Inorganic-free radical — reversible addition-fragmentation
RAFT polymerization: addition-fragmentation equilibrium
How the RAFT polymerization works, in 3 steps. Each step shows the structures and the curved arrows that move the electrons.
In short. A 3-step mechanism. Typical reagents: Monomer, radical initiator (AIBN), thiocarbonylthio RAFT agent Z–C(=S)S–R. Conditions: 60–80 °C, degassed solvent. Bonds formed: Pₙ–S bond; C=S (reformed); R•; bonds broken: C=S π; S–R. Stereochemistry: Atom-economic chain transfer; narrow dispersity (Đ ≈ 1.1). Rate and selectivity: Rapid addition–fragmentation (k_add, k_β) makes chain transfer much faster than propagation.
At a glance
Step 1: Radical adds to sulfur
The propagating radical Pₙ• adds to the sulfur of a thiocarbonylthio RAFT agent's C=S bond — not the carbon — forming a new Pₙ–S bond. This is the step that gives RAFT its name: reversible addition.
Step 2: Stabilised intermediate in equilibrium
The result is a resonance-stabilized carbon-centered radical intermediate, flanked by both sulfurs and the Z group. This intermediate sits in equilibrium (k_add / k₋add) with the starting materials — most of the time it simply reverts, which is exactly what keeps radical concentration low and controls the polymerization.
Step 3: β-Scission releases R•
β-Scission (k_β) fragments the intermediate the other way: the C=S bond reforms toward the sulfur that carried R, expelling R• as a leaving-group radical while the Pₙ–S bond stays in place. R• then reinitiates a new chain (resumption), and this whole cycle repeats on every chain as the main equilibrium — constantly shuffling the thiocarbonylthio end-group between chains, which is what keeps every chain growing at nearly the same rate and gives RAFT its narrow molecular-weight distributions.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- Ph
- Phenyl. –C₆H₅.
- AIBN
- Azobisisobutyronitrile. Radical initiator: on heating releases N₂ and two 2-cyano-2-propyl radicals.
- RAFT
- Reversible addition–fragmentation chain transfer. Controlled radical polymerisation with thiocarbonylthio agents.
How it compares with related reactions
| RAFT polymerization | Fenton reaction | Jacobsen-Katsuki epoxidation | |
|---|---|---|---|
| Reagents | Monomer, radical initiator (AIBN), thiocarbonylthio RAFT agent Z–C(=S)S–R | Fe²⁺ (e.g. FeSO₄), H₂O₂ | Chiral Mn(III)–salen (2–10 mol%), NaOCl (bleach) or mCPBA, alkene |
| Conditions | 60–80 °C, degassed solvent | Aqueous, pH ≈ 3, 25 °C | CH₂Cl₂, 0 °C, optional pyridine N-oxide |
| Stereochemistry | Atom-economic chain transfer; narrow dispersity (Đ ≈ 1.1) | None (non-selective radical) | High ee for cis-alkenes; salen pocket selects the face |
| Rate / selectivity | Rapid addition–fragmentation (k_add, k_β) makes chain transfer much faster than propagation | k ≈ 63 M⁻¹ s⁻¹; Fe³⁺ + H₂O₂ regenerates Fe²⁺ slowly | Oxygen transfer to alkene is rate-limiting |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the RAFT polymerization?
Monomer, radical initiator (AIBN), thiocarbonylthio RAFT agent Z–C(=S)S–R. Conditions: 60–80 °C, degassed solvent.
Which bonds are formed and which are broken?
Formed: Pₙ–S bond; C=S (reformed); R•. Broken: C=S π; S–R.
What is the stereochemical outcome?
Atom-economic chain transfer; narrow dispersity (Đ ≈ 1.1)
What controls the rate or the selectivity?
Rapid addition–fragmentation (k_add, k_β) makes chain transfer much faster than propagation
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam