Welcome to Topic 5.10: Polymer Chemistry

Welcome to one of the most practical and high-scoring topics in CCEA A2 Unit 2 Chemistry! Look around you right now: the clothes you wear, the water bottle on your desk, the screen casing of your phone, and even the non-stick coating in your frying pan are all made from polymers. Don't worry if organic chemistry sometimes feels overwhelming—this topic follows very neat, logical patterns. Master a few drawing rules and functional group reactions, and you will secure full marks on these questions in your exam!

1. Fundamentals: What is a Polymer?

Let's break down the basic language used across this entire topic:

  • Monomer: A small molecule that can combine with other small molecules to form a polymer. Think of a monomer like an individual paperclip.
  • Polymer: A large molecule (macromolecule) built up from many repeating units linked by covalent bonds. Think of the polymer as a long paperclip chain.
  • Repeating Unit: The specific atomic arrangement in a polymer chain that repeats continually and is bounded by open end bonds passing through brackets.

In A2 Chemistry, there are two distinct types of polymerisation you must master:

  1. Addition Polymerisation: Monomers containing unsaturated \( \text{C=C} \) double bonds join together without forming any other products.
  2. Condensation Polymerisation: Monomers join together with the simultaneous elimination of small molecules, such as \( \text{H}_2\text{O} \) or \( \text{HCl} \).

Key Takeaway: Addition polymerisation makes one single product (100% atom economy). Condensation polymerisation makes two products: the polymer plus a small eliminated molecule like \( \text{H}_2\text{O} \) or \( \text{HCl} \).

2. Addition Polymers

Addition polymers are formed from substituted alkenes (\( \text{CH}_2=\text{CH-R} \)). During the reaction, the weak \( \pi \)-bond in the \( \text{C=C} \) double bond breaks, allowing the carbons to form strong \( \sigma \)-bonds to adjacent monomers.

Key CCEA Addition Polymers to Memorise

  • Poly(ethene): Formed from ethene (\( \text{CH}_2=\text{CH}_2 \)).
  • Poly(propene): Formed from propene (\( \text{CH}_2=\text{CH}-\text{CH}_3 \)).
  • Poly(chloroethene) / Polyvinyl chloride (PVC): Formed from chloroethene (\( \text{CH}_2=\text{CHCl} \)).
  • Poly(phenylethene) / Polystyrene: Formed from phenylethene (\( \text{C}_6\text{H}_5\text{CH}=\text{CH}_2 \)).
  • Poly(tetrafluoroethene) (PTFE / Teflon): Formed from tetrafluoroethene (\( \text{CF}_2=\text{CF}_2 \)).

How to Draw Repeating Units for Addition Polymers

Follow this simple 3-step foolproof method:

  1. Draw a 2-carbon backbone: Draw two carbon atoms connected by a single bond: \( -\text{C}-\text{C}- \).
  2. Attach side groups vertically: Arrange all 4 attached groups pointing straight up and straight down. (For example, in poly(propene), one carbon has two \( -\text{H} \) atoms, and the second carbon has one \( -\text{H} \) and one \( -\text{CH}_3 \) group).
  3. Add brackets and open bonds: Extend the horizontal bonds through the brackets and write the subscript \( n \) outside the bracket.

Examiner Warning: Never draw a \( \text{C=C} \) double bond in the backbone of an addition polymer repeating unit! The double bond breaks during polymerisation.

Key Takeaway: Addition polymer repeating units always have a 2-carbon backbone with a single \( \text{C–C} \) bond, side groups attached vertically, and bonds extending through brackets.

3. Condensation Polymers: Polyamides

Condensation polymerisation occurs when monomers with two functional groups react together, kicking out a small molecule every time a new link is formed.

Polyamides are joined by an amide linkage (also called a peptide link): \( -\text{CONH}- \) or \( -\text{C}(=\text{O})-\text{NH}- \).

