Direct C-H coupling polymerization of asymmetric monomer: Synthesis an properties of regioregular poly(alkyl thiophene-3-carboxylates)

A series of poly(alkyl thiophene-3-carboxylates), herein referred to as poly(3-alkylesterthiophenes) (P3ET) (P1-P6) with high head-to-tail (HT) coupled regioregularity up to similar to 92% were synthesized by a direct C-H coupling polymerization (DCHCP). The substitutions with various alkyl ester side-chains on the 3-position of the thiophene ring were performed by changing the length and size of alkyl groups, with a view to investigate the role of the alkyl ester side chain on DCHCP reaction as well as on the crystallinity, optical and electrochemical properties of these resultant polymers. It was found that the longer and bigger size alkyl ester side chains on 3-position of thiophene monomers, the higher HT regioregularity of the corresponding polymer products. X-ray diffraction (XRD) measurements manifested that all polymers (P1-P6) existed in not highly ordered state in solid films especially in the it-stacking direction. Red shift of the UV-Vis absorption maxima of the resultant polymers in the film state confirmed more effective electron delocalization along the polymer chain than that in solution state. Cyclic voltammetry (CV) study indicated that the introduction of 3-alkyl ester side chain on the thiophene ring lowered the products' highest occupied molecular orbital (HOMO) energy level and enhanced their electrochemical band gaps significantly in comparison with the corresponding HT regioregular poly(3-alkylthiophenes).