This study describes a one-step Passerini and esterification reaction strategy for the first time at the macromolecular level in the literature. Polythioether (P-T) including di-tert-butyl acetylenedicarboxylate (DTBADC) units is synthesized via a previously described rapid double thiol-Michael addition polymerization method, followed by hydrolysis of tert-butyl groups to produce dicarboxylic acid pendant polythioether (P-H). P-H is then utilized for postpolymerization modification via Passerini 3-component reaction (Passerini- 3CR). Interestingly, it is found that on the course of Passerini-3CR, the use of methanol (CH3OH) as a co-solvent not only dissolves P-H but also leads to the esterification reaction, which thereby further modifies the resultant polymer. This finding encourages the creation of a variety of ester functionality in the final polymer via Passerini-3CR, by solely varying alcohols. The ratio of both ester and Passerini products in the modified polymers is monitored by H-1 NMR analyses and a mechanistic aspect is also presented to product distribution. In addition, a monoacid containing polythioether (P-MH) is prepared and used for the modification with Passerini-3CR under the identical conditions described for P-H, only Passerini product is obtained with no sign related to the ester product.