A. Nylon-6,6

  • Monomers: 1,6-diaminohexane (\( \text{H}_2\text{N}(\text{CH}_2)_6\text{NH}_2 \)) and hexanedioic acid (\( \text{HOOC}(\text{CH}_2)_4\text{COOH} \)) or hexanedioyl dichloride (\( \text{ClOC}(\text{CH}_2)_4\text{COCl} \)).
  • Small molecule eliminated: \( \text{H}_2\text{O} \) (if using the dicarboxylic acid) or \( \text{HCl} \) (if using the diacyl chloride).
  • Repeating Unit: \( \text{⁅–NH–(CH}_2\text{)}_6\text{–NH–CO–(CH}_2\text{)}_4\text{–CO–⁆} \)
  • Memory Trick: Why is it called "6,6"? Because there are 6 carbons in the diamine and 6 carbons in the dicarboxylic acid!

B. Kevlar

  • Monomers: Benzene-1,4-diamine (1,4-diaminobenzene) and benzene-1,4-dicarboxylic acid (terephthalic acid) [or benzene-1,4-dioyl dichloride].
  • Repeating Unit: \( \text{⁅–NH–C}_6\text{H}_4\text{–NH–CO–C}_6\text{H}_4\text{–CO–⁆} \)
  • Properties and Uses: Used in bulletproof vests, helmets, and tyre reinforcement.
  • Why is Kevlar so strong? Kevlar chains are rigid and planar due to the aromatic benzene rings. The chains pack closely together, allowing extensive intermolecular hydrogen bonding between \( \text{C=O} \) and \( \text{N-H} \) groups on neighbouring chains.

C. Nylon-6

  • Monomer: 6-aminohexanoic acid (or caprolactam via ring-opening).
  • Because this monomer has both an amine group and a carboxylic acid group on the same molecule, it polymerises with itself!

Key Takeaway: Polyamides contain \( -\text{CONH}- \) linkages. Nylon-6,6 uses two aliphatic 6-carbon monomers, while Kevlar uses rigid aromatic 1,4-substituted monomers that form strong hydrogen-bonded sheets.

4. Condensation Polymers: Polyesters

Polyesters are joined by an ester linkage: \( -\text{COO}- \) or \( -\text{C}(=\text{O})-\text{O}- \).

A. PET (Polyethylene terephthalate / Terylene / Dacron)

  • Monomers: Ethane-1,2-diol (\( \text{HO–CH}_2\text{CH}_2\text{–OH} \)) and benzene-1,4-dicarboxylic acid (\( \text{HOOC–C}_6\text{H}_4\text{–COOH} \)).
  • Repeating Unit: \( \text{⁅–O–CH}_2\text{CH}_2\text{–O–CO–C}_6\text{H}_4\text{–CO–⁆} \)
  • Small molecule eliminated: \( \text{H}_2\text{O} \)
  • Uses: Plastic beverage bottles and clothing fibres.

B. Polylactic Acid (PLA)

  • Monomer: 2-hydroxypropanoic acid (lactic acid, \( \text{CH}_3\text{CH(OH)COOH} \)).
  • Repeating Unit: \( \text{⁅–O–CH(CH}_3\text{)–CO–⁆} \)
  • Nature: PLA is a renewable, biodegradable, and compostable thermoplastic polyester derived from plant starches.

Key Takeaway: Polyesters contain \( -\text{COO}- \) linkages. PET is made from a diol and a dicarboxylic acid, whereas PLA is formed from a single hydroxy-carboxylic acid monomer.

5. Biodegradability and Hydrolysis of Polymers

Why do some plastics pollute the earth for centuries while others break down naturally? The answer lies in their chemical bonds!

Addition vs Condensation Polymers

  • Addition Polymers: The polymer backbone consists entirely of non-polar \( \text{C–C} \) single bonds. These bonds are chemically inert and resistant to attack by acids, alkalis, and biological enzymes. Therefore, addition polymers are non-biodegradable.
  • Condensation Polymers: Contain polar \( \text{C=O} \), \( -\text{COO}- \), and \( -\text{CONH}- \) linkages. The partially positive carbonyl carbon (\( \text{C}^{\delta+} \)) is susceptible to nucleophilic attack by water molecules and enzymes. Therefore, condensation polymers are biodegradable via hydrolysis.

Hydrolysis Reactions: Acid vs Alkaline Conditions

Hydrolysis means breaking bonds using water. In the exam, you must predict the exact ionic forms of the products depending on whether conditions are acidic or alkaline.

