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Radical Polymerization

A single radical can grab a double bond and never let go — adding one alkene after another until thousands of them have stitched into a single chain. That runaway chain is how polyethylene, polystyrene, and PVC are born, and why a pinch of inhibitor can stop the whole thing dead.

From One Addition to a Thousand

Earlier in this rung you met the radical chain mechanism and watched a single radical add to a C=C double bond in radical addition — the move behind anti-Markovnikov HBr. The key fact there was that radical addition does not stop the radical; it just moves it. A radical attacks the alkene, grabs one of the two pi electrons to form a new C-C bond, and the OTHER carbon is left holding the unpaired electron. You consumed one radical but you made a new one. Radical polymerization is simply that observation taken to its logical extreme: if every addition makes a fresh radical, and there is more alkene floating around, why would it ever stop?

So picture a vat of ethylene, CH2=CH2. Drop in one radical and it adds to a molecule of ethylene; the radical is now sitting on the far end of a two-carbon fragment. That fragment-radical adds to the NEXT ethylene; now it is four carbons long with the radical still at the tip. Add again, and again, and again — each step lengthens the chain by two carbons and hands the unpaired electron forward to the new end, like a relay runner who never drops the baton. After a few thousand such handoffs you have a single molecule of polyethylene, a carbon chain tens of thousands of atoms long, built in a fraction of a second. This is why the family is called addition (or chain-growth) polymerization: the monomers simply add on, one at a time, and nothing is thrown away.

The Four Acts: Initiation, Propagation, Transfer, Termination

Like every chain reaction you have seen, this one has the same skeleton: a few radicals are born (initiation), the chain runs (propagation), and eventually the radicals find each other and die (termination). What is new for polymerization is that the propagation step is the one that builds the product, repeated thousands of times, and that there is an extra wrinkle — chain transfer — that quietly limits how long each chain gets. Let us walk all four, then put real plastics to each.

  1. Initiation. You cannot start a radical chain without a first radical, and a stable alkene will not make one on its own. So you add an initiator — a molecule with one deliberately weak bond, like an O-O peroxide or the N=N of AIBN. Gentle heat or UV light snaps that weak bond by homolysis (each atom keeps one electron, drawn with fishhook half-arrows), giving two radicals. One of them then adds to the first monomer's C=C, and the chain is lit.
  2. Propagation. This is the engine. The chain-end radical attacks the C=C of a fresh monomer, forms a new C-C bond, and the unpaired electron reappears on the new end. Repeat this single step thousands of times and the chain grows. Each repeat is identical, fast, and slightly exothermic, so once lit, propagation races ahead until the monomer near that chain end runs low.
  3. Chain transfer. Sometimes the chain-end radical, instead of grabbing another monomer, abstracts a hydrogen atom from something else — a solvent molecule, an additive, or even another polymer chain. The growing chain ends there (it is now a dead, closed molecule), but the radical is not destroyed; it has merely jumped to a new spot and starts a brand-new chain. Net effect: more chains, each one shorter. Transfer is how you tune the average chain length without killing the reaction.
  4. Termination. The chain truly ends only when two radicals meet and pair their electrons. Two chain ends can simply join head-to-head into one longer dead chain (combination), or one can hand a hydrogen to the other, capping both (disproportionation, which leaves a C=C at one new end). Because radicals are scarce and chains are everywhere, two radicals meeting is rare — which is exactly why each chain gets so long before it dies.
INITIATION   In-In  --(heat/UV)-->  2 In*        (homolysis)
             In* + CH2=CH2  ->  In-CH2-CH2*

PROPAGATION  ...CH2-CH2* + CH2=CH2  ->  ...CH2-CH2-CH2-CH2*
             ( repeat ~ thousands of times )

TRANSFER     ...CH2-CH2* + H-R  ->  ...CH2-CH3  +  R*
             (this chain dies; R* starts a NEW one)

TERMINATION  ...CH2*  +  *CH2...  ->  ...CH2-CH2...   (combination)
             ...CH2-CH2*  +  *CH2-CH2...  ->  ...CH=CH2 + CH3-CH2...
                                              (disproportionation)
The four acts of a radical chain polymerization. The asterisk marks the unpaired electron. Notice that initiation and termination each touch the radical count (one makes radicals, the other consumes them), while propagation and transfer keep it the same — they only move the radical along.

Which End? Why Plastics Look the Way They Do

Ethylene is symmetric, so its polymer is featureless: just -CH2-CH2-CH2- forever. But most useful monomers are NOT symmetric — they carry a group on one carbon, like the chlorine of vinyl chloride (CH2=CHCl) or the benzene ring of styrene (CH2=CH-C6H5). For these, the same question that ruled radical HBr addition comes back: which carbon does the chain-end radical bond to, and where does the new radical end up? The answer is the one you already know from radical stability: the chain adds so that the unpaired electron lands on the MORE substituted, more stabilized carbon — the one next to the chlorine or the ring.

