Addition polymers

One bond opens, thousands join

A carrier bag, a milk bottle, a crate, a drainpipe, the insulation on a cable and the coating of a non-stick pan are all made from alkenes. Each is an addition polymer: a chain of thousands of carbon atoms, made by opening the double bond in each of thousands of small molecules and joining them end to end.

The small molecule is the monomer, and the long molecule made from it is the polymer. In addition polymerisation the π bond of each monomer breaks, and its two electrons are used to form σ bonds to the monomers on either side. Nothing else is produced, so the polymer has the same empirical formula as the monomer and the atom economy is 100%.

nCH₂=CH₂(g) → –[CH₂–CH₂]ₙ–(s)Poly(ethene). The n on the left is the number of monomer molecules; the n on the right is the number of repeating units in one chain.

A chain of poly(ethene) has no double bonds. Every carbon in it has four single bonds, so the polymer is saturated, and in its chemistry it behaves like a very long alkane. It does not decolourise bromine water. The name keeps the monomer in brackets for that reason: poly(ethene) is a polymer made from ethene, not a polymer containing ethene.

A sample of polymer is not one compound. Its chains grow and stop at random, so they have a range of lengths, and the relative molecular mass quoted for a polymer is an average. A typical poly(ethene) chain holds somewhere between several thousand and a hundred thousand carbon atoms. Because the chains differ, a polymer softens over a range of temperatures instead of melting sharply.

The conditions vary with the polymer wanted. Low-density poly(ethene) is made at high pressure with a trace of oxygen to start a radical chain reaction, and gives branched chains that pack loosely. High-density poly(ethene) is made at low pressure with a Ziegler–Natta catalyst, and gives unbranched chains that pack closely, so it is stiffer and softens at a higher temperature. Neither mechanism is examined.

Monomer

A small molecule that joins with many others of the same kind to form a polymer.

Polymer

A long-chain molecule made by joining many monomer molecules together.

Addition polymerisation

The joining of many unsaturated monomer molecules, by opening their double bonds, to form a polymer and no other product.

Repeating units and monomers

A polymer chain is too long to draw, so it is written as its repeating unit: the smallest part of the chain that, repeated, gives the whole. For an addition polymer made from one alkene, it is two carbons long. They are the two carbons that formed the double bond of one monomer.

Describe the drawing

Left, propene drawn displayed with its carbon-carbon double bond, two hydrogens on the left carbon, and a hydrogen above and a methyl group below on the right carbon, with n in front. An arrow labelled addition polymerisation leads to the repeating unit: the same two carbons joined by a single bond, the same side groups, and a bond running out from each carbon through a square bracket, with n after the closing bracket. Beneath, a chain of three repeating units with methyl groups on alternate carbons.

Three steps take a monomer to its repeating unit. Draw the two carbons of the double bond side by side with a single bond between them. Put every other group on those carbons above or below, where it was, as a side group. Then draw a bond out from each carbon through a square bracket, and write n after the closing bracket. For propene, CH₂=CHCH₃, the repeating unit is –[CH₂–CH(CH₃)]–.

An answer that puts three carbons in the main chain, –CH₂–CH₂–CH₂–, has drawn the methyl group into the backbone. The CH₃ was never part of the double bond, so it cannot be part of the chain. In poly(propene) it hangs off every second carbon.

Going backwards uses the same rule. Find two neighbouring carbons in the main chain that, with their side groups, repeat along it; draw them with a double bond between them; keep the side groups where they were. A section reading –CH₂–CHCl–CH₂–CHCl–CH₂–CHCl– repeats every two carbons as CH₂–CHCl, so the monomer is CH₂=CHCl, chloroethene.

Repeating unit

The smallest group of atoms in a polymer chain that, repeated, gives the whole chain; for an addition polymer, the two carbons of the monomer's double bond with their side groups.

Describe the drawing

A section of poly(methyl 2-methylpropenoate) drawn as six main-chain carbons, alternately CH₂ and a carbon carrying a methyl group above and an ester group below, the ester drawn as a carbon double-bonded to oxygen and single-bonded to an oxygen carrying a methyl group. One CH₂ and its neighbouring substituted carbon are boxed as the repeating unit. An arrow leads down to the monomer, methyl 2-methylpropenoate, with the restored carbon-carbon double bond in cyan and the ester group unchanged.

Four polyalkenes and what decides their uses

The chains in a polyalkene are held to each other only by intermolecular forces. The chains are long, so each one touches its neighbours at thousands of points, and the total attraction is large even though each contact is weak. How closely the chains pack, and how polar their side groups are, decide how strong, stiff and heat-resistant the plastic is.

Poly(ethene) chains are non-polar, so the forces between them are van der Waals forces, the induced dipole–dipole attractions found between all molecules. In PVC each C–Cl bond is polar, because chlorine is more electronegative than carbon, so neighbouring chains also attract through permanent dipole–dipole forces. The chains hold each other more firmly and slide less easily, and unplasticised PVC is rigid enough for a window frame.

A plasticiser is a small molecule mixed into the polymer. Its molecules sit between the chains and push them further apart, which weakens the intermolecular forces between the chains, so the chains slide over each other and the material bends. Plasticised PVC is soft enough for a cable sheath or a raincoat. The plasticiser is not bonded to the chains, so over years it slowly migrates out, and old flexible PVC goes stiff and brittle.

