Controlled radical polymerization
Atom transfer radical polymerization (ATRP)
How the Atom transfer radical polymerization (ATRP) 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: Alkyl halide initiator (R–Br), Cu(I)Br/ligand (bipyridine, PMDETA), monomer. Conditions: 60–110 °C, degassed. Bonds formed: C–Br (dormant); Cu(II)–Br; bonds broken: C–Br (activation). Stereochemistry: Low [R•] (~10⁻⁸ M); controlled growth. Rate and selectivity: Keq = kact/kdeact ≈ 10⁻⁸–10⁻⁶; Đ ≈ 1.1–1.3.
At a glance
Step 1: Activation: Cu(I) abstracts Br
A Cu(I) complex abstracts a halogen atom from a dormant alkyl halide chain end, generating an active polymer radical and oxidizing copper to Cu(II)–X — a reversible version of the halogen-atom-abstraction step from radical halogenation.
Step 2: Brief propagation
The active radical propagates by adding monomer, exactly as in ordinary radical polymerization — but only briefly, since it's rapidly re-capped.
Step 3: Deactivation: Cu(II)–Br returns Br
Cu(II)–X transfers the halogen back to the growing radical, deactivating it to a dormant alkyl halide again — this rapid, reversible activation/deactivation equilibrium keeps radical concentration very low, suppressing termination and giving much narrower molecular weight distributions than ordinary radical polymerization.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- X
- Halogen / leaving group. Cl, Br or I (sometimes any leaving group).
- L
- Ligand. A Lewis-base group (often a phosphine) bound to a metal centre.
- M
- Metal. A generic metal centre.
- ATRP
- Atom-transfer radical polymerisation. Controlled polymerisation via reversible halogen-atom transfer to Cu(I).
How it compares with related reactions
| Atom transfer radical polymerization (ATRP) | Radical polymerization | Radical polymerization | |
|---|---|---|---|
| Reagents | Alkyl halide initiator (R–Br), Cu(I)Br/ligand (bipyridine, PMDETA), monomer | AIBN (or benzoyl peroxide), vinyl monomer (styrene) | Growing chain radical, vinyl monomer (styrene) |
| Conditions | 60–110 °C, degassed | 60–80 °C (or hν), inert atmosphere | 60–80 °C, bulk or solution |
| Stereochemistry | Low [R•] (~10⁻⁸ M); controlled growth | Attack at the less substituted CH₂ gives the benzylic radical | Head-to-tail addition; tacticity set by chain end |
| Rate / selectivity | Keq = kact/kdeact ≈ 10⁻⁸–10⁻⁶; Đ ≈ 1.1–1.3 | Initiator decomposition (kd ≈ 10⁻⁵ s⁻¹ at 70 °C) is rate-limiting | kp ≈ 10²–10⁴ M⁻¹ s⁻¹; far faster than initiation |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Atom transfer radical polymerization (ATRP)?
Alkyl halide initiator (R–Br), Cu(I)Br/ligand (bipyridine, PMDETA), monomer. Conditions: 60–110 °C, degassed.
Which bonds are formed and which are broken?
Formed: C–Br (dormant); Cu(II)–Br. Broken: C–Br (activation).
What is the stereochemical outcome?
Low [R•] (~10⁻⁸ M); controlled growth
What controls the rate or the selectivity?
Keq = kact/kdeact ≈ 10⁻⁸–10⁻⁶; Đ ≈ 1.1–1.3
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make