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

Abstract

Hammett-style free energy studies of (thio)urea/ MTBD mediated ring-opening polymerization (ROP) of δvalerolactone 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.

Solvent‐free ring‐opening polymerization of lactones with hydrogen‐bonding bisurea catalyst

“…The polarity of solvent played an important role in the mechanism of ROP. A polar solvent favors a (thio)‐imidate mechanism and a nonpolar solvent favors a classic H‐bond mediated ROP . Under the conditions of solvent‐free ROP, the bisurea/MTBD may perform a classic H‐bond mediated ROP mechanism.…”

“…Here, a synergistic catalytic mechanism was proposed, in which the bisurea activated the carbonyl group of monomer and MTBD facilitated the nucleophilic attack of the initiating/propagating alcohol by hydrogen bonding (Scheme ) . Kiesewetter expounded that (thio)urea/MTBD mediated ROP of VL proceeded through one of two mechanisms, (thio)‐imidate mechanism and classic H‐bond mediated mechanism.…”

“…Based on the previous work, the multi(thio)urea catalysts studied by Kiesewetter are connected by flexible groups, and the activation mechanism is activated‐urea or dual‐urea activation that all between intramolecular hydrogen bonding . We put forward an assumption that the catalytic performances of HBD with essential rigid groups enhance the activity by intermolecular hydrogen bonding.…”

Ultrasound‐Promoted Mild, and Efficient Protocol for Three‐Component Synthesis of 2,4,5‐Trisubstituted Imidazoles Using Urea and PPh3 as the Sources of Nitrogen and Organocatalyst

“…It is a white, odorless, cheap and plentiful solid that its main industrial application appears in agriculture as a fertilizer. Moreover, in synthetic chemistry, urea is commonly employed as a catalyst and/or cocatalyst, [33,34] reactant [35,36] or nitrogen source for diverse reactions. [37][38][39] In 2001, Hofmann and coworkers described an invention for decomposition of urea to ammonia through the microwave-assisted method.…”

A Recommender System for Inverse Design of Polycarbonates and Polyesters

“…The controlled ROP of lactones and cyclic carbonates is a result of the interplay between monomer reactivity [20][21][22][23] and catalyst activity. 13,[24][25][26][27] While the thermodynamics and kinetics of ROP have been wellstudied, [20][21][22][23] these insights do not provide a robust theoretical framework to enable a priori catalyst selection. Particularly since even relatively small changes in monomer structure can have an outsized influence on reactivity.…”