**1. Introduction**

Aromatic polyimides (PIs) are well known as high-performance polymeric materials having excellent thermal, mechanical, and electrical properties. As a result of these properties, many PIs have been commercialized and used widely in microelectronic and aerospace engineering [1, 2]. Recently, aromatic PIs are considered as a strong candidate for flexible plastic substrates applicable to flexible electronics, including flexible solar cell arrays and flexible organic light-emitting diode (OLED) displays [3]. Despite the outstanding results associated with the use of aromatic PIs, they also have a number of drawbacks, one of which is their poor processability caused by their limited degrees of solubility in organic solvents due to strong interchain interactions. Another shortcoming is the pale yellow or a deep brown color of PI films due to their highly conjugated aromatic structures and/or the formation of an intermolecular charge-transfer complex (CTC) between alternating electron-donor (diamine) and electron-acceptor (dianhydride) moieties, thus narrowing their applicability [4, 5].

To overcome these problems, much research effort has focused on the synthesis of soluble and transparent PIs in a fully imidized form without deterioration of their excellent properties [4, 6]. Several successful approaches to synthesize soluble and transparent PIs, including the insertion of flexible or unsymmetrical linkages or bulky substituents on the main chain and the use of noncoplanar or alicyclic monomers, have been introduced over the last few decades [4–8].

Among many approaches, the incorporation of trifluoromethyl (CF3) groups onto polymer chains is considered as an effective means of realizing soluble and transparent PIs without deteriorating their excellent properties, not only because bulky CF3 groups disturb the interactions and chain packing between the polymer chains, but also because the strength of the carbon-fluorine chemical bond is the one of the strongest single bonds [6, 9–30]. It is also possible to give the corresponding PIs have many attractive features, such as a low refractive index as well as low optical loss, dielectric constant, surface energy, and moisture absorption characteristics, due to the high electronegativity and low electric polarity of fluorine atoms [31–39].

Recently, we reported new soluble PIs which were prepared from 4-(4<sup>0</sup> aminophenoxy)-3,5-bis(trifluoromethyl)aniline to introduce two CF3 groups unsymmetrically onto the repeating units of the chain [40, 41]. Unsymmetrical incorporation of the substituents into the main chain of PIs can improve the solubility and optical transparency because increasing the irregularity of the microstructure of PIs disrupts the interchain interactions [42–47]. The PIs synthesized in earlier work showed good solubility while retaining their useful thermal and optical properties due to the unsymmetrical presence of CF3 groups as substituents. Furthermore, the good solubility of the PIs led them to show lower critical solution temperature (LCST) behavior in organic solvents. This unprecedented phenomenon of the PIs may stem from a change of the interaction strength in the vicinity of CF3 between the polymer chains and the acetyl-containing solvents [41].

Subsequently, we designed another monomer, 2,6-bis(trifluoromethyl) benzidine, which has two CF3 groups at the 2,6-positions of the benzidine unit [48]. Although this monomer has more rigid structure compared to 4-(4<sup>0</sup> aminophenoxy)-3,5-bis(trifluoromethyl)aniline, a series of poly(amide-imide)s synthesized from the monomer exhibited good solubility as well as good thermal and optical properties. Meanwhile, in terms of the structure, the new benzidine monomer has an isomeric relationship with 2,2<sup>0</sup> -bis(trifluoromethyl)benzidine, well known as a rigid/linear benzidine unit containing the CF3 group and frequently employed in the synthesis of PIs having a high thermal resistance, a high *T*<sup>g</sup> value, a low degree of thermal expansion, a low refractive index, and low water absorption capabilities [16–30]. Therefore, we envisioned that the PIs obtained from the new benzidine monomer would exhibit high thermal and mechanical properties while maintaining good solubility in organic solvents, as they have twisted structures while retaining the rigidity of the chains. The chemistry and the physical properties of the PIs prepared from the twisted benzidine monomer containing two trifluoromethyl (CF3) groups on one aromatic ring are described herein.
