Polymers with carboxylic acid functionalities are widely used in adhesives, absorbents, dispersants, drug delivery systems, and hydrogels. Unlike common radical polymerization, it is difficult to synthesize polymers with carboxylic acid groups via anionic ring-opening polymerization because of the harsh reaction conditions. Although a carboxylic acid-functionalized polyether, poly(glycidoxy acetic acid), was previously prepared by using monomer-activated ring-opening polymerization, this approach often suffers from a low initiation efficiency and is limited to homopolymerization. Herein, we present a novel functional epoxide monomer bearing oxazoline as a protecting group, 4,4-dimethyl-2-oxazoline glycidyl ether, for a controlled synthesis of poly(4,4-dimethyl-2-oxazoline glycidyl ether) by overcoming the aforementioned limitations. The stepwise syntheses of the monomer and polymers were carefully analyzed via 1 H NMR, GPC, FT-IR spectroscopy, and MALDI-ToF analysis. Furthermore, copolymerization with another monomer and a macroinitiator yielded well-defined polymers. We anticipate that this study will provide a new platform for the synthesis of polyethers with carboxylic acid functional groups.