Green-Rooney

Another possibility is that carbene species are generated via the dissociative adsorption of ethylene onto two adjacent chromium sites [71]. A second ethylene molecule then forms an alkyl chain bridge between the two chromium sites this can subsequently propagate via either the Cossee or the Green-Rooney mechanism. 

A key question remains how is the olefin formed in the overall process Molecular tantalum complexes are known to undergo facile a- and transfer processes, leading to tantalumalkylidene and tantalum tt-olefin complexes, respectively (mechanism 9, Scheme 29) [98]. Moreover, olefin polymerization with tantalum complexes belongs to the rare case in which the Green-Rooney mechanism seems to operate (Eq. 10, Scheme 29) [102]. Finally, intramolecular H-transfer between perhydrocarbyl ligands has been exemplified (Eq. 11, Scheme 29) [103,104]. 

Figure 3. Modified Cossee mechanism for the polymerization of olefins with early transition metals. Green, Rooney and Brookhart introduced the presence of the adjuvant a-agostic interaction in the transition state.

Many questions remain about the initiation, propagation, and termination steps of the ethene polymerization mechanism. The most important models proposed to date are the Cossee model, which requires a vacant coordination site on the metal center in the position adjacent to the growing alkyl chain, where ethene is coordinated before insertion into the chain (628), and the Green-Rooney model, which requires the presence of a metal-carbene species and a vacant site where ethene is coordinated prior to insertion (629). 

Two major mechanisms have been proposed for alkene polymerization. These are the Cossee-Arlman mechanism and the Green-Rooney mechanism. A modified version of the latter has also been considered to explain the behavior of homogeneous, metallocene catalysts. The original Cossee-Arlman mechanism was proposed for the TiCl3 based heterogeneous catalyst. In the following sections we discuss these different mechanisms in some detail. In the following discussion in accordance with the results obtained from the metallocene systems, the oxidation states of the active surface sites are assumed to be 4+. 

The heterogeneous character of the conventional Ziegler-Natta catalyst makes studies directed towards mechanistic and structural elucidation at a molecular level extremely difficult. Experimental evidence is therefore sought from homogeneous metallocene and other related catalysts (see Section 6.5). Such evidence does not support the Green-Rooney mechanism. 

Green and Rooney81 proposed an alternative mechanism (Scheme 11.13b) that also accounted for Z-N catalysis. The mechanism resembles a metathesis-like pathway by starting with a-elimination to give a metal-carbene hydride followed by cycloaddition with the alkene monomer to form a metallacyclobutane. Reductive elimination finally yields a new metal alkyl with two more carbon atoms in the growing chain. The Green-Rooney mechanism, although plausible overall, requires an a-elimination, a process that is difficult to demonstrate. 

The experiments just described point out the feasibility of the 1,2-M-C insertion described by the Cossee mechanism, but they fail to distinguish between it and the Green-Rooney pathway. Grubbs85 reported definitive evidence in support of the Cossee mechanism when he measured the rate of polymerization (in the presence of catalyst 42) of a 1 1 mixture of H2C=CH2and D2C=CD2 (equation 11.27). There was no kinetic isotope effect, thus supporting the Cossee mechanism. 

The Grubbs Stereochemical Isotope Experiment The Green-Rooney Mechanism... 

The debate on the mechanism of polymerization, whether an insertion mechanism (Cossee-Arlman) [6], or a metathesis-type mechanism initiated by a-H elimination from the alkyl complex to give a hydrido-carbene intermediate (Green-Rooney) [108], was solved in favor of the former on the basis of the absence of isotope effect on the rates of insertion, and on the stereochemistry of alkene intramolecular insertion, when a-D alkyls were used in the cyclizafion reaction shown in Eq. 6.21 [109]. 

Schrock has demonstrated that the Ta-based ethylene polymerization system shown in Figure 19 is likely to proceed via the Green-Rooney alkylidene mechanism. They based this on their observation of the metallacyclobutane intermediate at low temperatures in the NMR. However, the alkylidene-metallacyclobutane mechanism does not appear to operate for most metal-based polymerization catalysts. 

Figure 18 Green-Rooney alkylidene mechanism for alkene polymerization.

There is another mechanism for polymerization related to the Green—Rooney alkylidene pathway that can operate when one deals with cyclic alkenes. This is called ring-opening metathesis polymerization (ROMP) (cf. Ref 28, 28a), and the first commercial product was prepared by CdF Chimie from norbornene (Equation (18)) using a heterogeneous catalyst based on M0O3 supported on alumnia. 

Modified Green-Rooney mechanism (ground and transition state a-agostic interaction)... 

An example of an alkyne insertion involving an early metal complex is shown in Equation 9.72. The insertion of dimethylacetylene into the permethylscandocene-alkyl complex occurs in a manner similar to that for the insertion of olefins into d metal-alkyl complexes. This reaction gave the product of a cis addition. The competition experiment shows that there is no measurable isotope effect of the a-hydrogen, implying that the modified Green-Rooney mechanism is not followed in this case. 

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