Ionic polymerization

In polymer chemistry, ionic polymerization is a chain-growth polymerization in which active centers are ions or ion pairs. It can be considered as an alternative to radical polymerization, and may refer to anionic polymerization or cationic polymerization. As with radical polymerization, reactions are initiated by a reactive compound. For cationic polymerization, titanium-, boron-, aluminum-, and tin-halide complexes with water, alcohols, or oxonium salts are useful as initiators, as well as strong acids and salts such as . Meanwhile, group 1 metals such as lithium, sodium, and potassium, and their organic compounds (e.g. sodium naphthalene) serve as effective anionic initiators. In both anionic and cationic polymerization, each charged chain end (negative and positive, respectively) is matched by a counterion of opposite charge that originates from the initiator. Because of the charge stability necessary in ionic polymerization, monomers which may be polymerized by this method are few compared to those available for free radical polymerization. Stable polymerizing cations are only possible using monomers with electron-releasing groups, and stable anions with monomers with electron-withdrawing groups as substituents.…

In polymer chemistry, ionic polymerization is a chain-growth polymerization in which active centers are ions or ion pairs. It can be considered as an alternative to radical polymerization, and may refer to anionic polymerization or cationic polymerization. As with radical polymerization, reactions are initiated by a reactive compound. For cationic polymerization, titanium-, boron-, aluminum-, and tin-halide complexes with water, alcohols, or oxonium salts are useful as initiators, as well as strong acids and salts such as . Meanwhile, group 1 metals such as lithium, sodium, and potassium, and their organic compounds (e.g. sodium naphthalene) serve as effective anionic initiators. In both anionic and cationic polymerization, each charged chain end (negative and positive, respectively) is matched by a counterion of opposite charge that originates from the initiator. Because of the charge stability necessary in ionic polymerization, monomers which may be polymerized by this method are few compared to those available for free radical polymerization. Stable polymerizing cations are only possible using monomers with electron-releasing groups, and stable anions with monomers with electron-withdrawing groups as substituents. While radical polymerization rate is governed nearly exclusively by monomer chemistry and radical stability, successful ionic polymerization is as strongly related to reaction conditions. Poor monomer purity quickly leads to early termination, and solvent polarity has a great effect on reaction rate. Loosely-coordinated and solvated ion pairs promote more reactive, fast-polymerizing chains, unencumbered by their counterions. Unfortunately, molecules that are polar enough to support these solvated ion pairs often interrupt the polymerization in other ways, such as by destroying propagating species or coordinating with initiator ions, and so they are seldom utilized. Typical solvents for ionic polymerization include non-polar molecules such as pentane, or moderately polar molecules such as chloroform. ==References==

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