A-Level Chemistry Study Notes (9701): Condensation Polymerisation
Welcome to one of the most exciting topics in organic chemistry: polymers! We've already met addition polymers, but now we dive into a different, more versatile type. Condensation polymerisation is absolutely vital to modern materials, covering everything from the clothes you wear to the plastics used in sophisticated engineering.
Don't worry if the long names seem intimidating. We will break down the two main types—polyesters and polyamides—by focusing on the key functional groups and the small molecule that is always *kicked out* during the reaction. Let's get started!
1. Introduction to Condensation Polymerisation
1.1 Defining the Process
Condensation polymerisation is a reaction in which monomers join together, eliminating a small molecule—usually water (\(H_2O\)) or hydrogen chloride (HCl)—at each step.
This process is fundamentally different from addition polymerisation, where monomers simply add together without losing any atoms.
Key Feature: Bifunctional Monomers
For condensation to occur, the monomers must be bifunctional. This means they must contain two functional groups, one at each end of the molecule. This allows the chain to grow continuously.
Analogy: Think of two paperclips (monomers) linking together. In condensation, every time you link them, a tiny little metal shaving (the eliminated molecule) falls off.
Quick Review: Condensation vs. Addition
- Condensation: Monomers link + Small molecule (like \(H_2O\)) eliminated. Requires bifunctional monomers.
- Addition: Monomers link (by breaking C=C double bonds) + No molecules eliminated. Requires unsaturated monomers.
2. Polyesters: Forming the Ester Linkage
2.1 What is a Polyester?
A polyester is a polymer formed when monomers link together via ester linkages (\(–COO–\)).
The ester linkage is formed through the reaction between an alcohol group (–OH) and a carboxylic acid group (–COOH) or an acyl chloride group (–COCl).
2.2 The Monomers Used to Make Polyesters
The syllabus requires you to understand two main ways polyesters are formed:
Method A: Two different bifunctional monomers
You react a molecule with two alcohol groups (a diol) with a molecule possessing two acid-related groups (a dicarboxylic acid or a dioyl chloride).
1. Diol + Dicarboxylic acid (e.g., HO–R–OH + HOOC–R'–COOH)
This reaction is a standard esterification, typically requiring an acid catalyst (like concentrated \(H_2SO_4\)) and heat. It eliminates \(H_2O\).
2. Diol + Dioyl chloride (e.g., HO–R–OH + ClOC–R'–COCl)
This reaction proceeds much faster at lower temperatures. It eliminates HCl.
Method B: A single bifunctional monomer
You can use a monomer that contains both functional groups needed, such as a hydroxycarboxylic acid (HO–R–COOH).
When this monomer reacts with itself, the –OH end of one molecule links to the –COOH end of another, eliminating \(H_2O\).
2.3 Example: Polyethylene Terephthalate (PET) / Terylene
This is a classic example and is used extensively for plastic bottles and synthetic fibers.
Monomers:
1. Ethane-1,2-diol (a diol): \(HOCH_2CH_2OH\)
2. Benzene-1,4-dicarboxylic acid (a dicarboxylic acid): \(HOOC-C_6H_4-COOH\)
Reaction: The –OH group from the diol reacts with the –COOH group from the acid, eliminating water.
Deducing the Repeat Unit:
The reaction occurs at the ends of the molecules. To find the repeat unit, mentally remove the atoms that form water:
From the diol: \(HO - R - OH\) becomes \( - O - R - O - \)
From the acid: \(HOOC - R' - COOH\) becomes \( - OC - R' - CO - \)
The repeat unit is:
\( - OCH_2CH_2O - OC - C_6H_4 - CO - \)
Did you know? The material Terylene is famous for its wrinkle resistance, making it perfect for permanent-press fabrics!
Key Takeaway for Polyesters: Look for the ester link (\(–COO–\)) in the polymer chain. It is formed by linking alcohol (diol) and acid (dicarboxylic acid or dioyl chloride) fragments, losing \(H_2O\) or HCl.
3. Polyamides: Forming the Amide Linkage
3.1 What is a Polyamide?
A polyamide is a polymer formed when monomers link together via amide linkages (\(–CONH–\)).
The amide linkage is formed through the reaction between an amine group (–NH₂) and a carboxylic acid group (–COOH) or an acyl chloride group (–COCl).
3.2 The Monomers Used to Make Polyamides
Polyamides are also formed in two main ways, conceptually identical to polyesters:
Method A: Two different bifunctional monomers
You react a molecule with two amine groups (a diamine) with a molecule possessing two acid-related groups (a dicarboxylic acid or a dioyl chloride).
1. Diamine + Dicarboxylic acid (e.g., \(H_2N–R–NH_2\) + HOOC–R'–COOH)
This reaction eliminates \(H_2O\).
2. Diamine + Dioyl chloride (e.g., \(H_2N–R–NH_2\) + ClOC–R'–COCl)
This is the faster reaction and eliminates HCl.
Method B: A single bifunctional monomer
You use a monomer that contains both functional groups, such as an aminocarboxylic acid (\(H_2N–R–COOH\)).
The –NH₂ end of one molecule links to the –COOH end of another, eliminating \(H_2O\).
3.3 Example: Nylon 6,6 (The Classic Polyamide)
Nylon is a synthetic fiber often used in ropes, carpets, and clothing.
