Transparent Aromatic Polyimides Derived from Thiophenyl-Substituted Benzidines with High Refractive Index and Small Birefringence

2,2'-Bis(thiophenyl)benzidine (BTPB) and 2,2'-bis(4-chlorothiophenyl)benzidine (BCTPB) were synthesized via a benzidine rearrangement reaction of the corresponding hydrazobenzene derivatives obtained after the reduction of (3-nitrophenyl)(phenyl)sulfane and (4-chlorophenyl)(3-nitrophenyl)sulfane, respectively. Transparent polyimides (PIs) with high refractive indices and small birefringences as well as good thermomechanical stabilities were synthesized by the conventional two-step thermal polycondensation of BTPB and BCTPB with five different dianhydrides in N-methyl-2-pyrrolidone. 3,3',4,4'-Biphenyltetracarboxylic dianhydride (BPDA)/BCTPB PI, a fully aromatic and non-fluorinated PI, with a highly distorted noncoplanar conformation of the main chain was transparent and colorless with a transmittance of similar to 83% and >88% at 450 nm and in the visible region, respectively. In addition, all six non-fluorinated sulfur-containing aromatic PIs exhibited high average refractive indices (1.7112-1.7339), and small birefringences (0.0007-0.0022) at 633 nm, because of the high atomic polarizability of sulfur and the rigid bulky molecular structure of BTPB and BCTPB. The more effective steric effect caused by bulky 4-chlorothiophenyl in BCTPB improved the optical transparency but decreased the average refractive indices of BCTPB-based PIs compared to BTPB-based PIs.