## 4 Supported Catalysts in Ring-opening Metothesis Polymerization (ROMP)
The molybdenum complex **51** was immobilized onto silica at 70 \({}^{\circ}\)C. The loss of neopentane in **52** was postulated to be involved in the generation of an active silica-supported catalyst **53**. While the structure of **53** was inferred, the presence of a complex like **53** was confirmed by elemental analyses that yield a Mo/N/C ratio of 1/1/10 (Scheme 4). This immobilized complex **53** can prepare ROMP products of norbornene and _cis_-cycloocene at 25 \({}^{\circ}\)C in high yield (74% and 85%, respectively). These ROMP products had \(M_{\rm n}\) values of _ca._ 310,000 Da with a PDI of _ca._ 1.8. The metal analysis of molybdenum in ROMP products was not described. In attempting to develop a greener metathesis reaction process, the [PERSON] group reported several syntheses of highly active PEG-bound ruthenium complexes that can be used in aqueous media to catalyze ROMP reactions. One example is the PEG\({}_{5000}\) conjugated \(N\)-heterocyclic carbene-containing ruthenium benzylidene catalyst **54**[60] (Scheme 5). This catalyst initiated the ROMP of **55** to give polynorbornene **56** in 73% conversion after 24 h, as measured by \({}^{1}\)H NMR spectroscopy in D\({}_{2}\)O. The rate of ROMP reaction with a catalyst like **55** depends on the rate of phosphine dissociation. In this case the dissociation of phosphine from catalyst **54** may be disfavored in water. However, the protonation of free phosphine by HCl could in principle inhibit re-association of the phosphine ligand. In this case, the rate of polymerization was dramatically increased to 95% conversion in 15 min when 1 equiv of HCl, relative to catalyst **54**, was present in the reaction. This chemistry is similar to the Phase Transfer Activation (PTA) principle used by the [PERSON] and [PERSON] groups where a phosphine that is more soluble in a fluorous or aqueous phase is used with a Grubbs catalyst under biphasic conditions where the phosphine separation into a fluorous or aqueous phase precludes re-association [61; 62]. In the [PERSON] work, the PEG-bound ruthenium catalyst **54** was shown to catalyze a ROMP reaction of the monomer _endo-_**57** with 86% conversion to polymer **58** within 14 h. It is known that this _endo-_monomer is a more challenging ROMP substrate than the _exo-_monomer **55**[60]. The extent to which the ROMP products were contaminated by residues of the PEG-bound ruthenium complex **54** was not reported. The complex **59** prepared with a PEG\({}_{2000}\) support that was used in RCM and CM reactions of substrates like 2-allyl-\(N\),\(N\),\(N\)-trimethylpent-4-en-1-aminium chloride and allyl alcohol in water has also been used in the ROMP reaction of **57** (Scheme 6) [63]. The polymerization of **57** occurred in 100% conversion in 5 h. The removal process of the PEG-bound ruthenium complex **59** from the products **58** was not discussed at least in terms of the catalyst and products in a ROMP reaction. The immobilization of a highly active Grubbs 3 rd generation catalyst using a PEG\({}_{350}\) support has also been reported [64]. [PERSON] and co-workers synthesized a PEG-supported Grubbs 3 rd generation by ligand exchange between pyridine ligands and PEG-bound pyridine ligands. This PEG-supported catalyst **60** was used to catalyze ROMP reactions in water (Scheme 7). The results showed that this catalyst works well at pH 1.5 with quantitative conversion of monomer **61** to polymer **62** in 2 h at room temperature. It was noted that the activity of **60** decreased with an increasing in pH, e.g., 23% conversion at pH 7 in 2 h at room temperature. However, with an addition of a Bronsted acid, _i.e._, CuSO\({}_{4}\) and CuBr\({}_{2}\), the conversion of monomer **61** was improved from 23% to 70%. The PDI of the polymer products prepared from **60** ranged from 1.3 to 1.5. The amounts of Ru content in the products were not reported. Figure 5: ROMP Reactions of **55** and **57** Catalyzed by **54**.