Preparation of new hole transport polymers via copolymerization of N,N′-diphenyl-N,N′-bis(4-alkylphenyl)benzidine (TPD) derivatives with 1,4-divinylbenzene

Monomers containing N,N′,N,N′-tetraphenylbenzidine (TPD) were prepared by introducing two alkyl or alkoxy substituents into the p-position of two the four phenyl groups in the triphenylamine unit. The monomers were copolymerized with 1,4-divinylbenzene (DVB) in the presence of p-toluenesulfonic acid and Lewis acids, such as SnCl₄ and BF₃. The polymerization conditions and the catalyst types showed a marked influence on yield, molecular weight and even polymer structure. ¹H NMR measurements revealed that, by using p-toluenesulfonic acid as a catalyst, the linkage between the TPD unit and DVB occurs at the p-position of the phenyl group and at the m-position of the tolyl group in the TPD unit, whereas, with SnCl₄ or BF₃ catalyst, substitution occurs exclusively at the p-position of TPD. All polymers are soluble in common organic solvents, which allows the preparation of thin films of good quality. The rigid structure of the polymeric backbone results in a high thermal stability up to 400°C. The results of ionization potential measurements and the redox behavior showed that the introduction of the TPD unit into this kind of polymer backbone does not change its electronic structure. On the basis of high electroactivity, the polymers were successfully used as hole transport layers in two-layer electroluminescence devices.