Our work combines polymerization-induced phase separation and a "doubly reactive" block copolymer to make robust nanoporous membranes with a well defined pore structure. Gas and liquid transport measurements confirm the pore structure spans the films. Transport across these films agrees with predictions based on BET and SAXS so macroscopic cracks do not dominate transport. The demonstrated molecular weight cut-off of dextrans is significantly sharper for our films than a commercial membrane said to have a similar cut-off. Fouling studies will determine if further development of these materials is warranted.