Insights into tungsten catalyzed ring expansion polymerization

The pincer tungsten (formal IV) alkylidene complex depicted in Figure 1 is a potent catalyst for ring-expansion polymerization of alkynes, e.g. reaching TONs of > 17,000 and activities in excess of 5,000 kg mol-1 h-1 (with phenylactylene) and degrees of polymerization, Pn, of up to 1000-2000 (with propene and 1-decene).1-4 Monomer dependent polymer dispersity and the known sensitivity of the system to changes in the alkylidene R-group hint at the complexity of the polymerization mechanism. Several mechanistic pathways are possible, including an initial insertion of the monomer into the catalyst backbone tungsten-aryl bond, which for example has been noted to lead to the active species in Hf(IV) pyridylamido catalyzed olefin polymerization.5 The key question is how the catalyst avoids unproductive benzene formation. In this regard, the flexibility of the alkylidene ligand to change from being part of the backbone to being part of the growing ring appears crucial. Using a combination of DFT mechanism exploration and experimental insight we will highlight recent developments.