Learn first: Amino acids and proteins
Polymers are substances made of huge molecules (macromolecules) built from thousands of repeating units. They are everywhere around us: poly(ethene) bags and pipes, poly(propene) containers, PVC flooring and window frames, nylon and polyester clothing and rubber tyres are synthetic polymers; cotton, wool, silk, starch, proteins and DNA are natural polymers. Polymers are light, resist corrosion and are easily shaped, so in many fields they have replaced metal, glass and wood. Exam questions ask you to calculate the degree of polymerisation, to distinguish addition from condensation polymerisation, and to classify fibres. In Uzbekistan, poly(ethene) and poly(propene) are produced at the Ustyurt gas-chemical complex.
M(polymer) = n · M(repeating unit)
A polymer is a mixture of macromolecules of different lengths, so its molar mass and degree of polymerisation are average values; polymers have no sharp melting point and soften over a temperature range.
By structure: linear (cellulose, poly(ethene)), branched (amylopectin, glycogen) and cross-linked or network (vulcanised rubber, phenol–formaldehyde resins). Linear polymers are flexible and can dissolve; network polymers are hard and insoluble.
By behaviour on heating: thermoplastics (poly(ethene), PVC, polystyrene) soften when heated and can be reshaped and recycled; thermosets (phenol–formaldehyde resin) set irreversibly on heating.
A sample of poly(chloroethene) (PVC) has an average molar mass of 62 500 g/mol. Find the degree of polymerisation.
Addition polymerisation joins monomer molecules containing a multiple bond without releasing any by-product. The π-bond breaks and the monomers link into a chain through σ-bonds; the repeating unit has the same composition as the monomer.
nCH₂=CH₂ → (–CH₂–CH₂–)ₙ — poly(ethene)
nCH₂=CH–CH₃ → (–CH₂–CH(CH₃)–)ₙ — poly(propene)
nCH₂=CHCl → (–CH₂–CHCl–)ₙ — poly(chloroethene), PVC
nCF₂=CF₂ → (–CF₂–CF₂–)ₙ — PTFE (Teflon)
Others include polystyrene (monomer styrene C₆H₅–CH=CH₂), poly(methyl methacrylate) (acrylic glass, Perspex) and polyacrylonitrile (acrylic fibre). PTFE is extremely chemically resistant, unaffected even by concentrated acids and alkalis — hence non-stick pan coatings. Polymerising two different monomers together is copolymerisation (styrene–butadiene rubber).
Addition polymerisation often proceeds by a free-radical chain mechanism: an initiator (such as a peroxide) splits to form a radical, the radical adds to the monomer’s double bond to give a new radical, the chain grows rapidly and stops when two radicals meet. It can also be carried out with Ziegler–Natta catalysts, which give polymers with a regular structure and greater strength.
Condensation polymerisation joins monomers while releasing a small molecule (usually water, sometimes HCl or ammonia). Each monomer must have at least two functional groups. The mass of the repeating unit is less than that of the monomers by the mass of the molecules eliminated.
Phenol–formaldehyde resin (phenol + methanal), nylon-6 (from 6-aminohexanoic acid), PET polyester (benzene-1,4-dicarboxylic acid + ethane-1,2-diol), proteins (from amino acids), starch and cellulose (from glucose)
In nylon and proteins the units are linked by amide (peptide) bonds –CO–NH–, in polyesters by ester bonds, so they are called polyamides and polyesters. Condensation polymerisation is step-wise: dimers and trimers form first, then ever longer chains.
The repeating unit of nylon-6, –NH–(CH₂)₅–CO–, comes from 6-aminohexanoic acid (C₆H₁₃NO₂). Find the degree of polymerisation of nylon-6 with a molar mass of 11 300 g/mol.
Each unit of rubber keeps a double bond, so rubber adds bromine and slowly oxidises ("perishes") in air. Vulcanised rubber is used for tyres, hoses, medical gloves and seals; carbon black is added to tyre rubber to strengthen it.
Isoprene rubber containing 3.01·10²¹ macromolecules was partly brominated with 600 g of 3.2 % bromine water (no chain breaking; all the bromine was used). The mass fraction of bromine in the product is 32 %. Find the degree of polymerisation.
Fibres are made by forcing a polymer solution or melt through very fine holes (a spinneret) and letting it set. Among natural fibres, cotton is almost pure cellulose while silk and wool are proteins, so wool and silk smell of burnt feathers when burnt, whereas cotton burns like paper — a simple way to tell them apart.
Synthetic fibres are strong, do not rot and are not eaten by moths, but they absorb little moisture, so they are often blended with natural fibres. For Uzbekistan the most important natural fibres are cotton and silk.
A polymer’s properties depend on chain length, structure and the forces between chains. Longer chains give greater strength; many ordered (crystalline) regions make a polymer harder and less transparent. For example, branched low-density poly(ethene) made at high pressure is soft and flexible (bags), while linear high-density poly(ethene) made at low pressure with a catalyst is stiffer (pipes, containers). Most polymers are electrical insulators, so they are used to insulate wires.
Uses: poly(ethene) — films, pipes, containers; poly(propene) — containers, ropes, car parts; PVC — flooring, window frames, cable insulation; polystyrene — expanded foam (thermal insulation); acrylic glass — transparent panels; PTFE — non-stick coatings and laboratory ware; phenol–formaldehyde resins — electrical fittings.
The environmental problem: most synthetic polymers break down extremely slowly in nature and remain as litter for decades, while burning them releases toxic gases (burning PVC gives off HCl). Solutions include recycling thermoplastics (melting them into new products), separating waste, producing biodegradable polymers and using fewer single-use plastic items.