31.1 Chain-Growth Polymers

Chain-Growth Polymerization

Chain-growth polymerization builds polymers by adding monomers one at a time to an active chain end, rather than linking monomers together all at once (that's step-growth polymerization). The type of active species at the chain end determines the mechanism and dictates which monomers work best, how termination happens, and what kind of polymer you get.

Process of Chain-Growth Polymerization

Every chain-growth polymerization follows three stages: initiation, propagation, and termination.

1. Initiation — An initiator generates a reactive species (a free radical, carbocation, or carbanion). That species reacts with the first monomer, creating an active chain end.
2. Propagation — The active chain end attacks the $\pi$ bond of another monomer, forming a new $\sigma$ bond and moving the active site to the newly added monomer. This repeats many times.
3. Termination — The active chain end is destroyed, stopping growth. How this happens depends on the mechanism:
   • Radical: combination (two radical ends couple) or disproportionation (hydrogen atom transfer between two radical ends, producing one saturated and one unsaturated chain end)
   • Cationic: reaction with a nucleophile, or proton transfer to monomer or solvent
   • Anionic: reaction with an electrophile, or proton transfer from solvent

Chain transfer is a side event where the active site jumps to another molecule (solvent, monomer, or a deliberately added chain-transfer agent). The original chain stops growing, and a new chain starts. This lowers the average molecular weight and broadens the molecular weight distribution.

Radical vs. Cationic vs. Anionic Polymerization

A few things worth noting:

• Styrene is unusually versatile. Its phenyl group stabilizes radicals, carbocations, and carbanions through resonance, so it can undergo all three mechanisms.
• Living anionic polymerization means the chain ends stay active indefinitely (no inherent termination step). You can add a second batch of a different monomer to make block copolymers with well-defined architectures. Termination only occurs when you deliberately quench the reaction (e.g., by adding water or $CO_2$).

Monomer Structure Effects on Reactivity

The key principle: a monomer is reactive in a given mechanism if its substituents stabilize the intermediate that mechanism produces.

• Radical polymerization — The growing chain end is a carbon radical. Groups that delocalize the unpaired electron through resonance (phenyl, vinyl) or hyperconjugation (alkyl) increase reactivity. Purely electron-withdrawing groups like $\text{-CN}$ or $\text{-COOR}$ also stabilize radicals through resonance with the lone pair or $\pi$ system, which is why monomers like acrylonitrile and methyl methacrylate polymerize well by radical mechanisms too.
• Cationic polymerization — The growing chain end is a carbocation. Electron-donating groups ($\text{-OR}$, $\text{-R}$, $\text{-NR}_2$) stabilize the positive charge and make the monomer more reactive. Electron-withdrawing groups destabilize the cation, so monomers like acrylonitrile do not undergo cationic polymerization.
• Anionic polymerization — The growing chain end is a carbanion. Electron-withdrawing groups ($\text{-CN}$, $\text{-COOR}$, $\text{-C}_6\text{H}_5$) stabilize the negative charge and increase reactivity. Electron-donating groups make the carbanion less stable and the monomer less reactive.

Polymerization Kinetics and Polymer Properties

The rate of polymerization in radical chain-growth depends on three rate constants: initiation ($k_i$), propagation ($k_p$), and termination ($k_t$). Under steady-state conditions (rate of radical generation ≈ rate of radical destruction), the overall rate is:

$R_p = k_p [M] \sqrt{\frac{f \cdot k_d [I]}{k_t}}$

where $[M]$ is monomer concentration, $[I]$ is initiator concentration, $f$ is initiator efficiency, and $k_d$ is the rate constant for initiator decomposition. The practical takeaway: increasing initiator concentration speeds up polymerization but also increases the rate of termination, which lowers the average molecular weight.

Degree of polymerization ($\bar{X}_n$, the average number of monomer units per chain) is governed by the ratio of propagation rate to termination rate. Faster initiation means more chains growing at once, so each chain gets fewer monomers before terminating.

Stereochemistry of the polymer backbone matters for physical properties:

• Isotactic — all substituents on the same side of the extended chain. Tends to crystallize, giving stiffer, higher-melting materials.
• Syndiotactic — substituents alternate sides regularly. Also can crystallize.
• Atactic — substituents placed randomly. Usually amorphous and softer.

Radical polymerization typically gives atactic polymer because the radical chain end is planar and has no strong facial selectivity. Anionic and coordination (Ziegler-Natta) catalysts can control stereochemistry to produce isotactic or syndiotactic polymers with tailored mechanical properties.