Graft copolymerization free radical generation

In its simplest form the direct grafting method involves the irradiation of polymeric substrate in the absence or presence of oxygen. Graft copolymerization of the monomer to the polymer is then initiated through the free radicals generated in the latter. The reaction can be schematically written as ... 

A number of metal chelates containing transition metals in their higher oxidation states are known to decompose by one electron transfer process to generate free radical species, which may initiate graft copolymerization reactions. Different transition metals, such as Zn, Fe, V, Co, Cr, Al, etc., have been used in the preparation of metal acetyl acetonates and other diketonates. Several studies demonstrated earlier that metal acetyl acetonates can be used as initiators for vinyl polymeriza-... 

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. 

We shall consider here graft copolymerization only by free-radical processes. There are three main techniques for preparing graft copolymers via a free-radical mechanism. All of them involve the generation of active sites along the backbone of the polymer chain. These include (i) chain transfer to both saturated and unsaturated backbone or pendant groups (ii)radiative or photochemical activation and (iii) activation of pendant peroxide groups. 

When a polymer chain is ruptured mechanically, terminal-free radicals can be generated, and these can be utilized to initiate block copolymerization. Under an inert atmosphere, block copolymers can be produced by cold milling, or mastication of two different polymers or of a polymer in the presence of a second monomer. This generally results in the formation of graft copolymers in addition to the block copolymers since radicals can be located in nonterminal positions by chain transfer. However, predominant yield of block copolymers is obtained by milling monomer-swollen polymers. The success of this technique depends on the physical state of the polymer. Generation of radical is favored if the polymer exists at or near the glassy state otherwise, polymer flow rather than bond rapture will occur. Table 5.5 shows some block copolymers prepared by this technique. 

This is still the most popular method for graft copolymerization of elastomers via free radicals. Free radicals (I) are generated from the same types of initiators which are used for free radical polymerization and copolymerization (see Section 2.4). In general, these radicals are formed in the presence of a polydiene elastomer and a monomer therefore, there are several possible reactions of these initiator-derived radicals which can occur as shown in Eqs. (2.93)-(2.96). The competition between initiation of monomer polymerization (Eq. 

Block and graft copolymerization can also be initiated in indirect modes. Here, light is absorbed by independent initiator molecules that are present in the reaction system but are not incorporated into the polymer. Reactive species formed in this way interact with the polymer so as to generate free radical sites... 

Grafting reactions are carried out conventionally through free radical addition copolymerization mechanism, where free radicals are generated on a polymeric backbone by direct oxidation of certain transition metal ions (e.g., Ce ", Cr +, Co ). The redox... 

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. 

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