Monomers, vinyl cellulose grafting methods

The use of ceric ions to initiate graft polymerization was first discussed by Mino and Kaizerman in 1958 [12]. Schwab and coworkers [13] were among the first to extend this method to the grafting of cellulose. Following their work, numerous papers have appeared in the literature on the grafting of vinyl monomers onto cellulose by this technique. 

With adequate swelling vinyl monomers can be satisfactorily grafted to cellulose and its derivatives by the three main methods developed for other polymers (98). These are (1) the mutual or direct method where the polymer is irradiated in the presence of the monomer or monomer vapor, (2) the pre-irradiation (sometimes called post-irradiation grafting) method where the polymer is first irradiated and then brought into contact with the monomer, and (3) the peroxide method in which the polymer is irradiated in air and the resulting polymeric peroxide used to initiate graft polymerization. 

Armstrong, and Rutherford have reported extensive studies on the vapor phase grafting of vinyl monomers to cellulosic fibers both mutual and pre-irradiation methods have been used (97, 120). Again, water or another swelling agent was found to be necessary for effective grafting to rayon and cotton for all the monomers studied. In the case of cellulose acetate water was helpful but not necessary except for styrene. Acetic acid and methanol vapors were also found to be effective promotors of vapor phase grafting to cotton and cellulose acetate fibers. 

A graft or block copolymer of cellulose is defined here as a combination of cellulose and polymer that is difficult to separate by solvent extraction without first degrading the cellulose. Furthermore, only proposed mechanisms and reaction methods of free-radical initiated graft and block polymerizations of vinyl monomers with cellulose are discussed. 

The grafting reaction of vinyl monomer onto cellulosic fiber is a heterogeneous reaction system. Cellulosic fibers are in a solid phase, and vinyl monomers are in a vapor phase or, as a solute, in a solution phase. Two general methods for initiating grafting reactions are, as follows (1) one-step method monomer is in contact with cellulose when the initiating macrocellulosic radicals are formed or (2) two-step method after formation of the radicals, monomer is contacted with the activated cellulose. 

Manganic ions (Mn +) in aqueous solution have been developed in our laboratories as an initiator for grafting vinyl monomers onto various polysaccharides, e.g. starch (1), cellulose (2) and guaran (3). This research is part of a general program on graft copolymerization onto polysaccharide substrates as described in a review paper (4). The Mn "1" grafting method has been applied as... 

This and the following subsection will compare heterogeneous and homogeneous synthetic methods, with emphasis on cellulosics. Fibrous cellulosic graft copolymers, prepared by radiation-induced free-radical copolymerization reactions of vinyl monomers with cellulose, retain some... 

Figure 19 Effects of monomer and grafting method on morphological structures of cotton-cellulose copolymers, as shown by electron microphotographs of fibrous copolymer cross-sections (a) immersion of cellulose in solution of styrene in methanol then high-energy IR radiation (b) immersion of cellulose in solution of vinyl acetate in 70% aqueous ZnClj then high-energy irradiation (c) immersion of cellulose in aqueous solution of butyl methacrylate then high-energy irradiation.

If a suitable monomer is present, grafts are grown from these initiating sites. This method has been applied by Kukani and Mehta to grafting polyacrylonitrile onto cellulose and by Iwakura and Imai to the preparation of poly(vinyl alcohol-gfra/t-methyl methacrylate). More recently, Parker et al have synthesized poly(vinyl alcohol- rq/t-vinylpyridine hydrochloride) and McCormick and Lin have grafted polyacrylamide chains onto starch. 

It was found that the sulfate radical anion S04 produced photochemically in Scheme (46) is responsible for generating the cellulose derivative macroradicals by hydrogen abstraction, which added the vinyl monomer to produce the grafted copolymer. The main disadvantage of this method is the production of large quantities of undesirable homopolymers in addition to the grafted copolymers. 

An effective method of NVF chemical modification is graft copolymerization [34,35]. This reaction is initiated by free radicals of the cellulose molecule. The cellulose is treated with an aqueous solution with selected ions and is exposed to a high-energy radiation. Then, the cellulose molecule cracks and radicals are formed. Afterwards, the radical sites of the cellulose are treated with a suitable solution (compatible with the polymer matrix), for example vinyl monomer [35] acrylonitrile [34], methyl methacrylate [47], polystyrene [41]. The resulting copolymer possesses properties characteristic of both fibrous cellulose and grafted polymer. 

A method of grafting vinyl monomers to substrates of cellulose xanthate was invented by Faessinger and Conte-. The initiation is a reaction of ferrated (12) or acidic (13) cellulose xanthate with hydrogen peroxide according to the following scheme (HO-OH and Fe + give HO- radicals) ... 

Recently, the methods of anionic graft polymerization of vinyl monomers have been adopted for graft pol3nnerization to cellulose, and a few papers have appeared in the literature.All of these involve the "grafting from" process. Alkali metal cellulosates are used as initiators for the anionic graft polymerization of vinyl monomers (Scheme 1). Although this method seems more versatile it does not allow adequate structural control, and it often yields rather polydisperse samples. 

Desirable and targeted properties for cellulose fibers can be obtained through graft copolymerization, in order to meet the requirements of specialized applications. Graft copolymerization is one of the best methods recommended for modif5dng the properties of cellulose fibers. Different binary vinyl monomers and their mixtures have been graft copolymerized onto ceUulosic materials for modifying the properties of numerous polymer backbones (Kaith et al. 2005 Kalia et al. 2009). 

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. ... 

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