Radical and Ionic Polymerization

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Klein 27.3 and 27.4. These are chain-growth mechanisms.

Radical polymerization of an alkene: initiation, propagation by addition to the double bond, and termination.
Why the more stable radical forms, which sets the regiochemistry of the chain.
Cationic polymerization of an alkene that can make a stable carbocation, and anionic polymerization of an alkene with electron-withdrawing groups.
Polystyrene and a cyano- or ester-substituted alkene as the two concrete cases.

Radical and Ionic Chain-Growth Polymerization

Building upon our introduction to chain-growth polymers, we now examine the detailed reactive intermediates that drive these rapid polymerizations. Because chain-growth operates through active ends—specifically radicals, carbocations, or carbanions—the electronic nature of the monomer dictates both the mechanism and the regiochemistry of the growing chain.

[ Insert Image: Active Intermediates in Radical, Cationic, and Anionic Chain-Growth ]

Figure 1. Divergent chain-growth pathways mediated by radical, carbocation, or carbanion active centers.

1. Radical Polymerization of Alkenes

Radical polymerization is one of the most widely used industrial methods for producing commercial plastics. It proceeds through three distinct stages:

Initiation: A thermal or photochemical initiator (such as a peroxide, $\text{R-O-O-R}$, or an azo compound like AIBN) undergoes homolytic cleavage to generate free radicals. These radicals then add across the $\pi$-bond of the first alkene monomer to create a carbon-centered radical active species.
Propagation: The carbon radical adds rapidly and sequentially to the $\pi$-bond of incoming alkene monomers. Each addition regenerates a new radical at the chain end, allowing the chain to grow by thousands of units in milliseconds.
Termination: Growth stops when two active radical chains collide and destroy each other, either via radical coupling (combining two ends to form a single long chain) or disproportionation (transferring a hydrogen atom to yield a saturated and an unsaturated chain end).

[ Insert Mechanism: Three Stages of Radical Polymerization: Initiation, Propagation, Termination ]

Figure 2. Stepwise radical mechanism showing homolytic initiation, monomer addition propagation, and radical coupling termination.

2. Regiochemistry and Radical Stability

When an unsymmetrical alkene (like a monosubstituted ethylene) undergoes radical addition, regiochemistry is strictly controlled by intermediate stability:

The More Stable Radical Wins: During propagation, the incoming radical attacks the less substituted end of the alkene ($\text{CH}_2$) so that the new radical forms on the more substituted carbon (where it is better stabilized by hyperconjugation and inductive electron donation from substituent groups or aromatic rings).
Head-to-Tail Regioselectivity: Because the more stable radical intermediate forms preferentially at every propagation step, monomers add in a remarkably regular head-to-tail orientation along the polymer backbone, minimizing head-to-head or tail-to-tail defects.

[ Insert Mechanism: Regiochemical Control via Intermediate Radical Stability ]

Figure 3. Preferential formation of the more stable carbon radical directing head-to-tail chain propagation.

3. Ionic Polymerization: Cationic and Anionic

When alkenes bear substituents that can stabilize ionic charges, polymerization can proceed via ionic chain-growth:

Cationic Polymerization: Requires alkenes with electron-donating groups (such as alkyl groups or phenyl rings) that can stabilize a developing carbocation. Initiated by strong protic or Lewis acids (like $\text{BF}_3$ or $\text{H}_2\text{SO}_4$), the carbocation adds to monomers via electrophilic attack.
Anionic Polymerization: Requires alkenes substituted with strong electron-withdrawing groups (such as cyano, ester, or phenyl groups) that can delocalize and stabilize a negative charge. Initiated by strong nucleophiles (like organolithium reagents or alkoxides), the carbanion attacks monomer $\pi$-bonds in a living polymerization pathway that lacks spontaneous termination steps.

[ Insert Diagram: Cationic vs. Anionic Chain-Growth Propagation Pathways ]

Figure 4. Stabilizing positive charge via electron donors (cationic) or negative charge via electron withdrawers (anionic).

4. Concrete Case Studies: Polystyrene and Substituted Alkenes

Concrete Example 1: Polystyrene (Radical or Cationic)

Styrene ($\text{C}_6\text{H}_5\text{-CH}=\text{CH}_2$) polymerizes readily via free-radical or cationic mechanisms. The pendant phenyl ring provides powerful benzylic resonance stabilization to both the radical and carbocation intermediates, ensuring smooth, highly stereoregular propagation to yield rigid, transparent polystyrene.

Concrete Example 2: Polyacrylates and Polyacrylonitrile (Anionic)

Monomers like methyl acrylate ($\text{CH}_2=\text{CH}-\text{COOCH}_3$) or acrylonitrile ($\text{CH}_2=\text{CH}-\text{CN}$) possess ester or cyano groups that withdraw electron density through resonance. These groups stabilize carbanionic intermediates, making these monomers exceptionally well-suited for anionic polymerization to form strong acrylic fibers and polymers.

[ Insert Structure: Representative Structures of Polystyrene and Polyacrylonitrile ]

Figure 5. Polymer repeating units for polystyrene (benzylic stabilization) and polyacrylonitrile (EWG stabilization).