Polymerisation

Addition polymerisation: many monomers, one giant molecule

Addition polymerisation is the joining together of many unsaturated monomer molecules to form one very large saturated molecule, the polymer. The monomers must contain a C=C double bond, because that double bond is the bond that opens: one of the two pairs of electrons in the pi bond breaks, each carbon of the double bond forms a new single bond to a carbon of the next monomer, and the chain grows. No small molecule is lost, so every atom of every monomer ends up in the polymer. That is the key difference from condensation polymerisation, and it means the empirical formula of the polymer is identical to the empirical formula of the monomer.

The industrial reaction is a free-radical chain reaction. An initiator, usually an organic peroxide, splits into radicals; a radical adds to the pi bond of a monomer, turning the pair of electrons in that pi bond into a new sigma bond and leaving the other carbon carrying the unpaired electron. That new radical attacks the next monomer, and the sequence repeats thousands of times until two radicals meet and combine, which terminates the chain. Because the chain ends are saturated, the finished polymer contains no C=C bonds and does not decolourise bromine water.

The shape around each carbon changes as the double bond opens. In ethene, a carbon of the C=C is joined to three atoms, so the shape is trigonal planar with bond angles of about 120 degrees. In poly(ethene) each carbon is bonded to four atoms, so the shape is tetrahedral with bond angles of about 109.5 degrees. The chains are long and flexible, held to their neighbours only by weak intermolecular forces, which is why a poly(alkene) softens on heating rather than melting sharply like a small molecule.

Poly(ethene): the conditions decide the structure

Poly(ethene) is made by heating ethene under pressure with a catalyst or an initiator, and the conditions chosen decide how the chains are built. In the high-pressure process, ethene at roughly 2000 atmospheres and about 200 degrees C with a small amount of an organic peroxide initiator gives chains that are branched, because a growing radical can attack a carbon in the middle of its own chain. The product is low-density poly(ethene), LDPE: the branches stop the chains packing closely, so the material is soft, flexible and lower in density, and it is used for carrier bags, food wrap and squeeze bottles.

In the low-pressure process, ethene at only a few atmospheres and about 60 degrees C with a Ziegler-Natta catalyst, titanium(IV) chloride together with triethylaluminium, gives chains that are almost perfectly linear with very few branches. This is high-density poly(ethene), HDPE. Linear chains pack closely together, so the intermolecular forces between them are greater, the density is higher, and the material is more rigid, stronger and higher melting. It is used for milk bottles, water pipes, crates and bottle caps.

The pattern is general: more branching means less close packing, weaker intermolecular forces, lower density, lower softening temperature and less strength. Exam questions often ask you to explain a difference in properties, and the chain of reasoning from branching to packing to intermolecular forces to a property is what scores the marks.

A third route uses a chromium(VI) oxide catalyst on silica, known as the Phillips catalyst, and also gives a high-density linear polymer. All three routes produce an addition polymer of the same empirical formula, CH2, and differ only in how the chains are arranged.

• High density = linear chains, made at low pressure with a Ziegler-Natta catalyst, used for rigid bottles and pipes.
• Low density = branched chains, made at high pressure with a peroxide initiator, used for flexible film and bags.

Poly(chloroethene), PVC, and what a plasticiser does

Chloroethene, also called vinyl chloride, is CH2=CHCl. Its C=C opens in the same way as ethene's, giving poly(chloroethene), whose repeat unit is -CH2-CHCl-. Every second carbon of the chain carries a chlorine atom, and the C-Cl bond is polar, so there are additional permanent dipole-dipole attractions between neighbouring chains. These extra attractions make PVC stronger and more rigid than poly(ethene) and give it a higher softening temperature, and they are the reason unplasticised PVC is used for drainpipes, guttering, window frames and roofing sheets.

Rigid PVC is hard and brittle, so a plasticiser is often added to soften it. A plasticiser is a small ester molecule, such as a phthalate ester, that wedges itself between the polymer chains. The plasticiser molecules push the chains apart, so the attractions between chains are weakened and the chains can slide over one another. The result is a flexible, rubbery material used for cable insulation, flooring, garden hoses, blood bags and hospital tubing.

The proportion of plasticiser therefore controls whether the finished article is rigid or flexible. A drainpipe and a length of electrical cable can be made from the same polymer, and it is the additive, not the polymer, that decides which is which.

PVC is worth a mark in every disposal question, because it is the one common poly(alkene) that contains a halogen. When it burns, the chlorine it contains is released as hydrogen chloride, so PVC has to be burned in a controlled incinerator with a scrubbing system rather than simply dumped or burned in the open.

Deducing the repeat unit from a given monomer

The repeat unit is the smallest part of a polymer chain that repeats along its length. To deduce it from a monomer, take the C=C bond and open it: turn the double bond into two single bonds, one coming out of each of the two carbons that were doubly bonded. Those two bonds are the bonds that join each unit to the next, and they must be shown crossing the brackets.

Write it in the standard way: put the repeat unit inside brackets, drawn with the two joining bonds sticking out through the brackets on each side, and put an n outside the brackets. The n is the number of repeat units in the chain, so the relative molecular mass of the polymer is n multiplied by the relative formula mass of the repeat unit. For the polymer from propene, that mass is 42, because the repeat unit is C3H6 (3 x 12.0 + 6 x 1.0 = 42).

Worked method, using chloroethene as the monomer. Chloroethene is CH2=CHCl, so the two carbons of the double bond are one CH2 and one CHCl. Opening the double bond gives -CH2-CHCl-, with one free bond on the CH2 carbon and one on the CHCl carbon, and the repeat unit is (-CH2-CHCl-)n. Note that the chlorine stays exactly where it was: the monomer is chloroethene, not chloroethane, and the C=C must be present for the monomer to polymerise at all.

