Question
Distinguish between addition and condensation polymerization, give one example of each, and explain why nylon-6,6 has higher melting point than polyethene.
Solution — Step by Step
Monomers add to each other across multiple bonds (C=C or C=O) without losing any small molecule.
Example: polyethene from ethene.
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Mechanism: free-radical, cationic, or anionic chain growth.
Two functional groups react to form a bond, losing a small molecule (HO, HCl, CHOH).
Example: nylon-6,6 from hexamethylenediamine + adipic acid:
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Two reasons:
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Hydrogen bonding: The amide N-H and C=O groups form intermolecular H-bonds between adjacent polymer chains. Polyethene chains, being non-polar, have only weak London forces.
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Chain stiffness: Amide groups create planar regions and limit rotation, leading to better crystalline packing.
Both raise the energy needed to separate the chains, so a higher temperature is needed to melt.
Higher intermolecular forces and crystallinity in nylon-6,6 give it a much higher melting point — by about over polyethene.
Final answer: Nylon-6,6 melts higher because of (1) intermolecular hydrogen bonding between amide groups and (2) increased crystallinity. Polyethene has only weak van der Waals forces.
Why This Works
Polymer properties trace back to chain-to-chain interactions, not just chain length. Two polymers with similar molecular weights can have wildly different melting points if one has H-bonding and the other doesn’t. Nylon, polyester, and proteins exploit H-bonding for strength; LDPE and rubber don’t.
This is why polar groups in the backbone (amide, ester, urethane) consistently produce engineering polymers, while purely hydrocarbon polymers (polyethene, polypropene) are softer and lower-melting.
Alternative Method
Compare crystallinity directly: nylon-6,6 is ~50% crystalline; polyethene varies from ~30% (LDPE) to ~80% (HDPE). Even HDPE, despite high crystallinity, lacks H-bonding and melts at ~135°C — still well below nylon’s 265°C.
Common Mistake
Students often think “nylon is stronger because the chain is longer”. Chain length matters secondarily — the key driver is type of intermolecular force. Nylon-6,6 with still has a higher melting point than a polyethene with , because H-bonds beat London forces by orders of magnitude per interaction.