Liquid Polymers
Nonvolatile solvents are environmentally-benign because they don't contribute to airborne emissions and can not harm workers by inhalation. However, it is important to ensure that the nonvolatile liquid is not harmful in other ways. Liquid polymers such as PEG, PPG, and PTHF (in most of its forms) are nonvolatile, nonflammable, biodegradable and nontoxic to humans, animals and aquatic life. They can be used as solvents for chemical reactions and the products can be removed from the liquid polymer by either distillation (for volatile products) or extraction with supercritical CO2. The liquid polymers are tunable over a wide range of polarities by modification of the repeating unit and by expansion of the polymer with dissolved CO2.
We have performed homogeneously-catalyzed and yeast-catalyzed reductions in a range of liquid polymer solvents. The best choice of polymer was different from reaction to reaction; PPG was best for CO2 hydrogenation, PMPS was superior for yeast-catalyzed reduction, and several polymers performed equally well for tiglic acid hydrogenation. Extraction of the product with scCO2 followed by recycling of the catalyst/polymer solution has been demonstrated for styrene hydrogenation and tiglic acid asymmetric hydrogenation but was difficult for the yeast-catalyzed reduction.
Even better, some of the polymers (eg. PEG-1500, see photo) freeze around the air-sensitive catalyst after each run, protecting the catalyst from air!
