End-group stannylation of regioregular poly(3-hexylthiophene)s

Polythiophenes have been intensively studied as semiconducting polymers for various applications such as in organic solar cells (OSCs)¹ and organic field-effect transistors.² To modify the intrinsic properties of polythiophenes, such as crystallinity and optoelectronic properties, and to improve their device performance, much work has been performed on the modifications of their side chains³ and end groups.^{4, 5} Concerning these end groups, it has been reported that hydrogen-terminated regioregular poly(3-hexylthiophene) (P3HT) shows a slightly higher absorption coefficient, stronger photoluminescence, and more highly ordered interchain packing than bromine-terminated P3HT, resulting in better performance in OSCs. Cho et al.⁴ reported that the surface energy of P3HT can be varied by the introduction of different end groups and that an improved phase separation of the active layer can be realized with a matched surface energy between P3HT and [6,6]-phenyl-C₆₁ butyric acid methyl ester, leading to enhanced photovoltaic efficiency.⁵ Besides these simple modifications, end-group functionalization has been used for the initiation of polymerization, leading to the synthesis of various P3HT-based block copolymers.^{6, 7}

There are three possible strategies for functionalizing the end groups of P3HT: the quenching of polymerization with a functionalized quencher,⁸ the initiation of polymerization with a functionalized initiator,⁹ and the post-functionalization of purified polymer. The first two strategies rely on the chain-growth nature of Ni-catalyzed Grignard metathesis (GRIM) polymerization of P3HT, in which the Ni complex serves as the initiator and the growing P3HT chains have living ends during the reaction process. Although both the initiator and quencher methods are effective for introducing functionality into one of the ends, a prerequisite of both approaches is that target end groups should be chemically tolerant to Grignard reagents, thus limiting their applicability to a certain extent. On the other hand, even though the post-functionalization of purified P3HT has certain drawbacks, such as lower introduction rate and selectivity, it is free from the limitations of functionalities and can be used to functionalize both ends of chains.^{10, 11, 12} Taking advantage of this, reactive groups such as carboxylic acids^{10, 11} and aldehydes¹² have been introduced into the ends of P3HT by post-functionalization. However, this approach has been relatively less studied.

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