Polymers: addition and condensation polymerisation

Polymers: Addition and Condensation Polymerisation

Learning Outcomes (Cambridge International AS & A Level Chemistry 9701 – Topic 20)

Syllabus Code	Learning Outcome (LO)	What you should be able to do
20.1 LO 1	Distinguish between addition (chain‑growth) and condensation (step‑growth) polymerisation.	State the key features, give typical monomers and name at least two polymers of each type, and produce a concise comparison table.
20.2 LO 2	Describe the three elementary steps of radical chain‑growth polymerisation (initiation, propagation, termination).	Write the corresponding equations, explain the role of initiators, and discuss rate laws, chain‑transfer and inhibition.
20.2 LO 3	Explain cationic and anionic chain‑growth polymerisation.	Give one example of each, indicate the catalyst/initiator, comment on stereochemical control and on “living” anionic polymerisation.
20.3 LO 4	Describe the step‑growth (condensation) mechanism, including the equilibrium nature of the reaction.	Write the overall reaction, the equilibrium constant expression and state why >95 % conversion is required for high DP.
20.4 LO 5	Identify factors that influence polymer properties (DP, branching, intermolecular forces, cross‑linking).	Relate each factor to a real polymer (e.g., HDPE vs LDPE, Nylon‑6,6, Kevlar).

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1. Introduction to Polymers

• Polymers are macromolecules built from repeating units (monomers) linked by covalent bonds.
• Cambridge A‑Level concentrates on two broad classes of polymerisation:
  • Addition (chain‑growth) polymerisation
  • Condensation (step‑growth) polymerisation

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2. Addition (Chain‑Growth) Polymerisation

2.1 Comparison of Addition vs. Condensation (LO 1)

Feature	Addition (Chain‑Growth)	Condensation (Step‑Growth)
Monomer type	Contains a C=C double bond or a strained ring (e.g., ethene, vinyl chloride, oxirane)	Contains two complementary functional groups (–OH, –COOH, –NH₂, –COCl, –NCO, etc.)
Growth mechanism	Polymer chain grows from an active centre (radical, carbocation or carbanion); only the chain end reacts.
Growth mechanism	Any two reactive species (monomer, dimer, oligomer, polymer) can combine; growth is not limited to chain ends.
By‑product	None (atoms are retained in the polymer backbone).	A small molecule is eliminated each step (H₂O, HCl, CH₃OH, NH₃, etc.).
Molecular‑weight build‑up	Very rapid; high DP can be reached at low conversion.	DP = 1/(1 – p); high DP only when conversion (p) > 0.95‑0.99.
Typical polymers	PE, PP, PS, PVC, PMMA, poly(isobutene), poly(ethylene oxide).	PET, Nylon‑6,6, polyurethanes, polycarbonates, phenol‑formaldehyde resin.

2.2 Radical Chain‑Growth Polymerisation (LO 2)

Elementary steps

1. Initiation – generation of a reactive radical.
2. Propagation – addition of monomer to the growing radical.
3. Termination – two radicals combine (combination) or transfer hydrogen (disproportionation).

Typical equations (using ethene as example)

Initiation:
  RO–OR  →  2 ·OR      (thermal/photolytic cleavage of a peroxide)
  ·OR + CH₂=CH₂  →  ·CH₂CH₂–OR   (radical adds to monomer)

Propagation (repeated):
  ·CH₂CH₂–OR  +  CH₂=CH₂  →  –CH₂CH₂–CH₂CH₂·
  –CH₂CH₂–CH₂CH₂·  +  CH₂=CH₂  →  –[CH₂CH₂]₂–CH₂CH₂·   (chain length +1)

Termination:
  Combination: ·R + ·R′ → R–R′
  Disproportionation: ·CH₂CH₂–R + ·CH₂CH₂–R′ → CH₃CH₂–R + CH₂=CH–R′

Kinetic description (exam‑relevant)

• Rate of polymerisation, \(R_p = k_p [M][R^\bullet]\).
• Steady‑state radical concentration, \( [R^\bullet] = \sqrt{\dfrac{2f k_d[I]}{k_t}} \) where:
  • \(k_d\) – rate constant for initiator decomposition,
  • \(f\) – efficiency of radical formation,
  • \([I]\) – initiator concentration,
  • \(k_t\) – termination rate constant.
• Overall: \(R_p = k_p [M] \sqrt{\dfrac{2f k_d[I]}{k_t}}\). This shows why polymerisation rate increases with monomer concentration and initiator amount.

