Free radical reactions, graft chemical initiation

The free radical polymerization of styrene initialized by iniferter is influenced by chemical binding of iniferter on the surface of the silica." This reaction is used for grafting the polymer onto the surface of the silica. A similar approach is used when carbon whisker is incorporated during the graft-polymerization of methyl methacrylate. Depending on how the whisker is prepared, surface conversion can be increased up to twelve times compared to a polymerization with no whisker present. The addition of graphite to the poly esterification reaction doubles the molecular weight of the polymer. ... 

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... 

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. 

Co+3, Mn+2 and Fe+2 have been found to be effective in producing free radical sites on the polymer backbone through the alcohol groups present on them [75]. In an alternative method, free radical initiators like BPO and AIBN are thermo-chemically activated to give rise to macro-radical sites on polymer backbone to initiate grafting of desired vinylic monomer. The efficiency of these initiators was found to be predominantly dependent on the nature of monomer while the course of reaction depended on the relative reactivity of monomer versus that of the macro-radical. 

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. 

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. 

Chemical evidence for trapped free radicals has been obtained by the observation of various reactions initiated by these radicals after irradiation. These are those of grafting, oxidation, isotope exchange and post-irradiation crosslinking. 

The grafting from procedure requires the generation of active sites on the main polymer chain which are capable of initiating the polymerization of a second monomer. Free radicals can be created by several methods such as irradiation of a polymer in the presence of oxygen [31,32], chain transfer to the backbone [33,34] or redox reaction [35]. Several commercial products have been produced by these methods because they are simple and rather easy to perform. Nevertheless, significant amounts of homopolymers are produced, and, in combination with the poor control of radical polymerization, the final products are characterized by chemical heterogeneity. 

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