CHEM2522 · Sustainable Chemical Manufacture
Radical Chain-Growth Polymerisation
Week 9 covers free-radical chain-growth polymerisation (FRP): the initiation/propagation/termination kinetic chain, and how the mode of termination — combination versus disproportionation — sets the molar mass. You also meet chain transfer, the gel (Trommsdorff) effect, copolymerisation controlled by reactivity ratios, and why FRP cannot make block copolymers (leading to controlled methods such as RAFT). Exam questions test the termination-mode molar-mass logic, copolymer sequence from reactivity ratios, and the FRP-vs-step-growth molar-mass-vs-conversion contrast.
What this chapter covers
- 01The FRP kinetic chain: initiation (initiator → radicals), propagation (fast monomer addition), termination
- 02Termination by combination (two chains join, doubling molar mass) vs disproportionation (H transfer, molar mass unchanged)
- 03Methacrylates terminate mostly by disproportionation (~60%) because of available alpha-hydrogens
- 04Chain transfer (to monomer, solvent or another chain) lowers molar mass and can branch the chain
- 05The gel / Trommsdorff (Norrish) effect: rising viscosity slows termination, auto-accelerating the rate
- 06Copolymerisation and reactivity ratios r1, r2: r1 = r2 = 1 random; r → 0 alternating; r > 1 blocky
- 07Propagation-rate ordering (kp): methyl acrylate > MMA ≈ vinyl acetate >> styrene (reactivity inverse to radical stability)
- 08Why FRP can't make blocks (short-lived chain ends) → controlled radical polymerisation (RAFT)
Combination vs disproportionation: what termination does to molar mass
- +1Molar mass of one chain: DP × repeat mass = 100 × 100.12 = 10 012 g/mol (end groups neglected).
- +1Combination: the two radical chain ends couple into a single chain, so the molar mass roughly doubles: 2 × 10 012 = 20 024 g/mol.
- +1Disproportionation: one chain abstracts a hydrogen from the other, giving two separate dead chains of unchanged length — each stays at ~10 012 g/mol (one saturated, one with a terminal C=C).
- +1Dominant mode: methacrylates such as MMA terminate mostly by disproportionation (~60%) because the alpha-methyl group provides abstractable hydrogens; the bulky, hindered chain ends also resist the head-to-head coupling that combination requires.
Key terms
- Free-radical polymerisation (FRP)
- Chain-growth polymerisation via radical active centres: an initiator makes radicals, monomer adds rapidly in propagation, and the chain dies in termination. High molar mass forms almost immediately, unlike step-growth.
- Termination by combination
- Two growing radical chains couple end-to-end into one dead chain, roughly doubling the molar mass.
- Termination by disproportionation
- One radical chain abstracts a beta-hydrogen from another, giving two dead chains of unchanged length — one saturated, one with a terminal double bond. Favoured by methacrylates.
- Chain transfer
- Transfer of the radical to monomer, solvent, initiator or another polymer chain; it stops one chain and starts another, lowering molar mass and (if to a chain) causing branching.
- Gel (Trommsdorff) effect
- Auto-acceleration late in a bulk polymerisation: rising viscosity slows the diffusion-controlled termination step while propagation continues, so rate and molar mass spike.
- Reactivity ratios (r1, r2)
- The ratios that set copolymer sequence: r1 = r2 = 1 gives a random copolymer, r values near 0 give alternating, r > 1 gives blocky runs of one monomer.
Radical Chain-Growth Polymerisation FAQ
How does the termination mode change the molar mass?
Combination joins two live chain ends into one dead chain, so the product molar mass is about the sum of the two — roughly double a single growing chain. Disproportionation instead transfers a hydrogen from one chain to the other, ending both without joining them, so each stays at its own length (one gets a saturated end, one a terminal double bond). Methacrylates favour disproportionation because of their abstractable alpha-hydrogens and hindered ends, whereas styrene tends to combine.
Why can't ordinary FRP make block copolymers?
Because a radical chain end lives only a fraction of a second before it terminates, there is no way to keep it 'alive' to add a second monomer block later. As soon as you stop feeding the first monomer, the chains die. Controlled/living radical methods such as RAFT solve this by capping the chain end reversibly (a dormant-active equilibrium), so chains stay dormant and can be reactivated to grow a second block, giving narrow dispersity and true block architectures.
How do reactivity ratios control copolymer sequence?
They compare how much each growing radical prefers its own monomer versus the other. If r1 = r2 = 1, both monomers add indiscriminately and you get a random copolymer; if both r values approach 0, each radical prefers the other monomer, forcing an alternating sequence; if an r value exceeds 1, that radical prefers its own monomer and you get blocky runs. Given r1 and r2 you can sketch a representative A/B sequence, a common tutorial and exam task.
Can Sia help me with FRP kinetics and copolymer questions?
Yes. Sia can walk the initiation/propagation/termination chain, quiz you on combination-versus-disproportionation molar-mass effects, and help you turn a pair of reactivity ratios into a copolymer sequence. It explains the method and checks your reasoning step by step; it does not do graded assessment for you, and University of Sydney academic-integrity rules apply.
Exam move
Fix the one high-yield fact first: combination doubles molar mass, disproportionation leaves it unchanged — and know that methacrylates favour disproportionation while styrene favours combination. Be able to draw the full FRP chain (initiation/propagation/termination) and to explain chain transfer and the gel effect in a sentence each. For copolymerisation, practise converting r1/r2 into random, alternating or blocky sequences, and remember the kp ordering (reactivity runs opposite to radical stability, so styrene propagates slowly). Keep the FRP-vs-step-growth contrast ready as a molar-mass-vs-conversion sketch (FRP high molar mass immediately and flat; step-growth only near p = 1; controlled/RAFT linear). These sketches and the termination logic are recurring Week 9 exam items.
Working through Radical Chain-Growth Polymerisation in CHEM2522? Sia is AskSia’s AI Chemistry tutor — ask any CHEM2522 Radical Chain-Growth Polymerisation question and get a clear, step-by-step explanation grounded in how CHEM2522 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
