Introduction to Polymers
Welcome to the world of polymers! You might not realize it, but polymers are everywhere—from the plastic bottle on your desk to the DNA inside your body. In this chapter, we will explore how these massive molecules are made, why they are so useful, and how we can deal with them once we are finished using them. Don't worry if the structures look a bit strange at first; once you see the patterns, it becomes much easier!
What is a Polymer?
A polymer is a substance with a high average relative formula mass. They are made up of very large molecules consisting of many repeating units.
Think of a polymer like a long train. Each individual train carriage is a small molecule called a monomer. When you join hundreds or thousands of these carriages together, you get the train, which represents the polymer.
Key Takeaway: Monomers are the small "building block" molecules, and polymers are the long chains created when they join together.
Addition Polymerisation
In addition polymerisation, many small molecules (monomers) join together to form one very large molecule (the polymer). For this to happen, the monomers must have a double carbon-carbon bond \( (C=C) \). These monomers are called alkenes.
During the reaction, the double bond "opens up" and allows the carbon atoms to bond to the next monomer in the chain. No other products are made; you just get the long polymer chain.
Common Addition Polymers and Their Uses
The name of a polymer is easy to figure out: just put "poly" in front of the monomer name in brackets. Here are the ones you need to know for your exam:
1. Poly(ethene)
Made from: Ethene monomers \( (C_2H_4) \).
Properties: Flexible, cheap, and a good electrical insulator.
Uses: Plastic bags, plastic bottles, and cling film.
2. Poly(propene)
Made from: Propene monomers \( (C_3H_6) \).
Properties: Flexible, strong, tough, and heat resistant.
Uses: Buckets, crates, and ropes.
3. Poly(chloroethene) - also known as PVC
Made from: Chloroethene monomers.
Properties: Tough, cheap, and long-lasting.
Uses: Window frames, water pipes, and insulation for electrical wires.
4. Poly(tetrafluoroethene) - also known as PTFE or Teflon
Made from: Tetrafluoroethene monomers \( (C_2F_4) \).
Properties: Very unreactive, non-stick, and resistant to high temperatures.
Uses: Non-stick coating on frying pans and burette taps.
Deducing Structures
In the exam, you might be asked to draw the polymer from the monomer, or vice-versa. Here is a simple trick:
To draw the polymer from a monomer:
1. Change the double bond \( (C=C) \) to a single bond \( (C-C) \).
2. Draw two long bonds sticking out of the sides of the carbons.
3. Put large square brackets around the unit.
4. Add a small \( n \) at the bottom right corner to show it repeats many times.
To find the monomer from a polymer:
1. Identify the repeating unit (the bit inside the brackets).
2. Remove the side bonds and the brackets.
3. Change the single bond between the carbons back into a double bond \( (C=C) \).
4. Remove the \( n \).
Natural Polymers
Not all polymers are man-made in a lab! Nature is an expert at building them too. You need to know these three examples:
1. DNA: This is a polymer made from four different monomers called nucleotides. It carries the genetic instructions for all living organisms.
2. Starch: A natural polymer used by plants to store energy. It is made from sugars (like glucose) joined together.
3. Proteins: These are polymers made from monomers called amino acids. Proteins are vital for many processes in your body, like building muscle and making enzymes.
Did you know? Even though DNA is a single molecule, if you stretched out the DNA from just one of your cells, it would be about 2 metres long!
Disposal and Recycling of Polymers
Because synthetic polymers (plastics) are often very unreactive, they don't break down easily in the environment. This is good for their use (like a water pipe that doesn't rot), but bad for the planet. There are three main ways to deal with them:
1. Landfill: Plastics are buried in the ground. This is cheap but takes up a lot of space, and most plastics are non-biodegradable, meaning they won't rot away for hundreds of years.
2. Incineration (Burning): Burning plastics releases energy that can be used to generate electricity. However, it can also release toxic gases and carbon dioxide, which contributes to climate change.
3. Recycling: This involves melting down old plastic to make new products. It saves crude oil (the raw material for plastic) and reduces landfill waste. However, it can be difficult and expensive because different types of plastic must be carefully sorted first.
Higher Tier Only: Condensation Polymerisation
Higher Tier students need to know about a second type of polymerisation called condensation polymerisation.
In addition polymerisation, you only get one product (the polymer). In condensation polymerisation, two different monomers react together, and for every bond made, a small molecule (usually water, \( H_2O \)) is "spit out" or lost.
A common example is making a polyester. To make a polyester, you need two types of monomers:
1. A dicarboxylic acid (a molecule with a \( -COOH \) group at both ends).
2. A diol (a molecule with an \( -OH \) group at both ends).
When the carboxylic acid group reacts with the alcohol group, an ester link is formed, and a molecule of water is released. This happens at both ends of the monomers, allowing them to join together into a long chain.
Summary of Condensation:
\( \text{dicarboxylic acid} + \text{diol} \rightarrow \text{polyester} + \text{water} \)
Quick Review
1. Polymers are large molecules made of repeating units called monomers.
2. Addition polymers need monomers with a \( C=C \) double bond.
3. Poly(ethene), Poly(propene), PVC, and PTFE have specific properties that make them useful for products like bags, ropes, pipes, and pans.
4. DNA, starch, and proteins are natural polymers.
5. Disposal methods include landfill, incineration, and recycling—each has its own pros and cons.
6. (Higher Tier) Condensation polymers like polyesters form when monomers join and lose a small molecule like water.