Donor and acceptor (D-A) type conjugated polymers (CPs) with large and rigid p-backbones enabled tunable energy levels, high charge transport property, great mechanical flexibility, and solution processability, which are essential materials for the plastic electronics and optoelectronics. To date, various thermally activated cross-coupling polycondensation have been developed to synthesize these CPs, such as Stille, Suzuki-Miyaura, and the direct arylation polycondensation (DArP). Thanks to the low toxicity profile, good site selectivity, and excellent functional group tolerance, the Suzuki-Miyaura cross-coupling has been widely used in the polymerization of phenyl nucleophiles. However, the usage of heteroaryl nucleophiles in Suzuki-Miyaura cross-coupling are very challenging due to the competitive protodeboronation from the hydrolysis of heteroaryl boronic acid. Moreover, these thermal-activation Suzuki-Miyaura cross-coupling usually led to undetectable and inseparable structural defects in CPs due to the homocoupling reactions of the aromatic halides, which not only resulted in batch-to-batch differences, but also impaired their optoelectronic properties. Since the homocoupling of the electrophiles normally occurs under high temperatures, exploring an environmentally friendly and room temperature cross-coupling method may be an ideal choice to achieve well-defined alternating CPs.

In recent years, our group has been committed to developing new synthetic methods based on C-S bond activation (Nat. Chem. 2017, 9, 188; Inorg. Chem. 2018, 57, 9266; Angew. Chem. Int. Ed. 2019, 58, 5044; Chem. Eur. J. 2022, 28, e202200869.). The latest research has found that using aryl sulfide as a substrate to replace aryl halides can prepare alternating conjugated polymers with more regular structures and better charge transfer properties (Nat. Commun. 2022, 13:144; Angew. Chem. Int. Ed. 2023, e202306307.), which is named CASP (carbon and sulfur bond activation based polymerization). Here, an anhydrous room temperature Suzuki-Miyaura cross-coupling reaction between (hetero)aryl boronic esters and aryl sulfides was explored (CAS-Suzuki-Miyaura polymerization), of which universality was exemplified by thirty small molecules and twelve CPs. Meanwhile, the mechanistic studies involving with capturing four coordinated borate intermediate revealed the direct transmetalation of boronic esters in the absence of H₂O suppressing the protodeboronation. Additionally, the room temperature reaction significantly reduced the homocoupling defects and enhanced the optoelectronic properties of the CPs.

Aryl sulfides were selected as the principal test electrophiles to react with (hetero)aryl boronic esters catalyzed by Pd(PPh3)4 at room temperature. Importantly, the capture of the four coordinated borate intermediate, which was confirmed by the 1H NMR and HRMS analysis, allows to propose a sequential oxidative addition-transmetallation-reductive elimination mechanism.

To demonstrate the universality of this methodology, various aryl boronic esters were screened under the optimal conditions. The results showed that the reaction gives high yields for heteroaryl boronic esters with substituents at different positions, and it also has good compatibility for substituted aryl sulfides, giving cross-coupling products in moderate yields.

Inspired by the success of the small molecule reactions, an unprecedented Suzuki-Miyaura-type polycondensation between aryl sulfides and aryl boronic acids or aryl boronic esters were explored, and the prepared polymers can have an average molecular weight of up to 38.6 kDa. Taking polymer P10 as an example, the author found that the polymer synthesized at room temperature had less homocoupling defects, less defect density of states, and higher hole mobility than at 70^oC。

In summary, this work develops the Suzuki-Miyaura method based on carbon-sulfur bond activation, which provides a new synthetic strategy for the synthesis of low-defect, high-performance organic/polymer semiconductor materials.

The first authors of this article are Wen Xuan, a master candidate in the research group, and Xie Wenbin, a special research assistant (post-doctors) of UCAS. The co-corresponding authors are professor Huang Hui, Shi Qinqin, Peng Aidong. The authors acknowledge the financial support from the NSFC, National Key R&D Program of China, Key Research Program of the Chinese Academy of Sciences, the Strategic Priority Research Program of Chinese Academy of Sciences, and Fundamental Research Funds for the Central Universities.

References：

Room Temperature Anhydrous Suzuki-Miyaura Polymerization Enabled by C–S Bond Activation

Xuan Wen,† Wenbin Xie,† Yawen Li, Xiaoying Ma, Zhaoying Liu, Xiao Han, Kaikai Wen, Fengjiao Zhang, Yuze Lin, Qinqin Shi,* Aidong Peng,* Hui Huang*

Angewandte Chemie International Edition

DOI: 10.1002/anie.202309922