MOFs Made From Flexible Polymers Buck Conventional Wisdom

A new study has shown that organic polymers can be used to make crystalline framework compounds, contradicting common chemical intuition. The finding may lead to a novel, broad class of porous materials with possible applications in gas separation and storage, catalysis, and drug delivery (Angew. Chem. Int. Ed. 2015, DOI: 10.1002/anie.201502733).

The burgeoning field of metal-organic framework (MOF) compounds owes some of its rapid growth to the simplicity of the rules that guide choosing ligands for MOF synthesis. By combining metal ions or clusters with short—and fairly rigid—organic linkers, researchers have synthesized thousands of high-surface-area MOFs and related crystalline compounds. Running the reaction instead with long, floppy polymers would likely generate an amorphous blob—or so chemists thought.

Stephen A. Miller of the University of Florida, Gainesville; Seth M. Cohen of the University of California, San Diego; and coworkers have now formed well-ordered crystals by reacting zinc ions with various polyethers containing aromatic dicarboxylic acid backbones. The products, dubbed polyMOFs, are mainly spherical, with diameters ranging from 1 to 10 μm.

Not only does the study buck conventional MOF wisdom, it shows unexpectedly that an amorphous, flexible, linear, nonporous polymer can be transformed into a three-dimensional, porous, crystalline network.

The researchers proved that the linkers in their MOFs were indeed the polymers originally added, rather than low-molecular-weight fragments, by decomposing the MOFs via acid digestion to recover the starting materials as well as by carrying out other tests. The team also showed that the morphology of the polyMOFs can be controlled by varying polymer type and reaction temperature. Now, the group is investigating polyMOF applications including separations of small molecules and gases.

University of Akron polymer specialist Kevin Cavicchi notes that the polymer ligand imparts many desirable properties to the polyMOFs, including hydrophobicity and pore stability. From a polymer perspective, what is “striking,” Cavicchi says, is that this result was achieved by using a straightforward condensation reaction with flexible polymers. He adds that the chemistry and chain architecture of polymeric ligands may be broadly used as new variables to further expand the design space of MOFs to achieve tailorable properties.