Bisurea and Bisthiourea H-Bonding Organocatalysts for Ring-Opening Polymerization: Cues for the Catalyst Design
Date of Original Version
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.
Hewawasam, Rukshika S., U. L.D.Inush Kalana, Nayanthara U. Dharmaratne, Thomas J. Wright, Timothy J. Bannin, Elizabeth T. Kiesewetter, and Matthew K. Kiesewetter. "Bisurea and Bisthiourea H-Bonding Organocatalysts for Ring-Opening Polymerization: Cues for the Catalyst Design." Macromolecules 52, 23 (2019): 9232-9237. doi:10.1021/acs.macromol.9b01875.