Chemistry (0620) | Polymers

Welcome to the World of Polymers!

Hello future chemists! This chapter is all about giant molecules called polymers. You encounter polymers every single day—they make up your plastic bottles, the fibres in your clothes, and even the proteins in your body!
Learning about polymers helps us understand why plastics are so useful, yet also why they cause huge environmental challenges.

Quick Definitions (The Building Blocks)

Imagine chemistry using tiny, repeated building blocks.

• Monomer: The small, simple molecule that acts as the basic unit (like a single LEGO brick).
• Polymer: A very large molecule (a macromolecule) built up from many thousands of smaller molecules (monomers) linked together in a long chain (like a whole LEGO structure). (Core 11.8.1)
• Polymerisation: The chemical reaction where monomers join together to form a polymer.

Memory Aid: Think "Poly" (meaning many) and "Mono" (meaning one).

Section 1: Addition Polymerisation

The Basics of Addition

Addition polymerisation is the simpler of the two types. It happens when monomers containing a Carbon-Carbon double bond (\(C=C\)) join together.

The key characteristic is that no other product is formed—the polymer is the only product (Supplement 11.8.5).

Analogy: Imagine a long line of people holding hands. Addition polymerisation is like everyone suddenly grabbing the hands of the people next to them to form one giant chain. No one dropped anything!

Example: Poly(ethene) (Polythene)

The most common example you need to know is the formation of poly(ethene), which is used to make plastic bags and bottles. (Core 11.8.2)

The monomer is ethene (\(C_2H_4\)), which is an alkene (unsaturated hydrocarbon).

Step-by-Step Process:

1. The ethene monomer has a \(C=C\) double bond.
2. During polymerisation, the double bond breaks open (or "opens up").
3. The carbon atoms can then form single bonds with the next monomer units, linking into a long chain.

Chemical Representation:

The monomer ethene:

      H   H
      |   |
    C = C
      |   |
      H   H
    

The polymer poly(ethene):

        H   H
        |   |
      [- C - C -]n
        |   |
        H   H
    

(The ‘n’ indicates that there are ‘many’ repeat units joined together.)

Extended: Identifying Repeat Units (Supplement 11.8.6, 11.8.7)

The repeat unit is the smallest section of the polymer chain that, when repeated, makes the entire polymer.

• To find the repeat unit of an addition polymer, look at the monomer (the alkene).
• The repeat unit looks exactly like the monomer, but the double bond is replaced by a single bond, and it has "connecting arms" sticking out to show it joins to the next unit.

Section 2: Condensation Polymerisation (Extended Syllabus)

Don't worry if this seems tricky at first! Condensation polymerisation is slightly more complex than addition, but you only need to focus on two key features: the reactants and the linkage formed.

The Basics of Condensation

Condensation polymerisation occurs when monomers join together by losing a small molecule, usually water (\(H_2O\)). (Supplement 11.8.9)

The monomers needed for this reaction must each have two functional groups (one on each end), which react together to form a “linkage” and release water.

Key Difference (Supplement 11.8.9)

• Addition: Monomers have C=C, one product.
• Condensation: Monomers have two functional groups, polymer + small molecule (e.g., water) are produced.

Type 1: Polyamides (e.g., Nylon)

Polyamides are formed by reacting two types of monomers: (Supplement 11.8.8a)

1. A dicarboxylic acid (a molecule with -COOH groups at both ends).
2. A diamine (a molecule with -NH₂ groups at both ends).

When these two meet, the \(-OH\) from the acid and the \(-H\) from the amine are removed, forming \(H_2O\), and creating a bond called the amide linkage.

The Amide Linkage: \(-\boldsymbol{C}(=O)\boldsymbol{-N(H)}-\)

Structure of Nylon (A synthetic polyamide): (Supplement 11.8.10a)

      O       H         O       H
      ||      |         ||      |
    [- C - R - C - N - R' - N -]n
                                 (R and R' are carbon chains)
    

Did you know? Nylon was originally developed as a synthetic replacement for silk for use in parachutes and stockings!

Type 2: Polyesters (e.g., PET)

Polyesters are formed by reacting two types of monomers: (Supplement 11.8.8b)

1. A dicarboxylic acid (a molecule with -COOH groups at both ends).
2. A diol (an alcohol molecule with -OH groups at both ends).

When these react, the \(-OH\) from the acid and the \(-H\) from the alcohol are removed, forming \(H_2O\), and creating a bond called the ester linkage.

The Ester Linkage: \(-\boldsymbol{C}(=O)\boldsymbol{-O}-\)

Structure of PET (Polyethylene terephthalate, a synthetic polyester): (Supplement 11.8.10b)

          O         O
          ||        ||
    [- R - C - O - R' - O -]n
                                 (R and R' are carbon chains)
    

Type 3: Natural Polyamides (Proteins) (Extended Syllabus)

Nature uses condensation polymerisation too!

Proteins are natural polyamides. (Supplement 11.8.12)

• Monomers: Amino acids.
• Amino acids are special because they contain both a carboxylic acid group (-COOH) and an amine group (-NH₂). They can react with themselves!
• The linkage formed is the amide linkage (also called the peptide bond in biology).

General Structure of an Amino Acid Monomer: (Supplement 11.8.12)

        R
        |
      H - N - C - C - O - H
          |   |   ||
          H   H   O
    

(R represents the side-chain which differs depending on the specific amino acid.)

Structure of Proteins (The Polyamide Chain): (Supplement 11.8.13)

When two amino acids join, they lose \(H_2O\) and form the amide linkage:

        H    O   H
        |    ||  |
    [- N - C - C - N -]n
          |
          R (side chain)
    

Section 3: Plastics and the Environment

Plastics are polymers. Their properties make them incredibly useful, but these same properties pose serious challenges when we dispose of them. (Core 11.8.3, 11.8.4)

A. Environmental Challenges (Core 11.8.5)

1. Disposal in Landfill Sites (Core 11.8.5a)

Most synthetic polymers (plastics) are non-biodegradable. This means they are not broken down by bacteria and micro-organisms in the soil.

• When dumped in landfill sites, they take up huge amounts of space and remain there for hundreds or thousands of years.

2. Accumulation in Oceans (Core 11.8.5b)

Because plastics are stable and non-biodegradable, large quantities of waste plastics end up floating in oceans.

• This harms marine life (e.g., animals may mistake plastic for food and choke).
• The plastic breaks down into tiny pieces (microplastics), which can enter the food chain, potentially affecting human health.

3. Formation of Toxic Gases from Burning (Core 11.8.5c)

Burning plastics (incineration) seems like an easy solution, but it can create serious pollution.

• Plastics often contain elements like Chlorine (Cl). When materials like poly(chloroethene) (PVC) are burned, they release toxic gases, such as hydrogen chloride gas, which contributes to acid rain and respiratory problems.
• Burning plastics also releases greenhouse gases like \(CO_2\).

B. Solutions and Recycling (Extended Syllabus)

To combat these issues, we need to think about how to reuse or break down polymers.

Recycling PET (Supplement 11.8.11)

Some modern recycling techniques can take condensation polymers, like PET (polyester), and convert them back into their original monomers.

• This process is called depolymerisation.
• Once they are monomers, they can be purified and used to make new, high-quality polymers. This is known as chemical recycling and is often better than just melting and reforming the plastic, which can degrade its quality.

Key Takeaway: Environmental Implications

The chemical stability (non-biodegradability) of most plastics is both their biggest advantage (they last forever!) and their biggest environmental problem. We must manage their disposal carefully.