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ChemistryPolymers
Which of the following is an addition polymer?
Options
1
Nylon-6,6
2
Terylene (Dacron)
3
Natural rubber
4
Bakelite
Correct Answer
Option 3 : Natural Rubber
Step-by-Step Solution
1

Addition polymers — form by addition of monomer units with NO loss of small molecules. Monomer must have a double bond (C=C). No byproduct formed.

Condensation polymers — form by condensation between monomers with LOSS of small molecules (H₂O, HCl, CH₃OH etc.). Monomers have two functional groups.

2

Check each option:

Option 1 — Nylon-6,6: formed from hexamethylenediamine + adipic acid → loses H₂O at each step → Condensation polymer ✗

Option 2 — Terylene: formed from ethylene glycol + terephthalic acid → loses H₂O → Condensation polymer ✗

Option 3 — Natural rubber: formed by addition polymerisation of isoprene (2-methylbuta-1,3-diene) → no byproduct → Addition polymer ✓

Option 4 — Bakelite: formed from phenol + formaldehyde → loses H₂O → Condensation polymer ✗

Theory: Polymers
1. Classification of Polymers

Polymers are large molecules (macromolecules) formed by joining many smaller monomer units. Based on mode of polymerisation, they are classified as addition polymers (no byproduct) and condensation polymers (small molecule lost per step). Based on source: natural (rubber, starch, proteins) and synthetic (nylon, PVC, polythene). Based on structure: linear, branched, and cross-linked (network).

2. Addition Polymers — Key Examples

📌 Polyethylene (polythene): n CH₂=CH₂ → (−CH₂−CH₂−)n, monomer: ethene

📌 PVC: n CH₂=CHCl → (−CH₂−CHCl−)n, monomer: vinyl chloride

📌 Polystyrene: monomer styrene (C₆H₅CH=CH₂)

📌 Teflon (PTFE): monomer tetrafluoroethylene (CF₂=CF₂)

📌 Natural rubber: monomer isoprene (2-methylbuta-1,3-diene) → cis-1,4-polyisoprene

📌 Neoprene: monomer chloroprene (2-chlorobuta-1,3-diene) — synthetic rubber

📌 Buna-N: copolymer of butadiene and acrylonitrile

📌 Buna-S: copolymer of butadiene and styrene

3. Condensation Polymers — Key Examples

📌 Nylon-6,6: hexamethylenediamine + adipic acid → loses H₂O, amide linkage (−CO−NH−)

📌 Nylon-6: ε-caprolactam (ring opening) → loses H₂O, amide bond

📌 Terylene/Dacron: ethylene glycol + terephthalic acid → loses H₂O, ester linkage (−COO−)

📌 Bakelite: phenol + formaldehyde → loses H₂O, cross-linked (thermoset)

📌 Glyptal: glycerol + phthalic acid → ester linkage

📌 Melamine-formaldehyde: loses H₂O, used in crockery

4. Natural Rubber — Structure and Properties

Natural rubber is cis-1,4-polyisoprene — an addition polymer of isoprene (2-methylbuta-1,3-diene). The cis configuration of the double bonds in the polymer chain is responsible for the elastic (rubbery) properties. The molecules are coiled and can stretch by uncoiling. The trans form (gutta-percha) is hard and non-elastic.

Isoprene: CH₂=C(CH₃)−CH=CH₂

n isoprene → cis-1,4-polyisoprene (natural rubber)

No byproduct — pure addition polymerisation

5. Vulcanisation of Rubber

Natural rubber is soft and sticky, loses elasticity in cold, and becomes brittle. Vulcanisation (Charles Goodyear, 1839) involves heating rubber with 5–8% sulphur at 150°C. Sulphur forms cross-links between polymer chains at the double bond positions, making rubber harder, stronger, and non-sticky. Car tyres use highly vulcanised rubber (30% S) called ebonite — a hard, rigid material.

6. Biodegradable vs Non-biodegradable Polymers

Biodegradable polymers are broken down by microorganisms. Examples: PHBV (poly-β-hydroxybutyrate-co-β-hydroxyvalerate) — used for surgical sutures and drug delivery; Nylon-2-nylon-6 (copolymer of glycine and aminocaproic acid). Non-biodegradable: PVC, polythene, polystyrene — persist in environment for centuries, causing pollution. PLA (polylactic acid) is a biodegradable synthetic polymer made from lactic acid (derived from corn starch) — used for biodegradable packaging.

