Polymers that possess dynamic covalent bonds activated at ambient conditions are ideal platforms for smart, responsive materials. Herein, a class of dynamic covalent polymerizations is developed based on the thiol-thioester exchange, that is, transthioesterification, reaction. Shifts in the equilibrium extent of the exchange reactions are deliberately utilized to drive the formation of oligomers and polymers. In particular, a series of AB and A2-B2 monomers, including amino acid derivatives, were polymerized rapidly by catalytic amounts of mild bases in various organic solvents under ambient conditions. Thioester backbone oligopeptides, including cysteine and glycine, were obtained with an average repeating unit of four and a PDI of 1.8. Further, structurally dynamic hydrogels were obtained by such reactions between four-armed poly­(ethylene glycol)-10000 based monomers in neutral water. These hydrogels showed dynamic, frequency-dependent flow behavior between sol and gel states.