Since its discovery in the 1960s, alkyne metathesis has evolved from early heterogeneous systems to well-defined molecular catalysts. However, much less progress has been made in the field of living ring-opening alkyne metathesis polymerization (ROAMP) until this decade. The ill-defined nature and instability of known living systems hinders their investigation, and presents a demand for a more stable and more selective molecular catalyst. Appending a pincer donating ligand to a known alkyne cross metathesis catalyst leads to a decrease in electrophilicity, reduces the number of open sites, and stabilizes the catalyst, suppressing chain back-biting events and enhancing catalyst lifetime. Addition of the deprotonated ONO-pincer ligand to [p-TolC=Mo(OR)3(dme)] in toluene quantitatively converted to the desired product via 1H and 19F NMR. The neutral complex equilibrates rapidly with its ligand saturated -ate form, with both species observed at room temperature by NMR. The rate of the dissociation was measured using signal inversion recovery (SIR) experiments. The dissociation constant is 0.00078 M and the rate of dissociation is 7.9 s-1 at 30 �C. Upon addition of 3,8-hexyloxy-5,6-dihydro-11,12-didehydrodibenzo[a,e]-[8]annulen, fast quantitative initiation of the catalyst is observed at room temperature, with much slower propagation. Analogous to the uninitiated complex, the active species is at an equilibrium between its -ate and free form with a dissociation constant KD of 0.0081 M and rate of dissociation of 6.4 s-1. This monomer/catalyst system accesses both high monomer conversion and PDIs as low as 1.02 at 90 �C. The rate is first order in initial catalyst and in monomer concentration, and inversely proportional to the concentration of alkoxide. Well-behaved kinetics, irreversible propagation, the absence of chain termination, and complete initiation of all catalytic centers indicates that this polymerization is living. This study opens new opportunities for the expansion of ROAMP, such as well-controlled semiconducting polymers and self-assembled block copolymers.

Highly selective catalysts for ring-opening alkyne metathesis polymerization

248th National Meeting (2014)

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