Polymers

Polymers are very large molecules (macromolecules) made by linking together many small repeating units called monomers; the reaction that joins them is polymerisation. Because the same few monomers can be arranged and bonded in different ways, polymers are classified along several axes — and NEET often gives a polymer and asks which category it falls in.

By source, polymers are natural (made by living things — natural rubber, cellulose, starch, proteins, nucleic acids), semi-synthetic (natural polymers chemically modified — cellulose acetate, vulcanised rubber, rayon), or synthetic (man-made — polythene, nylon, PVC, bakelite). By structure, they are linear (straight chains that pack closely, e.g. high-density polythene), branched (side chains that prevent close packing, e.g. low-density polythene), or cross-linked / network (chains joined by covalent bridges into a rigid 3-D network, e.g. bakelite, vulcanised rubber).

A particularly useful classification is by intermolecular forces, because it predicts behaviour. Elastomers have weak forces and coiled chains, so they stretch and spring back (rubber). Fibres have strong forces (often hydrogen bonds) and so are strong and thread-like (nylon, terylene). Thermoplastics have intermediate forces: they soften on heating and can be remoulded repeatedly (polythene, PVC). Thermosetting polymers are heavily cross-linked and set permanently on first moulding — they cannot be softened again (bakelite, melamine). The thermoplastic-vs-thermosetting distinction is a recurring NEET question.

Finally, by mode of synthesis, polymers are addition (formed by repeated addition of unsaturated monomers, with no by-product) or condensation (formed when bifunctional monomers join with the loss of a small molecule). This last classification is so important that the next topic is devoted to it. Polymers may also be homopolymers (one kind of monomer, e.g. polythene) or copolymers (two or more kinds, e.g. Buna-S) — another small but examinable point.

The two great families of polymerisation differ in how the monomers join. In addition (chain-growth) polymerisation, unsaturated monomers add to one another across their double bonds, so the polymer has the same empirical formula as the monomer and no small molecule is lost. The chain typically grows by a free-radical mechanism with three stages — initiation (a radical from a peroxide adds to a monomer), propagation (the growing radical adds monomer after monomer), and termination (two radicals combine). Ionic (cationic/anionic) mechanisms also exist.

Common addition polymers come from alkene-type monomers: polythene (from ethene; low-density LDPE is branched and flexible, high-density HDPE is linear and tough), PVC (from vinyl chloride, $\text{CH}_2{=}\text{CHCl}$, used for pipes and insulation), polystyrene (from styrene), polypropene (from propene), and Teflon/PTFE (from tetrafluoroethene, $\text{CF}_2{=}\text{CF}_2$, a non-stick, heat-resistant coating). These are NEET staples: you are expected to match each polymer to its monomer.

In condensation (step-growth) polymerisation, monomers each carrying two functional groups react with one another, and a small molecule (usually water, sometimes HCl or methanol) is eliminated at each link. Growth occurs in steps between any two molecules with reactive ends. The two big classes here are polyamides (with amide links, like proteins) and polyesters (with ester links).

Key condensation polymers include: nylon-6,6 (a polyamide from adipic acid + hexamethylenediamine, losing water), nylon-6 (from caprolactam), terylene / dacron (PET) (a polyester from ethylene glycol + terephthalic acid, used as a fibre and in bottles), and bakelite (a cross-linked thermosetting polymer from phenol + formaldehyde). The single most testable contrast is this: addition = unsaturated monomers, no by-product; condensation = bifunctional monomers, small molecule eliminated. Spotting which mechanism a given polymer uses — often from whether its monomer has a C=C or two functional groups — is a guaranteed mark.

NEET reliably asks you to connect a polymer with its monomer(s) and its everyday use, so it is worth knowing the common synthetic polymers as a compact set. They split naturally into the addition polymers (from a single unsaturated monomer) and the condensation polymers (from bifunctional monomers).

Among addition polymers: polythene (monomer ethene) is used for bags, bottles and insulation; polypropene (propene) for ropes, pipes and packaging; PVC / poly(vinyl chloride) (vinyl chloride) for pipes, cable insulation and raincoats; polystyrene (styrene) for packaging, cups and insulation foam; Teflon / PTFE (tetrafluoroethene) for non-stick cookware and gaskets; and Orlon / polyacrylonitrile (PAN) (acrylonitrile) for synthetic wool and acrylic fibres. Each of these comes from one alkene-type monomer and is made without losing any by-product.

Among condensation polymers: Nylon-6,6 (adipic acid + hexamethylenediamine) is a tough polyamide fibre used in textiles, ropes and brushes; Nylon-6 (from the single monomer caprolactam) is used in tyre cords and fabrics; Terylene / Dacron (PET) (ethylene glycol + terephthalic acid) is a polyester used as a wrinkle-resistant fibre and in drink bottles; Bakelite (phenol + formaldehyde) is a hard, heat-resistant thermosetting plastic used in electrical switches and handles; and Melamine–formaldehyde resin is used for unbreakable crockery and laminates.

A few memory hooks help in the exam. 'Nylon' and 'amide' both contain the idea of the amide link, so nylons are polyamides; 'terylene/PET' is a polyester. Fluorine in the name (Teflon) signals tetrafluoroethene; chlorine (PVC) signals vinyl chloride. Phenol + formaldehyde always means bakelite (thermosetting). With these associations, you can answer most monomer–polymer matching questions quickly and reliably — exactly the kind of one-mark item that adds up in NEET.

Natural rubber is a natural addition polymer of isoprene (2-methyl-1,3-butadiene); chemically it is cis-1,4-polyisoprene. The cis arrangement of the double bonds stops the chains packing closely, giving rubber its coiled, springy, elastic character. But raw natural rubber is soft, sticky when warm, brittle when cold, and not very strong — limitations that need fixing for practical use.

The fix is vulcanisation: heating rubber with sulphur (about 3–5% for tyres, more for hard rubber). The sulphur creates cross-links between the polyisoprene chains, locking them so they can no longer slip past each other. Vulcanised rubber is therefore much stronger, harder, more elastic and far more resistant to heat and abrasion — which is why it is used for tyres, tubes and seals. Vulcanisation is one of the most frequently asked applied facts in this chapter.

Several synthetic rubbers improve on or replace natural rubber. Neoprene (polychloroprene, from chloroprene) is resistant to oil and heat, used in hoses and gaskets. Buna-S (SBR) is a copolymer of butadiene and styrene, widely used for tyres. Buna-N is a copolymer of butadiene and acrylonitrile, prized for oil resistance. ('Bu' = butadiene, 'na' = sodium catalyst, 'S' = styrene, 'N' = nitrile — a handy way to remember the components.)

Two further ideas round off the chapter. Biodegradable polymers are designed to be broken down by microbes or natural processes, addressing the pollution caused by ordinary, non-degradable plastics; examples are PHBV (a polyester of 3-hydroxybutanoic and 3-hydroxypentanoic acids) and nylon-2–nylon-6. Finally, because polymer chains vary in length, a polymer does not have one single molar mass but an average molecular mass — quoted as a number-average or a weight-average, with their ratio (the polydispersity index) describing how uniform the chains are. For NEET, the must-knows are the structure of natural rubber, the purpose of vulcanisation, the make-up of Buna-S/Buna-N, and the idea (and examples) of biodegradable polymers.