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State Board · Class 12 · Chemistry

Lesson 15: Introduction to Polymer Chemistry

7 concepts · 56 practice questions with answers · Free to read

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Polymers: origin and chain structure

From daily life

A plastic bucket, a thread, a tyre — all are long chains built by joining small molecules.

Simple definition

A polymer is a very large molecule (molar mass 10³–10⁷) made of repeating units; a monomer is the small starting molecule.

ChainsChain structures of polymersLinear: HDPE, PVCBranched: LDPECross-linked: bakelite, vulcanised rubberpoly (many) + meros (part) = many units; unit = monomer

In the figure : Types of chain.

Rule 1: Terms — poly = many, meros = part; in –(CH₂–CH₂)ₙ–, CH₂–CH₂ is the repeat unit and n is the degree of polymerisation.

  • Easy : Monomer ethene ⇒ polythene.
  • Medium : Monomer ≠ repeat unit in condensation.
  • Exam-type : n = 1000 ⇒ M = 28000.

Rule 2: Linear and branched — Linear: long straight chains, close packing (HDPE, PVC); branched: side chains, lower density (LDPE).

  • Easy : HDPE is denser.
  • Medium : LDPE is softer.
  • Exam-type : Branches ⇒ poorer packing.

Rule 3: Cross-linked — Chains joined by covalent links ⇒ hard, brittle (bakelite); few links ⇒ elastic (vulcanised rubber).

  • Easy : Bakelite: 3-D network.
  • Medium : Rubber: a few –S–S– links.
  • Exam-type : Does not remelt.

Calculation: polythene M = 28000, repeat unit CH₂–CH₂ (M = 28)

  • Step 1: DP = M(polymer)/M(unit).
  • Step 2: DP = 28000/28.
  • Step 3: DP = 1000.

CET level (MCQ)

  • CET 1) Structure of LDPE? 1) branched 2) linear 3) cross-linked 4) cyclic — Answer: 1).
  • CET 2) Structure of bakelite? 1) cross-linked 2) linear 3) branched 4) spherical — Answer: 1).

Real-life use: Bags, pipes, tyres, clothes.

Remember

Monomer → polymer; DP = M(polymer)/M(unit); linear, branched, cross-linked.

Common mistake

Treating monomer and repeat unit as always identical.

CET trap

In condensation the repeat unit can differ.

Shortcut

Memory clue: "mono = one, poly = many".

Spot the difference

StructureExample
LinearHDPE
BranchedLDPE
Cross-linkedbakelite

Practice questions

Easy
  1. 1What is a polymer?
  2. 2What is a monomer?
  3. 3Three structures.
Medium
  1. 4HDPE vs LDPE.
  2. 5Why is bakelite hard?
  3. 6What is DP?
Challenging
  1. 728000 ÷ 28?
  2. 8Repeat unit.
Show answers (try them yourself first)
  1. 1Large molecule.
  2. 2Small unit molecule.
  3. 3Linear, branched, cross-linked.
  4. 4Branches ⇒ lower density.
  5. 53-D network.
  6. 6n.
  7. 71000.
  8. 8Repeating part.

Modes of polymerisation

From daily life

Like threading beads one by one, monomers add one after another.

Simple definition

Addition: double bonds open, nothing is lost; condensation: a small molecule (H₂O, HCl) is lost on joining.

ModesModes of polymerisationAddition (chain-growth): CH₂=CH₂ → –(CH₂–CH₂)ₙ–1. Initiation: R· + CH₂=CH₂ → R–CH₂–CH₂·2. Propagation: …–CH₂CH₂· + CH₂=CH₂ → …–CH₂CH₂CH₂CH₂·3. Termination: two radicals combinefree-radical initiator: benzoyl peroxideCondensation (step-growth): two groups ⇒ polymer + small molecule (H₂O, HCl)n H₂N–R–NH₂ + n HOOC–R′–COOH → –[NH–R–NHCO–R′–CO]ₙ– + 2n H₂Oneeds two or more functional groups eachCopolymer: two monomers, e.g. Buna-S (butadiene + styrene)

In the figure : Addition and condensation.

Rule 1: Addition (chain-growth) — Three steps: initiation (radical), propagation, termination; initiator benzoyl peroxide; monomer needs C=C.

