Grafting free radical mechanism

Mn(III) is able to oxidize many organic substrates via the free radical mechanism [32], The free radical species, generated during oxidation smoothly initiate vinyl polymerization [33-35]. Mn(III) interacts also with polymeric substrates to form effective systems leading to the formation of free radicals. These radicals are able to initiate vinyl polymerization and, consequently, grafting in the presence of vinyl monomers. 

The selective oxidation of cellulose to dialdehyde by sodium periodate is well known. It has been postulated by Criegee (74) and by Waters (73) that this reaction proceeds by a free radical mechanism. Toda (76) and Morimoto, Okada, Okada, and Nakagawa (77) have concluded that sodium periodate oxidation should initiate graft polymerization. They succeeded in grafting methyl methacrylate and acrylonitrile onto cellulose substrates, such as rayon and paper. A similar procedure is recommended in a patent of Chemische Werke Huels (78) to graft vinyl monomers onto cotton, polyethylene oxide, copolymers of vinyl chloride-vinyl acetate, and others. 

A graft or block copolymer of cellulose is defined here as a combination of cellulose and polymer that is difficult to separate by solvent extraction without first degrading the cellulose. Furthermore, only proposed mechanisms and reaction methods of free-radical initiated graft and block polymerizations of vinyl monomers with cellulose are discussed. 

The modification of the properties of cotton cellulosic textile products, through free radical-initiated graft copolymerization reactions with vinyl monomers, has been investigated at the Southern Laboratory for a number of years (6, 9). In this chapter, we summarize the basic mechanisms and principles involved in free radical reactions of cellulose, initiated by high energy radiation, ceric ion in acidic solution, and aqueous solutions of ferrous ion and hydrogen peroxide. Some of the properties of fibrous cotton cellulose graft copolymers are also presented. 

Both methods are very general. They apply to any polymer which undergoes radiolysis (i.e., "any" polymer) and the only limitation with respect to the monomer is that it polymerize by a free radical mechanism. Ionic grafting was also initiated by radiation (28,29) but the yields of this process are generally quite low. 

Block and graft copolymerizations by free-radical mechanism are usually conducted in a mixture of the parent polymer, the monomer(s) to be grown on the parent polymer, and fresh initiator. However, the product obtained in this case is likely to be a mixture. Thus, in addition to the desired block or graft copolymer, it may contain homopolymer of fresh monomer and parent homopolymer molecules that did not take part in the copolymerization. 

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. 

It was found (11) that while acrylonitrile and methyl methacrylate polymerized by a free radical mechanism, styrene led to both cationic and free radical intermediates. Graft copolymers were formed in the process. 

During the past 20 years, there has been much interest in understanding the grafting of polar vinyl monomers to polyolefins (PO). The grafting process can be performed in an inert solvent or in a PO melt. A direct grafting of a monomer to chains in molten PO that follows the free-radical mechanism appears more preferable, and it has been studied more widely. It is most often done by means of reactive extrusion (RE), where an extruder is used as a reactor of continuous action. This technology permits the production of a variety of functionalized PO (1-8). 

Compositions of these adhesives are suggested in a number of recent patents (5- )- All of these reactive adhesive patents indicate essentially the same concept an elastomer is colloidally dispersed in a monomer, or a monomer/oligomer/polymer solution. The system is then polymerized using a free radical mechanism. What occurs is a rapid, "in situ" polymerization of a (typically) methyl methacrylate system, toughened by elastomeric domains which have beer, incorporated into the structure by grafting. 

There are essentially three approaches to the preparation of graft copolymers via free radical mechanism ... 

Cross-linking of the outer casing is achieved by grafting a vinyl silane to polyethylene by means of a peroxide as part of the cable extrusion process. The chemistry is shown in Scheme 2.1. When the cable is subsequently soaked in water, the alkoxy silanes hydrolyse and crosslink within the matrix. It is an unresolved question whether polymers that have been modified by a free radical mechanism will survive for the expected lifetime of underground cables (ca. 75 years). 

Many copolymers of styrene are ntanufactured on a large commercial scale. Because styrene copolymerizes readily with many other monomers, it is possible to obtain a wide distribution of properties. Random copolymers form quite readily by a free-radical mechanism. Some can also be formed by ionic mechanism. In addition, graft and block copolymers of styrene are also among commercially important materials. 

High degrees of grafting by the 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 the preparation of graft copolymers of polysty-... 

Mechanism of free radical initiated grafting reaction of maleic anhydride onto PE (from ref 11). 

The melt phase grafting reaction of acrylic acid onto polypropylene proceeds by a free radical mechanism (10). Radicals generated by thermal decomposition of an initiator abstract hydrogen from the polypropylene backbone and initiate homopolymerization of acrylic acid. Acrylic acid also adds to the sites on the backbone, with the result that the product contains acrylic acid grafted polypropylene (AA-g-PP) and poly(acrylic acid) homopolymer. 

Monomers, which polymerize via a free radical mechanism, can be polymerized on the activated support to produce coatings of various thicknesses and depths of penetration. Ionizing radiation has been extensively used for modifying the surfaces of biomaterials via surface grafting reactions. " ... 

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