Single layers of covalently linked organic materials in the form of two-dimensional (2D) polymers constitute structures complementary to inorganic 2D materials. The electronic properties of 2D polymers may be manipulated through a deliberate choice of the organic precursors. Here we address the changes in electronic CO structure-from precursor molecule to oligomer by scanning tunneling spectroscopy and ultraviolet photoelectron spectroscopy. For this purpose, we introduce the polymerization reaction of 1,3,5-tris(4-carboxyphenyl)benzene via decarboxylation on Cu(111), which is thoroughly characterized by scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations. We present a comprehensive study of a contamination-free on-surface coupling scheme and study how dehydrogenation, decarboxylation, and polymerization affect the electronic structure on the molecular level.