A new n-type conjugated polymer is prepared using aldol polycondensation. The backbone is designed to be linear, coplanar and to have a low LUMO energy level. A bisisatin monomer is made in four steps only under metal-free and ambient conditions. Key to the linear and coplanar structure is the bridging of two isatin motifs by a thiazolothiazole unit. Aldol copolymers with benzodifuranone afford polymers with low LUMO energy levels of ∼ −4.2 eV without using halogens. Defect formation during aldol polycondensation is studied quantitatively using monofunctional model compounds. Besides known defects such as homocouplings, fused analogs are observed along with other defects. Zinc stearate is identified as a superior catalyst, with defects being mostly absent in model reactions. This is in contrast to all other catalytic conditions used, including common literature protocols. Importantly, water must be continuously removed from the reaction to prevent the formation of defects. From these results, protocols are selected that lead to different defect densities in aldol copolymers. Spectroscopic methods confirm the defective nature of aldol polymers but offer limited structural insight into their nature and abundance. Electrospray deposition scanning tunneling microscopy enables direct identification and quantification of defect structures, revealing both similar trends and notable differences in their relative frequencies between small-molecule model systems and full polymers, rationalized by differences in reaction equilibria and chain mobility. Alongside introducing a new low LUMO n-type polymer synthesized via a metal-free route, this study opens pathways toward defect-free aldol polymers, which are otherwise intrinsically prone to defect formation.