Welcome to Polymers!
Welcome to this chapter of A2 2: Organic Chemistry. Polymers are all around us—from the plastic bottles we drink from to bulletproof vests and synthetic clothing fibres. Understanding how these massive molecules are made, represented, and disposed of is not only essential for your exam success, but also gives you great insight into materials science and environmental sustainability.
Don't worry if organic synthesis seems intimidating at first. We will break down every mechanism, repeat unit, and reaction step by step so you feel confident answering any exam question!
Quick Analogy: Think of a polymer like a long train. Each individual carriage is a monomer. When hundreds or thousands of carriages couple together, you get the entire train—the polymer. The smallest repeating pattern of carriages along the train is the repeat unit.
1. Core Definitions
Before diving into reactions, let's nail down the three fundamental terms you must be able to define accurately:
Monomer: A small reactive molecule capable of chemically bonding to other monomers to form a macromolecule.
Polymer: A large molecule (macromolecule) composed of many repeating structural units derived from monomers.
Repeat Unit: The specific arrangement of atoms and bonds that repeats continuously along the polymer backbone chain.
2. Addition Polymerisation
Addition polymerisation happens when unsaturated monomers (molecules containing a carbon-carbon double bond, \( \text{C=C} \)) react together.
How Addition Polymerisation Works
The Mechanism: Under suitable conditions, the weaker \( \pi \)-bond in the \( \text{C=C} \) double bond opens up. Each carbon atom can now form a new single covalent bond to a neighbouring monomer unit. This links thousands of monomers together into a long, saturated carbon chain.
Crucial Rule: In addition polymerisation, no atoms are lost and no secondary byproducts are formed. The polymer is the sole product.
How to Draw an Addition Polymer Repeat Unit
Examiners look for three specific things when you draw an addition polymer:
Step 1: Change the central \( \text{C=C} \) double bond of the monomer into a single \( \text{C-C} \) bond in the polymer backbone.
Step 2: Draw single covalent bonds extending outwards horizontally from the carbons on both ends.
Step 3: Place square brackets around the unit so that the extension bonds pass straight through the brackets. Add a subscript \( n \) at the bottom right outside the brackets to represent the repeating nature: \( [-(\text{CH}_2-\text{CHR})-]_n \).
Key Addition Polymers to Know
1. Poly(ethene): Formed from ethene monomers (\( \text{CH}_2\text{=CH}_2 \)). It gives a repeat unit of \( -[\text{CH}_2-\text{CH}_2]_n- \).
2. Poly(propene): Formed from propene (\( \text{CH}_3\text{CH=CH}_2 \)). Its repeat unit is \( -[\text{CH}_2-\text{CH}(\text{CH}_3)]_n- \).
3. Poly(chloroethene) (PVC): Formed from chloroethene (\( \text{CH}_2\text{=CHCl} \)). Its repeat unit is \( -[\text{CH}_2-\text{CH(Cl)}]_n- \).
4. Poly(phenylethene) (Polystyrene): Formed from phenylethene (\( \text{CH}_2\text{=CH(C}_6\text{H}_5) \)). Its repeat unit contains a benzene ring pendant group along the backbone chain.
5. Poly(tetrafluoroethene) (PTFE): Formed from tetrafluoroethene (\( \text{CF}_2\text{=CF}_2 \)). Its repeat unit is \( -[\text{CF}_2-\text{CF}_2]_n- \).
Key Takeaway for Addition Polymers: Look for the \( \text{C=C} \) bond in the monomer. Break it to a single bond, draw extension bonds through square brackets, and write \( n \). Never leave a double bond in the backbone of an addition polymer!
3. Condensation Polymerisation
Unlike addition polymerisation, condensation polymerisation involves monomers with two reactive functional groups (bifunctional molecules) joining together with the elimination of a small molecule (usually water, \( \text{H}_2\text{O} \), or hydrogen chloride, \( \text{HCl} \)).
A. Polyesters
Polyesters are formed when dicarboxylic acids (or diacyl chlorides) react with diols.
The Reaction: A carboxylic acid group (\( -\text{COOH} \)) reacts with an alcohol group (\( -\text{OH} \)) to form an ester link (\( -\text{COO}- \) or \( -\text{C}(=\text{O})-\text{O}- \)), releasing a molecule of \( \text{H}_2\text{O} \).
Key Example: PET (Polyethylene terephthalate)
PET is made from two monomers:
1. Benzene-1,4-dicarboxylic acid (terephthalic acid): Contains a benzene ring with \( -\text{COOH} \) groups at positions 1 and 4.
2. Ethane-1,2-diol: \( \text{HO-CH}_2\text{-CH}_2\text{-OH} \).
When they react, the \( -\text{OH} \) is lost from the acid and the \( -\text{H} \) is lost from the diol, joining them via ester links and eliminating \( \text{H}_2\text{O} \).
B. Polyamides
Polyamides are formed when dicarboxylic acids (or diacyl chlorides) react with diamines (or from amino acids that contain both an amine and a carboxylic acid group).