Chain‑transfer and inhibition (AO2)

• Chain‑transfer agents (e.g., thiols, halogenated compounds) react with a growing radical to give a new, less reactive radical, reducing DP: \[ R^\bullet + X\!-\!H \rightarrow R\!-\!H + X^\bullet \]
• Inhibitors (e.g., quinones, oxygen) react rapidly with radicals, preventing polymerisation. In the lab, oxygen must be excluded.

2.3 Cationic & Anionic Chain‑Growth (LO 3)

Type	Typical Monomer	Catalyst / Initiator	Key Features
Cationic	Isobutene (CH₂=C(CH₃)₂)	Lewis acid (BF₃·OEt₂) or strong protic acid (H₂SO₄)	Propagation by carbocation; stereospecific (often gives isotactic poly‑isobutene). Termination by chain‑transfer to monomer or solvent.
Anionic	Styrene (C₆H₅CH=CH₂)	Organolithium (n‑BuLi) or alkoxide	Propagation by carbanion; “living” polymerisation possible (no termination step). Enables block‑copolymer synthesis such as PS‑b‑PMMA.

2.4 Ring‑Opening Addition Polymerisation

• Strained cyclic monomers open to give linear polymers without a by‑product.
• Examples:
  • Oxirane (ethylene oxide) → poly(ethylene oxide) – radical or anionic initiation.
  • ε‑Caprolactone → poly(ε‑caprolactone) – coordination polymerisation (Sn(II) octoate).

2.5 Representative Addition Polymers (LO 1)

• Polyethylene (PE) – from ethene (radical, Ziegler‑Natta).
• Polypropylene (PP) – from propene.
• Polystyrene (PS) – from styrene (radical or anionic).
• Poly(vinyl chloride) (PVC) – from vinyl chloride.
• Poly(methyl methacrylate) (PMMA) – from methyl methacrylate (radical).
• Poly(isobutene) – cationic polymerisation of isobutene.

2.6 Catalysts / Initiators (LO 2 & LO 3)

Category	Typical Reagent	Polymers Produced
Peroxides / Azo compounds (radical)	Benzoyl peroxide, AIBN	PE, PS, PMMA, PVC (radical)
Ziegler‑Natta (radical/coordination)	TiCl₄ / Al(C₂H₅)₃	HDPE, isotactic PP (stereospecific)
Lewis acids (cationic)	BF₃·OEt₂, H₂SO₄	Poly(isobutene), poly(vinyl ether)
Organometallics (anionic)	n‑BuLi, NaNH₂	Polystyrene, poly(tert‑butyl acrylate); living systems for block copolymers.

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3. Condensation (Step‑Growth) Polymerisation

3.1 General Features (LO 1 & LO 4)

• Monomers contain at least two complementary functional groups (e.g., –OH & –COOH, –NH₂ & –COOH, –NCO & –OH).
• Any two reactive species (monomer, dimer, oligomer, polymer) can combine; growth is not limited to chain ends.
• A small molecule (H₂O, HCl, CH₃OH, NH₃, etc.) is eliminated at each step.
• The reaction is reversible; high molecular weight is obtained only when the fractional conversion (p) is very high.

3.2 Step‑Growth Equilibrium

The equilibrium for a generic condensation of A‑X + B‑Y → A‑Y + B‑X (where X and Y are leaving groups) can be written as:

\[ K = \frac{[A\!-\!Y][B\!-\!X]}{[A\!-\!X][B\!-\!Y]} \]

For a simple diacid–diol polyester the equilibrium constant is:

\[ K = \frac{[Ester][H_2O]}{[Acid][Diol]} \]

Because water (or another by‑product) is continuously removed (vacuum, azeotropic distillation, inert‑gas sweep), the reaction is driven to the right, effectively increasing the conversion, p.

Degree of polymerisation (DP) – conversion relationship (LO 4)

\[ \text{DP} = \frac{1}{1-p} \]

• To reach DP ≈ 100, p ≈ 0.99 (99 % conversion).
• For DP ≈ 1000, p ≈ 0.999 (99.9 % conversion).
• Industrial processes therefore operate under conditions that continuously remove the by‑product and often employ high temperatures or catalysts to increase K.

