🌟 Welcome to Polymerisation! 🌟

Hello future chemist! This chapter, Polymerisation, is where we learn how small organic molecules hook up to create the giant molecules that make up everything from plastic bottles to your DNA. It connects fundamental organic chemistry (like alkenes and functional groups) with real-world materials science.
Don't worry if the structures look long—polymers are just long chains made of simple, repeating units!

What exactly are Polymers?

  • Mononer: A small molecule that can join together with others to form a long chain. (Think of a single LEGO brick).
  • Polymer: A very large molecule (macromolecule) formed by joining many monomers together. (Think of a giant chain built from those LEGO bricks).
  • Polymerisation: The chemical reaction process in which monomers are converted into polymers.

1. Addition Polymerisation (AS Level)

Addition polymerisation is the simpler of the two types. It happens when monomers containing a carbon-carbon double bond (i.e., alkenes) join together.

1.1 The Mechanism: Adding Up!

When addition polymerisation occurs, the double bond in the alkene monomer breaks, and the free electrons are used to form new single bonds with adjacent monomers.

Key features:

  • The reaction involves only addition—no atoms are lost.
  • The polymer formed has the exact same empirical formula as the monomer.
  • This process often requires high pressure, high temperature, and a catalyst (often a free radical initiator).
Step-by-Step Deduction of the Repeat Unit

This is a vital skill for the exam. The repeat unit is the smallest section of the polymer chain that, when repeated, makes up the whole polymer.

  1. Find the C=C double bond in the monomer.
  2. Replace the C=C double bond with a C–C single bond.
  3. Draw two lines extending outside the carbons (these are the bonds that connect to the rest of the chain).
  4. Place the side groups (R groups) exactly as they were on the carbons.

Analogy: Imagine breaking the C=C handcuffs between the two carbons and giving each carbon a new, external hand to hold onto the next monomer.

1.2 Key Examples of Addition Polymers

Example 1: Poly(ethene) (Polythene)
  • Monomer: Ethene (\(\text{CH}_2=\text{CH}_2\))
  • Use: Plastic bags, packaging, buckets.
  • Repeat Unit: \( \text{[-CH}_2\text{-CH}_2\text{-]}_n \)
Example 2: Poly(chloroethene) (PVC)
  • Monomer: Chloroethene (vinyl chloride) (\(\text{CH}_2=\text{CHCl}\))
  • Use: Drain pipes, window frames, electrical insulation.
  • Repeat Unit: \( \text{[-CH}_2\text{-CHCl-}\text{]}_n \)

1.3 Environmental Concerns (The Downside of Plastics)

The great stability that makes plastics useful also causes major environmental problems.

  • Non-Biodegradability: Poly(alkene)s (like poly(ethene)) have a strong, non-polar, continuous carbon-carbon single bond backbone. This structure is chemically inert and is not easily broken down by enzymes or microorganisms. They sit in landfills for centuries.
  • Harmful Combustion Products: When some addition polymers are burned (incinerated), they release toxic gases.
    Example: Burning PVC (Poly(chloroethene)) releases highly poisonous hydrogen chloride gas (\(\text{HCl}\)).
🔥 Quick Review: Addition Polymers

Addition polymers come from alkenes (monomers must have a C=C bond). They join head-to-tail, and nothing is lost. Environmental issue: they are inert and don't rot away.

2. Condensation Polymerisation (A Level Focus)

In condensation polymerisation, monomers join together by eliminating a small molecule, usually water (\(\text{H}_2\text{O}\)) or hydrogen chloride (\(\text{HCl}\)).

2.1 The Requirement for Condensation

For condensation to occur, each monomer must have two functional groups. These functional groups must be able to react with each other.

  • This is why we often need two different types of monomers (e.g., a diamine and a dicarboxylic acid).
  • Or, we can use a single monomer that contains both necessary functional groups (e.g., a single amino acid).

Memory Aid: Condensation = Con-D-N - meaning the monomers Conjoin and Drop a small Nuisance molecule (like water).

2.2 Polyesters (The Ester Link)

Polyesters are polymers containing the ester linkage (\( \text{-COO-}\)). They are typically formed in one of two ways:

A. Diol + Dicarboxylic Acid (or Dioyl Chloride)
  • Reactants: A diol (a molecule with two -OH groups) and a dicarboxylic acid (a molecule with two -COOH groups).
  • Reaction: The -OH group from the diol reacts with the -COOH group from the acid, forming an ester link and eliminating water (\(\text{H}_2\text{O}\)).
  • Better Reagent: Using a dioyl chloride (\(\text{RCOCl}\) at both ends) instead of the dicarboxylic acid is faster and more effective, eliminating hydrogen chloride (\(\text{HCl}\)) instead of water.
B. Hydroxycarboxylic Acid Monomer
  • Reactant: A single monomer containing both a hydroxyl group (-OH) and a carboxyl group (-COOH).
  • Reaction: One molecule's -OH reacts with the next molecule's -COOH.