For a monomer with a side chain the same rule applies. Propene, CH2=CHCH3, gives -CH2-CH(CH3)-, and the methyl group hangs off every second carbon of the backbone. The backbone of a repeat unit always contains exactly two carbon atoms from each monomer, whatever else is attached to them.

• Open the C=C and turn it into two single bonds, one on each carbon that was doubly bonded.
• Brackets round the repeat unit, the two joining bonds drawn through the brackets, n written outside.
• The backbone of one repeat unit always has two carbon atoms; side groups hang from it unchanged.

Finding the monomer in a section of a polymer chain

The mark scheme question runs the other way: given a section of a polymer chain, name the monomer or monomers. The method is fixed. First find the backbone, which is the continuous zig-zag of carbon atoms joined by single bonds. Then break that backbone into groups of two carbon atoms, because every two carbons in the backbone came from one monomer. Finally, put a C=C double bond back between the two carbons of each group, and read off the monomer from the atoms attached to those two carbons.

For example, a section written -CH2-CH(CH3)-CH2-CH(CH3)- is broken into two-carbon units, each of which is CH2 and CH(CH3) with a bond between them. Re-forming the double bond gives CH2=CHCH3, which is propene, and every unit in that chain gives the same monomer, so the polymer is poly(propene), a homopolymer built from one monomer.

If the units are not all the same, the polymer is a co-polymer and more than one monomer must be named. A chain made of -CH2-CH2- units alternating with -CH2-CHCl- units is formed from two monomers: the -CH2-CH2- unit gives back CH2=CH2, which is ethene, and the -CH2-CHCl- unit gives back CH2=CHCl, which is chloroethene. Name every distinct unit and its monomer; a common error is to name only the first one.

Check the answer by re-polymerising it in your head: open the C=C of the monomer you have named and see whether you get back the unit you were given. If the unit in the question contains a chlorine atom on the second carbon and your monomer has it on the first, you have written the monomer back to front.

Why poly(alkene)s do not biodegrade

The backbone of a poly(alkene) is a long chain of C-C single bonds with no functional groups attached. A C-C bond is strong and, in a hydrocarbon, non-polar, so there is nothing on the chain for water to attack and nothing for an enzyme to grip. Microorganisms such as bacteria and fungi digest natural polymers because those polymers, such as cellulose, starch and proteins, contain ester, glycosidic or amide links that their enzymes can hydrolyse. No enzyme in nature is shaped to attack a saturated hydrocarbon chain, so poly(ethene), poly(propene) and PVC pass through the soil and the sea essentially unchanged.

That is what non-biodegradable means, and it is the whole reason waste poly(alkene)s are a problem. Buried in landfill they occupy space and stay there for decades or centuries, and they also release methane slowly as buried waste decays around them. Escaping into the environment they become litter, entangle wildlife and break up into microplastics that are eaten by fish and birds and enter the food chain.

Sunlight does something to them, but not enough. Ultraviolet light can start the free-radical oxidation that makes a polyethylene chain brittle, and the film then fragments, but the fragments are still the same polymer, only smaller. Fragmentation is not degradation: the carbon skeleton survives.

There is one chemical consolation. Because the chains are held together only by weak intermolecular forces and are not joined chemically to one another, poly(alkene)s are thermosoftening: heating them overcomes those forces, the chains slide past one another, and the material can be melted and remoulded. That is what makes mechanical recycling possible, and it is tested by comparing them with a cross-linked polymer, in which covalent bonds between the chains cannot be broken by heat.

• The backbone is a chain of strong, non-polar C-C bonds with no functional groups.
• There is no ester, glycosidic or amide link for a microorganism's enzyme to hydrolyse.
• The chains are not cross-linked, so the polymer softens on heating and can be remoulded.

Disposal: burning the waste and recycling it

Burning. In a good supply of air poly(alkene)s burn completely to carbon dioxide and water. Carbon dioxide is a greenhouse gas, so even complete combustion adds to global warming, and it is released from a material that was itself a fossil fuel product. In a limited supply of air the combustion is incomplete and produces carbon monoxide, a poisonous gas that binds to haemoglobin and prevents it carrying oxygen, together with soot and unburned hydrocarbons.

PVC brings a third product. Because it contains chlorine, burning it releases hydrogen chloride, an acidic, corrosive and toxic gas that dissolves in atmospheric water to form hydrochloric acid and contributes to acid deposition. Waste incinerators therefore pass the flue gases through an alkaline scrubber, where the hydrogen chloride is neutralised, for example by lime: Ca(OH)2 + 2HCl -> CaCl2 + 2H2O. Older plants that burnt PVC with other waste produced dioxins, so PVC is now separated from other plastics before incineration wherever possible.

Recycling. Mechanical recycling melts the sorted plastic and remoulds it, which works because poly(alkene)s are thermosoftening. The difficulty is sorting. Bottles and containers carry recycling codes, 1 for PET, 2 for HDPE and 3 for PVC, and different polymers do not mix: a little PVC in a batch of HDPE produces hydrogen chloride when the batch is melted and turns the whole batch brittle and discoloured. Food-contaminated and multi-layer packaging cannot be separated economically, and mixed plastic is often only good enough for low-grade products such as park benches and bin liners.

Energy recovery burns the waste in a power station instead of recycling it, and feedstock recycling breaks the chains back down to smaller hydrocarbons that can be fed into a refinery. Both are used where sorting is impossible. The comparison to remember is that recycling keeps the carbon in use, whereas burning releases it as carbon dioxide straight away.