23. Polymers: Structure, Drawing & Properties

Based on the video, here are the notes on polymers, their repeating units, and why they have specific physical properties.

Definition: A polymer is a large molecule made up of many smaller repeating units called monomers.
Scale: A single polymer molecule can be thousands of atoms long.
Example: Propine (C3H6) monomers join together to form the polymer polypropylene, a common plastic.

Drawing out a chain of thousands of atoms is impractical, so scientists use a shorthand method:

Repeating Unit: Identify the smallest section of the chain that repeats.
Brackets: Draw this unit inside curved brackets.
Bonds: Show the covalent bonds passing through the brackets to indicate they continue to the next unit.
The "n": A small "n" is written in the bottom right corner. This represents the large (and often variable) number of repeating units in the chain.
- Example: If you combined 400 monomers, you could write 400 in the corner instead of "n".

Bonding: The atoms within a polymer chain are held together by very strong covalent bonds.
Intermolecular Forces: To melt or boil a polymer, you don't break the covalent bonds. Instead, you break the intermolecular forces between the separate, long polymer chains.
Strength of Forces: While intermolecular forces are individually weak, polymers are so long that they have a huge surface area. This results in many intermolecular forces acting between the chains.
Comparison:
- Higher melting/boiling points than simple molecular substances (like O2 or Cl2) because they have more intermolecular forces.
- Lower melting/boiling points than giant covalent structures (like diamond) or giant ionic structures (like NaCl).
State at Room Temperature: Because of the cumulative strength of these many intermolecular forces, most polymers are solids at room temperature.