Ring-opening polymerization — lactone
ROP of a lactone (coordination-insertion)
How the ROP of a lactone (coordination-insertion) 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: ε-Caprolactone (or lactide), Sn(Oct)₂/ROH or Al(OiPr)₃ initiator. Conditions: Bulk or toluene, 110–130 °C (or 25 °C with Al catalysts). Bonds formed: ester C–O (chain); metal alkoxide at chain end; bonds broken: ring acyl C–O (ring-opening); M–OR. Stereochemistry: Stereoretentive for lactide; chain end remains a living M–OR. Rate and selectivity: Insertion is rate-limiting; driven by ring-strain relief.
At a glance
Step 1: Lactone binds the metal
A metal alkoxide catalyst (like a tin or aluminum initiator) coordinates the lactone's carbonyl oxygen, activating the ring toward nucleophilic attack — the same Lewis-acid activation strategy used throughout carbonyl chemistry.
Step 2: Alkoxide inserts: ring opens
The metal's own alkoxide ligand attacks the activated carbonyl carbon intramolecularly-adjacent, and the ring opens as the weak, strained C–O bond breaks — an ordinary nucleophilic acyl substitution, just relieving ring strain as its driving force instead of expelling a small leaving group.
Step 3: Chain propagates
This generates a new metal alkoxide at the chain end, which repeats the identical insertion on the next lactone monomer — this coordination-insertion mechanism is how biodegradable polyesters like poly(lactic acid) are manufactured, built one ring-opened monomer at a time.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- L
- Ligand. A Lewis-base group (often a phosphine) bound to a metal centre.
- M
- Metal. A generic metal centre.
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
How it compares with related reactions
| ROP of a lactone (coordination-insertion) | Atom transfer radical polymerization (ATRP) | Cationic polymerization | |
|---|---|---|---|
| Reagents | ε-Caprolactone (or lactide), Sn(Oct)₂/ROH or Al(OiPr)₃ initiator | Alkyl halide initiator (R–Br), Cu(I)Br/ligand (bipyridine, PMDETA), monomer | Isobutylene (monomer), BF₃ (co-initiator), trace H₂O (initiator) |
| Conditions | Bulk or toluene, 110–130 °C (or 25 °C with Al catalysts) | 60–110 °C, degassed | CH₂Cl₂, –80 to 0 °C |
| Stereochemistry | Stereoretentive for lactide; chain end remains a living M–OR | Low [R•] (~10⁻⁸ M); controlled growth | Atactic/none (isobutylene is symmetrical) |
| Rate / selectivity | Insertion is rate-limiting; driven by ring-strain relief | Keq = kact/kdeact ≈ 10⁻⁸–10⁻⁶; Đ ≈ 1.1–1.3 | Very fast; chain transfer and termination limit the molar mass |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the ROP of a lactone (coordination-insertion)?
ε-Caprolactone (or lactide), Sn(Oct)₂/ROH or Al(OiPr)₃ initiator. Conditions: Bulk or toluene, 110–130 °C (or 25 °C with Al catalysts).
Which bonds are formed and which are broken?
Formed: ester C–O (chain); metal alkoxide at chain end. Broken: ring acyl C–O (ring-opening); M–OR.
What is the stereochemical outcome?
Stereoretentive for lactide; chain end remains a living M–OR
What controls the rate or the selectivity?
Insertion is rate-limiting; driven by ring-strain relief
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make