Graft Copolymerization Using Chemical Initiators

However, if the grafting is carried out in the presence of ceric ion, it forms a complex with the polysaccharide  

In order to minimize the homopolymerization and chain transfer reactions, ceric ion initiated graft copolymerization reactions are usually carried out at lower temperatures. Ceric ion initiated reaction is highly specific in nature. 

During grafting using ceric ion as an initiator, the presence of nitric acid is found to play an important role. Ceric ion in water reacts in the following manner  

in water as the reaction medium, ceric ion exists as Ce, [Ce(OH)j] and [Ce-0-Ce] +. No grafting was found in the absence of nitric acid as 

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A number of methods have been used to prepare graft copolymers in the past few decades including both conventional chemical and radiation-chemical methods [20,86,87]. In the latter case, graft copolymerization is usually initiated by creating active radical sites on existing polymer chains. The advantages of radiation-chemical methods are (i) ease of preparation as compared to... 

Graft copolymerization is one of the most common tools in the hands of chemists to bring-out the desired changes in the backbone polymer. Different vinyl monomers using chemical initiators or radiations can be grafted onto backbone thereby incorporating properties such as water repellency water absorbency, acid-base resistance, thermal stability and dyeability in the backbone polymers [34-37]. 

Also other oxidants have been used successfully to initiate graft copolymerization onto cellulose substrates. Toyo Rayon (79) achieved grafting by pretreatment of viscose rayon fibers with potassium permanganate-sulfuric acid and contacting it subsequently with acrylonitrile. Methyl methacrylate is being grafted to rayon when heated together with potassium bromate, as claimed by Asahi Chemical Industry... 

Different types of chemical initiators such as FAS-KPS, benzoyl peroxide, azo-bis-iso-butylnitrile, H O, ascorbic acid-KPS, ammonium persulphate, ceric ammonium persulphate, ceric ammonium nitrate, etc., can be used for the graft copolymerization of various vinyl monomers such as methylacrylate, methyl methacrylate, acrylic acid, methacrylic acid, ethyl methacrylate, vinyl acetate, acrylamide, etc., onto polysaccharides [38-40]. Figure 2.1 shows the proposed mechanism through which the grafting can be explained [39]. 

Pre-activate polymer substrate (e.g., by peroxidation in air or ozone) using plasma discharge, ioniring radiation or chemical treatment, llien contact with monometfs) solvents and initiate using heat and/or catalyst to graft copolymerize monomer onto polymer substrate. Remove solvent and any unreacted monomer. ( Indirect Method )... 

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. 

The first attempts to produce a graft copolymer of poly(amino acids) onto the natural polymers, such as cellulose, starch, and their derivatives, were carried out by Zilka and Avny in 1965. Sodium methoxide was known to be an initiator of NCA polymerization. Therefore, they proposed to use alkoxide derivatives of polysaccharides as macroinitiators for the graft copolymerization of NCA s of a-amino acids. All known methods for production of alkoxide derivatives of natural polymers, such as the reaction of polyhydroxy polymers with sodium metal in liquid ammonia, or exchange reactions between lower alkoxides and polyhydroxy polymers, - were unsatisfactory for the subsequent graft copolymerization of NCAs, because any residual base would lead to homopolymerization. In addition, alkoxide derivatives of cellulose acetate and nitrocellulose could not be obtained by these conventional methods due to chemical degradation. Zilka et al., finally found that the reaction of alkali metal naphthalenes [20] with polyhydroxy polymers in either... 

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