This is the very same regioselectivity logic behind anti-Markovnikov addition: in radical chemistry you always build the most stable radical you can. The practical consequence is that the chain grows in a tidy, repeating 'head-to-tail' pattern — every substituent ends up on every other carbon, regularly spaced down the chain, rather than scattered at random. That regularity is not a cosmetic detail. It is part of why polystyrene is a clear, glassy solid and PVC can be made into rigid pipe: a regular chain can pack against its neighbors in a predictable way. (Honest caveat: free-radical chains are not perfectly regular — the random head-to-head defects and chain-transfer branches are exactly why ordinary radical-made polyethylene is softer and less crystalline than the metal-catalyzed kind you will meet in the next track.)

Three Everyday Plastics

Now meet the products by name. Polyethylene comes from ethylene (CH2=CH2): the plastic of grocery bags, milk jugs, and squeeze bottles, an inert all-carbon chain that is cheap, flexible, and famously hard to break down. Polystyrene comes from styrene (CH2=CH-C6H5): every other carbon carries a bulky benzene ring, and those stiff rings are why polystyrene is a hard, clear, glassy material — the disposable cup, the CD case, and, puffed full of trapped gas, the foam in packaging. Poly(vinyl chloride) — PVC — comes from vinyl chloride (CH2=CHCl): every other carbon carries a chlorine, a heavy polar atom that makes the rigid version tough enough for water pipe and window frames, and, with a plasticizer added, supple enough for hose and floor tiles.

Three radically different materials, one shared mechanism. Look closely and the only difference between them is what hangs off every other carbon: nothing (polyethylene), a ring (polystyrene), a chlorine (PVC). Everything else — the relay of initiation, propagation, and termination — is identical. That is the quiet power of chain-growth chemistry: change the substituent on one small monomer and you change the personality of the bulk material, while the machinery that assembles it stays exactly the same. Acrylics (from CH2=CH-CO2R, giving Plexiglas), Teflon (from CF2=CF2), and the polyacrylonitrile in acrylic sweaters all join the same family by the same logic.

Stopping the Chain: Inhibitors and Antioxidants

A chain that never stops on its own is a problem as much as a gift. A drum of pure styrene left warm would slowly polymerize itself into a useless solid block, because stray radicals — from a trace of peroxide, from light, from heat — keep finding the C=C. The fix is an inhibitor: a molecule that catches radicals and refuses to pass the relay on. A good radical scavenger reacts with a chain-end radical to give a NEW radical that is so stabilized it is unreactive — it sits there harmlessly instead of attacking the next monomer. Add a pinch of inhibitor and the chain dies on contact; the monomer can be stored and shipped safely until you deliberately remove or overwhelm the inhibitor and let polymerization run.

The very same trick protects far more than monomer drums — it protects your food, your skin, and your own cells. You met autoxidation earlier in this rung: the slow radical chain by which oxygen attacks the C-H bonds next to double bonds, turning fats rancid and oils gummy. An antioxidant is exactly an inhibitor aimed at THAT chain. Vitamin E in your cell membranes, BHT in a bag of chips, the hindered phenols molded into a car bumper — all do the same thing: they donate a hydrogen to the chain-carrying radical, quenching it and becoming a tame, resonance-stabilized radical that simply stops. 'Antioxidant' on a label is not vague wellness language; it names a specific job — breaking an oxygen-driven radical chain before it can spread.

Tying It Back, and Looking Ahead

Notice how little new machinery this whole guide needed. The single radical-addition step you already knew, run as a chain, IS polymerization. The same radical-stability argument that picked the anti-Markovnikov end picks the head-to-tail pattern in the chain. And the radical-trapping idea behind inhibitors is just termination, on demand. You did not learn a new reaction here so much as see one reaction's true scale — from a single C-C bond in a flask to the plastic in your hand and the rancidity you can smell.

Two threads run forward from here. First, into the next track of this rung: radicals are blunt instruments — they give branched, irregular chains and you cannot easily dictate exactly how each monomer attaches. The transition-metal catalysts you are about to meet do the same C-C bond-building with surgical control, which is how modern, high-strength, precisely-tailored polyethylenes are actually made. Second, into the final rung, where polymers get their own proper treatment: you will compare these addition polymers head-on with the condensation polymers (polyester, nylon) you have already glimpsed, and ask the harder questions — crystallinity, thermoplastic versus thermoset, recycling, and why an all-carbon backbone is so wonderfully durable and so stubbornly permanent in the environment.