![]() However, methyl methacrylate and vinyl acetate are terminated chiefly by disproportionation.Īnother reaction that diverts radical chain-growth polymerizations from producing linear macromolecules is called chain transfer. For acrylonitrile and styrene combination is the major process. The relative importance of these terminations varies with the nature of the monomer undergoing polymerization. monomers, solvents and terminated chains), the rate at which these radical-radical termination reactions occurs is very small, and most growing chains achieve moderate length before termination. Since the concentration of radical species in a polymerization reaction is small relative to other reactants (e.g. Note that in both types of termination two reactive radical sites are removed by simultaneous conversion to stable product(s). The growing polymer chains are colored blue and red, and the hydrogen atom transferred in disproportionation is colored green. These reactions are illustrated by the following equations. The most common termination processes are Radical Combination and Disproportionation. In practice, larger numbers of moderately sized chains are formed, indicating that chain-terminating reactions must be taking place. In principle, once started a radical polymerization might be expected to continue unchecked, producing a few extremely long chain polymers. Because radicals are tolerant of many functional groups and solvents (including water), radical polymerizations are widely used in the chemical industry. Since carbon radicals are stabilized by substituents of many kinds, the preference for head-to-tail regioselectivity in most addition polymerizations is understandable. Each monomer unit adds to the growing chain in a manner that generates the most stable radical. The first two equations illustrate the initiation process, and the last two equations are examples of chain propagation. One example of this radical polymerization is the conversion of styrene to polystyrene, shown in the following diagram. The formulas of some common initiators, and equations showing the formation of radical species from these initiators are presented below.īy using small amounts of initiators, a wide variety of monomers can be polymerized. When radical polymerization is desired, it must be started by using a radical initiator, such as a peroxide or certain azo compounds. Since this can be initiated by traces of oxygen or other minor impurities, pure samples of these compounds are often "stabilized" by small amounts of radical inhibitors to avoid unwanted reaction. Virtually all of the monomers described above are subject to radical polymerization. Coordination Catalytic Polymerization The initiator is a transition metal complex, and the propagating site of reactivity (*) is a terminal catalytic complex. ![]() Anionic Polymerization The initiator is a nucleophile, and the propagating site of reactivity (*) is a carbanion.Cationic Polymerization The initiator is an acid, and the propagating site of reactivity (*) is a carbocation.Radical Polymerization The initiator is a radical, and the propagating site of reactivity (*) is a carbon radical. ![]() ![]() It is useful to distinguish four polymerization procedures fitting this general description. Indeed, cases of explosively uncontrolled polymerizations have been reported. Since a pi-bond in the monomer is converted to a sigma-bond in the polymer, the polymerization reaction is usually exothermic by 8 to 20 kcal/mol. A general diagram illustrating this assembly of linear macromolecules, which supports the name chain growth polymers, is presented here. Many of these addition reactions are known to proceed in a stepwise fashion by way of reactive intermediates, and this is the mechanism followed by most polymerizations. ![]() The most common and thermodynamically favored chemical transformations of alkenes are addition reactions. \)Īll the monomers from which addition polymers are made are alkenes or functionally substituted alkenes. ![]()
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