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Aromatic polyimides are widely used in the semiconductor and electronic packaging industry because of their outstand­ ing thermal stability, good insulation properties with low dielectric constant, good adhesion to common substrates, and superior chemical stability.1,2 However, their applications were limited in many fields because the early polyimides were insoluble and intractable. Therefore, considerable research has been undertaken to identify new ways to circumvent these restrictions. To circumvent these problems, solutions of polyimides would be desirable, instead of precursors. Flexible links (e.g. O-, -SO2-, -CH2-) or bulky groups (fluorenylidene, hexafluoropropylidene, etc.) are commonly employed as solubi­ lizing moieties, but it often costs a desirable in thermal stability and chain stiffness of polyimides. Alternatively, the solubilizing units may be derived from substituted biphenyls, increasing entropy factors due to rotational isomerism of benzene rings relative to aryl-aryl bond.3,4 The relevant compounds of this class are biphenyltetracarboxylic dian­ hydrides. Among them, 3,3',4,4'-biphenyltetracarboxylic dianhydride (s-BPDA) was commercialized in the early 1980s and is widely used at present.5 It has been known that polymers containing t-butyl group have excellent gas permeability.6-14 Soluble polyimides con­ taining bulky t-butylphenyl group offer particular promise in gas separation membrane application due to their superior mechanical property, high permeability and permselectivity. Thus, we tried to synthesize new s-BPDA-based polyimides containing bulky p-t-butylphenyl group, which is expected to give good solubility and high permeability and perm­ selectivity with good thermal stability and mechanical pro­ perty. In this communication, we report the synthesis and characterization of new soluble polyimides derived from sBPDA containing bulky p-t-butylphenyl group, and conven­ tional aromatic diamines. Monomer Synthesis. The monomer was obtained by various organic reaction. 3,3',4,4'-Tetramethylbiphenyl was obtained by Ni-coupling of 4-bromo-o-xylene. The bromi­ nation of 3,3',4,4'-tetramethylbiphenyl gave 2,2'-dibromo4,4',5,5'-tetramethylbiphenyl, which was reacted with p-tbutylbenzene boronic acid to generate 2,2'-bis(4''-t-butylphenyl)-4,4',5,5'-tetramethylbiphenyl. 2,2'-Bis(4''-t-butylphenyl)biphenyltetracarboxylic dianhydride (BBBPAn) was obtained by oxidation and dehydration of 2,2'-bis(4''-t-butylphenyl)-4,4',5,5'-tetramethylbiphenyl. The yields in each steps were very high and the obtained products in each steps were confirmed by various spectroscopies. Polymer Synthesis. It is known that there are two general methods for the preparation of polyimides. One procedure involves two steps and proceeds via poly(amic acid)s inter­ mediate. The other is one-step solution polymerization. The one step method has some advantage over the two step method. Polyimides in bulk are more easily produced, and polyimides with higher crystallinity can be more readily obtained.15 The one step method is also useful for unreactive diamines and dianhydrides which can not form high mole­ cular weight polyamic acids by the two step method.16-17 The disadvantage of the one step method is that insoluble poly­ imides can not form high molecular weight polyimides because of premature precipitation. We tried two general methods. When polyimides were synthesized by the conven­ tional two step procedure involving a ring-opening poly­ addition and subsequent thermal cyclodehydration, the poly­ mer molecular weight was not enough for preparing of the free stand film. It may be suggested that dianhydride is deactivated by steric hindrance of t-butylphenyl substituents in 2,2' position of s-BPDA unit. Otherwise, the one-step polymerization at high temperature, as shown in synthetic route b, resulted in high viscosity. BBBPAn and diamines were reacted in m-cresol with 2% (w/w) isoquinoline as a

Synthesis of New Highly Organosoluble Polyimides Bearing a Noncoplanar Twisted Biphenyl Unit Containing t-Butyl Phenyl Group