2.3 Polyamides (The Amide/Peptide Link)

Polyamides are polymers containing the amide linkage (\( \text{-CONH-}\)). They are crucial in nature (proteins) and industry (Nylon).

A. Diamine + Dicarboxylic Acid (or Dioyl Chloride)
  • Reactants: A diamine (a molecule with two \(\text{-NH}_2\) groups) and a dicarboxylic acid (two \(\text{-COOH}\) groups).
  • Reaction: The \(\text{-NH}_2\) group from the amine reacts with the \(\text{-COOH}\) group from the acid, forming an amide link and eliminating water.
  • Better Reagent: Using a dioyl chloride eliminates \(\text{HCl}\).
B. Aminocarboxylic Acid Monomer (E.g., Amino Acids)
  • Reactant: A single monomer containing both an amino group (\(\text{-NH}_2\)) and a carboxyl group (\(\text{-COOH}\)).
  • Biological Connection: When amino acids polymerise, the resulting polyamide chains are called polypeptides or proteins. The amide link formed here is known as the peptide bond.
Step-by-Step Deduction of the Repeat Unit (Condensation)

You must identify where the small molecule (\(\text{H}_2\text{O}\) or \(\text{HCl}\)) is lost.

  1. Identify the two functional groups that react (e.g., -OH and -COOH).
  2. Mentally remove the atoms that form the small molecule (e.g., remove H from the OH group of the alcohol/amine, and OH or Cl from the carboxylic acid/acyl chloride).
  3. Draw the resultant bond linking the two monomer remnants.
  4. Enclose the linked structure (the repeat unit) in brackets, showing the two bonding sites extending outward, followed by the subscript \(n\).
✨ Did You Know?

Nylon-6,6 is a classic example of a polyamide made from two monomers: a diamine (6 carbons) and a dicarboxylic acid (6 carbons). This gives it the "6,6" designation!

3. Predicting Polymer Type (A Level Skill)

3.1 Predicting from Monomer Structure

To predict whether polymerisation will be addition or condensation, look at the monomer(s):

Rule 1: If the Monomer is an Alkene (contains C=C):
It undergoes Addition Polymerisation.
(Example: ethene, propene, chloroethene.)

Rule 2: If the Monomer(s) have Two Reactive Functional Groups (like -OH, -COOH, -NH₂, -COCl):
It undergoes Condensation Polymerisation (because a small molecule must be eliminated for bonding).
(Example: Diols and Dicarboxylic acids, or amino acids.)

3.2 Deduce the Monomer from the Polymer Structure

This is the reverse process:

  • If the polymer backbone is all C–C single bonds (like a saturated chain): It came from Addition Polymerisation. Put the double bond back between the two carbons of the repeat unit.
  • If the polymer contains ester (\( \text{-COO-}\)) or amide (\( \text{-CONH-}\)) links: It came from Condensation Polymerisation. Break the link and add back the small molecule (usually \(\text{H}_2\text{O}\) or \(\text{HCl}\)) to identify the original functional groups of the monomers.

4. The Fate of Polymers: Degradation (A Level Focus)

Given the scale of plastic waste, understanding how polymers break down is crucial.

4.1 Why Poly(alkene)s are a Problem

As discussed earlier, poly(alkene)s like polythene are chemically inert.

  • The carbon-carbon single bonds are very strong.
  • The structure is non-polar, so it is resistant to attack by polar reagents, acids, or bases (which are often required for breakdown).
  • This non-biodegradability means they persist in the environment.

4.2 Degradable Alternatives

While addition polymers rely on C-C bonds, condensation polymers offer a weakness that can be exploited: their linkages.

A. Biodegradation of Condensation Polymers

Polyesters (ester links) and polyamides (amide links) can be broken down because they are formed via condensation reactions, which are reversible.

  • Hydrolysis: They are biodegradable by acidic or alkaline hydrolysis. Hydrolysis is the chemical breakdown of a compound due to reaction with water.
  • In the environment, moisture, acids (like rainwater), or alkaline conditions (like in soil) can slowly break these ester or amide bonds back into the original monomers.

Example: A polyester film can be hydrolysed back into its diol and dicarboxylic acid monomers.

B. Photo-Degradable Polymers

Some polymers are designed to degrade when exposed to light.

  • These polymers contain additives that absorb UV radiation, triggering bond cleavage and chain fragmentation.
  • This helps break down the plastic into smaller pieces, though those smaller pieces (microplastics) still require further environmental breakdown.
🔑 Key Takeaway for Degradation

Addition polymers are tough due to the C-C backbone. Condensation polymers are generally more easily degraded (biodegradable) because their linking bonds (ester or amide) can be broken down by hydrolysis.