Insight into how molecular structures of thiophene-based conjugated polymers affect crystallization behaviors

Combining the structural characterization of solution crystals fabricated from thiophene-based conjugated polymers with different molecular structures and a theoretical investigation of the polymer conformational transformability leads to an interesting discovery of the relationship between the molecular structures and their crystallization behaviors. The chain folding or nonfolding behavior of thiophene-based conjugated polymers in crystallization, an important factor to shape polymer crystals, is determined by their molecular structures, and can be estimated by the inter-ring rotation energy barriers of the polymer backbones. A quantitative theoretical calculation is proposed to evaluate the inter-ring rotation energy barriers, and the values are correlated with the experimentally observed chain folding or nonfolding behavior. The higher percentage of type I inter-ring σ bond (CH3 and H are at 3 and 3′ position of adjacent aromatic rings, respectively) or the lower average rotation barrier in polymer backbones creates higher capability of polymer conformational transformation and higher tendency of chain folding. Our study provides a valid prediction of the crystallization behavior of thiophene-based conjugated polymers through a theoretical evaluation of conjugated polymer molecular structures, and offers an essential understand of the structure–property relationship of conjugated polymers.