Rigid Macroporous Polynorbornene Monoliths by Ring Opening Metathesis Polymerization

Rigid macroporous polymer monoliths are widely used as efficient stationary phases for chromatographic separations, flow-through reactors, catalyst supports etc. An efficient sol-gel bottom-up synthetic approach involves polymerization of soluble monomers into an insoluble polymer that phase-separates into colloidal particles that under the right conditions of molecular structure, concentration, solvent and temperature form gels. Early phase-separation, hence smaller colloidal particles, can be induced by molecular-level cross-linking in the growing polymer akin to that in thermoset resins. Alternatively, for highly soluble linear (thermoplastic) polymers, phase-separation can be controlled with non-solvents. This is demonstrated here with open-cell macroporous polynorbornene monoliths synthesized by ring opening metathesis polymerization (ROMP) of norbornene in toluene using isopropanol (iPrOH) as non-solvent. Wet-gels were solvent-exchanged with liquid CO2 taken out at the end as a supercritical fluid (SCF). Changes in the microstructure as a function of the iPrOH:toluene ratio was probed by SEM, N2 sorption porosimetry. Bulk densities range from 0.40 to 0.68 g cm-3. Porosity ranges from 30 to 60% v/v with average pore diameters in the 1.4-2.3 mm range, but the BET surface area is low (1-3 m2 g-1. The skeletal framework consists of agglomerated particles forming macroglobular structures (1.8 to 4.5 µm in diameter). The monoliths are robust with e.g., ultimate compressive strength under high strain rates (1224 s-1) of samples made with iPrOH:toluene = 7:3 v/v (0.51 g cm-3) equal to 50 MPa at 75% ultimate strain