## 2 Heterogeneous Supported Olein Metathesis Catalysts  
The use of a heterogeneous insoluble support is among the oldest and most widely used tool to effect separation and isolation of catalysts from the products. The first insoluble organic support explored by the scientific community that was later used for homogeneous catalysts was based on a paper that was published by [PERSON]--work that subsequently earned [PERSON] a Nobel Prize [11]. While [PERSON]'s work was directed at using cross-linked polystyrene resins ([PERSON]'s resin) in peptide and nucleotide synthesis [11], others recognized that the physical separation of a growing peptide bound to an insoluble organic support he described had potential applications in separation of homogeneous catalysts and products and that if catalysts had stable stationary states that such catalysts could be recycled. Thus, this discovery of [PERSON] that focused on peptide synthesis led to many studies using similar insoluble polymeric materials as supports for homogeneous catalysts. As is true in peptide synthesis, catalysts immobilized on these divinylbenzene (DVB)-cross-linked polystyrene supports have the principle advantage of allowing for separation of catalysts and their ligands from a solution of the product in the solvent phase of a reaction mixture via simple filtration. In some cases, heterogeneous supports can also improve catalyst stability and prevent bimolecular decomposition pathways via a phenomenon known as site isolation [12; 13; 14; 15]. However, in other cases, the heterogeneity of an otherwise homogeneous catalyst can change activity in undesirable ways. If the goal of immobilization is to replace a homogeneous catalyst with a separable catalyst with the same activity, this is undesirable. The discussion below first describes in a general way some examples where olefin metathesis catalysts have been immobilized on always phase separated supports (_i.e._, insoluble inorganic or cross-linked polymer supports). It then describes similar chemistry that uses sometimes phase-separated supports (_i.e._, biphasic catalysis or soluble polymers that can be phase separated after a catalytic reaction). Finally, it goes on to discuss in more detail issues associated with the specific example of immobilized ring-opening metathesis polymerization (ROMP) catalysis. Ru metathesis catalysts like those shown in Figure 2 have been immobilized on divinylbenzene cross-linked polystyrene (DVB-PS). An early example of such catalyst was **6** which was first reported by [PERSON] and [PERSON] in 1995 [16]. In this example, they used an established type of supported ligand--a phosphine modified DVB-PS. This supported catalyst had extended lifetimes. This was ascribed to the reduced diffusivity of the catalyst molecules on the polystyrene support, which prevents a decomposition pathway that occurs via a bimolecular reaction. However, while **6** was recycled three times in a metathesis reaction of _cis_-2-pentene to form cross metathesis products of 3-hexene and 2-butene, the catalyst lost 20% of its activity after each cycle. This catalyst also had a modest activity that was ascribed to (i) incomplete substitution of phosphine; (ii) the diffusion limit of olefin into the cavities of cross-linked DVB-PS support; and (iii) the local high concentration of phosphine on the support. The amount of Ru leaching in the products was not analyzed. However, given that the Ru complex is immobilized by a labile phosphine ligand, given that the Ru and phosphine ligand dissociate in the reaction, and that the recovered polymer-bound catalyst loses significant activity cycle to cycle, it is likely that there is a significant amount of Ru leaching. Several years later, [PERSON] and co-workers described an alternative scheme that also used DVB-PS as a support. In this case, they immobilized a Ru pre-catalyst onto DVB-PS using a benzylidine group. In this second approach, [PERSON] introduced a new concept called the \"boomerang\" effect [17].