Ci -symmetric zirconocene

Rieger prepared the Ci-symmetric zirconocenes rac-C2H4(9-Flu)(l-Ind)ZrCl2 and the two diastereoi-somers of rac-C2H3-l-(i ,5)Ph-l-(9-Flu)-2-((i ,5)-l-In-... 

Molecular weight measurements are far less sensitive to variables which are by their nature most affected by experimental error, such as catalyst amount or catalyst/monomer purity, and, more important, on the number of active centers, but are highly sensitive to the catalyst structure, monomer concentration, and polymerization temperature. Hence, reliable molecular weights can give much information on the nature of the active sites. Studies of this kind have been reported by Miilhaupt and Brintzinger on catalysts C2-T24 and 25, by Kaminsky and Werner on three Ci-symmetric zirconocenes, and... 

Deng, H. Winkelbach, H. Taeji, K. Kaminsky, W. Soga, K. Synthesis of high-melting, isotactic polypropene with C2- and Ci-symmetrical zirconocenes. Macromolecules 1996, 29, 6371-6376. Mise, T. Miya, S. Yamazaki, H. Excellent stereoregular isotactic polymerizations of propylene with C2-symmetric sUylene-hiidged metallocene catalysts. Chem. Lett. 1989, 1853-1856. 

Esteb, J. J. Chien, J. C. W. Rausch, M. D. Novel Ci symmetric zirconocenes containing substituted indenyl moieties for the stereoregular polymerization of propylene. J. Organomet. Chem. 2003, 688, 153-160. 

In 1999, Bercaw synthesized Ci-symmetric zirconocene 40c that has a (+)-menthyl substituent (lR,25,5R-menthyl) on the top cyclopentadienyl ring (Figure 4.19). Complex 40c was... 

FIGURE 4.19 Ci-symmetric zirconocene precatalysts where -SCH2CH(CH3)CH2CH3 is (5)-2-methyl-1-butanethiolate. 

The CGC catalysts are less active than Ci-symmetric zirconocene/MAO systems, but produce nearly perfectly alternating copolymers. For all three CGC catalysts, the product rErN(rir2) is <0.03. The 90 "C copolymerization parameters for the three catalysts are 16, ve = 14.6, = 5x 10 ... 

Broadening this comparison to include copolymers prepared by both early and late transition metal catalysts, the results discussed immediately above show that Ci-symmetric zirconocenes such as 9/MAO produce only copolymers with isolated norbornene units or alternating structures (at 30 C), mainly with isotactic (meso) configurations. C2-symmetric zirconocenes such as 2/MAO readily produce norbornene dyads that are exclusively meso-linkcd (isotactic). In accordance with their catalyst structures, Q-symmetric zirconocenes such as 8/MAO produce norbornene dyads with a rac-linkage (syndiotactic), although with a generally lower stereoselectivity. Palladium a-diimine catalysts, despite the homotopic nature of their coordination sides (that would be expected to give a mixture of meso and racemic blocks), produce norbornene dyads that are solely rac-connected. This behavior can be attributed to a chain-end control type polymerization mechanism. 

Isotactic poly(4-methyl-l-pentene) from -symmetric zirconocenes melts at lower temperature, < 230 °C,846,916 compared with the polymer from ZN catalysts, and has much lower molecular weights. Ci-symmetric isospecific zirconocenes can produce poly(4-methyl-l-pentene) of higher isotacticity and correspondingly higher melting points, but still with low molecular weights at the higher polymerization temperatures.916... 

Before discussing the combination of the catalytic performance of catalysts combining two different symmetries, we briefly discuss the behavior of the meso isomers of ansa-zirconocenes. This background is necessary to understand the performance of both Waymouth s oscillating catalysts discussed in this section and the Ci-symmetric systems discussed in section IV.C. 

It is clear that the most important aspect of C -symmetric zirconocenes is the wide variability of their isospecificity, which has allowed the preparation of novel PP materials. This avenue has been opened by Chien and Rausch, who reported the preparation of thermoplastic—elastomeric polypro-pene (TPE-PP) with CpI-S-anti and its more active isomer The Zr analogue of Ci-... 

Elastomeric Polypropene. To model the propene polymerization to elastomeric polypropene catalyzed by bis(2-arylindenyl)zirconocenes ° or Ci-symmetric Me2X(Cp)(l-Ind) metallocenes, the consecutive two-state model originally described by Coleman and... 

Highly isotactic PMMA has also been reported for a family of Cz-symmetric monoalkyl-substituted flnifl-zirconocenes (42-45). The isotactic triad content in the PMMA increases with the size of the ligand alkyl substituent, whereas the parent unsubstituted analogue 41 exhibits moderate syndioselectivity. However, C2-symmetry is not a prerequisite feature of an isoselective group 4 initiator, as shown by the behavior of the Ci-symmetric anifl-metallocene complex 34, the P3Ph4] variant of which has also been reported to afford PMMA with 94.7% mm triad content at —30 °C. ... 

The polymers with trans-fused five-membered rings linked with a diisotactic head-to-tail sequence have chirality, although the polymers composed of the cis-fused ring are achiral. Scheme 10 summarizes the structures of the stereoisomeric polymers. The optically active zirconocene complex with a C2 symmetric structure catalyzes the enantioselective cyclopolymerization of 1,5-hexadiene (Eq. 20) [98, 99]. Although the polymer contains not only trans-fused ring but also cis-fused ring units (ca. 68 32), it shows optical rotation due to the main chain chirality. 

Copolymers of ethylene and propylene show, in some cases, elastic properties (324,325). Further incorporation of dienes, such as ethylidenenorbomene or 1,4-hexadiene, produces ethylene-propylene-diene monomer (EPDM) elastomers. Kaminsky (326) and Waymouth (327) have reported the predominantly alternating copolymerization of ethylene and propylene, using Ci and Cg symmetric (cyclopentadienyD(fluorenyl) aasa-zirconocene-based catalysts (eq. 3), demonstrating yet another example of preparation of new materials through rational exploitation of SSC systems. 

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