Welcome to the World of Polymers!
Hello future chemist! This chapter on Polymers might seem like it belongs in the plastics factory, and you'd be right! Polymers are incredibly large molecules (macromolecules) that are essential to modern life, forming everything from the nylon in your clothes to the DNA in your cells. Don't worry if the structures look huge—we only need to focus on the small, repeating parts. By breaking down these giants into their building blocks, you'll master this topic easily. Let's get started!
1. The Fundamentals: Monomers, Polymers, and Polymerisation
Before we look at the types of polymers, we need to understand the basic vocabulary:
Key Definitions
- Monomer: A small molecule that can be joined together repeatedly to form a long chain. (Think of a single LEGO brick.)
- Polymer: A long-chain molecule made up of many repeating monomer units. (Think of the completed LEGO model.)
- Polymerisation: The chemical reaction in which monomers join together to form a polymer.
2. Addition Polymers (Review from AS)
Addition polymerisation is typically encountered when studying alkenes, which is why we cover it in the AS curriculum. It's the simplest type of polymerisation.
2.1 Formation of Addition Polymers
Addition polymers are formed when monomers containing a carbon-carbon double bond (alkenes or substituted alkenes) join together.
The double bond breaks, and the resulting free bonding sites link up to form a long saturated chain. Crucially, no atoms are lost during this process—it’s just addition.
How to Draw the Repeating Unit (Step-by-Step)
- Start with the monomer (e.g., ethene: \({\rm CH}_2={\rm CH}_2\)).
- Remove the double bond and replace it with a single bond.
- Draw extension bonds (dashes) coming out of the carbons on either side to show where the chain continues.
Quick Review: Addition Polymerisation
- Monomers must contain a C=C double bond.
- The polymer chain is saturated (only single C-C bonds).
- The repeating unit is simply the monomer with the double bond replaced by extension bonds.
2.2 Structure, Reactivity, and Properties
Addition polymers, like poly(ethene) or poly(propene), are essentially very long alkane chains.
Why Addition Polymers are Unreactive
Addition polymers are famously chemically inert (unreactive). This is why they are so useful for packaging, but also why they cause disposal problems (see Section 4).
The reason for their unreactivity lies in their structure:
- They consist of strong, non-polar C-C and C-H single bonds.
- There are no functional groups or polar bonds to attract nucleophiles (like water or enzymes).
Intermolecular Forces in Polyalkenes
Polyalkenes (like PVC or poly(ethene)) are non-polar molecules. The forces holding the polymer chains together are primarily induced dipole–dipole forces (also known as van der Waals or London dispersion forces).
The longer the polymer chain, the more contact area there is between chains, and therefore the stronger these van der Waals forces are, leading to higher melting points and greater strength.
Example: Poly(chloroethene) (PVC)
PVC is a common addition polymer. Its properties can be dramatically changed by adding substances called plasticisers.
- Unplasticised PVC (rigid): Used for window frames and pipes. The chains are held tightly by strong van der Waals forces.
- Plasticised PVC (flexible): A plasticiser is added, which fits between the polymer chains. This pushes the chains further apart, weakening the intermolecular forces and allowing the chains to slide over each other more easily, making the material softer and more flexible (e.g., electrical cable insulation).
3. Condensation Polymers (A2 Content)
Condensation polymerisation is more complex than addition polymerisation because it involves the reaction between two different functional groups, leading to the loss of a small molecule, usually water.
3.1 Formation and Linkages
Condensation polymers are formed by linking molecules (monomers) where each monomer has at least two functional groups (one at each end). When the groups react, a small molecule (like \({\rm H}_2{\rm O}\) or \({\rm HCl}\)) is eliminated (hence "condensation").
3.1.1 Polyesters (Ester Linkage)
Polyesters are formed from the reaction between dicarboxylic acids (or diacyl chlorides) and diols.
The linkage formed is the ester linkage, \({\rm -COO-}\).
Example: Terylene (Poly(ethylene terephthalate) or PET) is a polyester used in clothing and drink bottles.
3.1.2 Polyamides (Amide Linkage)
Polyamides are formed from the reaction between dicarboxylic acids (or diacyl chlorides) and diamines.
