Membrane-based separations have shown promise for a host of applications. Yet, the development of polymer materials suitable for the preparation of membranes with tunable pore sizes between 2 and 10 nm remains challenging. In this study, we utilized poly[styrene-co-oligo(lactide) methacrylate] copolymers containing equal mass fractions of styrene and oligo(lactide) methacrylate as membrane precursors. The side chain of the oligo (lactide) methacrylate served as a sacrificial pore template, and by controlling the degree of lactic acid polymerization, n, between n = 21 and n = 40 the size of the pore templating domains could be tuned. After films were treated with a basic NaOH solution to hydrolyze the oligo(lactide) side chains, the porous films had well-defined pore sizes despite evidence of pore collapse. Poly(ethylene oxide) was filtered from methanol based on molecular size. Furthermore, we demonstrated that pore-solute interactions play a significant role in membrane performance and can lead to the selective removal or enrichment of solutes that possess characteristic dimensions several times smaller than the pore size of the membrane. Specifically, using a membrane with a pore diameter of 16 nm, Naphthol Yellow S (358 g mol(-1)) was rejected similar to 62% due to electrostatic repulsion and Martius Yellow (234 g mol(-1)) was enriched 4-fold due to hydrogen bonding between the solute and membrane pore.