Vinyl monomers, reaction with cellulose

The steps in free-radical initiated reactions of vinyl monomers (M) with cellulose (C) in solvent (S) to yield copolymers (CM), with a possible intermediate or sensitizing initiator (I), may be represented as follows, where an asterisk indicates activated state. 

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. 

When copolymers are prepared by the free-radical initiation of reactions of irradiated cellulose (on which the free-radical site is located) with binary mixtures of vinyl monomers, fractionation of cellulose and... 

Grafting reactions alter the physical and mechanical properties of the polymer used as a substrate. Grafting differs from normal chemical modification (e.g., functionalization of polymers) in the possibility of tailoring material properties to a specific end use. For example, cellulose derivatization improves various properties of the original cellulose, but these derivatives cannot compete with many of the petrochemically derived synthetic polymers. Thus, in order to provide a better market position for cellulose derivatives, there is little doubt that further chemical modification is required. Accordingly, grafting of vinyl monomers onto cellulose or cellulose derivatives may improve the intrinsic properties of these polymers. 

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) ... 

Dispersion polymerization involves an initially homogeneous system of monomer, organic solvent, initiator, and particle stabilizer (usually uncharged polymers such as poly(A-vinyl-pyrrolidinone) and hydroxypropyl cellulose). The system becomes heterogeneous on polymerization because the polymer is insoluble in the solvent. Polymer particles are stabilized by adsorption of the particle stabilizer [Yasuda et al., 2001], Polymerization proceeds in the polymer particles as they absorb monomer from the continuous phase. Dispersion polymerization usually yields polymer particles with sizes in between those obtained by emulsion and suspension polymerizations—about 1-10 pm in diameter. For the larger particle sizes, the reaction characteristics are the same as in suspension polymerization. For the smallest particle sizes, suspension polymerization may exhibit the compartmentalized kinetics of emulsion polymerization. 

Schwenkkr, R. F., and E. Pacsu Graft polymerization onto cellulose reactions of sodium cellulosate and alkali cellulose with vinyl monomers. Tappi 46, 665 (1963). 

Graft and block copolymers of cotton cellulose, in fiber, yam, and fabric forms, were prepared by free-radical initiated copolymerization reactions of vinyl monomers with cellulose. The properties of the fibrous cellulose-polyvinyl copolymers were evaluated by solubility, ESR, and infrared spectroscopy, light, electron, and scanning electron microscopy, fractional separation, thermal analysis, and physical properties, including textile properties. Generally, the textile properties of the fibrous copolymers were improved as compared with the properties of cotton products. 

The modification of the properties of fibrous cotton cellulose through free-radical initiated copolymerization reactions with vinyl monomers has been investigated at the Southern Laboratory for a number of years. Both graft and block copolymers are formed. Under some experimental conditions the molecular weight of the polyvinyl polymer, covalently... 

Applications of ESR spectroscopy to investigations of free-radical initiated copolymerization reactions of cellulose with vinyl monomers have been reported (1, 2). NMR and infrared spectroscopy have been used to examine products obtained from degradation of cellulose copolymers to characterize the nature of the chemical bonds between cellulose and polyvinyl polymer (35, 42). Covalent bonds were indicated in most cases. 

An Interpretative review of the reactions initiated by macrocellulosic free radicals with vinyl monomers to yield block and graft copolymers of fibrous cellulose was made. Macrocellulosic radicals are usually formed by interactions with radiation or chemical redox systems. Important factors in these heterogeneous reactions are lifetimes and accessibilities of the radicals and interactions of solutions of monomer with fibrous cellulose. Changes in organochemical, macromolecular, and morphological structures in cellulosic fibers through formation of copolymers are made. 

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. 

E.s.r. evidence for a living (propagating) polymer radical in grafting reactions of vinyl monomers with irradiated cellulose has also been reported (26). 

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. 

When vinyl monomers are in a vapor phase, the extent and rate of the grafting reaction are increased, if the cellulosic fibers are wetted with solvents for the monomers. There is a very low extent of reaction between activated, dried cellulose and vinyl monomer neat (3). 

In the predominating reactions, the number of different types of monomer units and their sequences are determined by their relative molecular reactivities for the macrocellulosic radicals and the monomer reactivity ratios. These types of reactions are useful in that less reactive monomers can be included in copolymers to add selected organochemical and macromelecular properties to the modified cellulosic products. In cases where vinyl monomers have been reacted to form oligomers, these reactions are useful in increasing the reactivity of oligomers with macrocellulosic radicals (29, 30, 31). 

Various free radicals are generated in cellulose and cellulose derivatives by ultraviolet light, which may be capable of initiating graft copolymerization reactions with vinyl monomers. The graftability of these photoinduced free radicals in homogeneous and heterogeneous media was studied. 

Graft copolymerization reactions of fibrous cellulose with vinyl monomers were initiated at free radical sites formed on the cellulose molecule by interaction with radiation, by reaction with Ce4+ ions in acidic solution, or by H abstraction by OH radicals formed by reaction of Fe2+ ions with H202 in aqueous solution. The effects of experimental conditions on the location of these sites on the cellulose molecule and on the reactions were studied by ESR spectroscopy. The molecular weights of the grafted copolymers and the distribution of the polymers within the fibrous cellu-losic structure were determined. Some of the properties of the copolymers are discussed. 

Tihe preparation and properties of cellulose graft copolymers have A been of considerable interest in the textile, paper, and wood products industries for a number of years. Both free radical- and ionic-initiated graft copolymerization reactions of vinyl monomers with cellulose have been reported. The vinyl-cellulose copolymers have some of the properties of both the cellulosic fibers and the grafted polyvinyl copolymers (I, 3, 47). 

Two important factors in initiating graft copolymerization reactions of vinyl monomers with activated cellulose are (1) the lifetime of the free radical sites and (2) the accessibility of the free radical sites to the monomers. For ceric ion initiation or hydroxyl radical initiation the lifetimes of the free radicals on the cellulose molecule were short (5, 19) therefore, the monomer should be present when the free radicals were formed. For ionizing radiation both short lived and long lived (trapped) free radicals were formed (22, 26, 44). Consequently, the activation reaction and the copolymerization reaction could either be conducted... 

Vapor or Spraying Applications. Cyanoethylation. Reaction of cellulose with acrylonitrile, CH2=CHCN, a vinyl monomer, was selected for study because it is known to produce several desirable changes in the properties of cellulosic fabrics and yams. After cyanoethylation, undegraded cotton becomes more resistant to microorganism attack, heat, acid, and abrasion (3). Moisture absorption decreases and the electrical properties change. The extent of these changes varies considerably... 

The high-energy radiation forms macrocellulosic radicals that are stable in the crystalline areas of cellulose. These radicals can initiate reactions with vinyl monomers to yield grafted polyvinyl-cellulosic fibers with desired properties [522-524]. 

It was found that cellulose graft copolymers containing 30-50% PAN can, after the reaction medium has been washed out with water, initiate graft polymerization of various vinyl monomers (Table 2) without any additional initiators. No effect was observed with test samples of cellulose treated under the same conditions as those of PAN grafting but without AN. 

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