Elsevier

Carbohydrate Polymers

Volume 75, Issue 1, 5 January 2009, Pages 189-191
Carbohydrate Polymers

Short communication
HRP-mediated synthesis of starch–polyacrylamide graft copolymers

https://doi.org/10.1016/j.carbpol.2008.07.004Get rights and content

Abstract

Starch was reacted with acrylamide in water in the presence of horseradish peroxidase (HRP) catalyst/H2O2/2,4 pentanedione to give starch–polyacrylamide graft copolymers.

Introduction

Modified starch-based polymers can be engineered for specific properties by combining starch with synthetic polymers through graft copolymerization (Fanta, 1996, Fanta and Doane, 1986, Fanta and Bagley, 1977, Willett and Finkenstadt, 2005). Polyacrylamide grafted starch has been considered for applications in areas such as superabsorbent (Kiatkamjornwong et al., 2002, Wu et al., 2000) paper-making additives (Heath et al., 1975, Lu et al., 2003, Yeng et al., 2004), and textile sizing (Hebeish, El-Rafie, Higazy, & Ramadan, 1996). So far there are over a hundred papers and tens of patents (Luebke, 2000) issued for graft copolymerization. All of these used oxidants such as ceric ammonium nitrate or ammonium persulfate as catalyst. We would like to report a process where we replaced the inorganic oxidants by a natural enzymatic catalyst (HRP). At room temperature and in water HRP/H2O2/2,4 pentanedione catalyzed free radical grafting of acrylamide onto starch. Starch–polyacrylamide graft copolymers had graft M.Wt in the range of 100–200 K with good product recovery and a grafting efficiency of 33–65%.  HRP is shown to be a viable alternative to conventional inorganic free radical catalysts.

In the past two decades there has been tremendous interest to develop enzymatic polymerizations where enzymes catalyze the building of polymers from monomers (Gross et al., 1998, Gross et al., 2001, Kobayashi et al., 2001). Enzymes are environmentally friendly green catalysts that operates under mild condition, very often in water. HRP enzyme is an oxidoreductase acting on hydrogen peroxide as oxidant and on several reducing substrates such as hydroquinone, catechol and beta diketones (Teixeira, Lalot, Brigodiot, & Marechal, 1999) etc. HRP biocatalytic properties have been widely used to polymerize phenol (Ikeda et al., 1996, Ikeda et al., 1996), aniline (Bruno et al., 1995) and methyl methacrylate (Gross & Kalra, 2000) monomers.

Derango et al. utilized HRP and other oxidase to catalyze free radical polymerization of vinyl monomers (Derango, Chiang, Dowbenko, & Lasch, 1992). However, the polymers were not well characterized. Free radical polymerization of acrylamide in water initiated by the system HRP/H2O2/RH where RH is 2,4-pentanedione was reported by Lalot (Emery et al., 1997, Teixeira et al., 1999). Laccase catalyzed acrylamide polymerization was reported by Kobayashi (Ikeda, Tanaka, Uyama, & Kobayashi, 1998). Gross reported the copolymerization of acrylamide with sodium acrylate (Gross & Kalra, 2002). We would like to report for the first time the simultaneous polymerization and grafting of acrylamide onto starch catalyzed by HRP/H2O2/RH.

The polymerization of the acrylamide by HRP has been well known. However the formation of polyacrylamide chain on the starch backbone to form the graft copolymer is novel and has both academic and commercial interest. It is assumed that the grafts are prepared by first generating free radicals on starch and then allowing these free radicals to serve as macro initiators for vinyl or acrylic monomers.

Section snippets

Results and discussions

All HRP catalyzed reactions were carried out at a temperature of 30 °C and pH of 7. Thus, the variable parameters for the reactions were the amount of starch, acrylamide and hydrogen peroxide. The success of the reactions were defined by conversions, graft content, graft efficiency and M.Wt. Conversion implies the percentage of acrylamide (AAm) monomer that was converted into polyacrylamide (PAAm). Graft content defines how much PAAm is attached to starch. It is postulated that peroxide

Materials

Waxy maize starch (10.4% water content) was used. Peroxidase type 2 from horseradish was purchased from Sigma. All other chemicals were obtained from Aldrich Chemical Co. and used as is.

Reaction procedure

In a typical experimental procedure, 6.271 g waxy starch in 90 ml water was gelatinized via microwave (Ethos 1600) by heating the mixture up to 140 °C in 5 min and cooling to room temperature. Then 10 ml of 0.5 M potassium phosphate buffer pH 7.0 was added to produce a 0.05 M solution. A portion (26.56 g) of the mixture

Conclusions

In conclusion, starch–polyacrylamide graft copolymers were synthesized by the enzymatic oxidative polymerization of acrylamide and starch using HRP catalyst, H2O2 and pentanedione in water and acetate buffer. Further studies to compare the structure and properties of HRP catalyzed vs. conventional persulfate catalyzed graft copolymers are now underway. We are also looking into the mechanism of HRP catalyzed oxidation of starch.

Disclaimer on manuscript

  • “Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by USDA implies no approval of the product to the exclusion of others that may also be suitable.”

Reaction Scheme

Acknowledgement

The authors thank Kelly Utt and Janet Berfield, for their excellent technical assistance.

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