Question
Distinguish between addition polymerization and condensation polymerization. Give one example of each, and explain how the molecular weight grows in each case.
Solution — Step by Step
Monomers contain C=C double bonds. The double bond opens up and the monomers link directly without losing any atoms. Initiator (e.g., a free radical) attacks one monomer; the growing chain attacks the next.
Example: ethene → polyethene. Reaction: .
Monomers each have two functional groups (e.g., -OH, -COOH, -NH). They link via condensation reactions, expelling small molecules (HO, HCl, NH) at each step.
Example: terephthalic acid + ethylene glycol → polyethylene terephthalate (PET). Each linkage releases one water molecule.
Addition: very fast. A single growing chain adds thousands of monomers in milliseconds. Distribution of chain lengths is broad early on but narrows.
Condensation: stepwise. Two monomers form a dimer, two dimers form a tetramer, etc. The distribution stays broad. Molecular weight grows slowly with conversion.
Addition products have only C and H (or vinyl groups). Condensation products often have ester, amide, or ether linkages — making them more polar and degradable.
Final answer: addition uses C=C bond opening (polyethene); condensation uses functional groups losing small molecules (PET, nylon).
Why This Works
Both processes give long chains, but the chemistry is fundamentally different. Addition keeps every atom of every monomer; condensation loses small molecules. This determines whether the polymer is biodegradable, recyclable, polar, etc.
Alternative Method
Memorise classic examples:
- Addition: polyethene, polypropene, PVC, polystyrene, Teflon
- Condensation: nylon-6,6, nylon-6, PET (Dacron), Bakelite, Kevlar
Quick test: does the monomer have C=C? If yes, addition polymerization. Does it have two reactive functional groups (e.g., -OH and -COOH)? If yes, condensation polymerization.
Common Mistake
Saying “addition polymers don’t release any byproducts”. True for addition, false for condensation. Reverse: students sometimes apply condensation logic (loss of water) to addition.
Confusing nylon-6 with nylon-6,6. Nylon-6 comes from caprolactam (single monomer, ring-opening). Nylon-6,6 comes from adipic acid + hexamethylenediamine (two different monomers). Both condensation; different mechanisms.