Free-Radical Grafting by Chain-Transferring Process

This technique is probably one of the simplest ways to form graft copolymers. It consists of carrying out free-radical polymerizations of monomers in the presence of polymers preformed from different monomers. A prerequisite for this synthesis is that the active sites must form on the polymeric backbones during the course of the reactions. Ideally, this occurs if the steps of initiations consist only of attacks by the initiating radicals on the backbones:

Propagations then precede from the backbone sites:

Terminations can take place in many ways. Of course, termination by combination will lead to cross-linked insoluble polymers and that is undesirable. An ideal termination takes place by chain transferring to another site on a polymer backbone to initiate another chain growth. The above, however, is an ideal picture. In reality, the efficiency of grafting by this technique depends upon the following:

1. Competitions between the different materials present in the reaction mixture, such as monomer, solvent, and polymer backbone for the radical species. This includes competition between chains growth and chain transferring to any other species present.

2. Competition between the terminating processes, such as disproportionation and chain transferring.

The conditions can vary considerably and it is possible to carry out the reactions in bulk, solution, or in emulsion. When the reaction take place in emulsion, the success depends greatly on experimental techniques. The rate of diffusion is a factor and anything that affects this rate must be considered. Because grafting efficiency depends upon chain transferring to the backbone, knowledge of the chain transferring constants can help predict the outcome of the reactions. Sometimes, the information on the chain-transferring constants is not available from the literature. It may, however, be possible to obtain the information from reactions of low molecular weight compounds with similar structures [284–286]. One assumes equal reactivity toward attacking radicals. The validity of such an assumption was demonstrated on oligomers [281–283]. The reactivity of the initiating radicals toward the backbones can vary and this can also vary the efficiency of grafting. Benzoyl peroxide-initiated polymerizations of methyl methacrylate monomer, for instance, in the presence of polystyrene [284] yield appreciable quantities of graft copolymers. Very little graft copolymers, however, form when di-t-butyl peroxide initiates the same reactions. Azobisisobutyronitrile also fails to yield appreciable quantities of graft copolymers. This is due to very inefficient chain transferring to the polymer backbones by t-butoxy and isobutyronitrile radicals. A better approach is work by Chung and coworkers [288] who grafted maleic anhydride to polypropylene with the use of Borane/O2 initiator. This initiator is claimed to form in situ a monooxidized adduct (R–O* *O–BR2). These adducts then carry out hydrogen abstractions from the polypropylene chains at ambient temperature. This results in formation of stable tertiary polymer radicals that react with maleic anhydride to form graft copolymers: Not all chain transferring to the backbones results in formations of graft copolymers. An example is polymerization of vinyl acetate in the presence of poly(methyl methacrylate). No graft copolymers form and this is independent of the reactivity of the initiators [285]. In fact, grafts of poly (vinyl acetate) to poly (methyl methacrylate) and to polystyrene cannot be prepared by this technique [286]. Grafting efficiency may increase with temperature [287]. This could be due to higher activation energy of the transfer reaction than that of the propagation reaction [288]. Meaningful effects of temperature, however, are not always observed. In grafting polystyrene to poly (butyl acrylate) in emulsion, for instance, there is no noticeable difference between 60 and 90C by this technique [289]. The presence of sites with high transfer constants on the polymeric backbone enhances the efficiency of grafting. Such sites can be introduced deliberately. These can, for instance, be mercaptan groups [290] that can be formed by reacting H₂S with a polymer containing epoxy groups:

Free-radical polymerizations of acrylic and methacrylic esters in the presence of the above backbones result in high yield of graft copolymers [291, 292]. Another example is formation of mercaptan groups on cellulose in order to form graft copolymers [293]:

Pendant nitro groups are also effective in chain transfer grafting reactions. Thus, graft copolymers of polystyrene with cellulose acetate p-nitrobenzoate [294] and with poly (vinyl p-nitrobenzoate) [295] form readily. Nitro groups appear to be more effective in formations of graft copolymers by radical mechanism than are double bonds located as pendant groups [294].

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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

Initiations of Polymerizations from the Backbone of Polymers

Preparation of Graft Copolymers with the Aid of Macromonomers

Free-Radical Grafting Reactions to Polymers with Double Bonds

Vulcanization of Elastomers

Miscellaneous Exchange Reactions

Substitution Reactions of Poly (vinyl alcohol)