Both anionic and cationic polymerization of an alkenes undergoes by virtue of the flow of electrons during propagation’s. Anionic intiation step imvolves the formation of a negatively charge carbanion group. The cationic initiation involves the formation of positively charge carbocation. See below
Anionic Polymerization of an alkenes
Initiation of alkene polymerization by the anion-chain mechanism may be formulated as involving an attack by a nucleophilic reagent Y :⊖ on one end of the double bond and formation of a carbanion:
Attack by the carbanion on another alkene molecule would give a four-carbon carbanion, and subsequent additions to further alkene molecules would lead to a high-molecular-weight anion:
The growing chain can be terminated by any reaction (such as the addition of a proton) that would destroy the carbanion on the end of the chain:
Anionic polymerization of alkenes is quite difficult to achieve because few anions (or nucleophiles) are able to add readily to alkene double bonds. Anionic polymerization occurs readily only with alkenes substituted with sufficiently powerful electron-attracting groups to expedite nucleophilic attack. By this reasoning, alkynes should polymerize more readily than alkenes under anionic conditions, but there appear to be no technically important alkyne polymerization’s in operation by this or any other mechanism. Perhaps this is because the resultant polymer would be highly conjugated, and therefore highly reactive, and may not survive the experimental conditions:
Cationic Polymerization of an alkenes
Polymerization of an alkene by acidic reagents can be formulated by a mechanism similar to the addition of hydrogen halides to alkene linkages. First, a proton from a suitable acid adds to an alkene to yield a carbocation. Then, in the absence of any other reasonably strong nucleophilic reagent, another alkene molecule donates an electron pair and forms a longer-chain cation. Continuation of this process can lead to a high-molecular-weight cation. Termination can occur by loss of a proton. The following equations represent the overall reaction sequence:
Ethene does not polymerize by the cationic mechanism because it does not have sufficiently effective electron-donating groups to permit easy formation of the intermediate growing-chain cation. 2-Methylpropene has electron-donating alkyl groups and polymerizes much more easily than ethene by this type of mechanism. The usual catalysts for cationic polymerization of 2-methylpropene are sulfuric acid, hydrogen fluoride, or a complex of boron trifluoride and water. Under nearly anhydrous conditions a very long chain polymer called polyisobutylene is formed.
Polyisobutylene fractions of particular molecular weights are very tacky and are used as adhesives for pres sure-sealing tapes.
In the presence of 60% sulfuric acid, 2-methylpropene is not converted to a long-chain polymer, but to a mixture of eight-carbon alkenes. The mechanism is like that of the polymerization of 2-methylpropene under nearly anhydrous conditions, except that chain termination occurs after only one 2-methylpropene molecule has been added:
The short chain length is due to the high water concentration; the intermediate carbocation loses a proton to water before it can react with another alkene molecule.
The proton can be lost in two different ways, and a mixture of alkene isomers is obtained. The alkene mixture is known as “diisobutylene” and has a number of commercial uses. Hydrogenation yields 2,2,4-trimethylpentane (often erroneously called “isooctane”), which is used as the standard “100 antiknock rating” fuel for intemal-combustion gasoline engines:
The differences between anionic and cationic polymerization is that cationic polymerization is faster than anionic and radical polymerization discuss later.
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