Polymer: a large molecule built from many small repeating units called monomers joined by covalent bonds. Polymerisation: joining of many monomers.
Addition polymerisation: monomers with a C=C double bond join without losing any molecule; the double bond opens and becomes a single bond. Condensation polymerisation: monomers with two functional groups join and a small molecule (usually water) is lost at each link.
Plastics: synthetic materials made from organic polymers (Greek plastikos, mouldable).
Addition polymers (repeat unit = the monomer with the double bond turned to a single bond)
Ethene CH₂=CH₂ gives polyethene, repeat unit -CH₂-CH₂- (made at high temperature and pressure). Propene gives polypropene. Chloroethene (vinyl chloride) gives polyvinylchloride (PVC). To deduce the monomer from a polymer, change the single bond in the backbone back to a double bond.
Condensation polymers
Polyamide (nylon-6,6): dicarboxylic acid (adipic acid HOOC-(CH₂)₄-COOH) + diamine (hexamethylenediamine H₂N-(CH₂)₆-NH₂), amide linkage -CONH- (peptide bond), water released. Used in clothing, carpets, ropes that do not rot, toothbrushes; strong, elastic, resists abrasion.
Polyester (PET): dicarboxylic acid (terephthalic acid) + diol (ethylene glycol, ethane-1,2-diol), ester linkage -COO-, water released. Used for clothing, curtains, bed linen, rope, tyre cords, drink bottles; strong, flexible, quick drying, keeps colour and shape.
General equation: n HOOC-R-COOH + n H₂N-R′-NH₂ → [-OC-R-CO-NH-R′-NH-]ₙ + 2n H₂O (approximately; one water per link).
PET recycling: acid hydrolysis with H₂SO₄ or HCl at 200 to 250 °C breaks the ester bonds back into terephthalic acid and ethylene glycol, which can be re-polymerised. This saves raw material and reduces plastic waste.
Addition against condensation (give two points)
Monomer: addition uses unsaturated monomers with C=C; condensation uses monomers with two functional groups (-COOH and -OH or -NH₂).
By-product: addition gives none; condensation releases a small molecule such as water.
Examples: addition gives polyethene, PVC, polypropene; condensation gives nylon and PET (polyamides and polyesters). Repeat unit: in addition it equals the monomer; in condensation it contains parts of two different monomers.
Plastics and the environment
Properties that affect disposal: durable (last hundreds or thousands of years), lightweight (carried by wind and water), contain additives that leach out, limited recyclability.
Landfill: takes up a lot of space and resists compaction; leaches chemicals into soil and groundwater; breaks into microplastics.
Oceans: about 8 million tons enter the ocean every year; gather into garbage patches; animals eat or are entangled by plastic and die; microplastics enter the food chain and reach humans.
Burning: releases toxic gases (dioxins, furans), CO₂ and methane, causes air pollution and respiratory disease; leftover ash is toxic.
Past-paper questions with model answers
What is a polymer? Give two examples (B 3 marks); what is meant by monomer (B, 2+1), define monomer and polymer (D, M): monomer = small repeating unit; polymer = large molecule built from many monomers; examples polyethene, PVC, nylon, PET.
Sec C: describe the formation of a polyamide from a dicarboxylic acid and a diamine with a general equation and the type of linkage (B, 2+2): see condensation polymer above; linkage is the amide (peptide) bond -CONH-.
Sec C: two differences between addition and condensation polymerisation (D): use the points above.
Sec C: environmental challenges of plastic, landfill disposal and ocean accumulation (M, 2+2): use the points above.