Polymers are large molecules made of repeating units called monomers, and they are found in plastics, fibers, rubbers, proteins, and DNA. This cheat sheet helps students connect molecular structure to the materials they see in everyday life. It is useful for comparing polymerization reactions, recognizing repeating units, and predicting basic properties.
Grade 11-12 chemistry students need these ideas for organic chemistry, materials science, and reaction classification.
The main ideas are that monomers join to form long chains, and the structure of those chains controls strength, flexibility, melting behavior, and chemical resistance. Addition polymerization usually uses unsaturated monomers with bonds and produces no small molecule byproduct. Condensation polymerization joins monomers with functional groups and often releases a small molecule such as or .
Important formulas include degree of polymerization, number-average molar mass, and simple repeating-unit notation.
Key Facts
- A polymer is a macromolecule made from many repeating units, often written as , where is the repeating unit and is the number of repeats.
- The degree of polymerization is when the polymer chain mass and repeat-unit mass are known.
- In addition polymerization, alkene monomers open their bonds to form a saturated polymer chain with no small molecule byproduct.
- Polyethylene forms from ethene by .
- In condensation polymerization, two functional groups react repeatedly and usually release a small molecule such as , , or .
- A polyester linkage contains the ester group , and a polyamide linkage contains the amide group .
- Number-average molar mass can be written as for a polymer with one main repeat unit.
- Cross-linking connects polymer chains together, which usually increases rigidity, thermal stability, and resistance to flow.
Vocabulary
- Monomer
- A small molecule that can chemically bond with other similar or different molecules to form a polymer.
- Polymer
- A very large molecule made of many repeating structural units joined by covalent bonds.
- Repeating Unit
- The smallest structural pattern that repeats along the backbone of a polymer chain.
- Addition Polymerization
- A polymerization reaction in which unsaturated monomers add together without forming a small molecule byproduct.
- Condensation Polymerization
- A polymerization reaction in which monomers with functional groups join while releasing a small molecule such as .
- Degree of Polymerization
- The number of repeating units in a polymer chain, often calculated using .
Common Mistakes to Avoid
- Confusing monomers with repeating units, because the repeat unit in the polymer may not look exactly like the original monomer after bonds break or small molecules are removed.
- Writing addition polymerization with a byproduct, because addition polymerization joins monomers directly and does not release , , or another small molecule.
- Forgetting the brackets and subscript in polymer notation, because shows that the unit repeats many times along the chain.
- Using the monomer molar mass instead of the repeat-unit molar mass, because condensation polymerization often removes atoms when small molecules such as are formed.
- Assuming all polymers melt sharply like pure small molecules, because polymer samples contain chains of different lengths and often soften over a temperature range.
Practice Questions
- 1 Polyethylene has the repeating unit with molar mass . What is for a chain with molar mass ?
- 2 A polymer has and a repeat-unit molar mass of . Calculate using .
- 3 Classify the reaction as addition or condensation polymerization, and identify the repeating unit.
- 4 Explain why a highly cross-linked polymer is usually harder and less flexible than a mostly linear polymer.
Understanding Polymers & Polymerization Reference
Addition polymerization is often a chain reaction. An initiator first creates a highly reactive particle called a radical. This particle attacks the carbon carbon double bond in one monomer.
A new reactive end remains on the growing chain, so another monomer can attach. This propagation step can happen many thousands of times in a short time. Growth stops when reactive ends meet or when a reaction transfers the reactive site elsewhere.
Conditions matter greatly. Temperature, pressure, initiator amount, and impurities can change chain length, branching, and yield. Oxygen is a common problem because it can react with radicals and slow the process.
Condensation polymerization works differently because any two suitable molecules can react. At first, mostly small molecules join. Later, these small chains react with one another to make longer chains.
This is called step growth. Very high conversion is needed to make long chains. If even a small amount of one reactant is missing, many chains end early.
Chemists therefore use nearly equal numbers of each functional group. Removing the small product during the reaction can push the reaction toward more polymer formation.
For example, water removal is important during some polyester syntheses. The functional groups at chain ends remain important because they control whether further growth can occur.
A sample of polymer does not contain chains that are all the same length. It contains a distribution of molar masses. Number average molar mass gives each chain equal importance, while mass average molar mass gives greater weight to heavier chains.
Long chains often make a material tougher because they become tangled. Branching can prevent chains from packing closely. Straight, regular chains may pack into ordered regions called crystalline regions.
These regions can raise density and strength. More disordered regions usually make a material clearer or more flexible.
Heating can soften many polymers because chain segments gain motion. Cross linked materials cannot flow easily on heating because the chains are tied into a network.
These ideas explain common material choices. Polyethylene film bends easily because its chains can move relative to one another. High density polyethylene containers are stiffer because their chains have less branching and pack more closely.
Rubber becomes useful after vulcanization, which creates a small number of links between chains. The links help rubber return to its original shape after stretching. Nylon fibers are strong partly because amide groups on nearby chains attract each other.
When studying structures, first identify the reactive group in each monomer. Then mark the bond made during the reaction and identify any atoms lost as a small molecule.
For calculation problems, use the mass of the repeating unit, not automatically the mass of the original monomer. End groups and lost molecules can make those masses different.