C2-Symmetric Metallocenes

The C2-symmetric ansa metallocenes possess a C2 axis of symmetry, are chiral, and their two active sites are both chiral. The two sites are equivalent (homotopic) and enantioselective for the same monomer enantioface. The result is isoselective polymerization. C2 ansa metallocenes are one of two classes of initiators that produce highly isotactic polymer, the other class being the C ansa metallocenes (Sec. 8-5e). C2 ansa metallocenes generally produce the most isoselective polymerizations. 

A C2-symmetric ansa metallocene is a racemic mixture of an enantiomeric pair—an example is rac-(dimethylsilyl)bis(l-indenyl)zirconium dichloride (XXXIV), abbreviated as rac-(CH3)2SiInd2ZrCl2. The enantiomers are designated as (R, R) and (S, S) to describe the two coordination sites in each enantiomer. Actually, the synthesis of a C2 ansa metallocene usually produces a mixture of the racemic pair plus the meso compound (R, S). The meso compound, which is a diastereomer of the racemic pair, can be separated from the racemic mixture by physical techniques such as recrystallization. The meso stereoisomer possesses Cs symmetry, and its stereoselectivity is very different from that of the enantiomeric pair (Sec. 8-5a-3). 

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A special case of the chain back skip polymerization mechanism and therefore an entirely different polymerization behavior was observed for differently substituted asymmetric complexes (for example catalyst 3). Although asymmetric in structure, these catalysts follow the trend observed for C2-symmetric metallocenes [20], Chien et al. [23] reported a similar behavior for rac-[l-(9-r 5-fluorenyl)-2-(2,4,7-trimethyl-l-ri5-indenyl)ethane]zirconium dichloride and attributed this difference in the stereoerror formation to the fact that both sides of the catalyst are stereoselective thus isotactic polypropylene is obtained in the same manner as in the case of C2-symmetric metallocene catalysts. 

The possible occurrence of a back-skip of the chain for catalytic systems based on C2-symmetric metallocenes would not change the chirality of the transition state for the monomer insertion and hence would not influence the corresponding polymer stereostructure. On the contrary, for catalytic systems based on Cs-symmetric metallocenes, this phenomenon would invert the chirality of the transition state for the monomer insertion, and in fact it has been invoked to rationalize typical stereochemical defects (isolated m diads) in syndiotactic polypropylenes.9 376 60 This mechanism of formation of stereoerrors has been confirmed by their increase in polymerization runs conducted with reduced monomer concentrations.65 In fact, it is reasonable to expect an increase in the frequency of chain back-skip by reducing the monomer concentration and hence the frequency of monomer insertion. 

In general, these stereochemical events are completely independent. For instance, isospecific catalysts like the heterogeneous ones (based on TiCl3 or on TiCU supported on MgCI2) as well as C2 symmetric metallocenes [e.g., based on rac-C2H4(H4-l-Ind)2ZrCl2] can present extremely poor cis trans... 

Dependence on Metallocene Symmetry of E-Z Selectivity for 2-Butene Copolymerizations. We have seen in the Section 3.1.3 that opposite enantiofaces are favored for primary and secondary propene insertion on C2-symmetric metallocenes, whereas the same enantioface is favored for primary and secondary insertion on Cv-symmetric metallocenes. In this framework, if the same steric interactions which rule the enantioselectivity of primary and secondary propene insertions hold for 2-butene, the insertion of... 

Z)-butene should be favored with C2-symmetric metallocenes, whereas insertion of ( >butene should be favored with (/(-symmetric metallocenes. 

According to the reported calculations, (Z)-butene insertion on the C2-symmetric metallocene is favored relative to insertion of (//(-butene by 1.6 kcal/mol, while (//(-butene insertion on the ((-symmetric metallocene is favored relative to insertion of (Z)-butene by 1.8 kcal/mol. Finally, it is worth noting that (Z)-butene coordination would be preferred for both C2-and Cs.-symmetric metallocenes.94... 

Figure 12. Scheme of stereospecific 1-olefins polymerization with generic C2 and Cs symmetric metallocenes. In the framework of a regular chain migratory mechanism, the C2 and Cs symmetric catalysts lead to iso- and syndiotactic polymers, respectively. In fact, multiple insertions of the same enantioface occur with C2 symmetric metallocenes, while multiple insertions of alternating enantiofaces occur with Cs metallocenes. 

