Anionic addition polymerization

An anionic addition polymerization of vinyl monomers is an addition polymerization initiated by a strong base and anion, such as an alkali amide, or an organometallic compound, such as n-butyllithium.

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A nucleophilic carbanion is formed containing an unshared pair of electrons.

Confronted with a carbon molecule with a terminal double bond, such as ethene and a wide variety of monomer vinyl compounds as styrene or acrylonitrile, a conjugate addition occurs between the terminal carbon atom of the carbanion and the first carbon atom of the C = C bond. The unshared pair of electrons moves to the other carbon atom of C = C bond. A new but longer carbanion is formed that will react likewise.

A rapid chain-reaction polymerization reaction follows and a macromolecule is formed.

The reaction only stops when all monomer is consumed or when another reaction occurs with an impurity or a molecule that is not part of the polymerization process.

Vinyl polymers are widely used, such as synthetic elastomers, acrylic glass and polystyrene.

CH₃CH₂CH₂CH₂Li → CH₃CH₂CH₂CH₂: ⁻ Li⁺

n-butyllithium → n-butyl carbanion

CH₃CH₂CH₂CH₂: ⁻ Li⁺ + CH₂=CH₂ → CH₃CH₂CH₂CH₂CH₂CH₂: ⁻ Li⁺

n-butyl carbanion + ethene → n-hexyl carbanion

Anionic addition polymerization is not confined to vinyl compounds. It equally occurs when 1,2-epoxyethane (commonly known as ethylene oxide) reacts with a small amount of a base, such as sodium methoxide. It turns, through polymerization , into a polyether of high molecular weight.