A series of modified polyimides (PIs) exhibiting refractive indices higher than 1.74 were developed by the introduction of sulfur-containing moieties.First

two sulfurheterocyclic-bridged diamines

2

5-bis(4-aminophenylenesulfanyl)thiophene (APST) and 2

5-bis(4-aminophenylenesulfanyl)-1

3

4-thiadiazole (BATT) were synthesized.The two diamines were polymerized with a sulfurcontaining dianhydride 4

4′-［p-thiobis(phenylenesulfanyl)］ diphthalic anhydride (3SDEA) respectively via a two-step polymerization procedure to afford two polyimides (PIs)

PI-1 (3SDEA-APST) and PI-2 (3SDEA-BATT).The effects of the sulfur-heterocyclic structure on the properties of the PIs are investigated.It was found that the introduction of the heterocyclic structure decreased the thermal stability and the optical transparency of the PI films to some extent.The initial thermal decomposition temperatures of the PIs are 418℃ for PI-1 and 466℃ for PI-2.The transmittance of the PI films at 450 nm is lower than 50%.However

the sulfur-heterocyclic structure apparently increases the refractive indices of the PI films.The PIs show refractive index of 1.7527 for PI-1 and 1.7490 for PI-2.In addition

the PIs have birefringences lower than 0.01.