JEE Chemistry Polymers MCQs Set 17

Answer: (b) –[CO–(CH₂)₅–NH]ₙ–

Explanation:
1. Nylon-6 is made by ring-opening polymerisation of caprolactam, a six-carbon cyclic amide.
2. Each repeat unit therefore contains five CH₂ groups, one CO and one NH: –[NH–(CH₂)₅–CO]ₙ–.

Teacher's Note:
1. Nylon-6 gets its name from the six carbon atoms in its monomer unit.

Answer: (a) Nylon-6,6

Explanation:
1. Nylon-6,6 is formed from adipic acid and hexamethylenediamine with loss of water.
2. Neoprene, Teflon and Buna-S are formed by addition polymerisation.

Teacher's Note:
1. A condensation polymer forms with the loss of small molecules such as water.

Answer: (a) Bakelite

Explanation:
1. Thermosetting polymers are cross-linked and infusible once set.
2. Bakelite is the standard example; the others are thermoplastics or elastomers.

Teacher's Note:
1. Bakelite cannot be remoulded after it sets.

Answer: (b) –[O–(CH₂)₄–CO]ₙ–

Explanation:
1. Nitrous acid converts the primary amino group into a diazonium group, which is replaced by OH in water: HOOC–(CH₂)₄–OH.
2. A hydroxy acid undergoes self-condensation to a polyester: –[O–(CH₂)₄–CO]ₙ–.

Teacher's Note:
1. Primary aliphatic amines react with HNO₂ to give alcohols, and hydroxy acids form polyesters.

Answer: (d) 3-hydroxybutanoic acid and 3-hydroxypentanoic acid

Explanation:
1. β-Hydroxybutyrate is 3-hydroxybutanoic acid, CH₃CH(OH)CH₂COOH.
2. β-Hydroxyvalerate is 3-hydroxypentanoic acid, CH₃CH₂CH(OH)CH₂COOH.

Teacher's Note:
1. PHBV is a biodegradable polyester used in speciality packaging.

Answer: (c) It is used in the manufacture of buckets, dustbins etc.

Explanation:
1. LDPE is made by free radical polymerisation of ethene at high pressure with a trace of O₂ or a peroxide.
2. It is a flexible, insulating polymer used for films and bags, whereas buckets and dustbins are made from the more rigid high density polythene, so (c) is false.

Teacher's Note:
1. LDPE is flexible and used for films; HDPE is rigid and used for containers.

Answer: (b) Glyptal

Explanation:
1. Glyptal is a polyester of ethylene glycol and phthalic acid.
2. It is used in paints and lacquers, while Bakelite is used for electrical fittings, polypropene for textiles and packaging, and PVC for rain coats and hand bags.

Teacher's Note:
1. Glyptal is the polymer for paints and lacquers.

Answer: (c) AlCl₃

Explanation:
1. Cationic polymerisation is started by an electrophile or Lewis acid.
2. AlCl₃ is electron deficient and acts as the Lewis acid initiator; BuLi is an anionic initiator.

Teacher's Note:
1. Lewis acids start cationic polymerisation and strong nucleophiles start anionic polymerisation.

Answer: (b) nylon-6,6

Explanation:
1. Nylon-6,6 has amide (–CO–NH–) groups in its chains.
2. The N–H of one chain forms hydrogen bonds with the C=O of another chain, giving strong intermolecular forces.

Teacher's Note:
1. Polyamides are strongly hydrogen bonded, which gives nylon its high strength.

Answer: (d) HCHO

Explanation:
1. Phenol and formaldehyde (HCHO) condense in acidic or alkaline medium.
2. The product is the cross-linked resin Bakelite.

Teacher's Note:
1. Formaldehyde provides the CH₂ bridges between phenol rings.

Answer: (c) Nylon-6,6

Explanation:
1. A polyamide has repeating –CO–NH– links.
2. Nylon-6,6 is made from adipic acid and hexamethylenediamine and has amide links; terylene is a polyester.

Teacher's Note:
1. Nylons are polyamides and terylene is a polyester.

Answer: (b) condensation reaction between monomers

Explanation:
1. Condensation polymerisation joins monomers with the loss of small molecules such as H₂O.
2. Hydrolysis breaks polymers down and protonation does not make a polymer, so the answer is a condensation reaction.

Teacher's Note:
1. Joining monomers with loss of small molecules is condensation polymerisation.