Unveiling Autocatalytic Kinetics in the Short-Chain Polyaddition of Poly(amic acid) by <i>In Situ</i> FTIR

The short-chain polyaddition stage of poly(amic acid) (PAA) synthesis exhibits pronounced autocatalytic behavior and completes within minutes, yet reliable kinetic data for this rapid process have remained elusive. Herein, we employ in situ FTIR spectroscopy to quantitatively investigate the early-stage polyaddition of 4,4′-diaminodiphenylmethane (MDA) with three dianhydrides: 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA), 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA), and 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA). The isolated characteristic band of MDA at 818 cm–1 can serve as an effective probe for quantitatively monitoring the variation of the MDA concentration. Kinetic analysis of the normalized band intensities (A-818 cm–1, T-818 cm–1, and S-818 cm–1) reveals that the reaction follows second-order irreversible autocatalytic kinetics. This combined in situ FTIR and modeling approach provides the first quantitative insights into how dianhydride electrophilicity, molecular structure, and temperature synergistically govern the extent of autocatalysis, thereby offering critical experimental and theoretical guidance for the precise control and optimization of PAA synthesis.