Controlled Photoredox Ring-Opening Polymerization of O-Carboxyanhydrides Mediated by Ni/Zn Complexes

The overall goal of this experiment is to synthesize high molecular weight poly(alpha-hydroxy acids)with side chain functional groups in a controlled manner. This method can help answer key questions in the polyester synthesis field about controlled synthesis of polyesters from O-carboxyanhydrides. The main advantage of this technique is that the polymerization is highly efficient and does not affect the serial chemistry of the monomer.

Demonstrating the procedure will be Quanyou Feng, a postdoc from my laboratory. To begin the procedure, prepare solutions of bipy-nickel(COD)zinc bis(hmds)benzyl alcohol, and the iridium catalyst, an anhydrous THF. Store the solutions minus 35 degrees Celsius.

Then, place 72.2 milligrams of recrystallized L1 in a seven milliliter scintillation vial. Dissolve the L1 in 722 microliters of anhydrous THF. Transfer 200 microliters of the L1 solution to another 7 milliliter scintillation vial with the stir bar.

Add 100 microliters of anhydrous THF to this vial, and cap the vial. Place the dilute L1 solution in the cold well, along with a thermometer. Turn off the glove box light before proceeding.

Add about 500 milliliters of ethanol to the dewar for the cold well. Then, add liquid nitrogen to the dewar. Place the dewar on a stir plate and jack, and raise the dewar to cool the well.

Wait for the cold well to reach minus 50 degrees Celsius. Then, obtain a pair of safety goggles that block blue light, and place them within easy reach. Open the vial of chilled L1 solution.

Remove the catalyst solutions from the freezer. Over the course of 30 seconds, add to the vial 16.4 microliters of bipy-nickel(COD)24.4 microliters of zinc bis(hmds)22.5 microliters of benzyl alcohol, and 24.2 microliters of the iridium catalyst solution. Cap the vial when finished.

Put on the blue light-blocking safety goggles. Then, turn on a 34 watt blue LED light and a small cooling fan. Direct the light towards the vial.

Start the stir motor. Cover the light, cooling fan, and cold well with aluminum foil. Monitor the reaction temperature, and add liquid nitrogen every 15 to 20 minutes, to keep the reaction at about minus 15 degrees Celsius.

The critical part of this procedure, is to keep the reaction temperature around minus 15 degrees. If the temperature is higher than minus 10 degrees, liquid nitrogen should be added to the dewar. Monitor the monomer conversion with FDIR spectroscopy.

Once the desired degree of polymerization has been achieved, dry and wash the polymer with one milliliter of diethyl ether, containing one percent acetic acid, and one milliliter of methanol. The polymerization of phenyl O-carboxyanhydride was monitored by FDIR. The peak, at about 1805 reciprocal centimeters, corresponded to the anhydride bond stretch of L1.As the reaction progressed, the developing peak at 1760 reciprocal centimeters, indicated the formation of the ester bonds in the polymer.

The polymerization was finished when the 1805 peak had completely disappeared. Gel permeation chromatography was used to determine the molecular weights and dispersity of a series of L1 polymers with increasing monomer feed ratios. The number average molecular weight increased linearly with initial monomer catalyst ratios.

The dispersity values were all below 1.1. Proton NMR spectroscopy was used to evaluate the polymer stereo chemistry. A homodecoupling study showed a single peak at the alpha methine region, indicating that the polymer was isotactic, and therefore no epimerization had occurred.

Visual demonstration of this method is critical, as photoredox polymerization at low temperature is difficult to learn without it. Proper reaction setup and temperature control are both critical for controlled polymerization. When it’s mastered, this technique can be done within four to six hours if it’s performed properly.

Remember to wear safety goggles while attempting this procedure. After watching this video, you should have a good understanding of how to perform photoredox ring-opening polymerization.