Zirconocene-catalyzed polymerization

In the zirconocene-catalyzed polymerization of alkenes, Landis and coworkers [20] have reported in situ observation of a Zr-polymeryl species, 15, at 233 K (Figure 1.5). Complex 15 is formed by partial reaction of 14 with excess 1-hexene. Derivatives 16 and 17 are generated quantitatively from 15 by addition of ca. 10 equiv. of propene and ethene, respectively. No other intermediates, such as alkene complexes, secondary alkyls, diasteromers of 15 or 16, or termination products, accumulate to detectable levels. These NMR studies permit direct monitoring of the initiation, propagation and termination processes, and provide a definitive distinction between intermittent and continuous propagation behavior. 

Scheme 1.7 Active-site counting method based on H-labelling, in the zirconocene-catalyzed polymerization of 1-hexene. Lower left typical NMR of the quenched polymer according to method A. The integrals allow the quantification of the Zr-alkyl active sites. All labels are found in the terminal position. Lower right comparison of fractional active-site counts using Method A (open circles, o) or Method B (diamonds ).

Karanikolopoulos, G Batis, C. Pitsikalis, M. Hadjichristidis, N. The influence of the nature of the catalytic system on zirconocene-catalyzed polymerization of aUcyl methacrylates. Macromol. Chem. Phys. 2003, 204, 831-840. 

We note that there are NMR-based kinetic studies on zirconocene-catalyzed pro-pene polymerization [32], Rh-catalyzed asymmetric hydrogenation of olefins [33], titanocene-catalyzed hydroboration of alkenes and alkynes [34], Pd-catalyzed olefin polymerizations [35], ethylene and CO copolymerization [36] and phosphine dissociation from a Ru-carbene metathesis catalyst [37], just to mention a few. 

Proton/deuterium isotope effects on reaction rates are useful mechanistic probes. In the zirconocene-catalyzed alkene polymerization, the observed values of k .iH/feo(.2H determined by NMR fall in the range of 1.2-1.3 and support a transition state in which there is an a-agostic interaction (see 88) [132]. 

In zirconocene-catalyzed olefin polymerizations similar processes are involved. Here polymerization rates depend at least linearly on olefin concentrations an... 

An intramolecular example is provided by the zirconocene-catalyzed cyclization of epoxy esters 222 to give bicyclic products 223 (Equation 82) <1997T16575>. When the closely similar fluorinated epoxy esters 224 are formed in situ, polymerization occurs to give poly(dioxolanes) (Scheme 22) <1999T6311>. Tetrabutylammonium cyanide catalyzes the isomerization of 225 to afford benzodioxoles 226 (Equation 83) <2004T3825>. 

A well-known problem in the preparation of MAO is the inevitable presence of trimethylaluminum in the MAO product or trialkylaluminum species in the MMAO product. The quantity of residual R3-A1 has major effects on the catalytic activity of MAO. Very low activities have been reported in the literature when TMA is used alone as the cocatalyst for Cp2ZrR2-catalyzed ethylene polymerization. The effect of free TMA on polymerization activity and polymer molecular weight has been studied by altering [TMA]/[MAO] ratios in zirconocene-catalyzed ethylene polymerization. as well as by replacing TMA with TEA or TIBA in combination with the... 

Titanocene- and zirconocene-catalyzed alkene polymerization involves initial alkyl group transfer from alkylaluminum cocatalyst to Ti or Zr centers and subsequent multiple insertion of monomer into the metal-carbon bond. Zr complex catalyzed carbomagnesation shown in Eq. 5.33 [128-136] also involves alkyl ligand transfer between the main group metal and Zr. 

Leclerc, M. K. Brintzinger, H. H. an a-Metallocene derivatives. 31. Origins of stereoselectivity and stereoerror formation in anra-zirconocene-catalyzed isotactic propene polymerization. A deuterinm labeling study. J. Am. Chem. Soc. 1995, 777, 1651-1652. 

Prosenc, M.-H. Brintzinger, H.-H. Zirconium-alkyl isomeiizations in zirconocene-catalyzed olefin polymerization A density functional study. Organometallics 1997,16, 3889-3894. 

The zirconocene catalyzed dehydrogenative coupling of phenylsilane provides a mild, room temperature method for preparing silicon catenates with degrees of polymerization up to 40 silicon atoms. Attractive features of catalytic methods include the ability to influence the stereochemistry of the polymerization reaction(55, 66) and the reactive functionality provided by the Si-H bonds.(47,56-59, 67,68)... 

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. 

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. 

Table 10 Ethylene polymerizations catalyzed by the phosphine-borane zirconocenes 161a-d...

The dehydrogenating polymerization of hydrosilanes was found to be catalyzed by Group 4 metallocene alkyl and sdyl derivatives (equation 26). For zirconocene and hafhocene catalysts, the reaction was found to proceed via a-bond metathesis steps. ... 

In relation with the abihty of cationic ansa zirconocene complexes to affect stereocontroUed alkene polymerization reactions, it is possible to insert simple alkenes into the [(ebi)Zr(jj -pyrid-2-yl)]+ and [(ebthi)Zr(j7 -pyrid-2-yl)]+ systems. The derived three-membered azazirconacycles are formed with high levels of diastereoselectivity, depending on the nature of alkene substituents. Thus, the enantiopure ebthi complex catalyzes asymmetric bond-forming reactions (equation 72). 

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