## 3 Homogeneous Supported Olein Metathesis Catalysts
More recent work from our group on polymer-supported olefin metathesis catalyst explored the use of Polyethylene  
Figure 8: PEG-supported Hoveyda-Grubbs catalyst **42** and **43**. oligomer (PEOlig) (\(M_{\rm n}\) = 550) as catalyst supports [52]. The unique property of PEOlig is that it is like higher molecular weight polyethylene and does not dissolve in any solvent at room temperature. However, like high molecular weight polyethylene, it does dissolve at elevated temperature. Moreover, polyethylene oligomers that are commercially available as 500-2000 Da materials are soluble in toluene or THF at relatively modest temperatures like 65 \({}^{\circ}\)C. A PEOlig-supported Hoveyda-Grubbs catalyst **46** formed from PEOlig with an \(M_{\rm n}\) of 550 Da can be dissolved in a solvent with modest heating to form a monophasic solution. The complex **46** quantitatively precipitates on cooling. Thus, **46** can be added to substrate, the suspension that forms can be heated to form a monophasic reaction solution, and the catalyst can be separated from a solution of the product by simply cooling it back to room temperature. Filtration then effects phase separation between the solid catalyst species and the product solution. In reports describing this catalyst, the PEOlig-supported catalyst **46** was used in a variety of RCM reactions for at least eight cycles with the Ru leaching based on ICP-MS analysis that was less than 0.3% based on the amount of charged Ru. Examples of these polyolefin-supported Ru metathesis catalysts are shown in Figure 10. The effectiveness of the catalyst/product separation in these processes was further tested in sequential reactions where the same sample of **46** was used to carry out three successive RCM reactions. This experiment not only showed that Ru leaching was low but that the product of one reaction was not detectable in a subsequent cycle that used the same catalyst used in a prior different reaction. Figure 9: Schematic representation of (**a**) thermomorphic liquid/liquid separation where a biphasic mixture of **polar** and **nonpolar** solvent is heated to form a **miscible solvent mixture**; (**b**) a latent biphasic liquid/liquid separation where a room temperature mixture of **miscible solvents** is used to effect a reaction and the solvent mixture is perturbed (e.g., by addition of another solvent (e.g., water) or some salt) to form a separable biphasic mixture of **polar** and **nonpolar** solvents; and (**c**) a thermomorphic solid/liquid separation system where a mixture of substrate and **insoluble polymer-bound catalyst** is heated to form a **monphasic solution** that is in turn cooled to form a **precipitate of the polymer bound catalyst** that is then separated from the **product solution** by filtration. Up to this point, this review has focused on background examples of ring closing and cross metathesis reactions where metathesis catalysts are separated either by liquid/solid or liquid/liquid separations. In these applications, the need for a separation strategy depends on the reactivity of the Ru catalyst. Catalysts with TONs that approach \(10^{5}\) or \(10^{6}\) may not even need a support and a separation strategy. This however is not true in polymerization reactions where no chain transfer occurs. In such cases, the living character of a ring-opening metathesis polymerization in a polymerization leads to one metal center being used for each polymer molecule synthesized. In these cases metal or metal-ligand separations are more critical. The balance of this review will discuss metathesis catalysts and ligand separation from products in this more demanding context.