Polymerization Mechanisms: Radical, Ionic, Ziegler-Natta

30 September 2026Chemistry Try the interactive 3D model →

Polymerization joins hundreds or thousands of small molecules, called monomers, into one giant chain called a polymer. Plastics, rubber, nylon and starch are all polymers. How the monomers link up depends on the mechanism, and chemists use three main chain-growth mechanisms: free-radical, ionic (cationic and anionic), and Ziegler-Natta polymerization.

The mechanism decides the polymer's shape and strength. Polypropylene made with a Ziegler-Natta catalyst has a neat, regular structure strong enough for car parts, while the same monomer made differently gives a soft, messy product. Exams often ask which catalyst belongs to which mechanism, so the pairs below are worth memorising.

Key Concepts

Monomer / polymer: a monomer is one building block (e.g. ethene, CH2=CH2); a polymer is a long chain of repeating monomer units.
Free-radical: the chain grows through carbon radicals (one unpaired electron). Initiators like benzoyl peroxide or AIBN split into radicals on heating. Used for ethene (low-density polyethylene), styrene, vinyl chloride.
Cationic: the growing end is a positive carbon (carbocation), started by Lewis acids like BF3 or AlCl3 with a trace of water as co-catalyst. Suits electron-rich alkenes like isobutylene (butyl rubber).
Anionic: the growing end is a negative carbon (carbanion), started by strong bases like n-butyllithium (n-BuLi). Suits electron-poor alkenes like styrene and acrylonitrile; with no natural termination it can be "living".
Ziegler-Natta: a solid catalyst of TiCl4 with triethylaluminium, Al(C2H5)3. Builds chains stereospecifically — isotactic polypropylene has all methyl groups on the same side.
Chain vs step growth: chain growth adds monomers one by one to an active end (fast, high molecular weight early); step growth links any two molecules with loss of a small molecule like water (slow build-up, e.g. polyester, nylon).

Formulas

Initiation (benzoyl peroxide): (C6H5COO)2 → 2 C6H5· + 2 CO2. Heat (~60–80 °C) splits the peroxide into radicals (the · marks the unpaired electron); oxygen must be absent since it traps radicals.
Propagation: R· + CH2=CHX → R–CH2–CH·X, then repeat. One monomer adds per step; no small molecule is lost in addition polymerization.
Termination by combination: two radical chain ends join into one long chain; disproportionation (hydrogen transfer) is the other route.
Units: chain length as degree of polymerization (unitless count of units); molar mass in g/mol.

Worked Example

An initiator creates 0.010 mol of radicals, each starting one polystyrene chain. The average degree of polymerization is 5,000. How many moles of styrene are consumed?

Step 1: Chains = 0.010 mol of radicals = 0.010 mol of chains.
Step 2: Monomer = 0.010 × 5,000 = 50 mol.
Step 3: Styrene is 104 g/mol, so mass = 50 × 104 = 5,200 g = 5.2 kg.

A tiny amount of initiator builds kilograms of polymer — the power of chain growth.

Common Misconceptions

"All polymerization works the same." Wrong. Radical, cationic, anionic and Ziegler-Natta routes use different active species, and the monomer must suit the mechanism — isobutylene loves cationic, acrylonitrile loves anionic.
"Ziegler-Natta just speeds things up." Wrong. Its real magic is stereospecificity: it lines monomer units up in one orientation, giving strong isotactic polypropylene instead of a weak random mix.
"Benzoyl peroxide is a catalyst." Wrong. It is an initiator — it is consumed, with its fragments ending up on the chain ends, while a true catalyst is regenerated.

Real-World Applications

Free-radical polymerization makes low-density polyethylene bags and films.
Cationic polymerization of isobutylene with AlCl3 makes butyl rubber for inner tubes and sealants.
Living anionic polymerization makes SBS block copolymers for shoe soles and adhesives.
Ziegler-Natta catalysts produce isotactic polypropylene (car bumpers, bottle caps) and high-density polyethylene pipes.

Radical, cationic, anionic and Ziegler-Natta routes all grow chains through different active species, each suiting different monomers and structures — matching monomer to mechanism is the core exam skill.