Overview
This lecture introduces the basics of polymers, covering their types, methods of polymerization, structural variations, and how these factors influence polymer properties and uses.
Introduction to Polymers
- Polymers are large molecules made from repeated smaller units called monomers.
- Biological polymers include DNA, RNA, and proteins; synthetic examples are plastics like polyethylene and polystyrene.
- Teflon (PTFE) and Kevlar are notable polymers discovered accidentally.
Types and Naming of Polymers
- Addition polymers are made by repeatedly joining the same monomers, named by adding "poly" to the monomer's name (e.g., polystyrene).
- Copolymers are formed from more than one type of monomer (e.g., ABS in Lego bricks).
- Repeat units in structures are indicated with brackets and an "n" for the number of repetitions.
Polymerization Mechanisms
- Addition (chain growth) polymerization can occur via radical, cationic, or anionic mechanisms.
- Free radical polymerization starts with a radical initiator and ends by random chain termination.
- Cationic polymerization uses a cationic initiator and ends through chain transfer reactions.
- Anionic polymerization requires anions and monomers that stabilize negative charges, with termination via proton transfer.
Condensation (Step-Growth) Polymers
- Condensation polymerization involves monomers joining and releasing a small molecule, often water or HCl.
- Examples include Kevlar and polycarbonates (Lexan), which can lose molecules other than water during formation.
- No termination step is required; reaction stops when reactants are depleted or conditions change.
Polymer Structure and Morphology
- Polyethylene exists as high-density (HDPE, linear, strong) and low-density (LDPE, branched, flexible) forms.
- Structure affects properties: linear polymers pack closely and are stronger; branched ones are softer.
- Polymer strands can be crystalline (ordered) or amorphous (disordered); real samples may have both.
Thermal Properties of Polymers
- Glass transition temperature (Tg) is when a polymer becomes soft and rubbery.
- Polymers with Tg above room temperature are hard and brittle (e.g., polystyrene).
- Tg affects performance; for example, brittle O-rings below Tg contributed to the Challenger disaster.
Key Terms & Definitions
- Polymer — Large molecule of repeating smaller units (monomers).
- Monomer — Small molecule that bonds to form a polymer.
- Addition Polymer — Polymer formed by linking identical monomers without byproducts.
- Copolymers — Polymers made from more than one type of monomer.
- Radical Polymerization — Chain reaction started by a molecule with an unpaired electron.
- Condensation Polymerization — Polymerization involving loss of a small molecule (often water or HCl).
- Repeat Unit — The molecular segment that repeats in a polymer chain.
- Crystalline Polymer — Polymers with ordered, tightly packed regions.
- Amorphous Polymer — Polymers with disordered, tangled regions.
- Glass Transition Temperature (Tg) — Temperature where a polymer becomes soft and rubbery.
Action Items / Next Steps
- Review polymerization mechanisms and practice drawing repeat units.
- Prepare for next lecture on benzene, aromatic compounds, and NMR analysis.
- Complete any assigned readings on polymer structures and applications.