Free-radical polymerizations mediated by peroxidases

Enzyme-catalyzed methods to polymerize vinyl monomers by using peroxidase catalysts were developed. This enabled the performance of safe, low-temperature, free-radical polymerizations. The Horseradish Peroxidase (HRP) mediated polymerization of acrylamide in the presence of hydrogen peroxide and 2,4-pentanedione was conducted in aqueous medium and in concentrated emulsions (Kalra et al., 2002). In aqueous medium, at 25 oC, poly(acrylamide) in 98% yield was formed in 3 h. The HRP-mediated polymerization of acrylamide, conducted in a concentrated emulsion formed with sodium monooleate, gave nearly quantitative yields in only 1.25 hours (Fig 27). The polymers obtained in both of the above cases were atactic. HRP-mediated polymerizations of methyl methacrylate (MMA) were also studied (Kalra et al., 2000). Polymerizations were conducted at ambient temperatures in the presence of low concentrations of hydrogen peroxide and 2,4-pentanedione in a mixture of water and a water-miscible solvent. Polymers of MMA formed were highly stereoregular with predominantly syndiotactic sequences (syn-dyad fractions from 0.82 to 0.87). A general scheme for the polymerization mechanism was presented (Scheme 21). Based on the catalytic properties of HRP and research performed, we believe that the keto-enoxy radicals from 2,4-pentanedione are the first radical species generated. Then, initiation takes place through this radical or by radical transfer to another molecule.

Scheme 21. Mechanism in the HRP-Mediated Free Radical Polymerization of Acrylates

Figure 27. HRP-mediated acrylamide polymerization in aqueous medium.

References

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