7. Thermoplastics vs Thermosetting Plastics

📌 Thermoplastics: soften on heating, harden on cooling — recyclable. Linear/slightly branched chains. Examples: PVC, polythene, polystyrene, nylon, Teflon

📌 Thermosetting: set permanently on heating (cross-linked network) — cannot be remelted. Examples: Bakelite, melamine-formaldehyde, urea-formaldehyde, epoxy resins

📌 Bakelite has 3D cross-linked structure — rigid, good electrical insulator, heat resistant

📌 Thermoplastics are generally addition polymers; thermosets are condensation

8. Copolymers and Their Uses

Copolymers are formed from two or more different monomers. They combine properties of both parent polymers. Buna-S (SBR — styrene butadiene rubber): used in car tyres, better abrasion resistance than natural rubber. Buna-N (NBR — nitrile rubber): resistant to oils and petrol — used for fuel hoses, gaskets. Acrilan: copolymer of acrylonitrile and vinyl acetate — used as wool substitute. Polyester fibres (Terylene/Dacron): combined strength of polyester with fabric-like properties.

Frequently Asked Questions
1. What is the key difference between addition and condensation polymerisation? ⌄
Addition: monomers add to each other without losing any atoms. Monomer must have unsaturation (C=C). No byproduct. Molecular formula of repeat unit = molecular formula of monomer. Condensation: two monomers react, losing a small molecule (H₂O, HCl) at each step. Monomers need two functional groups each. Repeat unit has fewer atoms than combined monomers.
2. Why is natural rubber elastic? ⌄
Natural rubber (cis-1,4-polyisoprene) has long coiled chains due to the cis double bonds creating a specific geometry. When stretched, chains uncoil and extend. When released, they spontaneously re-coil to lower entropy state (coiled = higher entropy due to more conformations). The cross-links in vulcanised rubber prevent permanent deformation — chains can uncoil but always return to original position.
3. What is the monomer of Nylon-6,6 and what type of linkage? ⌄
Two monomers: hexamethylenediamine H₂N(CH₂)₆NH₂ and adipic acid HOOC(CH₂)₄COOH. They react to form amide bonds (−CO−NH−) with loss of H₂O at each step. The "6,6" refers to the 6 carbons in each monomer. Nylon-6 uses only one monomer: caprolactam (ring opens to give 6-carbon aminocaproic acid unit).
4. What makes Bakelite a thermoset? ⌄
Bakelite is formed by condensation of phenol and formaldehyde. It has a 3D cross-linked network structure — chains are covalently bonded in all three dimensions. Once formed, the cross-links cannot be broken by heating (without destroying the polymer). This makes it permanently rigid, a good electrical insulator, and heat-resistant. Used for switches, handles, radio casings.
5. What is the difference between Buna-S and Buna-N? ⌄
Buna-S (SBR): copolymer of butadiene (75%) and styrene (25%). Used for car tyres — better abrasion resistance and aging than natural rubber. Buna-N (NBR): copolymer of butadiene (75%) and acrylonitrile (25%). The nitrile groups make it resistant to oils, fuels, and solvents. Used for fuel hoses, gaskets, oil seals. The different second monomer gives completely different chemical resistance properties.
6. What is vulcanisation and what does sulphur do? ⌄
Vulcanisation adds sulphur bridges (cross-links) between polyisoprene chains at the double bond positions. Natural rubber has C=C bonds that are reactive. Sulphur reacts with these, forming −S−S− or −S− bridges between chains. More cross-links = harder rubber. 5–8% S: flexible rubber (gloves, toys). 30% S: ebonite — rigid, hard material used for combs, bowling balls.
7. What is PHBV and why is it important? ⌄
PHBV (poly-β-hydroxybutyrate-co-β-hydroxyvalerate) is a biodegradable addition copolymer produced by bacteria. It is biocompatible, biodegradable in soil and water, and has mechanical properties similar to polypropylene. Used for biodegradable packaging, drug delivery capsules, and surgical sutures that naturally dissolve. It represents the future of "green polymers" that solve plastic pollution problems.
8. What is the repeat unit of Terylene? ⌄
Terylene (polyethylene terephthalate, PET): repeat unit is −[−OCH₂CH₂OOC−C₆H₄−CO−]−. Formed from ethylene glycol (HOCH₂CH₂OH) and terephthalic acid (HOOC−C₆H₄−COOH) with loss of H₂O. Ester linkage (−COO−) connects monomers. Used for polyester fibres (clothes), PET bottles (water, soft drinks), and film (photography). Highly recyclable — marked with recycling code 1.
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