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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 β-sulfido-ar,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.