The operative mechanism in the ROP of δ-valerolactone mediated by urea H-bonding catalysts plus MTBD in polar/nonpolar solvents has been investigated. 1H NMR studies suggest that the cocatalysts in polar solvents tend to follow the imidate pathway whereas the cocatalysts in nonpolar solvent undergoes H-bond mediated ROP. Rates of the para- and meta- substituted urea catalysts are correlated with the Hammett σ values. In polar solvents, urea catalysts with stronger electron withdrawing groups (high, positive, σ value) display more imidate character. In nonpolar solvents, urea catalysts with stronger electron withdrawing substituents (high, positive, σ values) show faster kinetics and classic H-bond mediated ROP. Kinetic isotopic effect studies show that lower KIE values for ureas with stronger EWGs are associated with transition states closer to the ground state. KIE values in polar solvents are lower than those in nonpolar solvents. These mechanistic insights of urea catalysts promise new opportunities to the field of H-bond mediated ROP of lactones.

Urea H-bond donating catalysts for ring-opening polymerization of lactones: Mechanistic insights via Hammett relationship

256th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.