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 exclusively obtained with no formation of eight-membered dithioacetal. 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 beta-sulfido-alpha,beta-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.

Automated summary

A dynamic polymerization system was developed based on reversible nucleophilic Michael polyaddition of activated alkynes and dithiols, leading to the formation of poly(dithioacetal)s in high concentration but smaller ring molecules at low concentration.