1. Acid Hydrolysis (Reflux with aqueous acid, e.g., \( \text{HCl}\text{(aq)} \) or \( \text{H}_2\text{SO}_4\text{(aq)} \))
  • Polyester: Breaks into the original diol + dicarboxylic acid.
  • Polyamide: Breaks into the dicarboxylic acid + diammonium dication salt (e.g., \( {}^+\text{H}_3\text{N-R-NH}_3^+ \)).
    Why? In acidic conditions, the amine groups act as bases and accept protons (\( \text{H}^+ \)) to form ammonium ions!
2. Alkaline Hydrolysis (Reflux with aqueous alkali, e.g., \( \text{NaOH}\text{(aq)} \))
  • Polyester: Breaks into the original diol + dicarboxylate salt (e.g., \( -\text{COO}^-\text{Na}^+ \) or \( -\text{COO}^- \)).
    Why? In basic conditions, the carboxylic acid reacts with \( \text{OH}^- \) to form a carboxylate salt.
  • Polyamide: Breaks into the original diamine + dicarboxylate salt (e.g., \( -\text{COO}^-\text{Na}^+ \)).

Quick Summary Table: Hydrolysis Products

Polymer Type Acid Hydrolysis (\( \text{H}^+\text{/H}_2\text{O} \)) Alkaline Hydrolysis (\( \text{OH}^-\text{/H}_2\text{O} \))
Polyester Diol + Dicarboxylic acid Diol + Dicarboxylate salt (\( -\text{COO}^- \))
Polyamide Diammonium ion (\( -\text{NH}_3^+ \)) + Dicarboxylic acid Diamine (\( -\text{NH}_2 \)) + Dicarboxylate salt (\( -\text{COO}^- \))

Key Takeaway: Non-polar \( \text{C–C} \) chains do not hydrolyse. Polar ester and amide links do hydrolyse. Remember: Acid protonates amines to \( -\text{NH}_3^+ \); Alkali deprotonates acids to \( -\text{COO}^- \).

6. Disposal and Environmental Considerations

Because synthetic polymers are manufactured in massive quantities, disposing of polymer waste is a major environmental challenge:

  1. Landfill:
    • Addition polymers do not biodegrade and remain in landfill sites indefinitely.
    • Takes up finite land space and creates long-term environmental hazards.
  2. Incineration (Combustion for Energy):
    • Polymers can be burned to generate electricity, reducing the volume of waste.
    • Environmental Disadvantage: Burning carbon-based polymers releases greenhouse gases such as \( \text{CO}_2 \).
    • Hazardous Emissions: Burning halogenated polymers like PVC (poly(chloroethene)) produces toxic, corrosive hydrogen chloride (\( \text{HCl} \)) gas and toxic dioxins.
    • Solution: Incinerators must use flue-gas scrubbers containing basic substances (e.g., \( \text{CaO} \) or \( \text{Ca(OH)}_2 \)) to neutralise acidic gases like \( \text{HCl} \).
  3. Recycling:
    • Polymers are sorted, melted, and remoulded into new items (for thermoplastics) or cracked chemically back into monomer feedstocks.
    • Conserves non-renewable petroleum crude oil and reduces landfill volume.

Key Takeaway: Landfills consume space due to non-biodegradable plastics; incineration generates energy but releases \( \text{CO}_2 \) and requires base scrubbers (like \( \text{CaO} \)) to remove toxic \( \text{HCl} \) from chlorinated plastics like PVC.

7. Top Exam Pitfalls to Avoid

  • Missing Open End Bonds: When drawing repeating units, ensure the single bonds pass completely through the brackets. If they stop inside the brackets, you will lose the mark!
  • Leaving Double Bonds in Addition Polymers: Always ensure the backbone has only single \( \text{C–C} \) bonds.
  • Forgetting Charges in Hydrolysis:
    • Under acidic conditions, polyamides produce protonated ammonium groups: \( -\text{NH}_3^+ \) (not neutral \( -\text{NH}_2 \)).
    • Under alkaline conditions, both polyamides and polyesters produce carboxylate salt groups: \( -\text{COO}^- \) (not neutral \( -\text{COOH} \)).
  • Count Repeat Units Carefully: Read the question prompt! Check whether it asks for one repeating unit or two. For Nylon-6,6 or PET, one repeat unit contains one residue from each of the two monomers.