A new class of H-bond donating ureas was developed for the ring-opening polymerization (ROP) of lactone monomers, and they exhibit dramatic rate acceleration versus previous H-bond mediated polymerization catalysts. The most active of these new catalysts, a tris-urea H-bond donor, is among the most active organocatalysts known for ROP, yet it retains the high selectivity of H-bond mediated organocatalysts. The urea cocatalyst, along with an H-bond accepting base, exhibits the characteristics of a "living" ROP, is highly active, in one case, accelerating a reaction from days to minutes, and remains active at low catalyst loadings. The rate acceleration exhibited by this H-bond donor occurs for all base cocatalysts examined. A mechanism of action is proposed, and the new catalysts are shown to accelerate small molecule transesterifications versus currently known monothiourea catalysts. It is no longer necessary to choose between a highly active or highly selective organocatalyst for ROP.

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.

The antibacterial compound, triclocarban (TCC), is shown to be a highly effective H-bond donating catalyst for ring-opening polymerization (ROP) when applied with an H-bond accepting base cocatalyst. These ROPs exhibit the characteristics of “living” polymerizations. TCC is shown to possess the high activity characteristic of urea (vs thiourea) H-bond donors. The urea class of H-bond donors is shown to remain highly active in H-bonding solvents, a trait that is not displayed by the corresponding thiourea H-bond donors. Two H-bond donating ureas that are electronically similar to TCC are evaluated for their efficacy in ROP, and a mechanism of action is proposed. This “off-the-shelf” H-bond donor is among the most active and most controlled organocatalysts for the ROP of lactones.

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.

Dual mechanisms for H-bond mediated ring-opening polymerization allow for precise control, high activity and new applications.