Monomers:
1. Hexane-1,6-diamine (a diamine): \(H_2N(CH_2)_6NH_2\)
2. Hexanedioic acid (a dicarboxylic acid): \(HOOC(CH_2)_4COOH\)
(Note: The '6,6' denotes that each monomer contains six carbon atoms.)
Deducing the Repeat Unit:
From the diamine: \(H_2N - R - NH_2\) becomes \( - NH - R - NH - \)
From the acid: \(HOOC - R' - COOH\) becomes \( - OC - R' - CO - \)
The repeat unit is:
\( - NH(CH_2)_6NH - OC(CH_2)_4CO - \)
3.4 Special Polyamides: Polypeptides and Proteins
This is extremely important for biology students! Proteins are natural polyamides.
- Monomers: The monomers are amino acids.
- Functional Groups: Amino acids are bifunctional, containing both an amine group (\(–NH_2\)) and a carboxylic acid group (\(–COOH\)).
- Linkage: When two amino acids link, the condensation reaction eliminates water and forms an amide linkage, which, in this context, is specifically called a peptide bond.
Key Takeaway for Polyamides: Look for the amide link (\(–CONH–\)) in the polymer chain. It is formed by linking amine (diamine or amino acid) and acid/acyl chloride fragments, losing \(H_2O\) or HCl.
4. Examination Skills: Monomers and Repeat Units
In exams, you must be able to move fluently between the monomers and the polymer structure.
4.1 Deduce the Repeat Unit from Monomers
This involves identifying the functional groups that react and drawing the remaining structure.
Step-by-step:
- Identify the condensation linkage (Ester: O-H + O=C-O-H; Amide: H-N-H + O=C-O-H).
- Remove the atoms that form the small eliminated molecule (\(H_2O\) or HCl) from the reacting groups.
- Connect the fragments and enclose the structure in square brackets, with 'n' outside, showing the bonds extending outside the brackets.
Common Mistake to Avoid: When forming a polyamide from a diamine and a dicarboxylic acid, remember that the diamine fragment contributes two 'H's (one from each end) and the dicarboxylic acid contributes two 'O's. The four atoms lost are \(2 \times H\) and \(2 \times OH\), totalling \(2 H_2O\) per pair of monomers, but only one \(H_2O\) is lost per linkage formed.
4.2 Identify the Monomer(s) from a Given Polymer Section
This is the reverse process: Hydrolysis. You must break the condensation link by adding back the eliminated molecule.
Step-by-step:
- Identify the condensation linkages (ester or amide).
- Mentally 'cut' the polymer chain at these links.
- Hydrolyse the links by adding \(H_2O\) (or HCl/alcohol component if dioyl chloride was used).
For Polyesters (\(–COO–\)): Break the C–O bond. Add H to the alcohol oxygen (to get –OH) and OH to the carbonyl carbon (to get –COOH).
For Polyamides (\(–CONH–\)): Break the C–N bond. Add H to the amine nitrogen (to get –NH₂) and OH to the carbonyl carbon (to get –COOH).
5. Degradable Polymers (A Crucial Environmental Link)
Condensation polymers have a major environmental advantage over addition polymers, and the syllabus requires you to understand why (Syllabus 35.3).
5.1 The Problem with Poly(alkenes)
Addition polymers, like poly(ethene) (polythene) or poly(chloroethene) (PVC), are made entirely of strong, non-polar C–C and C–H bonds. They are chemically inert and therefore non-biodegradable. They persist for hundreds of years in landfills.
5.2 The Advantage of Condensation Polymers
Condensation polymers (polyesters and polyamides) contain polar linkages (ester or amide bonds) within their backbone.
- These bonds are susceptible to attack by nucleophiles.
- Crucially, they can be broken down by acidic hydrolysis or alkaline hydrolysis (the reverse of condensation).
- This hydrolytic mechanism allows natural microorganisms and enzymes (like proteases) to break them down, making them biodegradable.
The "Zipper" Analogy: An addition polymer is like a continuous chain of C-C bonds with no easy handle to break it. A condensation polymer is like a chain with specific, weak links (the ester or amide bonds) that are easy to unzip (hydrolyse) back into their original monomers.
Key Takeaway on Degradability: The presence of hydrolysable functional groups (ester or amide links) is what makes condensation polymers biodegradable, unlike inert poly(alkenes).
6. Summary Review Box
Condensation Polymers at a Glance
Required Monomers: Always Bifunctional (two reactive ends).
Key Reaction: Elimination of a small molecule (e.g., \(H_2O\)).
I. Polyesters
- Linkage Name: Ester Linkage (\(–COO–\))
- How Formed: Alcohol (–OH) + Carboxylic Acid (–COOH) or Acyl Chloride (–COCl)
- Examples: PET / Terylene
II. Polyamides
- Linkage Name: Amide Linkage (\(–CONH–\)) or Peptide Bond
- How Formed: Amine (\(–NH_2\)) + Carboxylic Acid (–COOH) or Acyl Chloride (–COCl)
- Examples: Nylon, Proteins (Polypeptides)
Bonus Tip: To remember the reactants for polyamides and polyesters, remember the general functional groups: Polymerisation requires an 'acid end' (COOH/COCl) and either an 'alcohol end' (OH) for polyesters OR an 'amine end' (NH₂) for polyamides.