Intramolecularly Cooperative Catalysis for Copolymerization of Cyclic Thioanhydrides and Epoxides: A Dual Activation Strategy to Well-Defined Polythioesters

Abstract

The copolymerization of epoxides and cyclic thioanhydrides provides a promising method to produce polythioesters and represents an alternative route to the step-growth condensation of dithiols and diacyl chlorides. However, typical binary metal–salen complex/ionic salt systems suffer from an unavoidable transesterification reaction, which degrades the polymer backbone and changes the polymer properties. We herein describe the precise synthesis of polythioesters from the copolymerization of cyclic thioanhydrides and epoxides by an intramolecular acid–base cooperative catalysis strategy, which can significantly suppress side reactions. The bifunctional catalyst composed of Lewis acidic Cr–salen complex with a covalently tethered 1,5,7-triazabicyclo­[4,4,0]­dec-5-ene (TBD, a sterically hindered strong Lewis base) successfully suppressed the transesterification reaction even at low concentrations, yielding polythioesters with completely alternating structures, controlled molecular weights, and low dispersities. Mechanistic studies revealed that polymer chain propagation was predominantly proceeded by intramolecular alternating enchainment between the Lewis acid-activated epoxide and the Lewis base-mediated cyclic thioanhydride.

Heterocycle/Heteroallene Ring‐Opening Copolymerization: Selective Catalysis Delivering Alternating Copolymers

“…Scrambling increased with temperature and influenced the polymer's glass transition temperature ( T g =62 °C from reactions at 100 °C, T g =70 °C from reactions at 25 °C, both with M n ≈18 kg mol −1 ). A tethered Lewis base/Cr III ‐salen catalyst showed fewer transesterifications (TOF 94–567 h −1 , 0.1–0.01 mol %, 25–70 °C, M n =15.9–49.7 kg mol −1 ) [193] . Highly regioregular poly(ester‐ alt ‐thioester) formed at 25 °C but, once again, higher temperatures resulted in scrambling.…”

“…Very recently the sulphur analogues of anhydride/epoxide ROCOP were reported to deliver poly(thioester) structures inaccessible by conventional routes. [191][192][193] Figure 29: ROCOP of thioanhydride with epoxides or thiiranes.…”

S‐Carboxyanhydrides: Ultrafast and Selective Ring‐Opening Polymerizations Towards Well‐defined Functionalized Polythioesters

“…Motivated by the importance of the targets, the laboratories of Gutekunst, [10] Bowman, [11] Suzuki, [12] Lu, [13] Ren [14] and Chen [15] have developed elegant strategies for more controlled sulfur‐based ROP towards polythioesters. However, the synthesis of precisely controlled polythioesters via ROP of thiolactones still faces formidable challenges, including the minimal functional diversity of available thiolactone monomers, as well as the trade‐off between selectivity and efficient chain propagation: running polymerizations to full conversion enhances the efficiency but also promotes competitive transthioesterification.…”

Heterocycle/Heteroallene Ring‐Opening Copolymerization: Selective Catalysis Delivering Alternating Copolymers

“…Very recently,t he sulfur analogues of anhydride/ epoxide ROCOP were reported to deliver poly(thioester) structures inaccessible by conventional routes. [191][192][193] Lu and co-workers reported ROCOP of thiiranes with stearic (STA) or glutaric thioanhydride (GTA) using Lewis base/PPNX catalysts (Figure 29). [191] Them ost effective was PPNOAc, which delivered polymers quickly with molar masses close to theoretical expectations (TOF 787 h À1 ,2 5 8 8C, 0.4 mol %, M n = 53.6 kg mol À1 ,for STA/PS;F igure 30).…”