  • Easy : CH₂=CH₂ ⇒ polythene.
  • Medium : No by-product.
  • Exam-type : Fast chain reaction.

Rule 2: Condensation (step-growth) — Each monomer has two functional groups (–NH₂ + –COOH); a small molecule leaves at each step; e.g. nylon-6,6, bakelite.

  • Easy : H₂O is lost.
  • Medium : Mass grows slowly.
  • Exam-type : Unit ≠ monomer.

Rule 3: Copolymers — Two or more monomers together: Buna-S (buta-1,3-diene + styrene), Buna-N (butadiene + acrylonitrile).

  • Easy : Properties blend.
  • Medium : Buna-S is used in tyres.
  • Exam-type : Homopolymer: one monomer.

Reasoning: addition or condensation?

  • Step 1: Is there a C=C? ⇒ addition.
  • Step 2: Two different groups (–NH₂, –COOH)? ⇒ condensation.
  • Step 3: Check whether H₂O or HCl is lost.

CET level (MCQ)

  • CET 1) An addition polymer? 1) PVC 2) nylon-6,6 3) bakelite 4) terylene — Answer: 1).
  • CET 2) Monomers of Buna-S? 1) butadiene + styrene 2) butadiene + acrylonitrile 3) chloroprene 4) isoprene — Answer: 1).

Real-life use: Plastics, textiles, rubber industry.

Remember

Addition = C=C, no by-product; condensation = two groups, H₂O; copolymer = two monomers.

Common mistake

Calling PVC a condensation polymer.

CET trap

PVC: C=C ⇒ addition.

Shortcut

Memory clue: "addition just joins; condensation joins and drops water".

Spot the difference

ModeBy-product
Additionnone
CondensationH₂O / HCl

Practice questions

Easy
  1. 1Steps of addition polymerisation.
  2. 2What is condensation?
  3. 3What is a copolymer?
Medium
  1. 4Monomers of Buna-S.
  2. 5An initiator.
  3. 6Two differences.
Challenging
  1. 7Type of nylon-6,6?
  2. 8Type of PVC?
Show answers (try them yourself first)
  1. 1Initiation, propagation, termination.
  2. 2Small molecule lost.
  3. 3Two monomers.
  4. 4Butadiene + styrene.
  5. 5Benzoyl peroxide.
  6. 6C=C vs two groups.
  7. 7Condensation.
  8. 8Addition.

Classification by intermolecular forces, monomers and biodegradability

From daily life

A rubber band stretches, a nylon thread does not, bakelite never melts — forces differ.

Simple definition

Four classes: elastomers (weak forces), fibres (strong H-bonds), thermoplastics, thermosetting.

ClassificationClassification of polymersTypeForcesExampleElastomerweak forcesbuna-S, neopreneFibrestrong H-bondsnylon, teryleneThermoplasticintermediatepolythene, PVCThermosettingcross-linkedbakeliteBy monomer: homopolymer (one) vs copolymer (two)Biodegradable: PHBV, nylon-2-nylon-6Thermoplastic softens on heating (reusable); thermosetting sets for good

In the figure : Classification.

Rule 1: By forces — Elastomers: weak van der Waals, coiled chains (Buna-S); fibres: strong H-bonds, crystalline (nylon, terylene); thermoplastics: intermediate (polythene, PVC).

  • Easy : Stretch and release ⇒ recoil (elastomer).
  • Medium : Fibres: high tensile strength.
  • Exam-type : Thermoplastics soften on heating.

Rule 2: Thermosetting — Heat gives a cross-linked, rigid solid that cannot be remoulded (bakelite, melamine); thermoplastics can be reused.

  • Easy : Bakelite: electrical switches.
  • Medium : PVC: pipes.
  • Exam-type : Remelt and reshape: thermoplastic.

Rule 3: Monomers and biodegradability — Homopolymer: one monomer (polythene); copolymer: two (Buna-S); biodegradable: PHBV, nylon-2-nylon-6 (broken down by microbes).

  • Easy : Polythene is not biodegradable.
  • Medium : PHBV: packaging.
  • Exam-type : Eco-friendly.

Reasoning: why fibres do not stretch but elastomers do

  • Step 1: Fibres have strong H-bonds and aligned chains.
  • Step 2: Elastomers have weak forces and coiled chains.
  • Step 3: On release the coils return ⇒ the shape recovers.

