In order to synthesize semi-interpenetrating polymer networks (semi-IPNs) with molecular sieve characteristics, polyetherimide/bismaleimide (PEI/BMI) chromophore composites were made through sol-gel technique. A tunable and compatible chemical structure with fine morphology was obtained through in situ controlled sol-gel polymerization, crosslinking, chemical modification and membrane fabrication. The novel semi-IPN, synthesized and assembled by using ethanol as polar protic modifier and pore former, had a superior structure and morphology suitable for making gas separation membranes. These semi-IPN membranes gave fifteen times higher gas flux without significant decrease in their gas permselectivity than membranes prepared from pure PEL The chemical structures of these novel semi-IPNs were characterized by using FTIR, XPS and SEM. It was discovered for the first time that in situ simultaneous ethoxylation, anionic polymerization of BMI and imide modifications were responsible for creating the new chemical structure and molecular morphology that was different from traditional BMI resins. In addition to the superior structure, this chemical processes have the advantages of using simple green chemistry, ambient temperature and does not require initiators for polymerization.