Living Polymerization Caught in the Act: Direct Observation of an Arrested Intermediate in Metathesis Polymerization

Understanding the
stability and reactivity of the propagating species
is critical in living polymerization. Therefore, most living olefin
metathesis polymerizations require the stabilization of the catalyst
by coordination of external ligands containing Lewis basic heteroatoms,
e.g., phosphines and pyridines. However, in some cases, chemists postulated
that the propagating metal carbene could also be stabilized by olefin
chelation. Here, we disclose that stable 16-electron olefin-chelated
Ru carbenes play a key role in previously reported living/controlled
ring-opening metathesis polymerization of endo-tricyclo­[4.2.2.02,5]­deca-3,9-diene and cyclopolymerization of 1,8-nonadiynes
using Grubbs catalysts. We successfully isolated these propagating
species during polymerization and confirmed their olefin-chelated
structures using X-ray crystallography and NMR analysis. DFT calculations
and van ’t Hoff plots from the equilibrium between olefin-chelated
Ru carbenes and 3-chloropyridine (Py)-coordinated carbenes revealed
that entropically favored olefin chelation overwhelmed enthalpically
more stable Py-coordinated Ru carbenes at room temperature. Therefore,
olefin chelation stabilized the propagating species and slowed down
the propagation relative to initiation, thereby lowering polydispersity.
This finding provides a deeper understanding of the olefin metathesis
polymerization mechanism using Grubbs catalysts and offers clues for
designing new controlled/living polymerizations.