What a polymer is

Start with ethylene, CH₂=CH₂, a gas with two carbons. Heat it under high pressure with a trace of a peroxide, and the gas turns into a waxy solid: polyethylene, the plastic of shopping bags. Each ethylene has opened its C=C and bonded to the next ethylene, thousands of times over. One molecule of the solid is a chain of thousands of CH₂ groups.

That chain is a polymer: one very large molecule built by joining many small molecules end to end. The small molecule you start from is the monomer, which here is ethylene. The piece that repeats along the chain is the repeat unit. In polyethylene it is CH₂–CH₂, the two carbons that came from one ethylene. The number of monomer units in one chain is the degree of polymerization.

Chemists write the whole chain as its repeat unit in square brackets with a subscript n: –[CH₂–CH₂]–_n. For polyethylene, n is the degree of polymerization. IUPAC’s formal rules pick the smallest piece that repeats, so they write polyethylene as –[CH₂]–_n. Courses and exams use the unit that matches the monomer, and so does this chapter.

Nothing about ethylene’s reaction is new. Almost every polymer in this chapter is made by a reaction you already know: addition to a C=C, as in Addition reactions, or ester and amide formation, as in Nucleophilic acyl substitution. What is new is that each product can react again. A diacid or a diol has a reactive group at each end, so after one end reacts the other is still free. And when a radical adds to ethylene, the new radical goes on to add to the next ethylene instead of being capped by a hydrogen, as it would be in an ordinary addition.

Counting reactive sites

Take adipic acid, HO₂C(CH₂)₄CO₂H, which has a carboxyl group at each end, and heat it with three different alcohols.

• With ethanol, CH₃CH₂OH, which has one OH, each acid end makes one ester and the reaction stops. The product is diethyl adipate, a small molecule.
• With ethylene glycol, HOCH₂CH₂OH, which has two OH groups, each new ester leaves a free group at the far end, ready to react again. The pieces link into a long line: a chain.
• With glycerol, HOCH₂CH(OH)CH₂OH, which has three OH groups, each glycerol can join three chains. The chains tie into each other in every direction: a network.

The acid is the same in all three flasks. What changes the result is the number of reactive groups on one alcohol molecule. A reactive site is a group that reacts under the conditions used. Count the sites on each monomer: one site ends the growth, two build a chain, three or more build a network.

A C=C counts as two sites. When a growing chain adds to an alkene, one alkene carbon bonds to the chain and the other becomes the chain’s new active end, ready for the next monomer. So ethylene, with one C=C, builds a chain. Divinylbenzene is a benzene ring carrying two CH=CH₂ groups, so it has four sites. A little of it mixed into styrene, C₆H₅CH=CH₂, ties the growing polystyrene chains to each other. A bond that ties one chain to another is a cross-link.

The count decides what the finished material does when heated. Separate chains hold on to each other only by intermolecular forces, the attractions between separate molecules taught in Bond polarity. Heat overcomes those forces, the chains slide past each other, and the solid softens and can be molded again. A plastic that does this is a thermoplastic.

A network behaves differently. Covalent bonds join its chains, so the whole piece is one molecule. Heat cannot make those chains slide past each other. Strong heating breaks bonds instead, so the material decomposes rather than melts. A plastic like this is a thermoset, and it cannot be recycled by melting.

Two ways to build a chain

Compare propylene with the adipic acid and ethylene glycol pair. Propylene, CH₂=CH–CH₃, is C₃H₆, and its repeat unit in polypropylene is also C₃H₆. The acid and the glycol together hold C₈H₁₆O₆, but their repeat unit is C₈H₁₂O₄. Two waters are missing, one for each ester link.

These are the two kinds of polymerization. In an addition polymerization no atom leaves; most addition monomers are alkenes, whose π bond opens. In a condensation polymerization every link between monomers releases a small molecule.

