the next monomer insertion (alternating/chain migratory mechanism). But in some cases, the polymer chain can back skip to site A before the next monomer insertion (site epimerization). Norbornene, as a bulky olefin, can be rr-bonded only at site (see Chapter 2 for a further discussion of the alternating and site epimerization mechanisms with catalyst 9). 

It seems that the site epimerization mechanism, complex as it is, can be related to three main factors. Optimization of each would cOTitribute to the lowering of its frequency of occurrence and to improvement in the stereoregularity and physical properties of the produced s-PP. Whereas a direct link could be estabUsh between site epimerization and the ligand substituents bulkiness and positions on one hand, and the anion s size and charge distribution on the other hand, the coimection between structural flexibility and site epimerization, though proven empirically, is not very clear. 

An extensive review appeared on the configurational stability of enantiomeric organolithium reagents and the transfer of the steric information in their reactions. From the point of view of the present chapter an important factor that can be evaluated is the ease by which an inversion of configuration takes place at the metallation site. It happens that H, Li, C and P NMR spectra of diastereotopic species have been central to our understanding of the epimerization mechanism depicted in equation 26, where C and epi-C represent the solvated complex of one chiral species and its epimer, respectively. It has been postulated that inversion of configuration at the Li attachment site takes place when a solvent-separated ion pair is formed. This leads to planarization of the carbanion, its rotation and recombination to form the C—Li bond, as shown in equation 27, where Li+-L is the solvated lithium cation. An alternative route for epimerization is a series of... 

On the other hand, the above phenomena do not apply to Cj-symmetric (nor to syndiospecific (7,-symmetric) metallocenes for these, decreasing monomer concentration either increases the isotacticity and melting point of iPP or has no relevant effect. This is due to the mechanism of site epimerization (also referred to as chain backskip, Scheme 28), in which the chain, at the lower monomer concentrations, has a higher chance to migrate to the less hindered site, which is usually also the more stereoselective. For the same reason, increasing the polymerization temperature either increases the melting point of an isotactic poly(a-olefin), or has no relevant effect.725... 

A microstructural analysis suggested the presence of 10 atactic blocks per chain, each of which contained 100 propylene units, while the isotactic blocks between have only 50 monomer units.This structure was postulated to arise from an alternation of polymer growth between isospecific and aspecific sites, where site epimerization occurs on a time scale much slower than monomer enchainment (Scheme 8 Figure 8, pathway 7). This mechanism is a topic of considerable debate (vide infra). 

Collins has reported the synthesis of a related class of metallocenes (43), some of which form elastomeric, stereoblock polypropylene when activated by The elastomeric properties of the polymer formed using 43 (M = Hf, X = SiMe2 7)xn = 25 °C) were far superior to those formed by the other metallocenes in the study. The polymers made using 42 and 43 have similar microstructures, as well as physical and mechanical properties. However, after detailed microstructural analysis of the polymer the authors proposed an alternate mechanism of stereocontrol to Chien s site epimerization model (Scheme... 

In addition to low monomer concentration and high polymerization temperatures, the addition of methylene chloride has been observed to increase the relative rate of the site epimerization process for 5-1/MAO Fink, G. Herfert, N. Montag, P. The relationship between kinetics and mechanisms. In Catalysts, Recent Scientific Innovations and Technological Improvements, Fink, G, Miilhaupt, R., Brintzinger, H.-H., Eds. Springer Berlin, 1995 pp 159-179. 

SCHEME 4.2 Mechanisms for stereoerror formation in propylene polymerization enantiofacial misinsertion, site epimerization, and chain epimerization (with or without site epimerization). (P represents the polymer... 

At low propylene concentrations, isolated m stereoerrors were also observed for 35a-c/MAO and found to be more numerous than mm stereoerrors. The isolated m stereoerrors were found to have a strong dependence on propylene concentration. This gave further support for site epimerization as the major stereoerror-forming mechanism for 35a-c/MAO. However, since chain epimerization occurring with simultaneous site epimerization can also lead to isolated m stereoerrors, the authors undertook a deuterium labeling study to probe the importance of chain epimerization as a potential mechanism for forming both single m and double mm stereoerrors. 

In summary, experiments at reduced propylene concentration and at elevated temperatures support the assertion that site epimerization is the major stereoerror-forming mechanism for precatalysts 35a-c. Chain epimerization also appears to play a role as an error-forming mechanism, though to a lesser extent. 

Site epimerization is used to account for the isoselectivity of 40b at 25 °C at dilute propylene concentrations. When site epimerization is competitive with olefin insertion for a Ci-symmetric catalyst, it follows that olefin coordination and insertion from one side of the metallocene wedge will be energetically favored in comparison to the other side. Repetitive olefin insertions from one side of the metallocene wedge with the same enantioface will lead to a sequence of m dyads. The mechanism for site epimerization is illustrated in Scheme 4.3 (vide supra) this concept illustrated for Ci-symmetric 37 can be extrapolated to this case. 

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