Polymers with a strong size-sieving ability and superior H-2/CO2 selectivity at 100 degrees C or above are of great interests for pre-combustion CO2 capture. Polyimides (such as Matrimid and 6FDA-durene) have been crosslinked using diamines and show superior H-2/CO2 selectivity. However, these cross-linked polymers cannot be used for the pre-combustion CO2 capture because of the lack of thermal stability at 100 degrees C. Herein we demonstrate that commercial P84 (TM) can be chemically cross-linked using 1,4-butanediamine (BuDA) to achieve robust H-2/CO2 separation properties at 100-150 degrees C. The cross-linked P84 were thoroughly evaluated using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). The effects of the cross-linking time on the physical properties and H-2/CO2 separation properties at various temperatures were determined and interpreted using a free volume model. An exemplary sample based on P84 cross-linked by BuDA for 6 h exhibits a H-2 permeability of 47 Barrers (1 Barrer = 3.35 x 10(-16) mol m/m(2) s Pa) and H-2/CO2 selectivity of 14 at 100 degrees C, which is on the Robeson's upper bound, indicating their potential for practical applications.