Photochemical Initiation Of Radical Polymerization Reactions

Overview

Source: David C. Powers, Tamara M. Powers, Texas A&M

In this video, we will carry out the photochemically initiated polymerization of styrene to generate polystyrene, which is an important commodity plastic. We will learn the fundamentals of photochemistry and use simple photochemistry to initiate radical polymerization reactions. Specifically, in this module we will examine the photochemistry of benzoyl peroxide and its role as a photo-initiator of styrene polymerization reactions. In the described experiments, we will investigate the role of wavelength, photon absorption, and excited state structure on the efficiency (measured as quantum yield) of photochemical reactions.

Procedure

1. Measure the absorption spectrum of benzoyl peroxide.

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.
Add 0.5 mL of the prepared solution to a UV-vis cuvette using a volumetric pipette. Add 3.5 mL of toluene.
Prepare a second cuvette, filled only with pure toluene.
Measure the absorption spectrum (300–800 nm wavelength range

Results

Based on the UV-vis absorption spectra that we collected, benzoyl peroxide does not display substantial absorption in the visible spectrum. The lack of visible-light-absorption is consistent with the lack of polymerization chemistry that is observed when a sample of styrene is photolyzed in the presence of benzoyl peroxide. The residue left behind following evaporation of the photoreaction described in step 2 contains only benzoyl peroxide; no styrene-derived products have been generated.

Application and Summary

In this video, we have seen the impact of structure on the reactivity of radical initiators for olefin polymerization chemistry. We have examined photochemical conditions that: 1) did not contain appropriate absorbers, 2) contained appropriate absorbers but not appropriate initiators, and 3) contained both initiator and absorber molecules. These systems highlight the role of photon absorption and the importance of quantum efficiency on photochemical reactions.

Radical initiators are important

References

Greene, F. D.; Kazan, J. Preparation of Diacyl Peroxides with N,N'-Dicyclohexylcarbodiimide. J Org Chem. 28, 2168-2171 (1963).