What is a Polymer?
A polymer is a large molecule (macromolecule) composed of many repeated smaller units called monomers.
- High molecular weight (typically 10,000 to several million g/mol)
- Repeating units (monomers) linked by covalent bonds
- Long chain structure that can be linear, branched, or cross-linked
Examples
- Natural: DNA, proteins, cellulose, starch, rubber
- Synthetic: Plastics, nylon, polyester, Teflon
General Equation: n (monomer) → [—monomer—]ₙ (polymer)
Addition Polymers
Key Features
- Form from unsaturated monomers containing C=C bonds (see Alkenes)
- No by-products formed during polymerization
- Only one type of monomer typically used
- Result in saturated polymers (no C=C bonds in final product)
Mechanism
- Initiation: Free radical or catalyst starts the reaction
- Propagation: Chain grows as monomers add one by one
- Termination: Chain growth stops
Common Addition Polymers
| Monomer | Structure | Polymer | Polymer Structure | Uses |
|---|---|---|---|---|
| Ethene | CH₂=CH₂ | Polyethene (PE) | [—CH₂—CH₂—]ₙ | Plastic bags, bottles |
| Propene | CH₂=CH—CH₃ | Polypropene (PP) | [—CH₂—CH(CH₃)—]ₙ | Food containers, carpets |
| Styrene | CH₂=CH—C₆H₅ | Polystyrene (PS) | [—CH₂—CH(C₆H₅)—]ₙ | Foam cups, insulation |
| Vinyl chloride | CH₂=CHCl | PVC | [—CH₂—CHCl—]ₙ | Pipes, flooring |
Drawing Addition Polymers
Example: Ethene → Polyethene
- Monomer: H₂C=CH₂
- Break the double bond: The C=C bond opens up
- Connect monomers: —CH₂—CH₂—CH₂—CH₂—CH₂—CH₂—
- Show repeating unit: [—CH₂—CH₂—]ₙ
Important: The lines extending outside brackets are continuation bonds - they show the chain continues. The subscript n indicates many repeat units.
Condensation Polymers
Key Features
- Form from two different monomers with complementary functional groups
- Small molecule eliminated (usually H₂O)
- Monomers must be bifunctional (reactive groups at both ends)
- Result in alternating structure
Types of Condensation Polymers
Polyesters
- Formed from: Dicarboxylic acid + Diol (see Carboxylic Acids and Alcohols)
- Linkage: Ester bond (—COO—) (see Esters)
- Eliminated: Water (H₂O)
Example: PET Formation
HOOC—C₆H₄—COOH + HO—CH₂—CH₂—OH →
[—OOC—C₆H₄—COO—CH₂—CH₂—]ₙ + H₂O
Polyamides (Nylons)
- Formed from: Dicarboxylic acid + Diamine (see Amines)
- Linkage: Amide bond (—CONH—) (see Amides)
- Eliminated: Water (H₂O)
Example: Nylon 6,6
HOOC—(CH₂)₄—COOH + H₂N—(CH₂)₆—NH₂ →
[—OOC—(CH₂)₄—CONH—(CH₂)₆—NH—]ₙ + H₂O
Cross-Linking
Definition
Cross-linking occurs when polymer chains are connected to each other by covalent bonds, creating a three-dimensional network.
Types of Cross-Links
Chemical Cross-Links
- Covalent bonds between polymer chains
- Permanent connections
- Examples: Sulfur bridges in vulcanized rubber
Physical Cross-Links
- Weaker interactions (hydrogen bonds, ionic bonds) (see H-Bonds)
- Temporary connections
- Examples: Hydrogen bonding in nylon
Effects of Cross-Linking
| Property | Linear Polymer | Cross-Linked Polymer |
|---|---|---|
| Solubility | Soluble in good solvents | Insoluble (swells only) |
| Melting | Melts when heated | Does not melt (degrades) |
| Flexibility | More flexible | Less flexible, more rigid |
| Strength | Lower strength | Higher strength |
| Recycling | Easily recycled | Difficult to recycle |
Polymer Classifications by Cross-Linking
Thermoplastics
- No cross-links or very few
- Can be melted and reshaped repeatedly
- Examples: Polyethene, PVC, polystyrene
- Uses: Recyclable plastics
Thermosets
- Heavily cross-linked
- Cannot be melted without decomposition
- Examples: Epoxy resins, vulcanized rubber
- Uses: Adhesives, tires, circuit boards
Elastomers
- Lightly cross-linked
- Flexible and elastic
- Examples: Rubber, silicone
- Properties: Can stretch and return to original shape
Polymer Problem Solving
Drawing Polymers from Monomers
For Addition Polymers:
- Identify the C=C bond in monomer
- Break the double bond
- Connect monomers through single bonds
- Draw repeating unit with continuation bonds
Example: CH₂=CH—CH₃ → [—CH₂—CH(CH₃)—]ₙ
For Condensation Polymers:
- Identify functional groups on each monomer
- Show elimination of small molecule
- Connect through new linkage
- Draw alternating structure
Identifying Monomers from Polymers
For Addition Polymers:
- Identify the repeating unit
- Add double bond between backbone carbons
- That’s your monomer!
Example: [—CH₂—CHCl—]ₙ → CH₂=CHCl
For Condensation Polymers:
- Identify the linkage (ester, amide, etc.)
- “Break” polymer at linkage
- Add H and OH to reform functional groups
- You have your two monomers
Example: Break polyester at ester bonds, add H and OH [—OOC—R—COO—R’—]ₙ → HOOC—R—COOH + HO—R’—OH
Key Patterns to Remember
Addition vs Condensation Recognition
- Addition: Look for C=C in monomer, no by-products
- Condensation: Two different monomers, water eliminated, alternating structure
Common Mistakes to Avoid
- Forgetting continuation bonds in polymer structures
- Missing the subscript n
- Not showing eliminated molecule in condensation reactions
- Confusing monomer → polymer vs polymer → monomer direction
Memory Aids
- Addition = Alkenes (needs C=C bonds)
- Condensation = Combines 2 monomers + Creates by-products
- More cross-links = More rigid, Less flexible