Polymers

11.8 Polymers

Definition of Polymers

• Polymers are large molecules (macromolecules) made by joining many smaller repeating units called monomers.
• Monomers are small molecules that can chemically bond together to form a long chain.
• Polymerisation: The chemical process by which monomers join together to form a polymer.

Types of Polymerisation

1. Addition polymerisation:

• Occurs when unsaturated monomers (containing C=C double bonds) join together without the loss of any small molecules.
• Example: Ethene → poly(ethene).
• The double bond in each monomer opens up and links to adjacent monomers.
• General reaction:
  n(CH₂=CH₂) → –[CH₂–CH₂]–ₙ

2. Condensation polymerisation:

• Occurs when monomers with two functional groups react together and a small molecule (such as H₂O or HCl) is eliminated for each linkage formed.
• Examples:
  • Polyesters from dicarboxylic acids and diols.
  • Polyamides from dicarboxylic acids and diamines.

Identifying Repeat Units and Linkages

• Repeat unit: The section of a polymer chain that repeats over and over.
• Linkage: The bond that connects monomers in a polymer chain.
• For addition polymers, the linkage is a single covalent bond between carbon atoms (–C–C–).
• For condensation polymers, the linkage is:
  • Ester linkage: –COO– in polyesters.
  • Amide linkage (peptide bond): –CONH– in polyamides and proteins.

Deducing Structures

1. From monomer to polymer:

• Addition polymer:
  Remove the double bond in the alkene monomer and join carbon atoms into a chain with repeating units.
• Condensation polymer:
  Identify functional groups on monomers (–COOH, –OH, –NH₂) and connect them with ester or amide linkages.

2. From polymer to monomer:

• Addition polymer:
  Identify the repeating unit and insert a double bond between carbons.
• Condensation polymer:
  Break the polymer chain at the linkage and add functional groups back to the ends to form original monomers.

Polyamides

• Formed by reaction of a dicarboxylic acid (HOOC–R–COOH) and a diamine (H₂N–R’–NH₂).
• Linkage formed: amide linkage (–CONH–).
• Example: Nylon-6,6:
  • Monomers: hexanedioic acid and hexane-1,6-diamine.
  • Reaction:
    –COOH + –NH₂ → –CONH– + H₂O.
  • Structure of repeating unit:
    –NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–

Polyesters

• Formed by reaction of a dicarboxylic acid (HOOC–R–COOH) and a diol (HO–R’–OH).
• Linkage formed: ester linkage (–COO–).
• Example: PET (polyethylene terephthalate):
  • Monomers: benzene-1,4-dicarboxylic acid and ethane-1,2-diol.
  • Structure of repeating unit:
    –O–CH₂–CH₂–O–CO–C₆H₄–CO–

Differences Between Addition and Condensation Polymerisation

Plastics from Polymers

• Plastics are synthetic materials made from polymers.
• Properties:
  • Lightweight
  • Durable
  • Resistant to chemicals and water
  • Easily moulded
• Common examples: poly(ethene), poly(propene), PVC, PET.

Implications for Disposal

• Plastics are chemically inert and non-biodegradable, making disposal challenging.
• Environmental challenges:
  1. Landfill sites:
  • Plastics do not decompose quickly, causing long-term waste accumulation.
  2. Accumulation in oceans:
  • Harmful to marine life (ingestion, entanglement).
  • Microplastics enter food chains.
  3. Burning:
  • Releases toxic gases (e.g., hydrogen chloride from PVC).
  • Contributes to air pollution.

Nylon – Structure

• Polyamide formed from hexane-1,6-diamine and hexanedioic acid.
• Repeating unit: –NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–
• Amide linkage: –CONH–
• Strong, durable, used in fibres, ropes, fabrics.

PET – Structure

• Polyester formed from ethane-1,2-diol and benzene-1,4-dicarboxylic acid.
• Repeating unit: –O–CH₂–CH₂–O–CO–C₆H₄–CO–
• Ester linkage: –COO–
• Transparent, lightweight, used in bottles, clothing fibres.
• PET can be depolymerised into monomers and re-polymerised.

Proteins – Natural Polyamides

• Monomers: amino acids (general formula: H₂N–CH(R)–COOH, where R is the side chain).
• Linkage: peptide bond (amide linkage) –CONH–.
• Structure:
  • Repeating –NH–CH(R)–CO– units.
  • Formed via condensation polymerisation, releasing water.
• Proteins fold into specific shapes for biological functions (enzymes, hormones, structural components).