Syntheses of semi-interpenetrating polymer networks (semi-IPNs) with polyetherimide/bismaleimide (PEI/BMI) chromophore composites showing molecular sieve characteristics are reported. 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 Using ethanol as polar protic modifier and pore former, had superior structure and morphology for making gas separation membranes. These semi-IPN membranes gave 15 times higher gas flux than those membranes prepared from pure PEI without significant decrease ill gas selectivity. The chemical Structures of these novel semi-IPNs were elucidated by FTIR, XPS and SEM. It appears 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, these chemical processes give Superior structures Using green chemistry techniques Such as ambient temperature reaction and polymerization without initiators. Crown Copyright (c) 2005 Published by Elsevier Ltd. All rights reserved.