Crystallization, experimental methods overview, 1, 207 Crystal mounting, experimental methods overview, 1, 209 Crystal selection, experimental methods overview, 1, 209 CSA, and apparent dynamic NMR, 1, 423 CSD, see Cambridge Structural Database C2-symmetric metallocenes, for polypropylene polymerization, 4, 1058 CT transitions, see Charge-transfer transitions Cubanes... 

The comonomer response (compare rac-catalysts 2 and 3 with the corresponding meso-catalysts 4 and 5 in Fig. 2) can be improved by using meso-isomers of C2 symmetric metallocenes [60]. In copolymerisation of ethene with 1-hexene the reactivity ratios of siloxy-substituted meso-isomers, catalysts 4 and 5, are comparable with those of Me2Si(2-Me-Benz(e)Ind)2ZrCl2 [52], which is considered to be a highly efficient copolymerisation catalyst. 

The experimental observation was that C2-symmetric metallocene complexes of zirconium (Fig. 6) produced isotactic polymers, while Cs-symmet-ric metallocene complexes (Fig. 6) produced syndiotactic polymers. Pure MM calculations with frozen core showed that the stereoselectivity is not related to direct interactions of the -ligands of the chiral metallocene with the entering monomer, but to interactions of the -ligands with the growing chain. It is therefore the chirally oriented growing chain which discriminates between the prochiral faces of the propene monomer. For C2-symmetric complexes, identical enantiofacial orientation in all insertion steps results in isotactic polymer formation for Cs-symmetric complexes the enantiofacial orientation alternates between insertion steps and leads to syndiotactic polymers. 

A joint experimental and computational DFT/MM study [56, 57] on the copolymerization of ethene and 2-butene provided further proof of the validity of the growing chain control of stereoselectivity mentioned above. The idea is that, if the same steric interactions postulated for propene hold for 2-butene, insertion of Z-butene should be favored with C2-symmetric metallocenes like Zr(Me2Si(l-indenyl)2CH3+, while E-butene should be favored with Cs-symmetric metallocenes like Zr(Me2Si(cyclopentadienyl-9-fluorenyl) CH3+. DFT/MM calculations confirmed this qualitative view, predicting the barrier for Z-butene to be 1.6 kcal/mol lower in the case of C2-symmetric complexes, and the barrier for E-butene to be 1.8 kcal/mol lower in the case of Cs-symmetric complexes. These results were corroborated by experiments, which showed molar compositions of 14% and 25% when the appropriate 2-butene isomer was copolymerized with ethene, while the molar percent was in the range of 1% when the wrong isomer was used. 

Under the same conditions, syndiospecihc (Cs-symmetric) metallocenes are more effective in inserting a-olefins into an ethylene copolymer than isospecific (C2-symmetric) metallocenes or unbridged metallocenes. In particular, hafnocenes are more efficient than zirconocenes. An interesting effect is observed for the polymerization with ethylene(bisindenyl)zirconium dichloride and some other metallocenes. The catalytic activity for the homopolymerization of ethylene is very high, and it increases when copolymerization with propylene occurs (114) (Fig. 12). Munoz-Escalona et al. (125) observed similar effects in the copolymerization of ethylene with 1-hexene. 

Spaleck et al. (154) reported a large number of chiral zirconocenes with different bridges and substitutions on the indenyl ligand (Table VII). Some C2-symmetric metallocenes give polypropylenes with a high melting point (162°C) and tacticities (mmmm pentades) of 97-99%, measured by 13C-NMR spectroscopy (155,156). 

The copolymerization parameter rt which indicates how much faster an ethene is incorporated in the growing polymer chain than an a-olefin, when the last inserted monomer was an ethene unit, lies between 1 and 60 depending on the kind of comonomer and catalyst. The copolymerization parameter r2 is the analogous ratio for the a-olefin. The product r r2 is important for the distribution of the comonomer and is close to unity when using C2 symmetric metallocenes, indicating a randomly distributed comonomer. It is less than unity with a more alternating structure for Cs-symmetric catalysts [62-65] (Table 5). 

Under the same conditions, syndiospecific (Cs-symmetric) metallocenes are more effective for inserting a-olefins into an ethene-copolymer than isospecific working (C2-symmetric) metallocenes, or unbridged metallocenes. In this case hafnocenes are more efficient than zirconocenes, too. 

