Addition Polymers

• How chain-growth (addition) polymerisation differs from step-growth
• The three stages of free radical polymerisation: initiation, propagation, termination
• Cationic and anionic polymerisation and their catalysts
• Structures and monomers of PVC, polythene, Teflon, and polystyrene
• Why LDPE and HDPE have different densities, structures, and uses
• How vulcanisation improves natural rubber, and what neoprene is

What is Addition Polymerisation?

In addition polymerisation (also called chain-growth polymerisation), monomers containing a C=C double bond add to each other repeatedly without losing any small molecules. The molecular formula of the repeating unit is identical to the monomer.

General equation: n CH₂=CHX → −(CH₂−CHX)ₙ−

The driving force is opening of the π-bond, which is weaker (~265 kJ/mol) than the σ-bond formed (~350 kJ/mol).

Free Radical Polymerisation

The most common mechanism. It has three stages:

Stage 1 — Initiation A free radical initiator (e.g., benzoyl peroxide) decomposes on heating to give free radicals (R•). The radical adds to a monomer molecule, creating a new radical:

R• + CH₂=CH₂  →  R−CH₂−CH₂•

Stage 2 — Propagation The radical chain grows by successive addition of monomer units:

R−CH₂−CH₂• + CH₂=CH₂  →  R−CH₂−CH₂−CH₂−CH₂•
(continues thousands of times)

Stage 3 — Termination Two growing chains combine (coupling) or disproportionate, destroying both radicals and stopping chain growth:

~CH₂• + •CH₂~  →  ~CH₂−CH₂~   (coupling)

Cationic Polymerisation

• Catalyst: Lewis acid (e.g., BF₃, AlCl₃) + co-catalyst (trace H₂O)
• The Lewis acid generates a carbocation that initiates the chain
• Works well with electron-rich monomers (isobutylene, vinyl ethers)
• Example: Polyisobutylene (used in inner tubes)

BF₃ + H₂O  →  H⁺[BF₃OH]⁻
H⁺ + CH₂=C(CH₃)₂  →  CH₃−C⁺(CH₃)₂   (initiation)

Anionic Polymerisation

• Initiator: n-butyllithium (n-BuLi), NaNH₂, or other strong bases/nucleophiles
• Generates a carbanion that adds to monomer
• Produces living polymers with very narrow molecular weight distribution
• Works with electron-poor monomers (acrylonitrile, styrene)

Key Vinyl Polymers

Mnemonic for vinyl polymers: "Please Tell Students Polymer Chemistry" → PP, Teflon, Styrene/PS, Polythene, PVC (matched to CF₂=CF₂, CH₂=CH₂, CH₂=CHC₆H₅, CH₂=CH₂, CH₂=CHCl)

LDPE vs HDPE

Both are polyethylene but made by very different processes:

Why branched = lower density? Branches prevent chains from packing closely, so LDPE is less dense and less crystalline than HDPE.

Ziegler-Natta Catalyst: A combination of TiCl₄ (transition metal halide) + triethylaluminium Al(C₂H₅)₃ (organometallic). It coordinates the monomer and inserts it in a controlled stereoregular fashion. Nobel Prize (Ziegler and Natta, 1963).

Rubber Vulcanisation

Natural rubber = cis-1,4-polyisoprene. It is elastic but soft, sticky in summer, and brittle in winter. The C=C double bonds make it reactive.

Vulcanisation (discovered by Charles Goodyear, 1839): Natural rubber is heated with sulphur (S₈) at 140–160 °C. Sulphur forms crosslinks (−S−S− bridges) between polymer chains at the sites of double bonds.

Rubber chains:  ~−CH₂−C(CH₃)=CH−CH₂−~
After vulcanisation:   −S−S− bridges crosslink adjacent chains

Effect of crosslinking:

• Increases elasticity and resilience (chains can stretch but snap back)
• Reduces stickiness and tackiness
• Improves tensile strength and wear resistance
• Makes rubber temperature-resistant

More sulphur → more crosslinks → harder rubber (ebonite = hard rubber, ~30–35% S).

Neoprene (Polychloroprene)

• Monomer: Chloroprene (2-chloro-1,3-butadiene), CH₂=CCl−CH=CH₂
• Polymer: Neoprene = polychloroprene
• Mechanism: Free radical addition

n CH₂=CCl−CH=CH₂  →  −(CH₂−CCl=CH−CH₂)ₙ−

Properties: Resistant to oils, ozone, weather, and flame (Cl atom provides flame retardance). Uses: Wetsuits, hoses, gaskets, cable jacketing.

Chain Transfer — Why Free Radical Gives Branched LDPE

In free radical polymerisation at high pressure, a growing chain radical can abstract a hydrogen atom from the middle of another chain (intramolecular or intermolecular chain transfer). This creates a radical in the middle of an existing chain, which then grows outward as a branch. This is why LDPE made by free radical mechanism is branched.

In Ziegler-Natta catalysis, monomer insertion is stereospecific at the metal centre, no chain transfer to polymer occurs, giving a purely linear HDPE.

