Cs-Symmetric Metallocenes

Rieger et al. described a heteroatom-containing C, symmetric metallocene 13 whose stereoselectivity depended on the activator [28, 29], The resulting PP contained fewer stereoerrors when activated with a combination of TiBA and trityl tetrakis(pentafluorophenyl)borate than with MAO. In addition, the molecular weight was lower with MAO. To explain this, it was proposed that some of the stereoerrors arise by reversible chain transfer to aluminum. 

Site-controlled syndioselectivity of C/-symmetric metallocenes is unprecedented and represents a major achievement of the rational molecular design of single-site catalysts. Recent highly syndioselective metallocenes are listed in Figure The flat fluorenyl ligand has been further spatially extended by substituents at the 2,6- and/or 3,7-... 

In the case of the unsubstituted C -symmetric metallocenes 8 and 11, copolymerization proceeds under control of the last inserted monomer unit (chain-end control), that is, it can be described by a second order Markov model. Ethylene is inserted with these zirconocenes three times faster than norbornene. No norbornene block sequences longer than two (NN units) are formed, in agreement with parameters calculated for 8 (rsE = 2.40, r E = 4.34, rEN = 0.03, and rm = 0.00). This result easily explains the maximum observed Xn = 0.66. 

Arndt, M. Beuhch, I. C -symmetric metallocenes for olefin polymerization. Part 1 Catalytic performance of [Me2C(3- BuCp)(Flu)]ZtCl2 in ethene/norbornene copolymerization. Macromol. Chem. Phys. 1998,199, 1221-1232. 

Syndio- and Nonsyndiospecific Catalyst Systems with C Symmetric Metallocene Structures... 

Other Types of C Symmetric Metallocene Catalysts with Syndio- and Nonsyndiospecific Behavior... 

In the early stages of the metallocene-catalyzed olefin polymerizations the focus of research lay on C2- and CY-symmetric complexes [3, 12, 13], Since the beginning of the 1990s C,-symmetric catalysts have had more and more impact. The reason is... 

Chien already postulated that C,-symmetric ansa-bridged complexes exist in two isomeric states, which interconvert during the course of the polymerization reaction [14, 15, 21, 22], Different stereoselectivities for monomer coordination and insertion are found for the two coordination sites of the asymmetric metallocene catalysts (Fig. 6,1 and IV). The migration of the polymer chain to the monomer, coordinated at the isoselective site f I—>11), followed by a consecutive chain back-skip (at higher temperatures) to the sterically less hindered side (II >111) leads to isotactic [mmmm] sequences [11],... 

Figure 1.12 QM/MM transition states for insertion into polyethylene chain of (Z) and (E) isomers of 2-butene for (a,b) Cj- and (c,d) Cs-symmetric metallocenes. Molecular modeling studies have predicted preference of Cj- and Cs-symmetric metallocenes for (Z) and (E) isomers of 2-butene, respectively.

Achiral, C -symmetric unbridged metallocenes, 16 104 Achiral hydrobora ting agents, 13 667 Achiral molecules, 6 73 Acicular reinforcement, 5 554 Acid acceptors, in VDC polymer stabilization, 25 719 Acid-activated bentonites, 6 680-681 Acid amide herbicides, 13 319-320 Acid anhydrides, 10 403-406, 484 reactions with alkanolamines from olefin oxides and ammonia, 2 127 Acid-base catalysis, 5 205-209... 

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. 

The XXX type of Cs-symmetric metallocene is the meso diastereomer of a corresponding pair of enantiomers. For example, meso-(C I3)2SiInd2ZrCl2 is the meso diastereomer of rac-(CH3)2SiInd2ZrCl2 (XXXIV). The meso type of Cs metallocene is referred to as meso Cs ... 

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). 

Table 10. Changing the polymerization behavior from syndiotaetic via hemiisotactie to isotactic by introducing substituents in Cs symmetric metallocene. Polymerizations were carried out at 60 °C in 11 of liquid propene...

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... 

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...

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 common feature of Ci-symmetric metallocenes is that their two coordination sites are diastereotopia Because of this property, and depending on the size of the substituent on the cyclopentadienyl ligand, C -... 

Isospecific Ci-Symmetric Metallocenes. Perfect hemiisotacticity requires that mmmm = 18.75%. Deviating from the structure of Ci-I-6 in general makes the complexes more isospecific. Fink studied the series Ci-I-6—Ci-I-9 where the R substituent on Cp increases from methyl to terf-butyl. While the ethyl derivative produces a polypropene very similar to that (prevailingly hemiisotactic) made with the methyl derivative Ci-I-6, the 3-i-PrCp derivative C -... 

Syndiospecific catalytic systems based on Cj-sym-metric metallocenes are more regioselective than the C2-symmetric metallocenes of class II. As a consequence, the enantioselectivity in regioirregular insertions have been experimentally investigated for propene-based copolymers only. In particular, NMR characterization of ethene-l- C/propene copolymers suggests that the very low amount (0.03—... 

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