General and Mild Method for the Synthesis of Polythioesters from Lactone Feedstocks

Abstract

Polythioesters are attracting increasing interest in applications requiring degradability or recyclability. However, few general methods exist for the synthesis of these polymers. This report presents a fast and versatile method for synthesizing polythioesters from readily available lactone feedstocks. The two-step process begins with the thionation of lactones to thionolactones, followed by the ring-opening polymerization of the thionolactones to polythioesters. Unlike previous methods that rely on harsh reagents to accomplish this transformation, we demonstrate that the mild tetrabutylammonium thioacetate is a competent initiator for polymerization. This method exhibits broad applicability, as demonstrated by the successful polymerizations of an unstrained 17-membered macrocycle and an N-substituted cyclic thionocarbamate. Furthermore, the generality of this scheme enables the synthesis of polythioesters with highly tunable properties, as demonstrated here by the synthesis of a set of polymers with glass transition temperatures spanning 180 °C. Finally, the polythioesters are efficiently depolymerized into the corresponding thiolactones.

Figure 1

Overview of the previous work. (a) Initial work by Endo demonstrating conversion of thionolactones to polythioesters in a polythionoester/polythioester copolymer. (b) Yuan et al.’s recent work showing full selectivity to polythioester products with phosphazene superbase initiator system. (c) Typical 1,2-addition–elimination mechanism expected in ROP of thionolactones which generates polythionoesters. (d) S_N2 of thionolactones to generate polythioesters. (e) This work: mild, general, and selective conversion of thionolactones to polythioester followed by depolymerization to thiolactone small molecules.

Figure 2

Depolymerization of polymers. (a) Example of depolymerization of polymer P6. Conditions: 1 equiv of P6, 20 equiv of TBD/dodecanethiol per chain end, 0.5 M with respect to moles of repeat units, MeCN, 80 °C, 2 h. (b) Comparison of ¹H NMR of polymer P6 compared to the thiolactone product 9 produced after depolymerization.