Look around and much of what you see is made of polymers: plastic bottles and bags, polyester and nylon clothing, foam packaging, the proteins in your muscles and the cellulose in paper. A polymer is a very large molecule built by joining many small molecules, called monomers, into a long chain, like beads on a string. The reactions that link monomers are the same organic reactions you have already met: addition across a double bond, and condensation reactions such as esterification. Understanding how polymers form explains why plastics are so useful, why they last so long in the environment, and how some of them could be designed to break down.
Addition Polymers
Addition polymers form from monomers that contain a C=C double bond. One bond of each double bond opens, and each monomer's two carbons bond to neighbouring monomers, building a chain with a backbone of single bonds. No atoms are lost, so the polymer has the same empirical formula as the monomer, and the chain's molar mass is simply the number of monomers times the monomer's molar mass. The repeating unit is drawn in square brackets with a subscript , which stands for the number of units (often thousands).
| Monomer | Polymer | Repeating unit | Common uses |
|---|---|---|---|
| ethene, | polyethylene (polyethene) | plastic bags, films, bottles | |
| propene, | polypropylene | food containers, rope, carpet fibres | |
| chloroethene (vinyl chloride), | polyvinyl chloride (PVC) | pipes, window frames, flooring | |
| phenylethene (styrene), | polystyrene | foam packaging, disposable foam cups | |
| tetrafluoroethene, | polytetrafluoroethene (PTFE) | non-stick coatings |
Condensation Polymers
Condensation polymers form when monomers with two functional groups each join end to end, releasing a small molecule (usually water) at every link. Two groups per monomer are essential: after one end reacts, the other end is still free to react, so the chain keeps growing. A polyester forms from a dicarboxylic acid and a diol, joined by ester links; PET, used in drink bottles and polyester fabric, is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol. A polyamide forms from a dicarboxylic acid and a diamine, joined by amide links; nylon-6,6 is made from hexanedioic acid and hexane-1,6-diamine, and each monomer has six carbons, which is where the "6,6" comes from. Nature makes condensation polymers too: proteins are polyamides built from amino acids joined by amide links called peptide bonds, and starch and cellulose are chains of glucose units, each link also releasing water.
- Monomers have a C=C double bond
- No by-product; every atom ends up in the polymer
- Polymer's empirical formula equals the monomer's
- Examples: polyethylene, polypropylene, PVC, polystyrene
- Each monomer has two functional groups (e.g. –COOH, –OH, –NH₂)
- A small molecule, usually water, is released at each link
- Links are ester bonds (polyesters) or amide bonds (polyamides and proteins)
- Examples: polyesters (PET), polyamides (nylon), proteins, starch, cellulose
Plastics and the Environment
The strong C–C and C–H bonds that make plastics durable also mean that most of them break down extremely slowly in nature. Discarded plastic builds up in landfills, and litter breaks into tiny fragments called microplastics that spread through soil, rivers and oceans and are eaten by wildlife. Most plastics are made from petroleum or natural gas, so producing them uses non-renewable resources. Burning plastic releases , and burning PVC also releases hydrogen chloride gas. Recycling helps, but plastics must be sorted by type because different polymers do not blend well. Resin identification codes (the number inside the arrow symbol) show the polymer type; they do not guarantee that a local program accepts the item. Bio-based polymers such as polylactic acid (PLA), made from plant starch, can biodegrade, but usually only under the hot, controlled conditions of an industrial composting facility, not in a landfill or backyard. Personal choices make a difference too: carry reusable bags, bottles and containers instead of single-use plastic, recycle by your municipality's rules (one wrong item can contaminate a batch), and take leftover solvents, paint thinners and pesticides to a household hazardous waste depot instead of the garbage or a drain.
| Code | Abbreviation | Polymer |
|---|---|---|
| 1 | PET | polyethylene terephthalate (a polyester) |
| 2 | HDPE | high-density polyethylene |
| 3 | PVC | polyvinyl chloride |
| 4 | LDPE | low-density polyethylene |
| 5 | PP | polypropylene |
| 6 | PS | polystyrene |
| 7 | Other | other plastics and mixtures |
Handling Organic Solvents Safely
Organic solvents such as hexane, propanone (acetone), ethanol and methylbenzene dissolve grease, paint and plastics, which makes them useful, but most are volatile and flammable, and many are harmful to breathe. Their vapours can spread along a bench to a flame or spark some distance away. Some cause long-term harm with repeated exposure: prolonged exposure to hexane can damage nerves, and benzene is a known human carcinogen, which is why school labs use less hazardous solvents in its place. In Canadian workplaces and labs, WHMIS 2015 requires a supplier label with hazard pictograms and a Safety Data Sheet (SDS) with 16 standard sections, including hazards, safe handling and storage, protective equipment, and disposal. Consumer products sold for home use carry a separate set of household hazard symbols.