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.
Understanding Polymer Types & Polymerization Mechanisms
Addition polymerization is usually a chain reaction. It begins when an initiator produces a reactive particle called a free radical. This particle attacks the double bond in an alkene monomer.
One bond of the double bond becomes part of a new single bond, leaving a reactive end on the growing chain. During propagation, that end adds monomer after monomer very quickly. Growth stops when reactive ends combine or when one end is transferred to another molecule.
Temperature, pressure, initiator amount, and impurities can change the chain length. Oxygen is especially important because it can stop radical reactions. This is why industrial polymer production needs careful control.
Condensation polymerization works differently because any short chain can react with another short chain. A diol and a diacid, for example, can form an ester link between them. As the chains grow, small molecules must be removed to help the reaction continue.
Manufacturers may use heat, reduced pressure, or a stream of inert gas to remove water or another byproduct. The ratio of the two monomers matters greatly. If one monomer is present in excess, it caps many chain ends and limits the final molar mass.
Students should track the functional groups, not just the names of the reactants. Each monomer needs two reactive sites to build long mostly linear chains.
The arrangement of chains explains why materials made from similar elements can feel very different. Straight chains can pack closely, which often raises strength, density, and melting temperature. Branches disrupt packing, making a material softer or more flexible.
Strong attractions between chains matter too. Polyamides can form hydrogen bonds, so nylon fibers are tough and useful in clothing, ropes, and engineering parts. Cross-links join separate chains by covalent bonds.
A small number can improve elasticity in rubber. Many cross-links create a rigid network used in items such as electrical plugs, cookware handles, and epoxy adhesives. Heating a cross-linked material may cause decomposition rather than flowing.
Copolymers contain more than one kind of repeat unit. The units may alternate regularly, appear in random order, form long blocks, or occur as branches attached to a main chain. Changing this pattern can balance hardness, flexibility, impact resistance, or chemical resistance.
For example, impact resistant plastics often combine a hard component with rubbery regions that absorb energy during a collision. When drawing a polymer from monomers, identify the atoms that remain in the chain and the atoms that leave in a byproduct.
Then check that bonds and functional groups are chemically sensible. In calculations, remember that real samples contain chains of many different lengths, so one average molar mass never describes every chain exactly.