Urea and Thiourea H‑Bond Donating Catalysts for Ring-Opening Polymerization: Mechanistic Insights via (Non)linear Free Energy Relationships

Hammett-style free energy studies of (thio)­urea/MTBD mediated ring-opening polymerization (ROP) of δ-valerolactone reveal the complicated interplay of reagents that give rise to catalysis through one of two mechanisms. The operative mechanism depends most greatly on the solvent, where polar solvents favor a (thio)­imidate mechanism and nonpolar solvents favor a classic H-bond mediated ROP. Data suggest that the transition state is only adequately modeled with ground state thiourea–monomer interactions in the H-bonding pathway, and elusive urea/reagent ground state binding interactions may be irrelevant and, hence, not worth pursuing. However, neither relationship is robust enough to be predictive in the absence of other data. Isotope effects suggest that the base/alcohol binding event is directly observable in the ROP kinetics. New opportunities for catalysis emerge, and a reason for the observed mechanism change is proposed

Organocatalytic ring-opening polymerization of thionolactones: Anything O can do, S can do better

The H-bond mediated organocatalytic ring-opening polymerizations (ROPs) of four new thionolactone monomers are discussed. The kinetic and thermodynamic behavior of the ROPs is considered in the context of the parent lactone monomers. Organocatalysts facilitate the retention of the S/O substitution as well as the synthesis of copolymers. The thionoester moieties in the polymer backbone serve as a chemical handle for a facile crosslinking reaction, and the porosity of the resulting crosslinked polymer can be tuned by altering the thioester density in the (co)polymer. The crosslinked polymers are shown to be degradable in water, and an Au3+ recovery application is demonstrated

Bisurea and Bisthiourea H-Bonding Organocatalysts for Ring-Opening Polymerization: Cues for the Catalyst Design

A series of conformationally flexible bis(thio)urea H-bond donors plus base cocatalyst were applied to the ring-opening polymerization (ROP) of lactones. The rate of the ROP displays a strong dependence on the length and identity of the tether, where a circa five methylene-unit long tether exhibits the fastest ROP. Any constriction to conformational freedom is deleterious to catalysis. For the ROP of δ-valerolactone (VL) and ϵ-caprolactone (CL), the bisurea H-bond donors are more effective, but for lactide, the bisthioureas are more active catalysts. The ROP reactions are rapid and controlled across a wide range of reaction conditions, including solvent-free conditions, exhibiting excellent weight control from low Mn to high polymers. The active mechanism is highly dependent on the identity of the base cocatalyst, and a mechanistic rationale for the observations is discussed. Implications for the design of future generation catalysts are discussed