Tacticity in Vinyl Polymers

Ziegler-Natta catalysts also control tacticity — the spatial arrangement of substituents along the chain:

• Isotactic: All substituents on same side → highly crystalline, high melting point (isotactic PP, m.p. ≈165 °C)
• Syndiotactic: Alternating sides → crystalline
• Atactic: Random arrangement → amorphous, lower m.p.

Free radical polymerisation gives atactic (random) polymers.

Why Teflon is Non-Stick

In PTFE, all H atoms are replaced by F. The C−F bond (bond energy ≈ 485 kJ/mol) is one of the strongest bonds in organic chemistry. F atoms completely shield the carbon backbone:

• Chemically inert to almost all reagents (including aqua regia!)
• Very low surface energy → nothing adheres (non-stick)
• High melting point (~327 °C)
• Excellent electrical insulator

Degree of Polymerisation and Molecular Weight

$\bar{M}_n = n \times M_0$

where n = degree of polymerisation (number of repeat units), M₀ = molecular weight of repeat unit.

For polythene: M₀ = 28 g/mol (−CH₂−CH₂−). If n = 50,000, then M̄ₙ = 1,400,000 g/mol.

Real polymers have a distribution of chain lengths. The polydispersity index (PDI) = M̄w/M̄n (living polymers from anionic polymerisation can give PDI ≈ 1.0).

Example 1: Identifying Monomer from Polymer Structure

Problem: A polymer has the repeating unit −(CH₂−CHCl)ₙ−. Identify the monomer,
name the polymer, and state the mechanism of polymerisation.

Step 1: The repeating unit is −CH₂−CHCl−
        Since this is addition polymerisation, the monomer has a C=C double bond.
        Monomer = CH₂=CHCl (vinyl chloride)

Step 2: Name the polymer:
        Poly(vinyl chloride) = PVC

Step 3: Mechanism:
        PVC is made by free radical addition polymerisation.
        Initiator: benzoyl peroxide or AIBN (azobisisobutyronitrile)
        Conditions: ~60 °C, aqueous suspension or emulsion

Answer: Monomer = CH₂=CHCl; Polymer = PVC; Mechanism = Free radical addition

Example 2: LDPE vs HDPE — Choosing the Right Polymer

Problem: A manufacturer needs a rigid pipe for carrying water under pressure.
Should they use LDPE or HDPE? Justify with structural reasoning.

Step 1: LDPE is made by free radical polymerisation at 1000–2000 atm.
        It has branched chains → low crystallinity → soft and flexible.
        NOT suitable for rigid pressure pipes.

Step 2: HDPE is made using Ziegler-Natta catalyst at low pressure (~6 atm).
        It has linear chains → high crystallinity (~90%) → rigid and strong.
        High tensile strength and chemical resistance.

Step 3: For a rigid water pipe, HDPE is the correct choice.

Answer: HDPE — linear chains pack closely, giving high density, rigidity,
        and strength needed for pressure piping applications.

Bonus fact: The "2" resin code on plastic bottles means HDPE.

1. Write the monomer of Teflon. What makes its C−F bond so strong?
2. How does vulcanisation improve the properties of natural rubber? Name the scientist who discovered it.
3. What is the role of Ziegler-Natta catalyst in making HDPE? Name its two components.
4. Distinguish between initiation and propagation in free radical polymerisation with an equation.
5. (Stretch) Polystyrene made by free radical polymerisation is atactic and amorphous. The same monomer polymerised with a Ziegler-Natta catalyst gives isotactic polystyrene with a melting point of ~240 °C. Explain why tacticity affects melting point, and predict which form would be better for making heat-resistant cups.

• NCERT Class 12 Chemistry, Chapter 15 — Polymers
• Sections 15.2 (Classification), 15.3 (Types of Polymerisation), 15.4 (Natural and Synthetic Rubbers)
• JEE Main: 1–2 questions from this section; typically monomer identification, LDPE/HDPE comparison, or vulcanisation
• Common MCQ formats: "which polymer is made from CF₂=CF₂?", "HDPE is made by…", "vulcanisation involves crosslinking with…"

Study strategy: Learn monomer→polymer pairs as a table (flashcard style). For mechanisms, remember: free radical = high pressure/peroxide initiator; Ziegler-Natta = low pressure/TiCl₄+AlEt₃. Vulcanisation = sulphur crosslinks = Charles Goodyear.

Practice MCQs on addition polymer monomers, LDPE/HDPE properties, and vulcanisation in the Addition Polymers topic bank — start at Easy to build the monomer table, then Medium for mechanism questions.

Confused about why branching makes LDPE less dense than HDPE? Ask the Drishti AI tutor to explain with a diagram of how chain branching disrupts crystalline packing.

Complete the Quick Check questions and mark them in your Drishti progress tracker. Aim for 4/5 before moving to condensation polymers.

• Read: Condensation Polymers — nylon-6, nylon-6,6, Dacron, Bakelite
• Then: Natural and Commercial Polymers — natural rubber, starch, cellulose, silicones
• Practice: Mixed polymer MCQs (Medium difficulty, focus on identification)