CET level (MCQ)

  • CET 1) A fibre? 1) nylon-6,6 2) Buna-S 3) PVC 4) bakelite — Answer: 1).
  • CET 2) Thermosetting? 1) bakelite 2) polythene 3) PVC 4) nylon — Answer: 1).

Real-life use: Rubber, clothing, switches, packaging.

Remember

Elastomer weak; fibre H-bonds; thermosetting cross-linked; PHBV biodegradable.

Common mistake

Calling polythene biodegradable.

CET trap

Polythene persists ⇒ pollution; PHBV is the alternative.

Shortcut

Memory clue: "rubber weak, thread H-bond, bakelite net".

Spot the difference

ClassExample
Elastomerbuna-S
Fibrenylon-6,6
Thermosettingbakelite

Practice questions

Easy
  1. 1Classify polymers.
  2. 2What is an elastomer?
  3. 3Why fibres do not stretch?
Medium
  1. 4Thermoplastic vs thermosetting.
  2. 5Homo vs copolymer.
  3. 6Biodegradable examples.
Challenging
  1. 7Uses of PHBV.
  2. 8Problem with polythene.
Show answers (try them yourself first)
  1. 1Four classes.
  2. 2Weak forces.
  3. 3Strong H-bonds.
  4. 4Softens vs does not.
  5. 5One vs two monomers.
  6. 6PHBV.
  7. 7Packaging, sutures.
  8. 8Persistent, non-degradable.

Natural and synthetic rubbers

From daily life

Cycle tyres and gloves are rubber; the latex of the rubber tree is its source.

Simple definition

Natural rubber = cis-1,4-polyisoprene; vulcanisation heats it with sulfur to form –S–S– cross-links.

RubberRubbers: natural and syntheticNatural rubber = cis-1,4-polyisoprene (from latex)CH₂=C(CH₃)–CH=CH₂ (isoprene) → –[CH₂–C(CH₃)=CH–CH₂]ₙ–Vulcanisation: S at ≈ 150 °C ⇒ –S–S– cross-links ⇒ stronger, elasticGutta-percha: trans-isoprene, hardRubberMonomersUseNeoprenechloropreneoil resistantBuna-Sbutadiene + styrenetyresBuna-Nbutadiene + acrylonitrileoil hoses

In the figure : Rubber.

Rule 1: Natural rubber — Latex (a colloid) coagulates with acid; monomer isoprene; all double bonds cis ⇒ coiled and elastic; gutta-percha is trans ⇒ hard.

  • Easy : cis is bent, trans is straight.
  • Medium : Gutta-percha: golf balls.
  • Exam-type : Monomer C₅H₈.

Rule 2: Vulcanisation — Raw rubber is sticky and softens; S with an accelerator at ≈ 150 °C gives –S–S– links ⇒ strong, durable, elastic; 40–45 % S ⇒ hard ebonite.

  • Easy : Tyres: about 5 % S.
  • Medium : Goodyear, 1839.
  • Exam-type : More S ⇒ harder.

Rule 3: Synthetic rubber — Neoprene: chloroprene (2-chlorobuta-1,3-diene), oil-resistant; Buna-S: butadiene + styrene, tyres; Buna-N: butadiene + acrylonitrile, oil hoses.

  • Easy : Neoprene resists oil.
  • Medium : Buna-S resists abrasion.
  • Exam-type : Buna-N: fuel hoses.

Reasoning: why vulcanisation strengthens rubber

  • Step 1: In raw rubber the chains slip past each other.
  • Step 2: S adds at double bonds and forms –S–S– bridges.
  • Step 3: Bridges stop slipping ⇒ strong and elastic.

CET level (MCQ)

  • CET 1) Monomer of natural rubber? 1) isoprene 2) chloroprene 3) styrene 4) ethene — Answer: 1).
  • CET 2) Monomer of neoprene? 1) chloroprene 2) isoprene 3) butadiene 4) vinyl chloride — Answer: 1).

Real-life use: Tyres, gloves, hoses, seals.

Remember

Rubber = cis-polyisoprene; vulcanisation = S; neoprene = chloroprene.

Common mistake

Calling gutta-percha cis.

CET trap

Gutta-percha is trans.

