Proton-Transfer Polymerization by <i>N</i>‑Heterocyclic Carbenes: Monomer and Catalyst Scopes and Mechanism for Converting Dimethacrylates into Unsaturated Polyesters

This contribution
presents a full account of experimental and theoretical/computational
investigations into the N-heterocyclic carbene (NHC)-catalyzed
proton-transfer polymerization (HTP) that converts common dimethacrylates
(DMAs) containing no protic groups into unsaturated polyesters. This
new HTP proceeds through the step-growth propagation cycles via enamine
intermediates, consisting of the proposed conjugate addition–proton
transfer–NHC release fundamental steps. This study examines
the monomer and catalyst scopes as well as the fundamental steps involved
in the overall HTP mechanism. DMAs having six different types of linkages
connecting the two methacrylates have been polymerized into the corresponding
unsaturated polyesters. The most intriguing unsaturated polyester
of the series is that based on the biomass-derived furfuryl dimethacrylate,
which showed a unique self-curing ability. Four MeO- and Cl-substituted
TPT (1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene)
derivatives as methanol insertion products, RxTPT­(MeO/H)
(R = MeO, Cl; x = 2, 3), and two free carbenes (catalysts), OMe2TPT and OMe3TPT, have been synthesized, while OMe2TPT­(MeO/H) and OMe2TPT have also been structurally characterized.
The structure/reactivity relationship study revealed that OMe2TPT, being both a strong nucleophile and a good leaving group, exhibits
the highest HTP activity and also produced the polyester with the
highest Mn, while the Cl-substituted TPT
derivatives are least active and efficient. Computational studies
have provided mechanistic insights into the tail-to-tail dimerization
coupling step as a suitable model for the propagation cycle of the
HTP. The extensive energy profile was mapped out, and the experimentally
observed unicity of the TPT-based catalysts was satisfactorily explained
with the thermodynamic formation of key spirocyclic species.