Initiations of Polymerizations from the Backbone of Polymers

High degrees of grafting by free-radical mechanism can be attained when polymerizations are initiated from the backbones of the polymer. One way this can be done is to form peroxides on the backbone structures. Decompositions of such peroxides can yield initiating radicals. Half of them will be attached to the backbones. An example is preparation of graft copolymers of polystyrene [305, 306]:

Thermal cleavage of the above peroxides leads to both macromolecules with free-radicals sites. Hydroxyl radicals also form and initiate formations of homopolymers. Decompositions of the peroxides by redox mechanisms, however, increase the yields of graft copolymers, but do not stop all formations of hydroxy radicals [303]:

Some homopolymers still form [307]. Air oxidation of polypropylene can result in formation of hydroperoxide units at the sites of the tertiary hydrogens [383]. The polymer can also be oxidized when dissolved in cumene that contains some cumene hydroperoxide at 70–80C. A product containing 0.8% oxygen by weight as a hydroperoxide [308] can be formed and can subsequently be reacted to form graft copolymers. Various monomers [309–311] can be used, such as vinyl acetate or 2-vinyl pyrrolidone. Many other hydroperoxidations of polymers were reported in the literature. The materials are used in formations of graft copolymers. One example is hydroperoxidation of poly (ethylene terephthalate) and poly(e-caproamide). The products yield graft copolymers with various acrylic and methacrylic esters and acrylic and methacrylic acids [312–314]. Ozone treatment of polymers can also cause hydroperoxidation of labile hydrogens. It can, however, also cause extensive degradation of the backbone polymers, because attacks by ozone on double bonds in the backbones convert them to unstable ozonides. Starch can be ozonized to form graft copolymers [315, 316]. The same is true of cellulose [317], poly(vinyl chloride) [318], and polyethylene [319]. Hydroperoxides form without noticeable degradation. This allows subsequent preparations of graft copolymers. In a similar manner, it is possible to start with copolymers of acryloyl or methacryloyl chloride and react them with hydroperoxides [332]. This can be illustrated as follows:

The decomposition of the pendant peroxide in the presence of vinyl monomers yields mixtures of graft copolymers and homopolymers. Preparation and subsequent decomposition of polymers with diazonium salts can also be used to form graft-copolymers. An example is nitrated polystyrene, reduced to the amine derivative and then diazotized [333]. The decomposition of the diazonium salt results in formation of radicals:

The radical sites are capable of initiating polymerizations of monomers. A similar approach can be taken with cellulose [334]. Mercerized cotton and sodium salt of carboxymethyl cellulose will react with p-amino phenacyl chloride:

The material can be converted to diazonium salts and then decomposed with ferrous ions in the presence of some vinyl monomers to form graft copolymers. Acrylonitrile forms graft copolymer readily without formation of any homopolymers. Styrene and vinyl acetate, however, do not. A modification of this technique is to conduct the diazotization reaction in the presence of emulsifiers [335]. The amounts of graft copolymers that form with acrylic and methacrylic monomers and N-vinylpyrrolidone depend upon the nature and pH of the emulsifiers, the reaction time, and the temperature. Ceric ions form graft copolymers with various macromolecules by a redox mechanism. The reactions can be illustrated as follows:

The almost exclusive formation of free radicals on the polymeric backbones results in formations of many products that are close to being free from homopolymers [346, 350]. The reactions are widely used in formations of graft copolymers of poly(vinyl alcohol) and particularly of cellulose and starch. The grafting reaction fails, however, when attempted on polysaccharides that lack free hydroxyl groups on the second and third carbons. This led to speculation [351] that the bond between these carbons cleaves. In the process, free radicals, presumably, form on the second carbons and aldehyde structures on the third carbons of the glucose units. This point of view, however, is not generally accepted. Instead, it was proposed that more likely positions for attacks by the ceric ions are at the C1 carbons of the glucoses at the end of the polysaccharide chains [352]. This is supported by observation that oxidation of cellulose is an important prerequisite for the formation of graft copolymers [353]. Graft copolymerizations by redox mechanism can also take place with the aid of other ions. This includes grafting on cellulose backbones with ferrous ions and hydrogen peroxide [354]. Redox grafting reactions can also take place on nylon and on polyester. For instance, graft copolymers of methyl methacrylate on nylon 6 can be prepared with manganic, cobaltic, and ferric ions [355]. Another example is grafting poly (glycidyl methacrylate) on poly (ethylene terephthalate) fibers with the aid ferrous ion–hydrogen peroxide. The reaction depends on the concentration of the monomer, hydrogen peroxide, time, and temperature [356].

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مواضيع ذات صلة

Block Copolyesters

Block Copolymers

Miscellaneous Graft Copolymerizations

Preparation of Graft Copolymers with Ionic Chain-Growth and Step-Growth Polymerization Reactions

Graft Copolymer Formation with the Aid of High-Energy Radiation

Photochemical Syntheses of Graft Copolymers

Preparation of Graft Copolymers with the Aid of Macromonomers

Free-Radical Grafting Reactions to Polymers with Double Bonds

Free-Radical Grafting by Chain-Transferring Process

Vulcanization of Elastomers

Miscellaneous Exchange Reactions

Substitution Reactions of Poly (vinyl alcohol)