Shortcut

Memory clue: "cis = stretchy rubber, trans = hard gutta".

Spot the difference

RubberMonomer
Naturalisoprene
Neoprenechloroprene
Buna-Sbutadiene + styrene

Practice questions

Easy
  1. 1Structure of natural rubber.
  2. 2What is vulcanisation?
  3. 3Why do it?
Medium
  1. 4Neoprene.
  2. 5Buna-S.
  3. 6Buna-N.
Challenging
  1. 7cis vs trans.
  2. 8What is latex?
Show answers (try them yourself first)
  1. 1cis-polyisoprene.
  2. 2Heating with S.
  3. 3Strong, durable.
  4. 4From chloroprene.
  5. 5Butadiene + styrene.
  6. 6Butadiene + AN.
  7. 7Bent vs straight.
  8. 8Rubber-tree sap.

Addition polymers: polythene, teflon, PVC, orlon

From daily life

Water pipes (PVC), non-stick pans (Teflon) and plastic bags (polythene) are addition polymers.

Simple definition

An addition polymer comes from C=C monomers giving –(C–C)ₙ–, using all atoms.

AdditionAddition polymersPolymerMonomerConditionspolythene (LDPE)CH₂=CH₂1000–2000 atm, 350–570 K, peroxidepolythene (HDPE)CH₂=CH₂Ziegler–Natta, 333–343 K, 6–7 atmTeflonCF₂=CF₂persulfate, high pressurePVCCH₂=CHClperoxideOrlonCH₂=CH–CNperoxideUses: bags, non-stick coating (Teflon), pipes (PVC), wool-like fibre (Orlon)100 % atom economy

In the figure : Addition polymers.

Rule 1: Polythene — LDPE: 1000–2000 atm, 350–570 K, peroxide ⇒ branched, soft; HDPE: Ziegler–Natta (TiCl₄ + Al(C₂H₅)₃), 333–343 K, 6–7 atm ⇒ linear, hard.

  • Easy : LDPE: bags.
  • Medium : HDPE: buckets, bottles.
  • Exam-type : Density HDPE > LDPE.

Rule 2: Teflon and PVC — Teflon: CF₂=CF₂ (tetrafluoroethene), persulfate, high pressure ⇒ chemically inert, non-stick; PVC: CH₂=CHCl ⇒ pipes, cables.

  • Easy : C–F bond is strong.
  • Medium : PVC: rigid, or soft with plasticiser.
  • Exam-type : Unit –CF₂–CF₂–.

Rule 3: Orlon — Polyacrylonitrile from CH₂=CH–CN gives a wool-like fibre (acrilan) for sweaters and blankets.

  • Easy : Monomer acrylonitrile.
  • Medium : Also in Buna-N.
  • Exam-type : Artificial wool.

Calculation: DP of PVC (M = 62500), monomer vinyl chloride M = 62.5

  • Step 1: DP = M(polymer)/M(unit).
  • Step 2: 62500 / 62.5.
  • Step 3: DP = 1000.

CET level (MCQ)

  • CET 1) Monomer of Teflon? 1) CF₂=CF₂ 2) CH₂=CHCl 3) CH₂=CHCN 4) CH₂=CH₂ — Answer: 1).
  • CET 2) Monomer of Orlon? 1) CH₂=CHCN 2) CH₂=CHCl 3) CH₂=CH₂ 4) CF₂=CF₂ — Answer: 1).

Real-life use: Pipes, non-stick pans, bags, sweaters.

Remember

LDPE high pressure; HDPE Ziegler–Natta; Teflon CF₂=CF₂; Orlon CH₂=CHCN.

Common mistake

Citing high pressure for HDPE.

CET trap

HDPE: low pressure, Ziegler–Natta.

Shortcut

Memory clue: "LDPE needs HIGH pressure, HDPE LOW — opposite!".

Spot the difference

PolymerMonomer
PVCCH₂=CHCl
TeflonCF₂=CF₂
OrlonCH₂=CHCN

Practice questions

Easy
  1. 1Making LDPE.
  2. 2Making HDPE.
  3. 3Properties of Teflon.
Medium
  1. 4Uses of PVC.
  2. 5Orlon.
  3. 6Ziegler–Natta catalyst.
Challenging
  1. 7DP of PVC.
  2. 8Feature of addition.
Show answers (try them yourself first)
  1. 1High pressure, peroxide.
  2. 2Ziegler–Natta.
  3. 3Inert, non-stick.
  4. 4Pipes, cables.
  5. 5Polyacrylonitrile.
  6. 6TiCl₄ + Al(C₂H₅)₃.
  7. 71000.
  8. 8100 % of atoms.

