Graft copolymerization chemical free radical initiator

Arthur, J.C., Jr., Free-radical initiated graft polymerization of vinyl monomers onto cellulose, in Graft Copolymerization of Lignocellulosic Fibers, Hon, D.N.-S., Ed., ACS Symposium Series, No. 187, American Chemical Society, Washington D.C., 1982, p. 21. 

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. 

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. 

The morphology of the fibrous cellulose graft copolymers depended on the method of initiation of free radical formation, experimental conditions during the copolymerization, chemical modification of the cellulose before reaction, and the type of monomer used (60). Variations in the shape of the fibrous cross section, in layering effects in the fiber, and in the location and distribution of the grafted copolymer in the fiber were observed by electron microscopy (61). Cotton cellulose—poly (acrylonitrile) copolymer was selected to show the possible variations in location and distribution of the grafted copolymer in the fiber. 

Since destruction of polymer materials is very important for practical purposes, a large number of investigations on fracture phenomena in polymers have been carried out from both the experimental and theoretical points of view. Several reports provide indirect evidence for main chain scissions, for example decreases in molecular weight or initiations of the graft or block copolymerization after mastication. Direct evidence for chemical bond scission can be obtained from ESR measurements on fractured polymer materials [21]. The high reactivity and high mobility of free radicals produced by mechanical fracture (mechano-radicals) can also be followed. The ESR application to mechanical destruction of polymer materials is presented below. Temperature-dependent ESR spectra of polymer radicals produced... 

Free radicals may be produced on polymer chains as a result of hydrogen abstraction when a chemical initiator is decomposed after mixing with the polymer substrate. Compared with this method, graft copolymerization occurs with a much higher probability if the chemical initiator is covalently bound to the polymer substrate. 

Grafting a second polymer to the NR molecule in the latex stage is one of the many routes to chemically modified NR. An olefinic monomer with unsaturated double bonds such as methyl methacrylate (MMA), styrene and acrylonitrile are important monomers used for such grafting. " For example, MMA monomer is first converted into an emulsion with some suitable emulsifiers and then mixed with NR latex to copolymerize the monomer in a seeded emulsion polymerization process. It is important to ensure the seed latex particles are saturated with the monomer supplied through diffusion from the emulsified monomer droplets. An oil- or water-soluble initiator can be used to start the reaction. With proper control of the system and reaction conditions, the free radical reaction can be made to propagate within the latex particles as far as possible, so that only grafted NR occurs, without the formation of free homopolymer from the monomer. In this way only chemically modified NR... 

The most widely used method for chemical initiation for graft copolymerization on polysaccharides has been with ceric salts such as CAN or ceric ammonium sulfate. Free radical sites are generated on a polymeric backbone by direct oxidation through ceric metal ions (e.g., Ce "). The ceric ion with low oxidation potential is the proper choice for the reaction. The proposed mechanism for such processes has been depicted by an intermediate formation of metal ion polymer complex (chelate type) [17, 21-24]. Such a complex formation is not restricted to all polymers. The plausible mechanism for ceric ion induced graft copolymerization by direct oxidation method is shown in Scheme 3.1. A series of four grades of Ag-g-PAM copolymers have been synthesized by the conventional method. 

Graft copolymerization is the most effective method utilized in the chemical modification of natural fibers. According to Gassan et al. [99], the reaction is initiated by free radicals on the fiber surface. Ionization polymerization of fibers is carried out in an aqueous solution, followed by exposure to a high-energy radiation. Fiber molecule cracks and radicals are subsequently formed. Next, the radical sites of the fiber are treated with a suitable solution compatible with a polymer such as vinyl monomer, acrylonitrile, methyl methacrylate, or polystyrene. Finally, this leads to the formation of a graft copolymer, which possesses fiber and graft polymer characteristics. 

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