High-Performance ProDOT-Based Electrochromic Polymers with an Ester Group in the Side Chains: Synthesis, Effect of Ester Number, and Electrochromic Properties

3,4-Propylene dioxythiophene (ProDOT)-based polymers are a typical class of electrochromic materials of wide interest and excellent performance due to their large range of color change (optical band gap), low oxidation potential, and high optical contrast. The introduction of solubilizing side chains paves the way for low-cost-scale commercial applications. However, the introduction of solubilizing side chains with different structures or lengths will undoubtedly change the aggregation structure of the polymer and then affect the physicochemical properties of the polymer. Based on ProDOT, three kinds of soluble electrochromic polymers, OSProZ1, OSProZ, and OSPro2Z, were designed based on the ProDOT group and synthesized by arylation polycondensation. The effects of the conversion, quantity, and percentage of ester side chain on the electrochemical and electrochromic properties of polymer were systematically studied. It was found that the electrochromic properties of polymer changed with the increase in the number of ester group side chains. Among these soluble polymers, the OSProZ with a moderate side-chain percentage had a faster switching speed (0.64 s/0.47 s), higher optical contrast (at 556 nm of 66.4%), excellent coloring efficiency (598 cm2/C) and good cycling stability (only 2.4% decrease after 360 cycles). These results indicate that different solubilizing side chains can be designed to regulate the solubility, film formation, and some electrochemical and optical properties of the polymers.