4.2 Anionic Polymerization

  Polymers

It is initiated by bases: $Li^+$, $NH_2^-$,or organometallic compounds like $n-$butyllithium. For example:

Active metals like $Na$ or $Li$ can be used; here the initiation becomes a little more complicated, as in the polymerization of styrene by the action of sodium metal and naphthalene. A sodium atom transfers an electron $(1)$ to naphthalene to form a radical-anion:

$1.$

$2.$

$3.$

The radical-anion then donates the electron to styrene $(2)$ to form the styrene radical-anion. Like many other free radicals, these dimerize $(3)$. The resulting dianion is the true initiator, and begins to grow at both ends

Anionic polymerization is not limited to the vinyl kind, involving addition to carbon-carbon double bonds. Ethylene oxide, for example, is converted by a small amount of base into a high molecular weight polyether.