In this lesson, we will learn:
- How condensation polymers (polyesters and polyamides) are made.
- To identify the repeating units of condensation polymers.
- Some important commercial examples of condensation polymers.
- How condensation polymers occur naturally with amino acids.
- Polyesters and polyamides are both examples of condensation polymers. As a type of polymer, these can have very desirable properties, and some important industrially-made materials are polyamides or polyesters.
- We saw polyesters in Carboxylic acids, acyl chlorides and esters when diols and dioic acids (AKA dicarboxylic acids) react. This is a simple esterification but instead of just a carboxylic acid and alcohol, the diol and dioic acid make it happen at both ends of each molecule, and so a large polymer chain emerges.
The repeating unit of a polyester will contain one complete ester linkage and the next ester linkages cut in half. This should contain one of the diols and one of the dioic acids. For an example see below:
This is a condensation polymerisation to make polyethylene terephthalate (PET). It is a common polyester used in making plastic bottles and some clothing.
Look at how the repeating unit is drawn: one complete ester linkage (which joins the diol and the dioic acid).
Ester linkages in small molecules are relatively strong chemical bonds, but dilute acid and alkali can hydrolyse them. In polyesters, acid hydrolysis is extremely slow and only alkali/basic solution can reasonably break it down .
- Polyamides are made the same way as polyesters, but they have a -CONH- amide linkage instead of a -COOR- ester linkage.
As we saw last time in Carboxylic acids, acyl chlorides and esters, the reaction of a carboxylic acid and an amine (or ammonia) produces an amide. In exactly the same way as diols and dioic acids make polyesters, diamines and dioic acids produce polyamides.
This is a condensation polymer product.
One important example is nylon-6,6. See the image below:
This condensation polymerisation has two units joining together: hexanedioic acid and hexane-1,6-diamine. These form the polyamide nylon-6,6.
The 6,6 is named because both monomers that form the repeating unit are 6 carbons in length (the hexanedioic acid and hexane-1,6-diamine).
Another example is Kevlar which is uses in bulletproof vests. The monomers for this polyamide are benzene-1,4-dioic acid and benzene-1,4-diamine. See the image below:
Amide linkages are more resistant to hydrolysis in general than ester linkages. This helps make the polymer relatively strong, but its intermolecular hydrogen bonding gives Kevlar its strength.
The many benzene rings in each long polymer chain keep them relatively flat and straight. This enables separate polyamide chains to align, both aside and on top of one another. This close alignment with other chains allows hydrogen bonding to occur between intermolecular amide groups. These attractive forces accumulate throughout the very long polyamide chains making the combined mass extremely strong and hard to tear or pull apart.
This is like creating a strong rope out of many individual fibres, which are weak on their own. The whole is greater than the sum of its parts!
- The amide linkage in polyamides is also present when amino acids combine to form ‘polymer’ chains. Amino acids are molecules containing both an amine and a carboxylic acid group, usually at opposite ends of the molecule. The naturally-occurring amino acids have four key components:
- An amino group (-NH2).
- A carboxylic acid group (-COOH).
- An R group attached to the carbon bonded to the amino group. This R group is what identifies an amino acid. It can be as simple as hydrogen (in glycine) or much larger, involving rings, heteroatoms or hydrocarbon chains.
- An H atom bonded to the carbon which is bonded to the R group. This is important because this carbon atom, except in glycine, is now a chiral centre.
Just like polyamides made from an amine and acid group, amino acids can polymerise using both of these groups. Polymers made of amino acid monomers are called proteins, and they are formed by condensation reactions just like how polyamides are formed. When we study it in proteins, the amide link is called a peptide bond. See the image below, where glycine (gly) and alanine (ala) form a peptide bond:
In this example, it is just a combination of two molecule. The product can be called “gly-ala” as it is a unit comprised of both amino acids with a peptide bond.
As said above, it is the same bond making as in polyamides, but there are many different amino acids and they form very complex sequences – they’re the building blocks of life.
We will learn more about amino acids in a later lesson!