3.0 Free-radical vinyl polymerization

  Polymers

We have already discussed briefly the polymerization of ethylene and substituted ethylenes under conditions where free radicals are generated - typically in the presence of small amounts of an initiator, such as a peroxide.

Reaction occurs at the doubly bonded carbons – the vinyl groups – and is called vinyl polymerization.

A wide variety of unsaturated monomers may be used, to yield polymers with different pendant groups (G) attached to the polymer backbone. For example:

Polymerzation involves addition of free radicals to the double bond of the monomer: addition, first, of the free radicals generated from the initiator, and then of the growing polymer molecule. This is an example of chain-reaction polymerization.

$1.$ Peroxide $ \to $ Rad. (chain initiating steps)

$2.$ Chain Propagating steps

Then steps like $(2)$ repeated, until finally.

$3.$ Chain terminating steps:

or

In each step the consumption of a free radical is accompanied by the formation of a new, bigger free radical. Eventually, the reaction chain is terminated by steps that consume but do not form free radicals; combination or disproportionation of two free radicals.