Polymers: Addition and Condensation

Detailed Explanations

A macromolecule contains about 10² or more atoms joined by covalent bonds. A typical polymer molecule contains roughly 10² or more repeating units and has a relative molecular mass of about 10³ or greater. These are scale descriptions, not a claim that every polymer chain has exactly the same length.

A. Addition polymer example (ethene → poly(ethene))

Monomer: CH₂ = CH₂

Repeat unit idea: [-CH₂-CH₂-]ₙ

What to write:

• “The C=C opens and forms single bonds between monomers; no by-product forms.”

Because addition polymerisation uses C=C monomers and no small molecule is eliminated, therefore you should not write H₂O or HCl as a by-product for addition polymers.

B. Condensation polymer examples

1) Polyester (diol + dicarboxylic acid)

General reaction idea:

• -OH reacts with -COOH to form an ester link -COO⁻ and water is eliminated.

2) Polyamide (diamine + dicarboxylic acid, or acyl chloride route)

General reaction idea:

• -NH₂ reacts with -COOH to form an amide link -CONH⁻ and water is eliminated.

C. Proteins and peptide bonds

A peptide bond forms between -NH₂ and -COOH of amino acids, eliminating H₂O.

Hydrolysis (concept):

• heat with aqueous acid or aqueous alkali to break peptide bonds by adding water. Acid hydrolysis gives protonated amino-acid products; alkaline hydrolysis gives carboxylate salts.

D. Workflow: drawing an addition polymer repeat unit (exam method)

1. Identify the C=C in the monomer.
2. Replace the C=C with a C–C single bond.
3. Keep the substituents on the same carbon atoms.
4. Put the repeat unit in brackets and add n.

Mini example: Ethene CH₂ = CH₂ → repeat unit [-CH₂-CH₂-]ₙ.

E. Workflow: identifying monomers for condensation polymers

1. Identify the link in the polymer: ester -COO⁻ or amide/peptide -CONH⁻.
2. “Cut” the link back into the functional groups that formed it:
   • ester link → -COOH + -OH
   • amide link → -COOH + -NH₂
3. Ensure each monomer is difunctional so the chain can continue.

F. Recycling decisions

Recycling polymers involves linked environmental, economic and social choices:

• Environmental: it can reduce demand for fossil feedstocks and landfill, but collection, transport, washing and remelting also use energy and may produce emissions.
• Economic: clean, single-polymer streams are more valuable; mixed or contaminated waste can cost more to separate than the recycled material is worth.
• Social: convenient collection and clear labelling improve participation, while deposit schemes or regulations can change behaviour and cost distribution.

Mechanical recycling usually needs sorting by polymer type because different polymers soften at different temperatures and mixed products can have poor properties. Chemical recycling may recover monomers or useful feedstocks but often needs more energy. A balanced exam answer applies these trade-offs to the named polymer and process instead of stating that recycling is always automatically beneficial.