H-Bonding Organocatalysts for the Living, Solvent-Free Ring-Opening Polymerization of Lactones: Toward an All-Lactones, All-Conditions Approach

Abstract

The developing urea class of H-bond donors facilitates the solvent-free ROP of lactones at ambient and elevated temperatures, displaying enhanced rates and control versus other known organocatalysts for ROP under solvent-free conditions. The ROPs retain the characteristics of living polymerizations despite solidifying prior to full conversion, and copolymers can be accessed in a variety of architectures. One-pot block copolymerizations of lactide and valerolactone, which had previously been inaccessible in solution phase organocatalytic ROP, can be achieved under these reaction conditions, and one-pot triblock copolymers are also synthesized. For the ROP of lactide, however, thioureas remain the more effective H-bond donating class. For all (thio)­urea catalysts under solvent-free conditions and in solution, the more active catalysts are generally more controlled. A rationale for these observations is proposed. The triclocarban (TCC) plus base systems are particularly attractive in the context of solvent-free ROP due to their commercial availability which could facilitate the adoption of these catalysts.

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

“…Four bisurea catalysts were prepared by the reaction between 3,5‐bis(trifluoromethyl)phenyl isocyanate and the corresponding bisamines. This approach was inspired by the use of monourea or bis(thio)urea catalysts in small molecule transformations . Neutral bisureas were readily prepared in one step from the commercially available isocyanates and bisamines at room temperature .…”

“…To evaluate the catalytic performance of bisurea catalysts for ROP of lactones (ε‐CL and δ‐VL), the bisurea catalyst was combined with MTBD as a cocatalyst (the molar ratio of bisurea/MTBD was 1/1) and BnOH as the initiator (Tables and ).…”

“…Compared to the bifunctional catalysts, the advantage of hydrogen bond donor (HBD) in cooperation with a hydrogen bond acceptor (HBA) is a straightforward synthetic access from the commercial compounds. Kiesewetter and coworkers reported that the HBDs—urea and thiourea stand out among the controlled ROP of lactones, because of their precise controls for polymerization over transesterification . The urea HBD has better activity and controllability than the thiourea in the ROP of lactones (lactide excluded).…”

“…The urea HBD has better activity and controllability than the thiourea in the ROP of lactones (lactide excluded). In addition, the activity and controllability of catalysts can be further improved by using multiurea instead of monourea . In general, HBDs are tailored by electronic and steric effects of catalyst substituents to manipulate their catalysis performance .…”

“…In coordination with the appropriate tertiary amines as HBAs, HBDs can synergistically catalyze ROP through the mechanism that HBD activates the carbonyl of monomers and HBA activates initiators or the propagating chain end. Moreover, conducting ROP under solvent‐free conditions is an attractive strategy for the industrial‐scale polymerizations as well as the laboratory‐scale preparation of highly pure polyesters …”