Chapter 3: Bifunctional and Supramolecular Organocatalysts for Polymerization

Bimolecular, H-bond mediated catalysts for ring-opening polymerization (ROP) - thiourea or urea plus base, squaramides and protic acid/base pairs, among others - are unified in a conceptual approach of applying a mild Lewis acid plus mild Lewis base to effect ROP. The bimolecular, and other supramolecular catalysts for ROP, produce among the best-defined materials available via synthetic polymer chemistry through a delicately balanced series of competing chemical reactions by interacting with substrate at an energy of \u3c4 kcal mol-1. These catalysts are among the most controlled available for ROP. Part of this arises from the modular, highly tunable nature of dual catalysts, which conduct extremely controlled ROP of a host of cyclic monomers. The broader field of organocatalytic polymerization is a bridge between the disparate worlds of the materials chemist (ease of use) and the synthetic polymer chemist (mechanistic interest). The cooperative and collegial nature of the organocatalysis for the ROP community has facilitated the synergistic evolution of new mechanism to new abilities - in monomer scope, polymer architecture and level of reaction control

H-Bonding Organocatalysts for Ring-Opening Polymerization at Elevated Temperatures

The ring-opening polymerization (ROP) kinetics of ϵ-caprolactone and lactide with various H-bonding organocatalysts, (thio)ureas paired with an amine cocatalyst, were evaluated at temperatures up to 110 °C. In nonpolar solvent, most cocatalyst systems exhibit decomposition at high temperatures, while only two, a monourea and bis-urea H-bond donor plus base cocatalyst, are stable up to 110 °C. The onset temperature of cocatalyst decomposition must be measured under reaction conditions. In polar solvent, when the more active imidate form of the (thio)urea is favored, most cocatalyst systems become thermally stable up to 110 °C, exhibiting linear Eyring behavior, including some that were unstable in toluene. The very progress of an ROP is shown to influence the nature of the catalysts as the solution polarity changes from highly polar (at 0% conversion) to less polar at full conversion. Activation parameters are discussed, and a mechanistic explanation of the observations is proposed

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

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

Bis- and Tris-Urea H‑Bond Donors for Ring-Opening Polymerization: Unprecedented Activity and Control from an Organocatalyst

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