Polymerization zirconocenes

An interesting new approach in this area has been reported with the synthesis of novel polymeric zirconocene-silsesquioxanes 134. " The condensation of zirconocene derivatives with polyhedral silsesquioxanes led to... 

An interesting new approach in this area has been reported with the synthesis of novel polymeric zirconocene-silsesquioxanes (4.15) with R=cyclohexyl [42, 43]. The condensation of zirconocene derivatives with polyhedral silsesquioxanes led to amorphous polymers in high yields ( 90%) that were characterized by NMR spectroscopy and elemental analysis. These high molecular weight (M =14,000) materials exhibited high thermal stability (to ca. 475-515 °C) and, surprisingly, 4.15 a was found to be stable to both air and hydrolysis. 

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Examine the sequence of structures corresponding to Ziegler-Natta polymerization of ethene, or more specifically, one addition step starting from a zirconocene-ethene complex where R=CH3. Plot energy (vertical axis) vs. frame number (horizontal axis). Sketch Lewis structures for the initial complex, the final adduct and the transition state. Indicate weak or partial bonding by using dotted lines. 

Zirconocene dichloride 121 derived from (l-phenylethyl)cyclopentadienyl ligand is formed as a mixture of diastereomers from which the racemic form can be isolated by fractional crystallization. This complex was studied by X-ray diffraction methods and revealed a virtually chiral C2-symmetrical conformation in which the chiral ring-substituents are arranged in a synclinal position relative to the five-membered ring. It was proposed that this conformation is preserved in solution. Using 121 as catalyst the influence of double stereodifferentiation during isospecific polymerization of propylene (Eq. 32) was demonstrated for the first time [142],... 

Throughout this chapter we will use in the different Schemes the abbreviation [Zr] = CpjZrCl- thus [Zr]-H = [Cp2Zr(H)Cl]n (1) and [Zr]-R = Cp2Zr(R)Cl. Most of the compounds described in this chapter have metal centers with 16 electrons, and it is important to emphasize that any coordinatively unsaturated zirconocene (IV) complexes can potentially exist as dimeric, oligomeric, or polymeric aggregates. 

Metallocene catalysis has been combined with ATRP for the synthesis of PE-fr-PMMA block copolymers [123]. PE end-functionalized with a primary hydroxyl group was prepared through the polymerization of ethylene in the presence of allyl alcohol and triethylaluminum using a zirconocene/MAO catalytic system. It has been proven that with this procedure the hydroxyl group can be selectively introduced into the PE chain end, due to the chain transfer by AlEt3, which occurs predominantly at the dormant end-... 

The isotacticities and activities achieved with nonbridged metallocene catalyst precursors were low. Partially isotactic polypropylene has been obtained by using a catalyst system of unbridged (non-ansa type) metallocenes at low temperatures [65]. A chiral zirconocene complex such as rac-ZrCl2(C5H4 CHMePh)2 (125) is the catalyst component for the isospecific polymerization of propylene (mmmm 0.60, 35% of type 1 and 65% of type 2 in Scheme Y) [161]. More bulky metallocene such as bis(l-methylfluorenyl)zirconium dichloride (126) together with MAO polymerized propylene to isotactic polypropylene in a temperature range between 40 and 70°C [162]. 

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

Similar polymerization of MMA using enolate-zirconocene catalysts has also been found [223]. The mechanism of this catalytic reaction is related to the process described in Scheme XI because the cationic enolate complex is isolobal to that of the corresponding lanthanide complex. Recently, similar cationic... 

Abstract Zirconocenes have been used for a long time in the field of olefin polymerization using MAO as cocatalyst. The equivalent hafnocenes were seldom used due to a lack of productivity while using MAO activation. In the last few years borane and borate activation has come into the focus of research for olefin polymerization. A variety of different hafnocenes were used to investigate the polymerization mechanism and the different cocatalysts. 

The exchange of zirconium in isostructural complexes leads to a new family of asymmetric metallocenes (Fig. 1) bearing a 2-methyl substituent and varied substituents in positions 5, 6, and 7 of the indenyl moiety. After borate activation all catalysts show an unexpected high and constant activity toward the polymerization of propylene and lead to significantly increased molecular weight products compared to the zirconocene species [9-11],... 

Rausch MD, Chien JCW, Thomas EJ (2000) Substituent effects on the stereospecificity of propylene polymerization by novel asymmetric bridged zirconocenes. A mechanistic discussion. Macromolecules 33 1546-1552... 

Inspired by the ability of cationic ansa-zirconocene complexes to effect stereocontrolled alkene polymerization reactions, Jordan has recently reported the stereoselective insertion of simple alkenes into both the (ebi)Zr(r 2 -pyrid-2 -yl) and (ebthi) Z r (r 2 -pyr id - 2 -yl) systems [113]. As shown in Scheme 6.36, treatment of rac-(ebi)ZrMe2 114 with nBu3NH+BPh4 in the presence of 2-picoline affords the (ebi)Zr(q2-pyrid-2-yl) complex 115 (the derived B(C6F5) derivatives may also be prepared and are in fact reported to be more convenient to use). 

Tetrasubstituted alkenes are among the most challenging substrates for catalytic hydrogenation reactions. Towards this end, Buchwald and co-workers recently reported efficient and highly enantioselective Zr-catalyzed hydrogenations of a range of styrenyl tetrasubstituted alkenes (Scheme 6.41) [123]. Precedents based on efficient polymerization reactions promoted by cationic zirconocenes led these workers to consider similar catalyst species, derived from dimethylzirconocene 107, for this purpose. 

In the following sections, we discuss reactions in which cationic zirconocenes are involved as reagents, intermediates, or catalysts. As already mentioned, polymerization reactions will not be considered. Section 8.2 deals with the use of the Cp2ZrCl2/AgC104 system (or similar combinations) as an activator in glycoside synthesis. In Section 8.3, nucleophi-... 

Cationic zirconocene species efficiently activate alkenes toward carbon—carbon bond formation via carbometalation, as has been demonstrated in studies of alkene polymerization. Today, some zirconocene catalysts are available that allow single additions of metal-alkyls (mainly aluminum-alkyls) to alkenes or alkynes, thereby forming stable alkyl- or alkenyl-metals that do not undergo any further oligomerization. On the other hand, carbozirconation with Cp2ZrRCl in the presence of stoichiometric or catalytic amounts of activators has also been realized. 

---