The table’s headings give each kind a second name, chain-growth or step-growth, and that name answers a different question. “Addition or condensation” asks what happens to the atoms: does a small molecule leave? “Chain-growth or step-growth” asks how the chains get long. In chain-growth, each chain has one active end that adds monomer after monomer. In step-growth, any two pieces can join: a monomer with a monomer, or one short chain with another.

For alkenes, and for polyesters and polyamides made from two partners (a diacid with a diol or a diamine), the two answers line up. Alkenes give chain-growth addition polymers, and those polyesters and polyamides are step-growth condensation polymers. That is why the two pairs of names are often used as if they meant the same thing. When a question asks “which kind”, say which of the two questions you are answering.

Polyurethanes are the first case where the answers split. They grow step by step, like a polyester, but no atom leaves, so by atom count they are additions. They are made from a diol and a diisocyanate, which has an isocyanate group, –N=C=O, at each end. Each link forms when an alcohol O–H adds across the C=N bond of an isocyanate group.

Caprolactam blurs the line in a different way. It is a ring of seven atoms, six carbons and one nitrogen, closed by an amide group. That makes it a lactam, a cyclic amide. Caprolactam is the lactam of 6-aminohexanoic acid, H₂N(CH₂)₅CO₂H. When the ring’s amide C–N bond opens, the opened ring joins the chain as –NH–(CH₂)₅–C(=O)–. That repeat unit is C₆H₁₁NO, exactly the formula of caprolactam, so by atom count it is an addition. Yet the chain is a polyamide, nylon 6, with the same amide links as nylon 6,6, the condensation polymer of adipic acid and a diamine.

How nylon 6 grows depends on how the reaction is run. With a strong base, each chain has one active end that opens ring after ring. That is chain-growth with nothing lost, the same pairing as an alkene. The industrial route starts with water instead. Water opens some of the rings to 6-aminohexanoic acid. Chains then grow in two ways at once: their amine ends open more rings, and an amine end can also join an acid end, releasing water. So the industrial route is step-growth with no net loss, the same pairing as a polyurethane. By either route, the repeat unit keeps caprolactam’s formula.

Addition polymerization: one active end

Return to ethylene and the peroxide. A peroxide, ROOR, has a weak O–O bond that breaks to give two RO· radicals, the same step that starts the peroxide effect in Markovnikov / anti-Markovnikov. A radical adds to one carbon of a C=C, which leaves the unpaired electron on the other carbon. That new radical adds to the next ethylene, and the chain grows one monomer at a time, always from its single active end.

The active end need not be a radical. There are three kinds of active end, each started a different way:

• Radical — an initiator starts it: a compound, such as the peroxide, that supplies the first radical. This is how low-density polyethylene (LDPE) and polystyrene are made. Most of the world’s polyethylene is not made this way. High-density polyethylene (HDPE) comes from metal catalysts that hold the growing chain on a metal atom. So does linear low-density polyethylene (LLDPE), made with a little of a longer alkene mixed in, which gives short branches of one length. The next section, Addition polymers, covers these catalysts.
• Cationic — an acid or a Lewis acid (BF₃ with a trace of water) starts it, and the active end is a carbocation. The chain grows only if that cation is stabilized, so the monomer needs donating groups. Isobutylene, CH₂=C(CH₃)₂, works because its growing end is a tertiary cation. Vinyl ethers, CH₂=CH–OR, work because an oxygen lone pair stabilizes the cation directly.
• Anionic — a strong base or an organolithium such as BuLi starts it, and the active end is a carbanion. The monomer needs withdrawing or delocalizing groups to hold that negative charge. Acrylonitrile, CH₂=CHCN, and methyl methacrylate, CH₂=C(CH₃)CO₂CH₃, work, and so does styrene, whose ring spreads the negative charge out.

Each ionic route works only for the right kind of monomer. Acrylonitrile will not polymerize cationically, because its nitrile would pull electron density away from the cation. Isobutylene will not polymerize anionically, because its methyls would push more electron density onto the carbanion.