Describe the drawing

Two panels of PVC chains drawn as wavy lines carrying small chlorine atoms. Left, unplasticised PVC: five chains packed close together, with many short dashed dipole-dipole attractions between C–Cl bonds on neighbouring chains. Right, plasticised PVC: the same five chains spaced further apart with small plasticiser molecules drawn as short rods between them, fewer and longer dashed attractions, and an arrow showing a chain sliding past its neighbour.

Poly(propene) has a methyl group on every second carbon. When the methyl groups all lie on the same side of the chain, the chains pack regularly, and the polymer is stiffer and tougher than poly(ethene) and can be spun into fibres strong enough for rope.

PTFE is poly(tetrafluoroethene): every hydrogen of poly(ethene) replaced by fluorine. The C–F bond is the strongest single bond that carbon forms, and the fluorine atoms make a tight sheath around the carbon chain. Little can react with it and almost nothing sticks to it, so it lines frying pans.

Van der Waals forces

Weak attractions between all molecules, caused by temporary dipoles inducing dipoles in neighbouring molecules.

Plasticiser

A small molecule added to a polymer that gets between the chains, pushes them apart and weakens the forces between them, so the polymer becomes flexible.

Why polyalkenes last, and what can be done with them

The properties that make polyalkenes useful make them hard to get rid of. The chains are saturated and contain only C–C and C–H bonds, which are strong and almost non-polar. There is no double bond for an electrophile, no polar bond for a nucleophile and no functional group that an enzyme can recognise, so bacteria and fungi cannot break the chains down. Polyalkenes are non-biodegradable: a poly(ethene) bag buried in the ground is still there decades later.

Describe the drawing

A grid comparing four routes for waste polymer, landfill, incineration, mechanical recycling and feedstock recycling, on four rows: space used, energy recovered, gases released, and cost and effort. Landfill uses much space and recovers no energy but is cheap. Incineration needs little space and makes electricity but releases carbon dioxide and, from PVC, hydrogen chloride unless scrubbed. Mechanical recycling saves energy but needs collection, sorting and cleaning. Feedstock recycling partly recovers fuels and monomers in a costly plant. Each cell has a tick or a cross and no route has ticks on every row.

Landfill is cheap and needs little processing, but the plastic occupies space for centuries, suitable sites are running out, and the energy and the carbon in the polymer are lost. Incineration releases the energy stored in the chains, which can generate electricity, and greatly reduces the volume of waste. It releases carbon dioxide, and burning PVC also gives hydrogen chloride, which is removed from the flue gases by passing them through a base such as calcium oxide before they leave the chimney.

CaO(s) + 2HCl(g) → CaCl₂(s) + H₂O(l)Removing hydrogen chloride from the gases made when PVC is incinerated.

Recycling saves crude oil and the energy needed to make new polymer. Mechanical recycling sorts the waste by polymer type, washes it, shreds it and melts it to be moulded again. Mixed or dirty plastic gives a weaker product, and each cycle shortens the chains. Feedstock recycling heats the polymer to crack it back into small molecules, which can be used to make new polymers or fuels.

Sorting comes first, and density does much of it. Shredded mixed plastic dropped into water separates: poly(ethene) and poly(propene), with densities of about 0.90 to 0.96 g cm⁻³, float, while poly(phenylethene), about 1.05 g cm⁻³, and PVC, about 1.4 g cm⁻³, sink. Automated lines also shine infrared radiation on each piece and read its absorption spectrum, which picks out, for example, the C–Cl bonds of PVC.

Biodegradable polymers avoid the problem rather than managing it. Polyesters such as poly(lactic acid), made from plant starch, have polar ester links in the chain that water and enzymes can hydrolyse. They are condensation polymers. They cost more, are weaker for some uses, and break down only in the warm, wet conditions of industrial composting. In an evaluation, name the bond that is broken, the ester link, rather than saying the plastic 'rots'.

Biodegradable

Able to be broken down by microorganisms in the environment into small molecules such as carbon dioxide and water.

Feedstock recycling

Heating waste polymer to crack it into small molecules that are used to make new polymers or fuels.

Other boards: London forces and photodegradable polymers

OCR and Edexcel call van der Waals forces London forces, and Cambridge calls them instantaneous dipole–induced dipole forces. The chemistry is the same: a temporary dipole in one chain induces a dipole in its neighbour. Use the board's own name in an answer.

OCR and Cambridge also examine photodegradable polymers, which break down in light. Poly(ethene) absorbs almost none of the ultraviolet in sunlight, so a few carbonyl groups, C=O, are built into the chain by copolymerising ethene with a little carbon monoxide or a ketone that carries a C=C bond. Each C=O absorbs ultraviolet, and the energy breaks a C–C bond beside it. Ultraviolet at 300 nm carries about 400 kJ per mole of photons, more than the 347 kJ mol⁻¹ of a C–C bond. The chains snap into shorter pieces, and the plastic turns brittle and crumbles.

Photodegradation has two limits. It needs light, so a bag buried in landfill or under other rubbish stays whole. And the pieces it gives are still poly(ethene), only shorter: unless microorganisms then digest them, they persist as small fragments of plastic. It suits items that are bound to end up outdoors, such as the rings that hold drinks cans together and mulch film on fields.

Photodegradable

Able to be broken down by light, usually ultraviolet absorbed by groups such as C=O built into the polymer chain.