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ALKENES

Dr Sukhbir Kaur

Organic Chemistry by Morrison and Boyd

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Most technically important polymerizations of alkenes occur by chain mechanisms and may be classed as anion, cation, or radical reactions, depending upon the character of the chain-carrying species. In each case, the key steps involve successive additions to molecules of the alkene, the differences being in the number of electrons that are supplied by the attacking agent for formation of the new carbon-carbon bond. For simplicity, these steps will be illustrated by using ethene, even though it does not polymerize very easily by any of them:

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Anionic Polymerization

Initiation of alkene polymerization by the anion-chain mechanism may be formulated as involving an attack by a nucleophilic reagent

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.

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Cationic Polymerization

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:

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Radical Polymerization

Ethene can be polymerized with peroxide catalysts under high pressure (1000atm or more, literally in a cannon barrel) at temperatures in excess of 100o. The initiation step involves formation of radicals, and chain propagation entails stepwise addition of radicals to ethene molecules.

Chain termination can occur by any reaction resulting in combination or disproportionation of free radicals.

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Allylic Carbocation

An allylic carbocation is a resonance-stabilized carbocation in each of the two resonance forms of which the formal charge of +1 is on an allylic carbon.

Alkenyl Carbocation

An alkenyl or vinylic carbocation is a carbocation in which the formal charge of +1 is on a carbon atom doubly bonded to another carbon atom