Grafting on Cellulose

The grafting methods can in principle be divided into three categories, namely, radical polymerization, ionic polymerization, and condensation or addition polymerization. Only the first case is discussed in the following since the most common grafting methods belong to this category. 

Chain transfer to cellulose RM + Cell—H — RMXH -I- Cell- (9-26) 

Graft copolymer chain initiation Cell + M — Cell—M- (9-27) 

Graft side chain growth (propagation) Cell—M- +M— Cel I—M2-,etc. (9-28) 

Graft side chain termination Cell—Mx- + Mi+n — Cell—(9-30) Cross-linking Cell—Mx-+Cell—Mj.+n — Cell——Cell (9-31) 

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Zaidan-Hojin Japan Radiation High Polymer Research Association Grafting on Cellulose, Japan. Pat. 3300 62) June 1, 1962 C. A. 58, 11, 544 (1963). 

Figure 12.33. (b) Shielding effectiveness (SE) obtained using multilayer polyaniline grafted on cellulose sheet using the ASTM-7 coaxial test method. 

HAN 12a] Hansson S., Tischer T., Goldmann A.S. et al, Visualization of poly(methyl methacrylate) (PMMA) grafts on cellulose via high-resolution FT-IR microscopy imaging , Polymer Chemistry, vol. 3, pp. 307-309, 2012. 

This reaction is utilized by Burrows et al. [70] to photoinitiate grafting of acrylamide, acrylic acid, methacrylamide, and acrylonitrile on cellulose triacetate in acidic aqueous solution. 

Graft reactions on cellulosics are well studied and are well known to incorporate desired properties in polymers [61,72,73,76,77,99-102], but commercialization of the processes on cellulosics are not increasing. (Table 4 gives a summary of the techniques of grafting.) A fresh imaginative approach is required to solve this problem. 

Following the findings of Mino and Kaizerman [51] that ceric ion can form a redox system with cellulose, grafting onto various natural polymers has been carried out by the ceric ion method. In the case of cellulose, the reaction between ceric ion and cellulose occurs to produce active sites on cellulose in the following manner ... 

This is a highly selective process and very good results on cellulose and starch grafting have been observed. Ceric ion initiated grafting is usually carried out at lower temperatures and, therefore, wastage of monomer in chain transfer reactions is minimal. 

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. 

By using this technique acrylamide, acrylonitrile, and methyl acrylate were grafted onto cellulose [20]. In this case, oxidative depolymerization of cellulose also occurs and could yield short-lived intermediates [21]. They [21] reported an electron spin resonance spectroscopy study of the affects of different parameters on the rates of formation and decay of free radicals in microcrystalline cellulose and in purified fibrous cotton cellulose. From the results they obtained, they suggested that ceric ions form a chelate with the cellulose molecule, possibly, through the C2 and C3 hydroxyls of the anhy-droglucose unit. Transfer of electrons from the cellulose molecule to Ce(IV) would follow, leading to its reduction... 

As in the case of ceric and vanadium ions, the reaction of organic compounds with Co(III) proceeds via formation of an intermediate complex. Such a complex decomposes and produces free radicals capable of initiating vinyl polymerization. However, only a few reports on Co(IIl) ion-initiated grafting onto cellulose fibers are available [38]. 

If a vinyl monomer is polymerized in the presence of cellulose by a free radical process, a hydrogen atom may be abstracted from the cellulose by a growing chain radical (chain transfer) or by a radical formed by the polymerization catalyst (initiator). This leaves an unshared electron on the cellulose chain that is capable of initiating grafting. As cellulose is a very poor transfer agent [10], very little copolymer results from the abstraction of hydrogen atoms by a growing chain radical. The... 

A large variety of polymers has been considered. In the beginning, polystyrene and styrene/ divinylbenzene copolymers (Merrifield resins) were by far the most used.73 Then others were tested such as polyvinyls,47-50,61-64 polyacrylates,72 4,75 and cellulose.76,77 Most commonly, diphenylphos-phane groups were grafted on the polymeric support, either directly or via one CH2 group. 

Graft copolymerization can be initiated by physical or chemical processes. This paper describes methods for chemical initiation of grafting onto cellulosic fibers with special emphasis on recent developments, i.e. studies from the last 10-15 years. The initiation reactions may be free radical or ionic processes. The grafting reaction may also be a condensation or addition process. 

Metal Ion Initiation on Cellulose Xanthate. Vinyl monomers can be grafted to cellulose at high rates by reaction with cellulose xanthate using v5+, Fe + and Cr + as redox initiators l. 

Deters (14) grafted acrylonitrile, methyl methacrylate and vinyl chloride on cellulose and cellulose triacetate. The first two monomers were put in the reactor as liquids, the last as a gas. The results are summarized on Table 1. Vinyl chloride did not graft to cellulose (14). 

The water absorption properties of cellulose may be increased by grafting on polar groups, such as carboxyl groups, as in Super Slurpers, and this absorption may be decreased by the introduction of nonpolar moieties, such as ester groups, as in cellulose acetate. 

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