Chain-growth polymerization — anionic
Living anionic polymerization
How the Living anionic 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: n-BuLi (initiator), styrene (monomer). Conditions: THF or cyclohexane, rigorously dry, O₂-free, −78 to 25 °C. Bonds formed: C–C σ bonds along the chain; bonds broken: C=C π bond of each monomer. Stereochemistry: Atactic in THF; narrow dispersity (Đ ≈ 1.05). Rate and selectivity: Fast initiation then steady propagation; no termination.
At a glance
Step 1: Initiation
A strong nucleophile like n-butyllithium attacks a monomer's electrophilic alkene carbon (e.g., styrene's vinyl group, activated by the adjacent phenyl ring) directly — the same nucleophilic addition logic as any organolithium attacking a Michael acceptor.
Step 2: Propagation
This generates a new, resonance-stabilized carbanion at the chain end, paired with a lithium counterion. That carbanion adds to the next monomer exactly the same way, propagating the chain — because there's no chain-transfer or termination step available under these rigorously dry, oxygen-free conditions, the carbanion stays 'living' indefinitely.
Step 3: Living chain, block copolymer
Since chains keep growing until monomer runs out, with no random termination events, anionic polymerization gives exceptionally narrow molecular weight distributions — and because the living chain end survives, a second monomer can be added afterward to build a block copolymer, impossible with ordinary radical polymerization's constant termination.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- THF
- Tetrahydrofuran. Cyclic ether solvent; its oxygen lone pairs solvate Li⁺/Mg²⁺ so organometallics stay dissolved and reactive.
- n-BuLi
- n-Butyllithium. Very strong base/nucleophile (pKa of butane ≈ 50); deprotonates alkynes and does lithium–halogen exchange.
How it compares with related reactions
| Living anionic polymerization | Cationic polymerization | Atom transfer radical polymerization (ATRP) | |
|---|---|---|---|
| Reagents | n-BuLi (initiator), styrene (monomer) | Isobutylene (monomer), BF₃ (co-initiator), trace H₂O (initiator) | Alkyl halide initiator (R–Br), Cu(I)Br/ligand (bipyridine, PMDETA), monomer |
| Conditions | THF or cyclohexane, rigorously dry, O₂-free, −78 to 25 °C | CH₂Cl₂, –80 to 0 °C | 60–110 °C, degassed |
| Stereochemistry | Atactic in THF; narrow dispersity (Đ ≈ 1.05) | Atactic/none (isobutylene is symmetrical) | Low [R•] (~10⁻⁸ M); controlled growth |
| Rate / selectivity | Fast initiation then steady propagation; no termination | Very fast; chain transfer and termination limit the molar mass | Keq = kact/kdeact ≈ 10⁻⁸–10⁻⁶; Đ ≈ 1.1–1.3 |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the Living anionic polymerization?
n-BuLi (initiator), styrene (monomer). Conditions: THF or cyclohexane, rigorously dry, O₂-free, −78 to 25 °C.
Which bonds are formed and which are broken?
Formed: C–C σ bonds along the chain. Broken: C=C π bond of each monomer.
What is the stereochemical outcome?
Atactic in THF; narrow dispersity (Đ ≈ 1.05)
What controls the rate or the selectivity?
Fast initiation then steady propagation; no termination
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make