Here we demonstrate a synthesis of novel poly(methylacrylate)-based polymers with tertiary amines by directly post-modifying the methyl ester side chains of PMA. The transesterification reactions involve the reaction of PMA with various alcohols ((2-[2-(dimethylamino)ethoxy]ethanol, 1-(2-hydroxyethyl)piperidine, 4-(3-hydroxypropyl)morpholine)) containing tertiary amines under 1,5,7-triazabicyclo[4.4.0]dec-5-ene as nucleophilic catalyst. The complete conversions of PMA into polymer products, RW-S-1202, RW-S-1203, and RW-S-1204, were confirmed by nuclear magnetic resonance spectroscopy, indicating the efficient transesterification reaction. The pH responsiveness of the novel cationic polymers was confirmed via titration assay. Observed buffering capacity exhibited by the novel polymers containing tertiary amines during titration underscore the potential as carriers for negatively charged biomolecules within the body, such as messenger RNA. We anticipate further utilization of the newly synthesized cationic polymers in this study, aiming for enhanced intracellular delivery of nucleic acids and contributing to the design of the next generation of gene therapy drugs.