BACKGROUNDA Block copolymers can be effective extractants in solid-liquid two-phase partitioning bioreactors (TPPBs) when selected on the basis of their partition coefficient toward target solutes, and their thermo-mechanical properties. A series of Pebax (R) block copolymers, containing varying proportions of soft poly(tetramethylene oxide) and hard poly(amide-12), was evaluated to determine the effect of hard segment proportion on solute affinity toward two biotransformation target molecules, carveol and carvone. Subsequently, representative homopolymers comprising the copolymers' soft segment were examined individually for the effects of molecular weight, crystallinity, and polymer end group polarity on affinity. RESULTS: Partition coefficients decreased with greater proportions of copolymer hard segments and homopolymer crystallinity, both of which act as non-absorptive domains, but which impart mechanical strength to the polymer. Partition coefficients increased with decreasing homopolymer molecular weight, which was ascribed to increased entropy of mixing. Hydroxyl vs. ether end group functionality had a variable effect on the partitioning of polar and non-polar solutes, providing a basis for the rational design of selective oligomeric absorbents. CONCLUSION: Block copolymers can provide an attractive, low-cost option for polymeric sequestering phases in TPPBs, but commercial grades of these materials are not optimized for these applications. TPPB performance can be improved by selecting/fabricating polymers that minimize the molecular weight of the soft component, the proportion of hard segment, and by considering the effects of end group composition. (C) 2014 Society of Chemical Industry