Take ethene. The π-bond of CH2=CH2 breaks; the two electrons become a new σ-bond to the next monomer:
nCH2=CH2⟶−repeat unit(CH2−CH2)−n
Why no by-product? Nothing is removed — the double bond simply "opens" and reconnects. So:
mass of polymer=n×mass of monomer
Monomer C2H4 (28) and repeat unit C2H4 (28) — identical. ✔
Difference between addition and condensation polymer
Addition: C=C monomers add without losing any atom (repeat unit = monomer formula). Condensation: bifunctional monomers join with elimination of a small molecule (H₂O/HCl/NH₃).
Why do thermosetting plastics not melt on reheating
They have a 3-D network of strong covalent crosslinks that heat cannot break, so they char/decompose instead of softening.
Why CAN thermoplastics be remoulded
Their chains are held only by weak intermolecular forces which loosen on heating, then reform on cooling.
Give one example each: natural, synthetic, semi-synthetic
What small molecule is eliminated in nylon-6,6 formation
Water (amide bond formation between –NH₂ and –COOH).
Why does addition polymerisation conserve mass
The π-bond merely opens to form new σ-bonds; nothing is removed, so monomer and repeat unit have the same formula.
Are 'addition vs condensation' and 'thermoplastic vs thermosetting' related
No, they are independent axes — one is about mechanism, the other about crosslinking/thermal behaviour.
Semi-synthetic polymer definition
A naturally occurring polymer that has been chemically modified, e.g. cellulose nitrate (gun cotton).
Recall Feynman: explain to a 12-year-old
Imagine LEGO. Addition plastics are like clicking identical bricks together — nothing falls off the table when you click. Condensation plastics are like gluing bricks where a tiny drop of glue squeezes out each time (that drop = the water lost).
Now about melting: thermoplastic is like a chocolate chain — warm it and it goes soft, you can reshape it again and again. Thermosetting is like baking a cake — once you bake the batter into a cake, heating it more just burns it; you can never turn it back into batter. The cake's ingredients got permanently locked together (that's crosslinking).
Dekho, polymer matlab ek bahut bada molecule jo chhote-chhote repeating units (monomers) ko jodke banta hai — bilkul beads ki mala jaisa. Inhe samajhne ke liye hum teen alag-alag sawaal poochte hain, aur ye teeno independent hain. Pehla: kahan se aaya — nature se (cellulose, rubber, protein = natural) ya lab mein bana (polythene, nylon, Bakelite = synthetic), ya natural ko thoda modify karke (rayon = semi-synthetic).
Doosra axis hai kaise bana. Addition mein monomer ka C=C double bond khulta hai aur seedha jud jata hai — koi atom bahar nahi nikalta, isliye monomer aur repeat unit ka formula same rehta hai (jaise ethene se polythene). Condensation mein har monomer ke do functional groups hote hain (–OH, –COOH, –NH₂) aur jab ye jude to ek chhota molecule (zyadatar H₂O) bahar nikal jata hai — jaise nylon ya polyester banta hai.
Teesra axis hai garam karne pe kya hota hai. Thermoplastic ki chains weak forces se judi hoti hain — garam karo to soft ho jata hai, dobara mould kar sakte ho (polythene, PVC). Thermosetting mein strong covalent crosslinks ka 3-D jaal hota hai — ek baar set ho gaya to garam karne pe melt nahi, balki char/jal jata hai (Bakelite). Yaad rakho: thermoplastic = "play-stic" (khelta rehta hai), thermosetting = "set in stone" (set forever). Exam mein sabse common galti yahi hai ki bachche sochte hain thermosetting bhi melt hota hai high temp pe — nahi! Wo decompose hota hai, melt nahi.
