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Tuesday, 2 June 2015

Polymerization

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In the chain polymerization reaction is triggered by the formation of a chemical species active, call initiator, which can be constituted by a radical, a carbocation or a carbanion. The initiator sum recursively upon himself a monomer molecule, shifting the center of reactivity (the unpaired electron, the electric charge) at the end of the chain As this grows. The production process of a polymer chain is divided into 3 phases: activation, propagation, and termination.

An example of a polymer obtained by chain polymerization is polyethylene.

In the presence of a high temperature or pressure the double bond between the carbon atoms breaks down and is replaced with a covalent single bond. At each carbon atom compete then 7 electrons in the outermost energy level. To meet the demands of bond, the monomer is combined with other monomers of ethylene, thus ensuring that each carbon atom shares four covalent bonds. The polymerization can take place thanks to the presence of a double covalent bond between the carbon atoms in the molecule of the reagent. The double bond (together with the triple bond) is said "unsaturated bond"; it is constituted by two links that have binding forces of different entities: the bond σ (stronger) and the bond π (weaker). The breaking of the bond π generates a chemical species unstable (called radical) that can be counted along additional monomers, lengthening the chain.


The concentration of monomer in the reaction in the case of chain polymerization decreases with constant speed. The yield for this type of polymerization is high, and the polymers that are obtained have a lower degree of polymerization than those obtained in the polymerization in stages.