Polyisobutylene, the cationic polymer of isobutylene, cannot be vulcanized on its own. Rubber is toughened by vulcanization: heating it with sulfur, which forms sulfur bridges between chains at their C=C bonds and the carbons beside them. A saturated chain like polyisobutylene has no C=C, so sulfur has nothing to attack. Butyl rubber, the airtight rubber of inner tubes, solves this. It is a copolymer, a chain built from two different monomers: isobutylene with one or two percent of isoprene, CH₂=C(CH₃)–CH=CH₂. Each isoprene unit keeps one C=C in the chain, and those C=C bonds give the sulfur somewhere to bond.

Water, acid, O₂ and CO₂ all destroy an anionic active end. In a dry flask kept free of air, none of them is present, so the active end has no easy way to stop. Two carbanions repel each other rather than pairing up, so the active ends stay reactive after the monomer runs out. This is living polymerization, and two things follow. When every chain starts at the same moment and grows at the same rate, the chains finish at nearly the same length. And adding a second monomer afterwards extends every chain with a run of the new monomer, giving a block copolymer: one long run of the first monomer joined to one long run of the second.

Vinyl chloride, CH₂=CHCl, has two different ends. Call the CHCl carbon the head and the CH₂ carbon the tail. A growing radical adds to the tail, because that puts the new unpaired electron on the head carbon. There it sits on the carbon that carries the chlorine, whose lone pairs help stabilize it. Adding the other way round would leave the unpaired electron on a bare CH₂ end, which is less stable. This is the same rule as in the peroxide effect: the radical adds to the carbon that leaves the more stable radical behind. Every monomer adds the same way, so the chlorines land on every other carbon. This is head-to-tail linking, and any monosubstituted alkene, CH₂=CHX, links the same way.

Condensation polymerization: everything joins everything

Take adipic acid and ethylene glycol again. The first ester joins one acid and one glycol into a dimer, which still has an acid at one end and an OH at the other. That dimer can react with a monomer, with another dimer, or with any longer piece. This is the step-growth described above.

Step-growth and chain-growth therefore run very differently. In a chain-growth polymerization, a chain that has started adds monomer very fast, so long chains exist from the start. Halfway through, the flask holds long chains and untouched monomer. In a step-growth polymerization everything grows together, slowly. Halfway through, the flask holds dimers, trimers and other short pieces, and almost no long chains.

In a step-growth polymerization, chains get long only at the very end. The conversion, p, is the fraction of the functional groups that have reacted. With exactly equal amounts of the two groups, the average chain holds 1/(1 − p) monomer units.

So a step-growth polymerization needs exactly equal amounts of the two groups, pure monomers, and continuous removal of the water or other small molecule. An excess of either group leaves chains with the same group at both ends, and they stop growing. An impurity with only one reactive group caps the end of a chain for good. Removing the water pulls the equilibrium toward the ester. The useful material appears only in the last fraction of a percent of reaction.

Same repeat unit, different material

Polyethylene comes both as a soft plastic bag and as a stiff milk bottle, and both are –[CH₂–CH₂]–_n. The difference is branching. A branch is a short side chain growing off the main chain. Radical polymerization at high pressure gives LDPE, whose chains carry many short branches. The metal catalysts give HDPE, whose chains have almost no branches; they are linear.

Straight chains lie close together along their whole length, so the London forces between them add up, just as they do between the straight chains of saturated fatty acids in Lipids. HDPE’s chains pack tightly into a dense, rigid solid. Branches hold the chains apart, so LDPE is less dense and more flexible.

Branching is one of several ways the same repeat unit can give different materials. Chain length, the arrangement of side groups along the chain, and how densely a network is cross-linked each get their own treatment later in this chapter.

What carries forward

Count the reactive sites on each monomer to know whether you get a small molecule, a chain or a network. Count the atoms to know whether the polymerization was an addition or a condensation. Then look at how branched the chains are to predict the material. Polyethylene is both a bag and a bottle, and what separates them is the shape of the chains, not the repeat unit.