Sequential self-repetitive reaction toward wholly aromatic polyimides with highly stable optical nonlinearity

Abstract

A sequential self-repetitive reaction (SSRR) based on carbodiimide (CDI) chemistry was utilized for preparing a high-yield wholly aromatic polyimide. The polyimide was synthesized with 4,4′-methylene-diphenylisocyanate (MDI) and a di(acid-ester) compound which was derived from the ring-opening reaction of 3,3′,4,4′-oxydiphthalic dianhydride (ODPA) at room temperature by the addition of equimolar methanol. Poly-CDI was first synthesized from MDI. The di(acid-ester) compound was then reacted with poly-CDI to form poly(N-acylurea). After curing process, N-acylurea moiety was converted to di(ester-amide) structure viaSSRR and further subjected to a ring-closure reaction to form the wholly aromatic polyimide with a T_g of 247 °C. This approach was further taken to prepare thermally stable nonlinear optical (NLO) materials. Similarly a diimide-diacid containing chromophore was reacted with poly-CDI to obtain an intermediate, poly(N-acylurea). The poly(N-acylurea) with the ester side groups would exhibit excellent organosolubility, which enabled the fabrication of high quality optical thin films. After in situ poling and curing processes, N-acylurea moiety was converted to di(ester-amide) structure viaSSRR and further subjected to a ring-closure reaction to form the wholly aromatic NLO polyimide with an electro-optical coefficient, r₃₃ of 25 pm/V (830 nm). Excellent temporal stability at elevated temperatures (200 °C) and a waveguide optical loss of 2.5 dB cm⁻¹ at 1310 nm were also obtained.