Condensation polymers: nylons, terylene, bakelite, rayon

From daily life

Parachute nylon cords, polyester shirts and electric switches are condensation polymers.

Simple definition

Nylon-6,6: hexamethylenediamine + adipic acid; terylene: ethylene glycol + terephthalic acid; bakelite: phenol + HCHO.

CondensationCondensation polymersPolymerMonomersTemp.Nylon-6,6hexamethylenediamine + adipic acid553 KNylon-6caprolactam (+ H₂O)533–543 KTeryleneethylene glycol + terephthalic acid420–460 KBakelitephenol + HCHOacid/baseNylon-6,6: –[NH(CH₂)₆NHCO(CH₂)₄CO]ₙ–; repeat unit M = 226Terylene = Dacron (polyester); Bakelite = cross-linked thermosetRayon: regenerated cellulose (artificial silk)

In the figure : Condensation polymers.

Rule 1: Nylons — Nylon-6,6: 553 K, high pressure, amide link –CONH–; nylon-6: caprolactam + water, 533–543 K; ropes, brushes, fabrics.

  • Easy : A polyamide.
  • Medium : Nylon-6,6: 6 C + 6 C.
  • Exam-type : H-bonds ⇒ strong fibre.

Rule 2: Terylene and rayon — Terylene (Dacron): glycol + terephthalic acid, 420–460 K, Zn(OAc)₂ + Sb₂O₃ ⇒ polyester (ester link); rayon: regenerated cellulose.

  • Easy : Polyester shirts.
  • Medium : Ester –COO–.
  • Exam-type : Rayon: artificial silk.

Rule 3: Bakelite — Phenol + formaldehyde (acid/base) ⇒ novolac (linear) ⇒ heating gives cross-linked bakelite; thermosetting, electrical insulator.

  • Easy : Switches, handles.
  • Medium : CH₂ bridges at o- and p-.
  • Exam-type : Does not remelt.

Calculation: nylon-6,6 repeat unit (M = 226) and polymer M = 22600

  • Step 1: C₁₂H₂₂N₂O₂ ⇒ 12×12 + 22 + 28 + 32 = 226.
  • Step 2: DP = 22600/226.
  • Step 3: DP = 100 units (200 monomers).

CET level (MCQ)

  • CET 1) Linkage in nylon-6,6? 1) amide 2) ester 3) ether 4) C–C — Answer: 1).
  • CET 2) Terylene is a? 1) polyester 2) polyamide 3) polyolefin 4) polyether — Answer: 1).

Real-life use: Ropes, clothes, switches, handles.

Remember

Nylon = amide; terylene = ester; bakelite = phenol + HCHO; rayon = cellulose.

Common mistake

Thinking rayon comes from synthetic monomers.

CET trap

Rayon comes from natural cellulose.

Shortcut

Memory clue: "nylon → N (amide); terylene → E (ester)".

Spot the difference

PolymerLink
Nylonamide
Teryleneester
Bakelite–CH₂–

Practice questions

Easy
  1. 1Preparing nylon-6,6.
  2. 2Nylon-6.
  3. 3Preparing terylene.
Medium
  1. 4Preparing bakelite.
  2. 5What is rayon?
  3. 6Uses of nylon.
Challenging
  1. 7DP for 22600.
  2. 8What is a polyester?
Show answers (try them yourself first)
  1. 1Diamine + adipic acid.
  2. 2Caprolactam.
  3. 3Glycol + acid.
  4. 4Phenol + HCHO.
  5. 5Regenerated cellulose.
  6. 6Ropes, fabrics.
  7. 7100 units.
  8. 8With ester links.

Molecular mass, degree of polymerisation and biodegradable polymers

From daily life

Chains in a sample are not equally long, so averages are used.

Simple definition

M̄n = ΣNM/ΣN; M̄w = ΣNM²/ΣNM; PDI = M̄w/M̄n ≥ 1; DP = M/M(unit).

