This study describes the use of polyisobutylene (PIB) to phase-anchor pyridine ligands that form a phase-separable Grubbs third-generation catalyst. We further show that this complex is useful in ring-opening metathesis polymerization (ROMP) reactions. These PIB-bound pyridine-ligated Grubbs catalysts provide the same benefits of control over polymer chain growth and polydispersity of the product as their low-molecular-weight analogs and reduce Ru leaching in ROMP products from approximately 16% (820 ppm residues) as seen with a similar pyridine-ligated catalyst to a value of approximately 3% (160 ppm residues). These labile ligands are shown to be as effective at generating separable metal complexes as less labile PIB-functionalized N-heterocyclic carbene catalyst ligands that are typically used for immobilization but that require a multistep synthesis.