## 1 Introduction
Originally, ring-closing metathesis (RCM) reactions required catalyst loadings that were 1-5 mol % or greater depending on specific application. However, this level of catalyst loadings seems to be an overestimate. For example, according to the studies by [PERSON] and [PERSON] in 2002, they found that catalyst loadings could be much lower in some cases. Their work shows that in suitable cases Ru-catalyzed olefin metathesis can be carried out effectively with catalyst loadings that are several orders of magnitude lower than normally reported [10]. In suitable cases, the effective turnover number for the Ru catalysts can be as high as  
Figure 1: Examples of olefin metathesis catalysts **1–5**. 600,000. While these studies focused on achieving the maximum turnover number, the reactions with very low catalyst loadings in some cases never reached 100% conversion. Nonetheless, their work demonstrated the potential of lower mol % loadings of Ru catalysts, a potential that has been realized by others such as the [PERSON] group who reported success in reducing the catalyst loadings to be as low as 25 ppm in ring-closing metathesis of diethyl diallymalonate, by employing more sterically demanding and reactive Ru-based catalysts [7]. An alternative strategy to address the issue of separation of transition metal complexes and products in metathesis that has been also used in other transition metal catalyzed chemistry is to design the catalyst and its ligands such that both can be efficiently separated, recovered, and recycled. If such separations recover an active catalyst, a reuse of this catalyst can in effect increase turnover numbers. In any case, a simpler way to separate catalyst residues from products can minimize the effects of catalyst or ligand contamination in products. Various methods to separate, recover and recycle the catalyst have been developed. The most classical approach is to design a catalyst such that it is coupled to an always insoluble support such as silica gel or a cross-linked and thus insoluble polymer. Such an approach leads to so-called heterogeneized catalysts on supports that are always insoluble before, during and after a reaction. A second approach is to design a catalyst such that it has some sort of phase selective solubility. Examples of this second approach have involved ligands that contain ionic liquid compatible functionality, or fluorocarbon groups that make metathesis catalysts soluble in fluorocarbon solvents, or soluble polymers that lead to catalysts whose solubility mirrors that of the polymer to which they are attached. Unlike cases where catalyst immobilization involves an insoluble support, these catalysts can typically be characterized by conventional spectroscopy. A further advantage of each of these approaches is that the catalysts that can be used as homogeneous catalysts avoiding potential problems of heterogeneous catalysts. However, in each of these three cases, some sort of biphasic separation is required after a reaction to separate the tagged catalyst and ligands from the products. Such separations can involve a simple filtration in the case of heterogeneous supports. Soluble supports usually require some sort of liquid/liquid separation or extraction to separate catalysts and products. The discussion below briefly summarizes some examples where heterogeneous and homogeneous supports are used in metathesis. It then describes in more comprehensive detail examples where phase separable metathesis catalysts are used in ring-opening metathesis polymerization (ROMP).