DPMolecular mass and DPMchainsM̄n = ΣNM / ΣNM̄w = ΣNM² / ΣNMPDI = M̄w / M̄n ≥ 1DP = M(polymer) / M(unit)10 × 10000 and 10 × 30000⇒ Mn 20000, Mw 25000, PDI 1.25Polythene 28000 ÷ 28 ⇒ DP = 1000Biodegradable: PHBV (3-hydroxybutanoic + 3-hydroxypentanoic acid)Nylon-2-nylon-6: glycine + aminocaproic acid

In the figure : M̄n, M̄w and DP.

Live figure

Molecular mass, degree of polymerisation and biodegradable polymers

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Rule 1: M̄n and M̄w — M̄n weights by number; M̄w by mass (big chains count more) ⇒ M̄w ≥ M̄n; PDI = 1 only if all chains are equal.

  • Easy : PDI = 1 is ideal.
  • Medium : Addition gives higher PDI.
  • Exam-type : PDI < 1 is impossible.

Rule 2: Degree of polymerisation — DP = M(polymer)/M(unit); polythene 28000 ⇒ 1000; nylon-6,6 22600 ⇒ 100.

  • Easy : PVC 62500 ⇒ 1000.
  • Medium : Larger DP ⇒ stronger.
  • Exam-type : Check the unit M first.

Rule 3: Biodegradable — PHBV: 3-hydroxybutanoic + 3-hydroxypentanoic acid; nylon-2-nylon-6: glycine + 6-aminohexanoic acid; microbes break the ester/amide links.

  • Easy : PHBV: packaging, drug release.
  • Medium : Eco-friendly.
  • Exam-type : Less waste.

Calculation: 10 chains of M = 10000 and 10 chains of M = 30000

  • Step 1: M̄n = (10×10⁴ + 10×3×10⁴)/20 = 20000.
  • Step 2: M̄w = (10×10⁸ + 10×9×10⁸)/(4×10⁵) = 25000.
  • Step 3: PDI = 25000/20000 = 1.25.

CET level (MCQ)

  • CET 1) PDI is always? 1) ≥ 1 2) < 1 3) = 0 4) < 0 — Answer: 1).
  • CET 2) Biodegradable? 1) PHBV 2) PVC 3) Teflon 4) polythene — Answer: 1).

Real-life use: Packaging, sutures, drug delivery.

Remember

M̄w ≥ M̄n; PDI ≥ 1; DP = M/M(unit); PHBV and nylon-2-nylon-6.

Common mistake

Thinking M̄w < M̄n.

CET trap

M̄w ≥ M̄n; PDI is never below 1.

Shortcut

Memory clue: "w (weight) is the bigger, n (number) the smaller".

Spot the difference

QuantityFormula
M̄nΣNM/ΣN
M̄wΣNM²/ΣNM
PDIM̄w/M̄n
DPM/M(unit)

Practice questions

Easy
  1. 1What is M̄n?
  2. 2What is M̄w?
  3. 3What is PDI?
Medium
  1. 4Define DP.
  2. 5DP for 28000.
  3. 6M̄n of the given chains.
Challenging
  1. 7Monomers of PHBV.
  2. 8Nylon-2-nylon-6.
Show answers (try them yourself first)
  1. 1Number average.
  2. 2Mass average.
  3. 3M̄w/M̄n.
  4. 4M/M(unit).
  5. 51000.
  6. 620000.
  7. 7Two hydroxy acids.
  8. 8Glycine + acid.

Quick revision of the whole lesson

ConceptKey formula / fact
DPDP = M(polymer)/M(unit)
Number avg.M̄n = ΣNM/ΣN
Mass avg.M̄w = ΣNM²/ΣNM
PDIM̄w/M̄n ≥ 1
Nylon-6,6diamine + adipic acid → polyamide
Teryleneglycol + terephthalic acid → polyester
Bakelitephenol + HCHO → network
Vulcanisationrubber + S (≈ 150 °C)
Live figure

🚀 Quick revision of the whole lesson

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🎯 Most important points for the exam

  • Chain structure.
  • Addition, condensation.
  • Classification.
  • Rubbers.
  • Addition polymers.
  • Condensation polymers.
  • DP, biodegradable.
Study mantra

understand the idea → remember the formula → solve the example → solve problems yourself.