The linkage formed is the amide linkage (or peptide link), \({\rm -CONH-}\).
Example: Nylon 6,6 and Kevlar are polyamides.
3.1.3 Amino Acid Polymers (Polypeptides)
Amino acids are monomers that contain both an acid group (${\rm -COOH}$) and an amine group (${\rm -NH}_2$). They can react with themselves to form polymers.
The linkage is also the amide linkage, referred to as the peptide link in biological contexts.
3.2 Intermolecular Forces in Condensation Polymers
Condensation polymers are generally much stronger than simple polyalkenes because their polar linkages allow for stronger intermolecular forces.
- Polyamides (Nylon, Kevlar): The amide linkage, \({\rm -CONH-}\), contains N-H and C=O groups. These allow for strong hydrogen bonding between adjacent polymer chains. This strong bonding gives polyamides immense strength (e.g., Kevlar is used in bulletproof vests).
- Polyesters (Terylene): The ester linkage, \({\rm -COO-}\), allows for permanent dipole-dipole forces and van der Waals forces, but typically does not form hydrogen bonds (as there is no N-H or O-H group directly involved in the link).
Key Takeaway for Condensation Polymers: They are formed by joining two different functional groups and losing a small molecule. Polyamides are especially strong due to hydrogen bonding.
4. Biodegradability and Disposal of Polymers
This section looks at the important differences in how we dispose of these materials.
4.1 Polyalkenes: Inert and Difficult to Dispose Of
As discussed earlier, polyalkenes (addition polymers) are chemically inert and non-biodegradable.
Why Polyalkenes Resist Breakdown
Microorganisms (bacteria and fungi) break down organic material using enzymes. However, polyalkenes only contain strong, non-polar C-C single bonds. Enzymes cannot efficiently catalyse the breaking of these bonds, meaning the plastic remains intact for hundreds of years.
4.2 Condensation Polymers: Hydrolysed and Biodegradable
Polyesters and polyamides are biodegradable, although the process can sometimes be slow.
Why Condensation Polymers Can Be Broken Down
The key difference is the presence of the ester (\({\rm -COO-}\)) or amide (\({\rm -CONH-}\)) linkages. These linkages are polar and susceptible to attack by polar molecules like water, a reaction known as hydrolysis.
Hydrolysis breaks the polymer back down into its original monomers (or their salts) by inserting a water molecule across the bond.
In the environment, organisms possess enzymes that can catalyse this hydrolysis reaction, breaking the polymer into smaller molecules that can be absorbed or further degraded.
4.3 Polymer Disposal: Recycling, Benefits, and Drawbacks
The syllabus requires you to understand the advantages and disadvantages of different disposal methods, particularly recycling.
Recycling of Polymers
Advantages:
- Conserves finite raw materials (crude oil).
- Reduces the amount of waste sent to landfill sites.
- Reduces emissions from incineration.
Disadvantages:
- Collecting, sorting, and cleaning plastics is expensive and energy-intensive.
- Many mixed plastics cannot be recycled together because different polymers melt at different temperatures.
- Recycled polymers are often of lower quality than the original (down-cycling).
Do NOT confuse hydrolysis (breaking a condensation polymer using water) with the chemical inertness of polyalkenes (addition polymers). Polyalkenes are alkane chains and are resistant to almost everything!
Key Takeaway: Addition polymers are inert and hard to break down. Condensation polymers have reactive linkages (ester/amide) that can be hydrolysed, making them biodegradable.
Chapter Summary Review
You've just tackled some macromolecules! Remember these core concepts:
- Addition Polymers (Polyalkenes) are formed by opening C=C bonds. They are saturated, non-polar, and unreactive (inert/non-biodegradable).
- Condensation Polymers (Polyamides, Polyesters) are formed by joining two different functional groups and losing a small molecule (like water).
- Condensation Linkages: Polyamides use the Amide link (\({\rm -CONH-}\)); Polyesters use the Ester link (\({\rm -COO-}\)).
- Strength: Polyamides are very strong due to hydrogen bonding between chains.
- Disposal: Condensation polymers can be broken down by hydrolysis, making them biodegradable, unlike the chemically inert polyalkenes.
Keep practicing drawing those repeating units—it’s the most common exam challenge in this topic! You've got this!