3.3 Typical Condensation Mechanisms

(a) Polyester formation (acid + diol)

HO–R–OH   +   HOOC–R'–COOH   ⇌   –O–R–O–CO–R'–CO–   +   2 H₂O

(b) Polyamide formation (diacid + diamine)

H₂N–R–NH₂   +   HOOC–R'–COOH   ⇌   –NH–CO–R'–CO–NH–R–NH–   +   2 H₂O

(c) Polyurethane formation (di‑isocyanate + diol)

\[ \text{OCN–R–NCO} + \text{HO–R'–OH} \;\rightleftharpoons\; \text{–OC(O)NH–R'–NHCO–O–} + \text{H}_2\text{O} \]

3.4 Ring‑Opening Step‑Growth Example – Nylon‑6 (LO 4)

• Monomer: ε‑Caprolactam (a seven‑membered cyclic amide).
• Mechanism: Heat opens the ring to a linear diamine; two such units then condense, releasing ammonia.
• Overall reaction: \[ n\;\text{ε‑Caprolactam} \;\longrightarrow\; \bigl[–(CH_2)_5–CO–NH–\bigr]_n \;+\; n\,\text{NH}_3 \]

3.5 Scope of Monomers (LO 1)

• Diacids + Diols → polyesters (PET, PBT, poly(butylene succinate)).
• Diacids + Diamines → polyamides (Nylon‑6,6; Kevlar – p‑phenylenediamine + terephthaloyl chloride).
• Di‑isocyanates + Diols → polyurethanes (flexible foams, elastomers, coatings).
• Phenol + Formaldehyde → phenol‑formaldehyde resin (thermosetting, Bakelite).
• Bisphenol A + Phosgene → polycarbonate (PC).

3.6 Catalysts / Reaction Conditions (LO 4)

• Acid catalysts (H₂SO₄, p‑toluenesulfonic acid) – accelerate esterification and trans‑esterification.
• Base catalysts (NaOH, K₂CO₃) – used for polyamide formation (neutralise the acid generated).
• Metal salts (Zn(OAc)₂, TiCl₄) – promote trans‑esterification in polyester synthesis and improve molecular‑weight control.
• Removal of by‑product – vacuum, azeotropic distillation, nitrogen sweep or continuous flow reactors are employed to push the equilibrium toward polymer.

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4. Factors Influencing Polymer Properties (LO 5)

Factor	Effect on Properties	Illustrative Example
Degree of polymerisation (DP)	Higher DP → higher tensile strength, higher melting point, lower solubility.	HDPE (DP ≈ 10⁵) vs LDPE (DP ≈ 10⁴) – HDPE is stronger and has a higher melting point.
Branching / side‑chains	Increases free‑volume → lower density, lower crystallinity, lower melting point.	LDPE contains many short branches; it is more flexible and has a lower density than HDPE.
Inter‑molecular forces	Stronger forces (hydrogen‑bonding, dipole‑dipole) raise Tₘ and T_g.	Nylon‑6,6 (hydrogen‑bonded amide groups) has a higher Tₘ (~ 260 °C) than PET (dipole‑dipole).
Cross‑linking	Creates a three‑dimensional network → insoluble, thermosetting, higher rigidity.	Vulcanised rubber (S‑S cross‑links) and phenol‑formaldehyde resin.
Stereochemistry (isotactic, syndiotactic, atactic)	Regular (isotactic/syndiotactic) chains pack efficiently → higher crystallinity & Tₘ.	Isotactic polypropylene (high Tₘ) vs atactic polypropylene (amorphous, low Tₘ).

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5. Summary Checklist for the Exam

• Be able to contrast addition vs condensation polymerisation in a two‑column table.
• Write the three radical steps, include the rate law and explain the effect of initiator concentration.
• Identify a cationic and an anionic system; note catalyst, stereochemical control and the concept of “living” polymerisation.
• Derive the DP‑conversion relationship for step‑growth and state why >95 % conversion is essential.
• Recall at least two commercial polymers from each class and the monomers from which they are prepared.
• Link polymer properties to DP, branching, intermolecular forces, cross‑linking and stereochemistry with real‑world examples.