Cellulose, oxidized grafting reactions

This paper examines two types of hydroxypropyl lignin based macromers, and these are illustrated schematically in Figure 1. Macromers with propylene oxide (PO) are formed by reducing the number of available hydroxyl groups on HPL followed by chain extension with PO and macromers with cellulose triacetate (CTA) are synthesized by attaching a monofunctional CTA chain to a limited number of terminal OH groups on HPL via a suitable grafting reaction. 

The grafting reactions initiated by free radicals discussed here have generally been identifiable, at least in part, by ESR spectroscopy. Other methods include chemical oxidation of cellulose by thermal decomposition of peroxides, ultrasonic radiation, electric-arc discharge (also called corona discharge), electrolysis, mechanical milling and oxidation of products of chemical reactions in the presence of vinyl monomers. Chemical modification of cellulose, e,g. by diazotization and thiocar-bonation, increases the rate of oxidation and of graft copolymer formation in the presence of vinyl monomers. ... 

The grafting reaction between oxidized cellulose substrates and amines was performed according to Bulpitt and Aeschlimann [18] with minor modifications [22], The typical run... 

Figure 17. Grafting reaction onto oxidized cellulose samples.

Through this work, some parameters are identified as key drivers to success. In order to validate the experimental results, some possibilities are suggested in open discussion concerning the reactivity of cellulose substrates. With totally oxidized cellulose such as polyglucuronlc acid, the grafting reaction is limited by the molecular weight of primary amines, their solubilities at reactional temperature and especially the unprotonated state of amine at the pH used. Indeed, on the basis of reactional mechanism, the active ester intermediate must obviously react with amine groups of molecule. 

Graft copolymers of nylon, protein, cellulose, starch, copolymers, or vinyl alcohol have been prepared by the reaction of ethylene oxide with these polymers. Graft copolymers are also produced when styrene is polymerized by Lewis acids in the presence of poly-p-methoxystyrene. The Merrifield synthesis of polypeptides is also based on graft copolymers formed from chloromethaylated PS. Thus, the variety of graft copolymers is great. 

Redox initiation is often an efficient method for graft polymerization. Hydroxyl-containing polymers such as cellulose and poly(vinyl alcohol) undergo redox reaction with ceric ion or other oxidizing agents to form polymer radicals capable of initiating polymerization... 

The selective oxidation of cellulose to dialdehyde by sodium periodate is well known. It has been postulated by Criegee (74) and by Waters (73) that this reaction proceeds by a free radical mechanism. Toda (76) and Morimoto, Okada, Okada, and Nakagawa (77) have concluded that sodium periodate oxidation should initiate graft polymerization. They succeeded in grafting methyl methacrylate and acrylonitrile onto cellulose substrates, such as rayon and paper. A similar procedure is recommended in a patent of Chemische Werke Huels (78) to graft vinyl monomers onto cotton, polyethylene oxide, copolymers of vinyl chloride-vinyl acetate, and others. 

An addition graft copolymerization is the reaction of /9-propiolactone with cellulose, which has been extensively studied by Daul, Reinhardt, and Reid [138). In a similar reaction cellulose adds ethylene imine, as shown by work by Cooper, and Smith (139), and ethylene or propylene sulfide, as described by Champetier (140). Also the well known hydroxy-ethylation of cellulose by reaction with ethylene oxide belongs to this class of reactions. 

Many other chemical redox systems have been reported as initiators of macrocellulosic radicals and as catalysts for graft polymerization. One variation has been to modify cellulose chemically to increase its reactivity with selected oxidizing and reducing agents which on reaction yield macrocellulosic radicals (14, 15). 

Preparation of Cellulose-Styrene-Sulfur Dioxide Graft Copolymer (Homogeneous Grafting). The procedure for preparation of cellulose-styrene copolymer in a S0 -diethylamine (DEA)-dimethylsulf-oxide (DMS0) medium was described in our previous paper (3). The grafting proceeds homogeneously throughout the reaction. 

However, once introduced on the cellulose molecule, the hydroxyethyl group is highly reactive and capable of further reaction with another molecule of ethylene oxide. This type of reaction, called graft etherification, can build up relatively long-chain substituents on the cellulose molecule without involvement of many additional cellulosic OH groups ... 

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