Site epimerization

A kinetic model has been proposed based on microstructural analysis, including both chain-epimerization and site-epimerization reactions in both C2- and C.-symmctric metallocenes, and rationalizing the observed pseudo-second-order kinetics of propylene polymerization promoted by C2-symmetric metallocene catalysts. This point has been extended to co-polymers.298 A thorough study of propylene polymerization with the Me2C(Cp)(9-Flu)ZrCl2 system in the presence of a large series of different counterions that rationalized the correlation between the nature of ion pair and the microstructure of the resulting PPs has been performed.104... 

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... 

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

Scheme 8. Site Epimerization Mechzinism for Synthesizing Stereoblock Poljmiers...

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... 

Under the conditions of a high monomer concentration, kinetic trapping of the two different coordination sites can reasonably be expected to be competitive with site epimerization and, therefore, conversely, the dynamic process of site epimerization would be expected to become more dominant under conditions of monomer starvation. 

SCHEME 2.2 Site epimerization and enantiofacial misinsertion are two principal sources of stereoerrors commonly observed in syndioselective polymerizations. The bridge substituents have been omitted for clarity P represents the growing polymer chain and M = cationic Zr+ or Hf ", generally. 

FIGURE 2.6 Site epimerization and enantiofacial misinsertion produce defective pentads in characteristic ratios. (Reprinted with permission from Miller, S. A. Bercaw, J. E. Organometallics 2004, 23, 1777-1789. Copyright 2004 American Chemical Society.)... 

Of all the hafnium compounds listed in Figure 2.7, none excels their zirconium counterparts in syndioselectivity. This observation is sometimes attributed to hafnium s decreased rate of bimolecular propagation relative to unimolecular site epimerization. Lower overall activities with hafnium catalysts corroborate this argument. The most syndioselective hafnium catalyst appears to be Cl-symmetric i-92-Hf, which produces syndiotactic polypropylene with [r] = 92.2% and... 

The zirconium-based catalyst 5 -136/MAO is highly syndioselective ([r] > 99%, Tp = 0 C) and apparently provides the highest-melting syndiotactic polypropylene known The unannealed Tm for the polypropylene is 164 °C, and the annealed Tm is 174 °C. Stereoerrors have not been detected in this polymer by NMR, but pentads presumably attributable to site epimerization (single m mistakes) can be found if polymerizations are conducted in dilute propylene at elevated temperatures. 

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... 

Efforts to differentiate between these two models through statistical analysis of the pentad distributions have been inconclusive. Figure 2.12 summarizes one such investigation with i-1, which concluded that the site epimerization model and the alternating model converged to the same enantiofacial selectivity parameter upon minimization of the RMS (root mean square) error in comparison with the observed pentad populations. If the site epimerization model applies, then... 

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... 

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