MATS3004 · Polymer Science and Engineering 1
Free-Radical Polymerization Mechanism
Lecture 3 walks through the four elementary steps of free-radical chain polymerization — initiation, propagation, termination (combination vs disproportionation) and chain transfer — using the styrene-with-AIBN scheme. The exams ask you to draw each step's structures for a named monomer/initiator and to identify which monomers polymerize by the radical route, so the bankable skill is reproducing a clean, correctly-labelled mechanism and knowing the two termination modes apart.
What this chapter covers
- 01Initiation as two sub-steps: initiator decomposition I → 2R• (rate constant kd) then first addition R• + M → M₁• (rate constant ki)
- 02Initiator families: organic peroxides (benzoyl peroxide), hydroperoxides, azo compounds (AIBN); radicals made thermally, photochemically or by redox
- 03Propagation Mₙ• + M → Mₙ₊₁• (rate coefficient kp), head-to-tail addition across the C=C
- 04Termination by combination (two radicals join → one dead chain) vs disproportionation (H-transfer → one saturated + one vinyl-ended chain); kt = ktc + ktd
- 05Chain transfer to solvent/monomer/initiator/transfer agent — starts a new chain and lowers molar mass
- 06Which monomers suit the radical route (vinyls: styrene, vinyl chloride, vinyl acetate, acrylates/methacrylates, acrylonitrile)
- 07Monomer geometry: monosubstituted or 1,1-disubstituted vinyls polymerize well; 1,2-disubstituted are sterically hindered
- 08The living-radical extension (RAFT) as a controlled variant
Write the free-radical mechanism for methyl methacrylate with benzoyl peroxide
- +1Initiation (two sub-steps). BPO decomposes thermally at its O−O bond into two benzoyloxy radicals: BPO → 2 (C₆H₅COO)• (rate constant kd). One radical then adds across the MMA C=C to make the first chain radical: (C₆H₅COO)• + CH₂=C(CH₃)COOCH₃ → (C₆H₅COO)−CH₂−•C(CH₃)COOCH₃ (rate constant ki).
- +1Propagation. The chain radical adds monomer repeatedly, head-to-tail, so the radical always sits on the more-substituted carbon: ~CH₂−•C(CH₃)COOCH₃ + CH₂=C(CH₃)COOCH₃ → ~CH₂−C(CH₃)(COOCH₃)−CH₂−•C(CH₃)COOCH₃ (rate coefficient kp).
- +1Termination by combination. Two growing radicals couple head-to-head to give one dead chain, joining the two chains into a single molecule (rate constant ktc): ~Mₙ• + •Mₘ~ → ~Mₙ−Mₘ~.
- +1Termination by disproportionation + chain transfer. In disproportionation an α-hydrogen transfers from one radical to another, giving two dead chains — one saturated, one with a terminal C=C (rate constant ktd). Separately, chain transfer moves the radical to solvent, monomer, initiator or a transfer agent, ending that chain and starting a new one (this lowers molar mass and is ignored in the ideal kinetics).
Key terms
- Initiation
- The two-step start of a radical chain: the initiator decomposes to primary radicals, I → 2R• (rate constant kd), then a radical adds to the first monomer, R• + M → M₁• (rate constant ki). Decomposition is the slow, rate-determining sub-step.
- Initiator
- A molecule that generates radicals — organic peroxides (benzoyl peroxide), hydroperoxides or azo compounds such as AIBN — by thermal, photochemical or redox decomposition. A peroxide contains an O−O bond that cleaves to two oxygen radicals.
- Propagation
- Repeated head-to-tail addition of monomer to the chain radical, Mₙ• + M → Mₙ₊₁• (rate coefficient kp). The active centre stays on the chain end and the chain grows one repeat unit at a time.
- Termination by combination
- Two growing radicals couple to form a single dead chain (rate constant ktc); the two chains merge, adding their degrees of polymerization. Radical polymers that terminate this way have a dispersity near 1.5.
- Termination by disproportionation
- One radical abstracts a hydrogen from another, giving two dead chains — one saturated and one with a terminal C=C (rate constant ktd). This mode drives dispersity toward 2. Overall kt = ktc + ktd.
- Chain transfer
- Transfer of the radical to another species (solvent, monomer, initiator or a deliberate transfer agent), which ends the current chain and begins a new one. It lowers molar mass and is assumed absent in the ideal steady-state kinetics.
Free-Radical Polymerization Mechanism FAQ
What is the difference between termination by combination and by disproportionation?
Both end two radicals, but differently. In combination the two radical chain-ends join directly, so two chains merge into one longer dead chain and their degrees of polymerization add. In disproportionation a hydrogen atom transfers from one radical to the other, giving two separate dead chains — one fully saturated and one carrying a terminal carbon-carbon double bond. Combination tends to give a dispersity near 1.5, disproportionation nearer 2, and many systems do a mix of both (kt = ktc + ktd).
How does chain transfer affect the polymer?
Chain transfer moves the radical off the growing chain onto another molecule — solvent, monomer, initiator or an added transfer agent — which stops that chain (capping it at a lower length) and starts a fresh one. The overall rate of polymerization is barely changed because a radical is still active, but the average molar mass drops. That is why the standard kinetic derivations explicitly assume no chain transfer, and why solvents are chosen to minimise it.
Which monomers polymerize by the free-radical route?
Most common vinyl (mono- or 1,1-disubstituted) monomers: styrene, vinyl chloride, vinyl acetate, the acrylates and methacrylates (such as MMA), acrylonitrile and dienes like butadiene. Monomers with a strongly electron-withdrawing or electron-donating substituent may prefer an ionic route, and 1,2-disubstituted vinyls are too sterically hindered to polymerize well. The mechanism you draw only needs a monosubstituted or 1,1-disubstituted C=C to work.
How is the mechanism examined in MATS3004?
You are asked to reproduce the four steps for a named monomer/initiator pair and to identify termination mode or suitable monomers. Marks go to correct structures at each step (radical on the right carbon, both termination modes shown) and correct labelling of the rate constants (kd, ki, kp, ktc, ktd). Practise redrawing the whole scheme from memory for a monomer other than styrene. Confirm the examinable set on the UNSW course outline / Moodle.
Exam move
Rehearse the four-step mechanism until you can draw it cold for any common vinyl monomer (MMA, vinyl acetate, acrylonitrile), not just the styrene example from lecture, because the exam swaps the monomer to test understanding rather than recall. Fix the head-to-tail rule (radical on the more-substituted carbon) and keep combination visibly different from disproportionation — the terminal double bond is the giveaway for disproportionation. Know the initiator families and how each makes radicals (thermal, photochemical, redox), and be able to say in one line why chain transfer lowers molar mass without much changing the rate. This mechanism is the setup for the kinetics chapter, so getting the species and rate constants right here pays off in the Rp derivation. Confirm the assessed monomer/initiator scope on the UNSW course outline / Moodle.
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