Polymers: TCE Chemistry (Tasmania)
“Distinguish addition and condensation polymerisation and relate polymer structure to properties”
Polymers are long chains built from many small monomers. Addition polymers form when alkene monomers join with no loss of atoms, while condensation polymers form when monomers join with the loss of a small molecule such as water.
What this dot point is asking
A polymer is a very large molecule, or macromolecule, made by joining many small repeating units called monomers. The process of joining monomers is called polymerisation. Polymers are everywhere, from natural materials such as cellulose, starch and proteins to synthetic materials such as polyethene, nylon and polyester. Understanding polymers means relating the small monomer building blocks to the structure and properties of the final material.
There are two main types of polymerisation, distinguished by what happens to the atoms during the reaction. Addition polymerisation involves unsaturated monomers, usually alkenes. The carbon-carbon double bond opens up and the monomers add to one another to form a long saturated chain. No atoms are lost in the process, so the empirical formula of the polymer matches that of the monomer.
A reaction in which unsaturated monomers, typically alkenes, join together when their double bonds open, forming a long-chain polymer with no loss of any atoms. The polymer and monomer share the same empirical formula.
Common addition polymers include polyethene (from ethene), polypropene (from propene), polyvinyl chloride or PVC (from chloroethene), and polystyrene (from styrene). To draw the repeating unit, take the monomer, change the double bond to a single bond, and place the structure inside brackets with a bond extending from each side and a subscript to show many repeats.
Condensation polymerisation joins monomers with the elimination of a small molecule, most often water. This requires monomers with two reactive functional groups, so that chains can grow at both ends. Two important classes are polyesters and polyamides. A polyester forms when a dicarboxylic acid reacts with a diol, linking through ester bonds and releasing water at each link. A polyamide, such as nylon, forms when a dicarboxylic acid reacts with a diamine, linking through amide bonds.
Proteins are natural condensation polymers (polyamides) made from amino acid monomers joined by peptide bonds, while carbohydrates such as starch and cellulose are condensation polymers of glucose. The same chemistry that builds synthetic plastics also builds the molecules of life.
Identifying the polymerisation type
Nylon is made from a diamine and a dicarboxylic acid, releasing water at each junction. Polyethene is made from ethene with no by-product. Classify each.
Step 1: Nylon loses a small molecule (water) when monomers join, and its monomers each have two functional groups. This is condensation polymerisation.
Step 2: Polyethene forms when the double bond in ethene opens and monomers add together with no loss of atoms. This is addition polymerisation.
Conclusion: nylon is a condensation polymer (a polyamide); polyethene is an addition polymer.
The properties of a polymer depend on its structure. Chain length affects strength and melting point, with longer chains generally giving stronger, higher-melting materials. The degree of branching matters: highly branched chains pack loosely, giving a low-density, flexible material, while unbranched chains pack closely, giving a high-density, more rigid material. This explains the difference between low-density and high-density polyethene.
Cross-linking, where covalent bonds connect adjacent chains, makes a polymer harder and more rigid because the chains can no longer slide past one another. The strength of intermolecular forces between chains, such as hydrogen bonding in nylon and proteins, also raises melting point and strength. Side groups influence both flexibility and the way chains pack together.
A common error is to think every polymer forms by addition from a double bond. Condensation polymers such as polyesters, nylon and proteins form from monomers with two functional groups, not from alkene double bonds, and they release a small molecule each time a bond forms. Always check whether a small molecule is lost before classifying the polymerisation.
In exams, draw the repeating unit with brackets and the subscript , identify the type of polymerisation with a reason, and connect any property of the material to a specific structural feature.
Exam-style questions
Questions in the style of TASC exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
Practice2 marksA water bottle is made of PET (polyethylene terephthalate). Given its repeat unit contains ester linkages, state whether PET is an addition or condensation polymer and justify your answer.Show worked answer →
PET is a condensation polymer. It is made by reacting a dicarboxylic acid (terephthalic acid, two groups) with a diol (ethane-1,2-diol, two groups). (1 mark)
Each ester linkage () forms with the loss of a small molecule (water), which is the defining feature of condensation polymerisation. An addition polymer would instead have a saturated carbon backbone formed from monomers with no small molecule lost. (1 mark)
Practice2 marksLactic acid contains both a hydroxyl group and a carboxylic acid group and can form polylactic acid. Name the type of polymer formed and justify your choice.Show worked answer →
Type: condensation polymer (specifically a polyester). (1 mark)
Lactic acid has two different functional groups, a hydroxyl () and a carboxylic acid (). The of one molecule reacts with the of the next to form an ester linkage, eliminating a small molecule (water) at each step. Because a small molecule is lost, this is condensation, not addition, polymerisation. (1 mark)
Practice2 marksPolylactic acid (PLA) has the repeating unit . Deduce the structure and systematic name of its monomer, lactic acid.Show worked answer →
Break the polymer at the ester linkages to recover the monomer. Each repeat unit comes from a single monomer with both an and a group on a three-carbon chain. (1 mark)
Structure: (a hydroxyl on carbon 2 and a carboxyl at carbon 1); systematic name 2-hydroxypropanoic acid. The presence of both and on the monomer is what allows condensation (polyester) polymerisation. (1 mark)