8.10 Radical Additions to Alkenes: Chain-Growth Polymers

Radical Polymerization of Alkenes

Steps of radical polymerization

Radical polymerization converts alkene monomers into long polymer chains through a free-radical chain reaction. The mechanism follows the same three-phase pattern you've seen in other radical reactions: initiation, propagation, and termination.

Initiation happens in two sub-steps:

1. An initiator (typically a peroxide like benzoyl peroxide, or an azo compound like AIBN) decomposes into two free radicals when heated or exposed to UV light. For a peroxide, homolytic cleavage of the weak O–O bond produces two oxygen-centered radicals.
2. One of those initiator radicals adds to the $\pi$ bond of an alkene monomer, forming a new C–C bond and generating a carbon-centered radical on the monomer.

Propagation is where the chain actually grows:

1. The carbon radical on the first monomer attacks the $\pi$ bond of a second monomer, forming another C–C single bond and moving the radical to the end of the two-unit chain.
2. This repeats thousands of times. Each addition extends the chain by one monomer unit, and the radical always sits at the growing end.

Termination destroys the radical and stops chain growth. Two common pathways:

1. Combination: Two growing chain radicals collide and couple, forming one large polymer molecule with a new C–C bond at the junction.
2. Disproportionation: One growing chain abstracts a hydrogen atom from another growing chain. This produces two dead polymer molecules, one with a terminal alkene and one with a saturated end.

Vinyl monomers in polymer formation

Vinyl monomers have the general structure $CH_2=CHR$, where $R$ is any substituent group. The identity of $R$ determines which polymer you get:

• $R = H$ → polyethylene
• $R = Cl$ → poly(vinyl chloride), PVC
• $R = C_6H_5$ (phenyl) → polystyrene
• $R = CN$ → polyacrylonitrile

During polymerization, each monomer's double bond breaks open and forms two new C–C single bonds to adjacent monomers. Because the $R$ group is attached to every other carbon along the backbone, the resulting polymer has a regular, repeating structure: $[-CH_2-CHR-]_n$.

Radical intermediates of vinyl monomers

For unsymmetrical vinyl monomers ($CH_2=CHR$), the initiator radical can add to either carbon of the double bond. This creates two possible radical intermediates, and their relative stability determines the polymer's structure.

1. Radical on the substituted carbon ($-CH_2-\dot{C}HR-$): The radical sits next to the $R$ group, which can stabilize it through resonance or hyperconjugation (depending on what $R$ is). A phenyl group, for example, delocalizes the radical by resonance. This is the more stable intermediate.
2. Radical on the unsubstituted carbon ($-\dot{C}H_2-CHR-$): The radical is isolated from the $R$ group and receives no extra stabilization. This is the less stable intermediate.

Because the more stable radical forms preferentially, the initiator radical adds to the $CH_2$ end of the monomer (the less substituted carbon). This places the radical on the carbon bearing $R$, and the pattern repeats with each propagation step. The result is a head-to-tail polymer: $[-CH_2-CHR-]_n$, where all the $R$ groups are spaced regularly along the chain.

Chain-growth polymerization

Radical polymerization is one type of chain-growth polymerization, a broader category that also includes cationic and anionic polymerization. What defines chain-growth polymerization is that monomers add one at a time to a reactive chain end (a radical, cation, or anion), and each addition regenerates the reactive site.

This contrasts with step-growth polymerization, where any two monomers (or oligomers) with complementary functional groups can react with each other at any time. Nylon and polyesters form by step-growth; polyethylene and polystyrene form by chain-growth.

In radical chain-growth polymerization specifically, the reactive chain end is a free radical, and the monomers are alkenes whose $\pi$ bonds provide the electrons for each new C–C bond.