Organocatalytic Conjugate-Addition Polymerization of Linear and Cyclic Acrylic Monomers by N‑Heterocyclic Carbenes: Mechanisms of Chain Initiation, Propagation, and Termination

This contribution
presents a full account of experimental and theoretical/computational
investigations into the mechanisms of chain initiation, propagation,
and termination of the recently discovered N-heterocyclic carbene
(NHC)-mediated organocatalytic conjugate-addition polymerization of
acrylic monomers. The current study specifically focuses on three
commonly used NHCs of vastly different nucleophilicity, 1,3-di-tert-butylimidazolin-2-ylidene (ItBu), 1,3-dimesitylimidazolin-2-ylidene (IMes), and 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene (TPT), and two representative
acrylic monomers, the linear methyl methacrylate (MMA) and its cyclic
analog, biomass-derived renewable γ-methyl-α-methylene-γ-butyrolactone
(MMBL). For MMA, there exhibits an exquisite selectivity of the NHC
structure for the three types of reactions it promotes: enamine formation
(single-monomer addition) by IMes, dimerization (tail-to-tail) by
TPT, and polymerization by ItBu. For MMBL,
all three NHCs promote no dimerization but polymerization, with the
polymerization activity being highly sensitive to the NHC structure
and the solvent polarity. Thus, ItBu is
the most active catalyst of the series and converts quantitatively
1000–3000 equiv of MMBL in 1 min or 10 000 equiv in
5 min at room temperature to MMBL-based bioplastics with a narrow
range of molecular weights of Mn = 70–85
kg/mol, regardless of the [MMBL]/[ItBu]
ratio employed. The ItBu-catalyzed MMBL
polymerization reaches an exceptionally high turnover frequency up
to 122 s–1 and a high initiator efficiency value
up to 1600%. Unique chain-termination mechanisms have been revealed,
accounting for the production of relative high-molecular-weight linear
polymers and the catalytic nature of this NHC-mediated conjugate-addition
polymerization. Computational studies have provided mechanistic insights
into reactivity and selectivity between two competing pathways for
each NHC-monomer zwitterionic adduct, namely enamine formation/dimerization
through proton transfer vs polymerization through conjugate addition,
and mapped out extensive energy profiles for chain initiation, propagation,
and termination steps, thereby satisfactorily explaining the experimental
observations.