Contrasting Roles of Counterions in Anionic Ring-Opening Polymerization Mediated by Heterocycle Organocatalysts

Highly Selective O-Phenylene Bisurea Catalysts for ROP: Stabilization of Oxyanion Transition State by a Semiflexible Hydrogen Bond Pocket

“…As shown in Table , the anions of the monoureas U-1 and U-2 exhibit selectivity parameters k p / k tr = 1000, whereas those of the thiourea anion TU-1 and urea anion U-3 ( k p / k tr = 190 and 360, respectively) are lower than those of U-1 and U-2 but higher than that of TBD ( k p / k tr = 70). The BU-1 dianion emerged as the most selective catalyst in our study, with a k p / k tr ratio of 8500, surpassing TBD by over 120-fold, the U-3 anion by 24-fold despite similar reactivity, and outperforming the recently reported indolocarbazole potassium system by 13-fold . Comparing OPBU to other bisureas lacking o -phenylene linkages, the o -phenylene bridged bisurea BU-2 monoanion is 10 times more selective than its meta-substituted counterpart, BU-4, indicating that the disposition of the ureas in the phenyl ring (ortho- vs meta-) significantly influences both the activity and selectivity.…”

“…The catalytic behaviors of both the mono- and di-deprotonated forms of the bisureas BU-1–6 were assessed by the ring-opening polymerization of ε-caprolactone (ε-CL) in THF at room temperature with bisurea catalyst loadings of 0.55 mol % and base loadings of 0.5 or 1.0 mol %. The polymerization behavior of the bisurea complexes was compared with those of other organic catalysts TBD, , (thio)urea ,, anions, and indolocarbazole potassium complexes (eq 1, Table ).…”

“…In this study, we report a class of OPBU (di)anions as a mild yet highly efficient catalytic system for fast, tunable, and ultraselective ROP of various cyclic monomers. Of the various classes of active and selective organic catalysts, − ,,, the OPBU (di)anions stand out both for their fast and tunable rates and high selectivities. On the basis of our studies to date, the BU-1 anion or dianion is the most versatile for high to moderately reactive monomers; for less reactive monomers (ε-CL, δ-CL), the more active BU-3 (di)anions are recommended.…”

Ring-Opening Polymerization of Cyclic Esters and Carbonates with (Thio)urea/Cyclopropenimine Organocatalytic Systems

“…Organocatalytic ring-opening polymerization (OROP) of various monomers such as cyclic esters, carbonates, and derivatives is a versatile synthetic strategy for functional, biocompatible, and biodegradable materials. − Over the last two decades the advancement of OROP has facilitated the discovery of new materials for a wide range of applications. − …”