Precision Polyketones by Ring-Opening Metathesis Polymerization: Effects of Regular and Irregular Ketone Spacing

The synthesis and characterization of regioregular aliphatic polyketones is reported. Poly(1-oxoheptamethylene), a semicrystalline polyketone, was prepared via ruthenium-catalyzed ring-opening metathesis polymerization (ROMP) of a ketal-protected 7-membered cyclic ketone followed by subsequent hydrogenation and deprotection. Temperature and catalyst studies of the ROMP reaction guided the preparation of polyketones with high monomer conversions, molecular weights as high as 30 kDa, and dispersities as low as 1.4. Because of the symmetric nature of the monomer, the polymer has ketones spaced every six methylene units apart. The thermal properties of this polyketone were investigated by differential scanning calorimetry, revealing a peak melting range of 160-165 degrees C. A related polymer, poly(1-oxooctamethylene), was also prepared in a similar fashion, and a peak melting range of only 130-133 degrees C was observed. This difference in melting range is attributed to the lack of the regioregularity in poly(1-oxooctamethylene), which was derived from an asymmetric 8-membered ring monomer and has ketones spaced every 6, 7, or 8 methylene units apart.