The Reaction: A carboxylic acid group (\( -\text{COOH} \)) reacts with an amine group (\( -\text{NH}_2 \)) to form an amide link (or peptide bond: \( -\text{CONH}- \) or \( -\text{C}(=\text{O})-\text{NH}- \)), releasing a molecule of \( \text{H}_2\text{O} \).
Key Example 1: Nylon-6,6
Nylon-6,6 gets its name because both of its reacting monomers contain 6 carbon atoms:
1. Hexanedioic acid: \( \text{HOOC}(\text{CH}_2)_4\text{COOH} \)
2. Hexane-1,6-diamine: \( \text{H}_2\text{N}(\text{CH}_2)_6\text{NH}_2 \)
When combined, they form repeating units joined by amide links (\( -\text{CONH}- \)), eliminating \( \text{H}_2\text{O} \) molecules.
Key Example 2: Kevlar
Kevlar is an exceptionally strong, rigid polyamide used in bulletproof vests and lightweight protective gear. It is synthesised from:
1. Benzene-1,4-dicarboxylic acid (terephthalic acid)
2. Benzene-1,4-diamine (1,4-diaminobenzene)
The rigid benzene rings in both monomers allow the polymer chains to pack tightly with strong intermolecular hydrogen bonding between adjacent chains, giving Kevlar its incredible tensile strength.
Memory Trick for Condensation Polymers:
"Condensation creates a droplet!" Remember that every time an ester or amide linkage is formed between two monomer residues, a small molecule (like \( \text{H}_2\text{O} \) or \( \text{HCl} \)) drops out.
Key Takeaway for Condensation Polymers: Polyesters contain the \( -\text{COO}- \) linkage (formed from diacid + diol). Polyamides contain the \( -\text{CONH}- \) linkage (formed from diacid + diamine). Both release a small molecule such as \( \text{H}_2\text{O} \) during formation.
4. Waste Management & Sustainability
Because synthetic polymers are durable and resistant to chemical attack, their disposal presents major environmental challenges. Life and Health Sciences covers three primary routes for handling polymer waste:
1. Incineration (Energy Recovery)
Process: Controlled combustion of polymer waste at high temperatures to generate thermal energy, which is used to produce electricity.
Environmental Consideration: Burning certain plastics releases toxic, hazardous gases. For example, incinerating chlorinated polymers such as poly(chloroethene) (PVC) releases acidic hydrogen chloride gas (\( \text{HCl} \)). Modern incinerators must use chemical flue-gas scrubbers to neutralize and remove \( \text{HCl} \) before releasing exhaust gases into the atmosphere.
2. Mechanical Recycling
Process: Waste plastics are collected, sorted by polymer type, cleaned, melted down, and remoulded into new plastic products.
Benefits: Reduces the volume of waste sent to landfill and conserves non-renewable crude oil reserves by decreasing the demand for newly synthesised virgin polymer.
3. Chemical / Feedstock Recycling
Process: Polymer waste undergoes thermal or catalytic cracking to break the long polymer backbones down into original monomers or short-chain hydrocarbon fractions.
Benefits: These recovered hydrocarbon fractions can be fed directly back into petrochemical refineries as raw feedstocks, effectively creating a closed-loop chemical cycle.
5. Common Pitfalls and Examiner-Reported Errors
Avoid these frequent mistakes in your examinations:
1. Missing Extension Bonds: When drawing repeat units, always ensure the single bonds pass completely through the square brackets on both the left and right sides. Without these open trailing bonds, it looks like a separate small molecule rather than a repeating unit of a long chain.
2. Keeping Double Bonds in Addition Polymers: Double bonds (\( \text{C=C} \)) are present in the monomer, but must never remain in the backbone of an addition polymer. The \( \pi \)-bond opens up to become single bonds.
3. Incomplete Repeat Units in Condensation Polymers: When two different monomers react (such as a diacid and a diamine), one full repeat unit must include the residue of both monomers joined together by the link, not just one half.
4. Forgetting the Eliminated Byproduct: In equations for condensation polymerisation, remember to balance the small byproduct molecules produced (e.g., \( + (2n - 1)\text{H}_2\text{O} \) or simplified as \( + n\text{H}_2\text{O} \)).
Quick Review Summary
Addition Polymerisation: Unsaturated monomers (\( \text{C=C} \)) link by breaking \( \pi \)-bonds. No atoms lost. Examples: Poly(ethene), PVC, PTFE, Polystyrene, Poly(propene).
Polyesters: Diacid + Diol \( \rightarrow \) Polyester (\( -\text{COO}- \)) + \( \text{H}_2\text{O} \). Example: PET.
Polyamides: Diacid + Diamine \( \rightarrow \) Polyamide (\( -\text{CONH}- \)) + \( \text{H}_2\text{O} \). Examples: Nylon-6,6 and Kevlar.
Sustainability: Incineration (needs scrubbers for \( \text{HCl} \)), Mechanical recycling (sort, wash, melt, remould), and Feedstock recycling (cracking polymers back into raw chemicals).