Condensation polymerisation releases a small molecule per link
Definition
Condensation polymerisation
A reaction in which monomers with two functional groups join to form a polymer, releasing a small molecule such as water each time a link forms.
Definition
Monomer
A small molecule that can join with many other molecules to form a polymer.
- Each monomer has two functional groups, one at each end of the molecule.
- The group at one end reacts with a monomer on one side and the group at the other end reacts with a monomer on the other side, so a chain grows.
- Most condensation polymers are made from two different monomers that join alternately.
- Each time two functional groups react, a new link forms and a small molecule, usually water, is released.
- Condensation polymerisation therefore gives two products, the polymer and the small molecule.
- Addition polymerisation, in contrast, uses monomers with a C=C\text{C}=\text{C}C=C double bond and makes the polymer as its only product.
Key Idea
Look for monomers with a reactive group at both ends, and expect a small molecule to be released each time a link forms.
A diol and dicarboxylic acid form a polyester
Definition
Diol
A molecule with two hydroxyl groups, OH, which can react at both ends to form a condensation polymer.
Definition
Dicarboxylic acid
A molecule with two carboxyl groups, COOH, which can react at both ends to form a condensation polymer.
- A diol has two hydroxyl groups, −OH-\text{OH}−OH, and a dicarboxylic acid has two carboxyl groups, −COOH-\text{COOH}−COOH.
- The −COOH-\text{COOH}−COOH group of the acid reacts with the −OH-\text{OH}−OH group of the diol to form an ester link, −COO−-\text{COO}-−COO−, and a water molecule.
- The water forms from the −OH-\text{OH}−OH of the acid group and the hydrogen atom of the alcohol group.
- Each monomer reacts at both ends, so the chain grows with acid and diol units alternating.
- Using a box and a circle for the carbon chains between the functional groups, the equation is: n HOOC−□−COOH+n HO−◯−OH→[−OC−□−COO−◯−O−]n+2n H2On\,\text{HOOC}{-}\square{-}\text{COOH} + n\,\text{HO}{-}\bigcirc{-}\text{OH} \rightarrow \left[{-}\text{OC}{-}\square{-}\text{COO}{-}\bigcirc{-}\text{O}{-}\right]_n + 2n\,\text{H}_2\text{O}nHOOC−□−COOH+nHO−◯−OH→[−OC−□−COO−◯−O−]n+2nH2O
- PET, made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid, is the polyester used for drinks bottles and polyester clothing.
Common Mistake
Do not write n H2On\,\text{H}_2\text{O}nH2O in the equation, because each repeating unit contains two ester links, so 2n2n2n water molecules form.
A diamine and dicarboxylic acid form a polyamide
Definition
Diamine
A molecule with two amine groups, NH2, which can react at both ends to form a polyamide.
- A diamine has an amine group, −NH2-\text{NH}_2−NH2, at each end of its molecule.
- The −COOH-\text{COOH}−COOH group reacts with the −NH2-\text{NH}_2−NH2 group to form an amide link, −CONH−-\text{CONH}-−CONH−, and a water molecule.
- The polymer formed is a polyamide, and nylon is the best-known example.
- Nylon-6,6 forms from hexane-1,6-diamine and hexanedioic acid, which each have six carbon atoms.
- Nylon is strong and hard-wearing, so it is used for ropes, fishing line and clothing.
- If the acid is replaced by an acid chloride such as hexanedioyl dichloride, with a −COCl-\text{COCl}−COCl group at each end, the small molecule released is hydrogen chloride, HCl\text{HCl}HCl.
- Proteins and DNA are also condensation polymers, made in living things from amino acids and nucleotides.
Common Mistake
Do not assume the small molecule is always water, because monomers with −COCl-\text{COCl}−COCl groups release HCl\text{HCl}HCl instead.
Making nylon where two solutions meet
Definition
Immiscible
Describes two liquids that do not mix, so they settle into separate layers.
- One monomer, hexane-1,6-diamine, is dissolved in water.
- The other, hexanedioyl dichloride, is dissolved in an organic solvent such as cyclohexane, which does not mix with water.
- The two solutions are immiscible, so they form two layers, with the less dense cyclohexane solution on top.
- The monomers can only meet at the boundary between the layers, so a thin film of nylon forms there.
- As the film is pulled out, fresh monomers meet at the boundary and more nylon forms, so a continuous thread can be drawn out.
- The reaction releases hydrogen chloride, so the diamine solution often contains a little sodium hydroxide to neutralise it.
Practical
- Aim: make a thread of nylon from two monomers.
- Apparatus: a 50 cm350\,\text{cm}^350cm3 beaker, two 10 cm310\,\text{cm}^310cm3 measuring cylinders, a glass rod, tweezers, a solution of hexane-1,6-diamine in water, a solution of hexanedioyl dichloride in cyclohexane, a beaker of water for washing, gloves and a fume cupboard.
Method
- Working in a fume cupboard, pour about 5 cm35\,\text{cm}^35cm3 of the hexane-1,6-diamine solution into the small beaker.
- Slowly pour about 5 cm35\,\text{cm}^35cm3 of the hexanedioyl dichloride solution down a glass rod on top, so that the two layers do not mix.
- Use tweezers to grip the film of nylon that forms at the boundary between the layers.
- Lift the film out slowly to form a thread.
- Wind the thread onto a glass rod, turning it steadily so that fresh nylon keeps forming at the boundary.
- Wash the nylon thoroughly in the beaker of water before touching it.
Accuracy and safety
- Pouring the second solution down a glass rod keeps the layers separate, so the nylon forms only at the boundary.
- The nylon is coated in unreacted chemicals until it has been washed.
- Wear gloves and eye protection, because the acid chloride is corrosive.
- Keep cyclohexane away from flames, because it is highly flammable.
Self review
- What is released each time a link forms in condensation polymerisation?
- Which two types of monomer react to form a polyester?
- What is the name of the link formed between a diamine and a dicarboxylic acid?
- Where does nylon form when it is made from two layers of solution?
- Which small molecule forms when nylon is made from hexanedioyl dichloride?
