Photochemical Initiation Of Radical Polymerization Reactions

1. Measure the absorption spectrum of benzoyl peroxide.

1. Benzoyl peroxide is commercially available. Prepare a solution of benzoyl peroxide in toluene. Using a 10 mL volumetric flask, add 10 mg of benzoyl peroxide. Fill the volumetric flask with toluene.
2. Add 0.5 mL of the prepared solution to a UV-vis cuvette using a volumetric pipette. Add 3.5 mL of toluene.
3. Prepare a second cuvette, filled only with pure toluene.
4. Measure the absorption spectrum (300–800 nm wavelength range) of the toluene-filled cuvette. This spectrum should be used to subtract the solvent background from the spectrum of benzoyl peroxide collected below.
5. Measure the absorption spectrum (300–800 nm wavelength range) of the benzoyl peroxide-containing cuvette. To obtain the absorption spectrum of benzoyl peroxide, subtract the spectrum of toluene obtained in step 1.4. Many UV-vis software packages perform background subtraction automatically. If not, background subtraction can be accomplished using standard database software, such as Excel.

2. Reaction of benzoyl peroxide and styrene in the absence of a photosensitizer.

1. Measure the tare weight of a 25-mL round-bottom flask.
2. Prepare a solution of benzoyl peroxide and styrene in toluene by combining 1 mL of the solution prepared above to 10 mL toluene and 3 mL styrene. Transfer the reaction solution to a 25-mL round-bottom flask, fit the flask with a rubber septum, and degas the reaction solution by bubbling nitrogen through the solution (see the “Synthesis Of A Ti(III) Metallocene Using Schlenk Line Technique” video in the Inorganic Chemistry series). It is not important to exclude water from the reaction solution, only to remove dissolved O₂.
3. In a ventilated fume hood, clamp the reaction flask fitted with the nitrogen filled balloon to a stir plate. Turn on the Hg-arc lamp and wait 10 minutes for the light bulb to warm up. With magnetic stirring, irradiate the solution with a Hg-arc lamp using a 350-nm long-pass filter for 10 minutes.
4. Concentrate the photoreaction on a rotovap. If non-volatile residue remains, obtain a mass of the flask. The weight of the non-volatile residue can be determined using the tare weight measured in step 2.1 Obtain a ¹H NMR spectrum of the residue in CDCl₃.

3. Measure the absorption spectrum of benzophenone.

1. Benzophenone is commercially available. Prepare a solution of benzophenone in toluene. Using a 10 mL volumetric flask, add 10 mg of benzophenone. Fill the volumetric flask with toluene.
2. Add 0.5 mL of the prepared solution to a UV-vis cuvette using a volumetric pipette. Add 3.5 mL of toluene.
3. Measure the absorption spectrum (300–800 nm wavelength range) of benzophenone.
4. Using the spectrum of toluene obtained in Step 1.4, perform a background subtraction to obtain the absorption spectrum of pure benzophenone.

4. Reaction of benzoyl peroxide and styrene in the presence of the photosensitizer benzophenone.

1. Measure the tare weight of a 25-mL round-bottom flask.
2. Prepare a solution of benzoyl peroxide, benzophenone, and styrene in toluene by combining 1.0 mL of the benzoyl peroxide solution prepared in step 1.1, with 1.0 mL of the benzophenone solution prepared in step 3.1 with 10 mL toluene and 3 mL styrene. Transfer the reaction solution to a 25-mL round-bottom flask, fit the flask with a rubber septum, and degas the reaction solution by bubbling nitrogen through the solution (see the “Synthesis Of A Ti(III) Metallocene Using Schlenk Line Technique” video in the Inorganic Chemistry series). It is not important to exclude water from the reaction solution, only to remove dissolved O₂.
3. In a ventilated fume hood, clamp the reaction flask fitted with the nitrogen filled balloon to a stir plate. Turn on the Hg-arc lamp and wait 10 minutes for the light bulb to warm up. With magnetic stirring, irradiate the solution with a Hg-arc lamp using a 350-nm long-pass filter for 10 minutes.
4. Concentrate the photoreaction on a rotovap. If non-volatile residue remains, obtain a mass of the flask. The weight of the non-volatile residue can be determined using the tare weight measured in step 4.1 Obtain a ¹H NMR spectrum of the residue in CDCl₃.

5. Control reaction of styrene in the presence of the photosensitizer benzophenone.

1. Measure the tare weight of a 25-mL round-bottom flask.
2. Prepare a solution of benzophenone and styrene in toluene by combining 1.0 mL of the benzophenone solution prepared in step 3.1 with 10 mL toluene and 3 mL styrene. Transfer the reaction solution to a 25-mL round-bottom flask, fit the flask with a rubber septum, and degas the reaction solution by bubbling nitrogen through the solution (see the “Synthesis Of A Ti(III) Metallocene Using Schlenk Line Technique” video in the Inorganic Chemistry series). It is not important to exclude water from the reaction solution, only to remove dissolved O₂.
3. In a ventilated fume hood, clamp the reaction flask fitted with the nitrogen filled balloon to a stir plate. Turn on the Hg-arc lamp and wait 10 minutes for the light bulb to warm up. With magnetic stirring, irradiate the solution with a Hg-arc lamp using a 350-nm long-pass filter for 10 minutes.
4. Concentrate the photoreaction on a rotovap. If non-volatile residue remains, obtain a mass of the flask. The weight of the non-volatile residue can be determined using the tare weight measured in step 5.1 Obtain a ¹H NMR spectrum of the residue in CDCl₃.