Dynamic Ring-chain Equilibrium of Nucleophilic Thiol-Yne “Click” Polyaddition for Recyclable Poly(dithioacetal)s

We report a dynamic polymerization system based on the reversible nucleophilic Michael polyaddition of activated alkynes and dithiols. Four poly(dithioacetal)s (P1-P4) were prepared via the base-catalyzed thiol-yne “click” polyaddition of two dithiols (1,4-butanedithiol (4S) and 1,5-pentanedithiol (5S)) and two alkynones (3-butyn-2-one (Y1) and 1-phenyl-2-propyn-1-one (Y2)) at high concentrations. We systematically investigated the base-catalyzed polymerization of 4S and Y1 (for polymer P1) under different conditions, and found that this polymerization was a highly concentration-dependent dynamic system: polymer P1 was formed at high concentration, while seven-membered dithioacetal, 1-(1,3-dithiepan-2-yl) propan-2-one (C1), was obtained at low concentration. The polymerization of 4S and Y2 (for polymer P4) displayed similar polymerization behavior, generating 2-(1,3-dithiepan-2-yl)-1-phenylethanone (C4) at low concentration. On the contrary, polymer P2 (from Y1 and 5S) was obtained with no formation of eight-membered dithioacetal and it was stable at any concentration. The polymerizations of Y1 with 1,2-ethanedithiol (2S) and 1,3-dimercaptopropane (3S) only afforded corresponding five-and six-membered dithioacetals, 1-(1,3-dithiolan-2-yl) propan -2-one (C2) and 1-(1,3-dithian-2-yl) propan-2-one (C3). This dynamic behavior of P1 and P4 was attributed to the concentration-dependent retro-Michael addition reaction between a thiol and a β-sulfido-α, β-unsaturated carbonyl compound catalyzed by bases. Furthermore, polymers P1 and P4 could be depolymerized into C1 and C4 in yields of 58% and 95%, respectively. The ring-opening polymerization of C1 at high concentration could successfully regenerate polymer P1. Thus, a new type of closed-loop recyclable poly(dithioacetal)s was developed.