Due to the fact that the polymer chain migrates during insertion, the symmetry of the metallocene is of fundamental importance to the tacticity of the polymer produced. C2-symmetric metallocenes such as the bridged bis(indenyl) compounds mentioned above have homotopic coordination sites and thereby always favor the same orientation of the prochiral monomer during the approach. This leads to the formation of an isotactic polymer (Figure 3). 

The misinsertions are responsible for the low melting points of the polymers produced at high temperatures. Also, the low molecular weights obtained at industrially favored temperatures (60-70 °C) caused the need for catalyst improvement. Since the mid-1980s about a hundred C2-symmetric metallocenes... 

Early metal-metallocene-alkene polymerization catalysts permit the synthesis of highly isotactic polypropylene . They rely on controlling the stereochemistry of alkene insertion by the use of chiral C2 symmetric metallocenes . Late metal systems for alkene polymerization , and copolymerization of alkenes and CO , have also been developed. 

Figure 14 The most relevant elementary steps observed at the (R,/ -enantiomer of a chiral, C2-symmetric, isospecific zirconium center with a primary growing chain end (top) and a secondary growing chain end (bottom). The (S,S)-enantiomer produces the opposite stereochemistry of each single event, but overall the same polymer chains and the same insertion mistakes. In practice, in the case of C2-symmetric metallocenes, the racemic mixture (R,R+S,S) is always used. P = growing polypropylene chain [C] = concentration of active primary centers pC] = concentration of active secondary centers.

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... 

Of the three classes designed for elastomeric or plastomeric PP, the most successful seems to be the class of C -symmetric structures. These are based on the bilaterally symmetric fluorenyl ligands, first developed by Ewen,209 and have received a great deal of attention due mainly to three facts (i) they are far simpler to synthesize than the chiral isospecific C2-symmetric metallocenes (ii) they can cover a very broad range of stereoselectivity by structural modification of one ligand only and (iii) due to the presence of two different active sites, they offer a more potent mechanistic tool and intellectual challenge. 

Figure 29 Melting point, 7"m (second melt, heating rate 10° min-1), of regioregular iPPs made with C- - and C2-symmetric metallocene catalysts, as a function of isotacticity ( mmmm % O m %,).725,742...

In general, C2-symmetric metallocenes are not exceedingly regioselective in propylene polymerization. 

Cl-symmetric metallocenes present a synthetic advantage over C2-symmetric ones in terms of their application for isospecific polymerization of propylene. As mentioned previously, a problem associated with the synthesis of anra-C2-symmetric metallocenes is that they are almost invariably generated along with their wew-isomers, which are difficult to remove from the catalyst mixture and often produce undesirable low molecular weight atactic PP with certain polymerization activity. The synthesis of pure C2-symmetric catalysts usually requires multiple purification steps with low yields. In contrast, a e o-form does not exist... 

It should be noted that Ci-symmetric metallocenes show a stereoselectivity increase with elevated polymerization temperature and lowered monomer concentration, a behavior opposite to that displayed by the C2-symmetric metallocenes. The regioregularity of PP samples prepared with most Ci-symmetric metallocene catalysts is fairly high. The predominant monomer insertion mode is 1,2. Isolated 2,1 and/or 3,1 units can be observed with <0.5 mol%o. 

Figure 17. Stereokinetic isotope effects in the polymerization of E- and 2 propene-l-c/i using a C2-symmetric metallocene. The transition state with an a-hydrogen agostic interaction is considered to be more stable than the transition state with an a-deuterium agostic interaction. Reprinted from ref 102. Copyright 1996 American Chemical Society.

The invention of syndiospecific Cj-symmetric metallocenes has marked the turning point in the understanding of the mechanism of stereocontrol with metallocene catalysts. Again, the presence of isolated insertion errors of the type rrrrmmrrr is consistent with site control (Scheme 27). In the case of the syndiospecific Me2C(Cp)(9-Flu)ZrCl2 catalyst, in which the two sites are enantiotopic, occasional skipped insertions produce a minor amount of insertion errors of the type rrrrmrrrr, which are identical to those produced by chain-end control. In the case of isospecific C2-symmetric metallocenes, skipped insertions would not be observable due to the presence of two homotopic sites. 

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