Nonstoichiometric Stille Coupling Polycondensation Via Intramolecular Pd(0) Catalyst Transfer Using Excess Imide-Based Dibromo Monomers
Authors: Terayama Kosuke , Higashihara Tomoya 
Published: February 2020

π-Conjugated polymers are widely used for the organic electronic devices, such as Organic Light Emitting Diodes (OLED), Organic Photovoltaics (OPV) and Organic Thin-film Transistors (OTFT). These polymers are typically synthesized by AA+BB type polycondensations based on a transition-metal-catalyzed cross-coupling reaction. Generally, the number-average degree of polymerization of polymers (X
n) synthesized by AA+BB-type polycondensations is generally determined by Carothers equation as X
n=(1+r)/(1+r-2rp), where r is the stoichiometric molar ratio between AA and BB monomers and p is the extent of the reaction. Thus, the molecular weight of the polymers relies significantly on the stoichiometry of the monomers. In our previous report, high-molecular-weight n-type polymers were exceptionally obtained from 2,5-bis(trimethylstannyl)thiophene (1) and 1~10 fold excesses of 4,9-dibromo-2,7-bis(2-decyltetradecyl)benzo[lmn][3,8]-phenanthroline-1,3,6,8-tetraone by the Stille coupling polycondensation. The reason for the successful nonstoichiometric Stille coupling polycondensation can be explained by the intramolecular Pd(0) catalyst transfer after the substitution reaction of a bromo moiety of the dibromo-monomer with one distannyl-monomer, resulting in the C-Pd(II)-Br terminal via Pd(0)-π association, which can be readily substituted with other distannyl-monomers to afford all-stannylated oligomeric/polymeric terminals. Therefore, the excess dibromo monomers are not consumed for capping the oligomeric/polymeric during polymerization. In addition, it was suggested that the intramolecular Pd(0) catalyst transfer was promoted by the electron-deficient NDI structure in the model reactions. On the other hand, Yokozawa group also reported nonstoichiometric Suzuki-Miyaura coupling polycondensation via intramolecular Pd(0) catalyst transfer almost at the same time. In this report, they concluded that the electron-rich monomers are suitable for the intramolecular Pd(0) catalyst transfer because of the strong Pd(0)-π interaction between Pd(0) and dibromo monomers. Therefore, determining the criteria for the Pd(0) catalyst-transfer systems using electron-deficient monomers is necessary to extend the applicable monomers for stoichiometry-independent polycondensations.

Here, we report the effect of monomer structures and ligand types suitable for the nonstoichiometric Stille coupling polycondensation. First, the model reactions were examined between 2-tributylstannylthiophene (2) and 1 molar equiv. 1,3-dibromo-5-octylthieno[3,4-c]pyrrole-4,6-dione (3), or N-octyl-3,6-dibromophthalimide (4) which have the imide-substituent and the compact thiophene/phenylene skeletons. The results implied that the intramolecular Pd(0) catalyst transfer occurred using 3 with tri(o-tolyl)phosphine (P(o-tolyl)3) as the ligand; however, no catalyst transfer occurred using 4 with P(o-tolyl)3, judged from the presence/absence of the monoadduct product. On the other hand, the employment of a more electron-rich ligand, tri(t-butyl)phosphine (P(t-Bu)3), led to the intramolecular Pd(0) catalyst transfer on 4. It was suggested that the strong electron-donating ligand promoted the intramolecular Pd(0) catalyst transfer. Based on the model reactions, we examined the polymerization between 1 and 1,3-dibromo-5-(2-hexyldecyl)thieno[3,4-c]pyrrole-4,6-dione (5) or N-(2-hexyldecyl)-3,6-dibromophthalimide (6) under the nonstoichiometric conditions. Unfortunately, only oligomers were obtained in stoichiometric and nonstoichiometric condition between 1 and 5. In contrast, the high-molecular-weight polymers could be synthesized using 6 with P(t-Bu)3 under the nonstoichiometric conditions ([6]0/[1]0 =2~10). All polymers were confirmed to have almost the same structure by 1H NMR spectroscopy. The model reaction was further investigated using another electron-deficient benzothiadiazole (BTz)-based monomer instead of 4. However, the catalyst transfer does not appear to happen on the BTz monomer with any of the tested Pd catalysts including Pd2(dba)3/P(o-tolyl)3, Pd/P(t-Bu)3 and PEPPSI-
i
Pr based on the fact that mono-substituted products were obtained from the reaction between 2 and an equimolar amount of 4,7-dibromo-2,1,3-benzothiadiazole under all conditions. This result indicate that the specific feature of the imide substituents is necessary for the successful nonstoichiometric Stille coupling polycondensation. In conclusion, the high-molecular-weight polymers could be obtained under the nonstoichiometric conditions between 1 and 6 ([6]0/[1]0 =2~10) via the intramolecular Pd(0) catalyst transfer. The strong-electron donating phosphine ligand and the imide-substituent is crucial for the success of the intramolecular Pd(0) catalyst transfer in nonstoichiometric Stille coupling polycondensation.

Figure 1