<p>Two pyrimidoisoindigo-based polymers were synthesized by copolymerizing thiophene-flanked pyrimidoisoindigo (T-PymII) with thiophene (T) or 3,4-difluorothiophene (2FT), and their structure-property correlations were investigated. Although both polymers exhibited ambipolar transport properties, P(PymII-TTT was dominated by hole transport, while P(PymII-T-2FT-T) was dominated by electron transport. Among them, the highest hole mobility up to 1.66×10⁻² cm²·V⁻¹·s⁻¹ was observed for P(PymII-TTT), while the highest electron mobility of 6.37×10⁻³ cm²·V⁻¹·s⁻¹ was observed for P(PymII-T-2FT-T). AFM and GIWAXS analyses revealed that their poor morphology and crystallinity may account for their inferior performance. Therefore, further side-chain engineering is needed to improve the crystallinity of PymII-based polymers.</p>