Tuning Mechanical and Optoelectrical Properties of Poly(3-hexylthiophene) through Systematic Regioregularity Control

While the regioregularity (RR) of conjugated polymers is known to have a strong influence on their inherent properties, systematic study of the RR effect has been limited due to the lack of a synthetic methodology. Herein, we successfully produced a series of poly­(3-hexylthiophene)­s (P3HTs) having a wide range of RR from 64 to 98%. Incorporation of controlled amounts of head-to-head (H–H) coupled dimer in modified Grignard metathesis polymerization allows a facile tuning of the RR of the P3HTs with comparable molecular weight and low polydispersity. Then, we investigated the effect of RR on structural, electrical, and mechanical properties of P3HTs in which a higher content of H–H regio-defects, namely lower RR, systematically lowered the degree of crystallinity. Although high RR P3HT (98%) had higher charge carrier mobility (1.81 × 10^–1 cm² V^–1 s^–1), its strong crystallinity induced high brittleness and stiffness, resulting in device failure under a very small strain, as shown in tensile and bending tests. The tensile modulus was reduced significantly from 287 MPa (RR 98%) to 13 MPa (RR 64%), and also the RR 64% P3HT film had much better mechanical resilience with an order of magnitude higher elongation at break than that of the RR 98% polymer. Our findings suggest that the mechanical and electrical properties of conjugated polymers can be systematically tuned by controlling the RR to meet the purposes of various organic electronic applications, i.e., flexible portable devices vs high-performance panels