This cheat sheet covers the main classes of polymers and the mechanisms used to make them. Students need it because polymer chemistry connects organic reactions, materials science, and real products such as plastics, fibers, adhesives, and biomolecules. It helps organize many examples into a few reliable patterns.
It also supports quick comparison of polymer structures, monomers, and reaction types.
The core ideas are that polymers are large molecules built from repeating monomer units. Addition polymerization usually joins alkene monomers without losing small molecules, while condensation polymerization joins bifunctional monomers and often releases small molecules such as or . Polymer properties depend on monomer structure, chain length, branching, cross-linking, and intermolecular forces.
Common shorthand uses to show many repeat units, such as .
Key Facts
- A polymer is a macromolecule made of many repeating units, often written as , where is the degree of polymerization.
- Addition polymerization joins unsaturated monomers such as alkenes, for example .
- Condensation polymerization joins monomers with two functional groups and often releases a small molecule, such as .
- Polyethene forms from ethene by opening the carbon carbon double bond: .
- Polypropene forms from propene with a methyl side group on every other carbon: .
- A polyester linkage contains the ester group , while a polyamide linkage contains the amide group .
- Thermoplastics soften when heated because chains can slide past each other, while thermosets do not melt easily because covalent cross-links hold chains in a network.
- The approximate number-average molar mass can be estimated by , where is the molar mass of the repeat unit.
Vocabulary
- Monomer
- A small molecule that can chemically bond to other similar or compatible molecules to form a polymer.
- Repeat unit
- The smallest structural unit that repeats along a polymer chain and is written inside brackets as .
- Addition polymerization
- A polymerization mechanism in which monomers add together, often by opening bonds, with no small molecule byproduct.
- Condensation polymerization
- A polymerization mechanism in which bifunctional monomers join while eliminating a small molecule such as or .
- Copolymer
- A polymer made from two or more different monomers arranged in patterns such as alternating, random, block, or graft structures.
- Cross-linking
- The formation of covalent bonds between polymer chains, creating a network that usually increases rigidity and heat resistance.
Common Mistakes to Avoid
- Confusing monomers with repeat units, which is wrong because the repeat unit may not have the same bonds as the original monomer after polymerization.
- Writing addition polymerization with a byproduct, which is wrong because alkene addition polymerization normally conserves all atoms in the polymer chain.
- Forgetting that condensation polymers need bifunctional monomers, which is wrong because each monomer must form links in two directions to make long chains.
- Ignoring side groups when drawing polymers, which is wrong because groups such as , , or strongly affect structure and properties.
- Assuming all plastics melt the same way, which is wrong because thermoplastics can soften repeatedly while thermosets are held by cross-links and usually decompose instead of melting.
Practice Questions
- 1 Write the repeat unit for the addition polymer formed from chloroethene, .
- 2 If a polymer has degree of polymerization and repeat unit molar mass , estimate using .
- 3 A condensation reaction forms ester linkage and releases molecule of each time a diacid reacts with a diol. How many molecules of are released when ester linkages form?
- 4 Explain why a highly cross-linked polymer is usually more rigid and less meltable than a linear polymer made from the same monomer.