Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Synthetic Polymers and Biopolymers
20.4 Free Radical Polymerization of Alkenes

The principle involved in free radical polymerization is one that we have utilized before when we examined free radical halogenation of alkanes. Like free radical halogenation of alkanes, free radical polymerization of alkenes involves three steps: an initiation step, a propagation step, and a termination step. The initiation step is the first step of the reaction mechanism and it is the step that produces radicals. Usually the radicals are generated by the homolytic cleavage of a weak nonpolar covalent bond to produce fairly stable radicals. The compounds that are used to produce such radicals are called radical initiators, and an example of a radical initiator reaction is shown in Reaction (20-8).

(20-8)

For Reaction (20-8), the O─O bond of the benzoyl peroxide is a very weak nonpolar covalent bond, and if this compound is heated, the O─O bond will break in a homolytic manner to produce radicals. As shown, these radicals are fairly stable owing to the resonance gained through conjugation with the adjacent carbonyl group. Owing to the favorable release of carbon dioxide, which is a gas, the initially formed radical often undergoes a further reaction to form CO₂ and another radical, the phenyl radical, as shown in Reaction (20-9).

(20-9)

These phenyl radicals, as well as the benzoyl radicals, in the presence of an alkene functionality, react to produce more radicals, a propagation step. The RO^. notation is typically used to represent such radicals.

Problem 20.2

The molecule shown below is often used as a radical initiator. By heating it slightly, a stable radical is produced, in addition to nitrogen gas. Give the structure of the radical produced and demonstrate its stability by drawing a resonance structure.

20.4.1 Free Radical Polymerization of Isobutylene

For the free radical polymerization of isobutylene, the first step of the reaction involves the generation of radicals using a radical initiator as shown in Reaction (20-10).

(20-10)

Once radicals are produced, and in the presence of an isobutylene, an addition will occur to generate another radical, which reacts with another mole of the alkene to form a coupled radical, as shown in Reaction (20-11).

(20-11)

Since new radicals are produced in this step, it is possible for this newly formed radical to react with another molecule of the alkene to produce yet another radical, which has a longer chain. This process will continue until the chain of the radical is fairly long as shown in Reaction (20-12), and n represents an integer, typically in the hundreds.

(20-12)

Polymers are not radical, but they are neutral molecules; thus, a terminating step is needed for this mechanism. Termination can occur in one of two ways, radical coupling or disproportionation. In a terminating step, which occurs by radical coupling, two radicals come together to form one molecule. Since there are two electrons in a single covalent bond, and each radical has one electron, coupling is a very favorable step. Radicals can form neutral molecules by another way, disproportionation. In this step, a radical abstracts a hydrogen atom (a hydrogen with one electron) from another radical to form two products. Reaction (20-13) shows a termination step where two radicals couple to form a neutral product.

(20-13)