RuPhos Pd Precatalyst and MIDA Boronate as an Effective Combination for the Precision Synthesis of Poly(3-hexylthiophene): Systematic Investigation of the Effects of Boronates, Halides, and Ligands

Abstract

Herein, we report detailed mechanistic studies of Suzuki-Miyaura catalyst-transfer polycondensation (SCTP) of thiophene. The effects of boronates, halides, ligands, and chain transfer agents (CTAs) on the control of polymerization were systematically investigated in detail by SEC, 1H NMR and MALDI-TOF analyses. Initially, we identified that the use of the slow-hydrolyzing N-methyliminodiacetic acid (MIDA) boronate in place of conventional pinacol boronate effectively suppressed side reactions such as protodeboronation, homocoupling, and chain transfer reactions, thereby improving control of SCTP. Screening halides revealed that the monomer containing bromide was optimal for SCTP, resulting in less side reactions. Moreover, screening several ligands and adding a CTA further supported our conclusion that the RuPhos-Pd system showed the best catalyst-transfer ability among the tested catalysts. We further elucidated that externally added ligands effectively stabilized living chain-ends and suppressed chain transfer, thereby achieving controlled polymerization.

Pairing Suzuki–Miyaura cross-coupling and catalyst transfer polymerization

“…91 Choi and coworkers then reported a controlled polymerization of a thiophene MIDA boronate with good control over M n and dispersity (17.6 kg mol −1 , Đ = 1.16). 31,39 Notably, polymerizing a thienyl-Bpin monomer resulted in lower yields as compared to the MIDA monomer due to protodeboronation. 31,39 Using the thienyl-MIDA monomer also narrowed dispersities and improved end-group fidelity, which was attributed to the lower concentration of active monomer for polymerization.…”

“…31,39 Notably, polymerizing a thienyl-Bpin monomer resulted in lower yields as compared to the MIDA monomer due to protodeboronation. 31,39 Using the thienyl-MIDA monomer also narrowed dispersities and improved end-group fidelity, which was attributed to the lower concentration of active monomer for polymerization. 31,39 Choi and coworkers also exploited reactivity differences between Bpin and BMIDA moieties to build a polythiophene block copolymer by combination of two monomers in one pot (Fig.…”

“…7). 31,33,39,40,44,45 Recent advances with allyl or indenyl precatalysts may prove to be beneficial for SM CTP in the future. [109][110][111][112] The externally initiated catalysts with a ligand, a halide and an arene (LM(Ar)Br) produce well-defined chain-ends and narrow molar mass distributions in SM CTP.…”

“…In CTP, two dialkylbiaryl ligands have received considerable attention, namely SPhos and RuPhos. 31,33,39,69,115,116 The first example of these ligands being used in SM CTP of 3-alkylthiophenes appeared in 2006 from Higgins. 69 Though chaingrowth polymerization was not the focus of that work, it is clear from the data obtained that the obtained P3HT was highly regioregular, with a narrow molecular weight distribution.…”

“…56 Following this work, Choi recently described an approach to synthesize poly(3-alkylthiophenes) via polymerization of MIDA boronates with externally initiated RuPhos and SPhos catalysts. 31,39 The authors noted that additional phosphine in the reaction mixture relative to catalyst was key to achieving a controlled polymerization. In our own work, we have also noted the importance of additional equivalents of phosphine and diphosphine ligand in chain-growth polymerizations (with both Pd and Ni).…”