Photochemical Initiation Of Radical Polymerization Reactions

Organic polymers are ubiquitous chemicals in everyday life. Polyolefins, which are generated by the polymerization of alkene monomers, comprise common plastics and rubbers found in drinking cups, soda bottles, car tires, and even some fabrics. Polystyrene, for example, is a polymer based on styrene monomers, and finds important applications in protective packaging (i.e., packing peanuts), water bottles, and disposable forks and knives. Polystyrene is generated on a scale of several million tons per year.

Polymer synthesis is a field of chemistry devoted to developing methods to initiate and control polymer growth in order to generate polymers with specific targeted applications. For example, the exact properties of polyolefin materials depend intimately on the polymer chain structure, and structural factors such as the extent of chain branching can completely change the properties of polymeric materials built from the same monomer units.

Simple olefins do not spontaneously participate in polymerization chemistry, despite the fact that these reactions are thermodynamically downhill. To get efficient polymerization to proceed, either initiators or catalysts, which lower the kinetic barrier to polymerization, are required. Benzoyl peroxide is an example of an effective radical initiator for polymer growth. Photochemically promoted homolytic cleavage of the O-O bond results initially in the generation of carboxylate radicals and, following decarboxylation, in phenyl radicals and CO₂ (Figure 1). These phenyl radicals react with styrene to generate a new C-C bond and a benzylic radical. This initially formed benzylic radical can react with additional styrene in a radical chain reaction to eventually afford long-chains of polystyrene.

The basis of photochemistry is that photon absorption generates an excited state in molecules, which increases the likelihood of molecular participation in chemical reactions. In order to access the required state of molecular excitation, the molecule of interest (in this case, benzoyl peroxide) needs to have absorbed light at a specific wavelength. Benzoyl peroxide absorbs only in the UV portion of the spectrum; O-O homolysis can be initiated by irradiation with ~250 nm light. This wavelength is too short for the human eye to see. To illustrate the importance of photon absorption for photochemistry, we will first examine the polymerization of styrene initiated by benzoyl peroxide directly under visible light irradiation. The absorption spectrum of benzoyl peroxide does not have significant absorption in the visible region (benzoyl peroxide is colorless), and photoinduced polymerization with visible light is not facile.

In order to overcome the poor absorptivity of many organic molecules towards visible light, photosensitizers are utilized. Photosensitizers are molecules that participate in photon absorption and then transfer energy to a different molecule to promote photochemical reactions. Benzophenone is a common photosensitizer; the photochemistry of this molecule is described in Figure 2. Photon absorption in the ground state generates a singlet excited state. Intersystem crossing affords the triplet ground state, which compared to the singlet excited state, is long lived. Energy transfer from the triplet excited state to benzoyl peroxide can lead to O-O bond cleavage and radical-chain initiation. The photosenstitizer is useful because unlike benzoyl peroxide, it has substantial absorption in the visible spectrum.

In competition with energy transfer to substrate, the triplet excited state can undergo relaxation to regenerate the singlet ground state; if this process is fast relative to energy transfer, then sensitization is not efficient. The efficiency of sensitization is measured by the quantum yield, which is a measure of the fraction of absorbed photons that are utilized productively in the targeted chemical reaction. In order for photosensitizers to work effectively, the excited state of the sensitizer must encounter the other reactant; in our case, the triplet excited state of benzophenone must encounter benzoylperoxide in order to accomplish energy transfer. The quantum yield of polymerization using bimolecular initiators is low if relatively few of the photogenerated benzophenone excited states result in generation of benzoyl radicals by O-O cleavage.

Quantum Yield = photoreactions accomplished / photons absorbed

Through the experiments that are outlined in this video, we will confront the topics of photochemistry, triplet sensitizers, and polymerization chemistry.

Figure 1. (a) O-O bond cleavage eventually leads to the formation of phenyl radicals, which can initiate polymerization. (b) Illustration of the radical-chain polymerization initiated by phenyl radicals.

Figure 2. Diagram of photosensitization. Photon absorption by benzophenone initially generates the first singlet excited state (S₁). Intersystem crossing provides access to the first triplet excited state (T₁). Energy transfer from the triplet excited state to benzoyl peroxide leads to productive photochemistry.

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.

In contrast to benzoyl peroxide, benzophenone does absorb a substantial amount of visible light (> 300 nm). Photolysis of a mixture of benzophenone, benzoyl peroxide, and styrene results in the formation of some polystyrene. The polymerization results in the formation of an oily, non-volatile residue that remains after evaporation of the photoreaction. In addition, ¹H NMR analysis of the residue indicates the presence of polystyrene. Polystyrene is characterized by diagnostic ¹H NMR signals: the aromatic protons appear as a broad multiplet from 7.2-6.4 ppm and the aliphatic protons appear as multiplets centered at 1.9 and 1.5 ppm that integrate in a 1:2 ratio.

Finally, note that benzophenone alone is not a competent photoinitiator. Only when the sensitizer, initiator, and substrate were all present did polymerization proceed.

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 tools for the production of polymer materials. Photo-initiated polymerization reactions find applications in a variety of areas. For example, photochemically initiated polymerization chemistry can be used to make designer materials in which the monomer that is incorporated is changed on demand, which allows precise control over the molecular structure of the material that results (Figure 3).

Figure 3. Photochemical control over block copolymer synthesis provides a strategy to making designer materials.

A second application is using photochemically initiated polymerization to generate 3-dimensionally patterned structures from polymers. Typically, such patterning is achieved by generating a mask, which prevents irradiation of areas of a surface that are covered with an appropriate monomer. Photoinduced polymerization is then carried out to generate a structure in which polymerization has been accomplished in the relief of the mask.

In this experiment, we saw the critical impact of photosensitizers on the efficiency of photochemical reactions. The concepts explored here underpin an important area of polymerization chemistry, which endeavours to identify highly efficient sensitizers and initiators. One example which we can understand based on our experiments is the use of unimolecular initiator-sensitizer hybrids.¹ By combining the sensitizer, which has strong absorption in the visible spectrum, and the radical initiation benzoyl peroxide in the same molecule, we effectively increase the quantum yield of sensitization and thus more efficiently initiate polymerization. These observations highlight the importance of molecular design in identifying highly efficient polymerization initiators.