Inorganic — ring-opening polymerization
ROMP: ring-opening metathesis polymerization
How the ROMP 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: Strained cyclic alkene (norbornene), Ru=CHR (Grubbs) or Mo/W alkylidene (Schrock). Conditions: CH₂Cl₂ or toluene, 25 °C, inert atmosphere. Bonds formed: C=C (new, in chain); M=C (new carbene); bonds broken: ring C=C π; M=C π. Stereochemistry: Cis/trans and tacticity depend on catalyst. Rate and selectivity: Driven by ring strain (≈ 27 kcal/mol for norbornene); reversible for low-strain rings.
At a glance
Step 1: [2+2] to the metallacyclobutane
A metal carbene catalyst (Grubbs- or Schrock-type) reacts with a strained cyclic alkene (like norbornene); the same [2+2] cycloaddition used in ordinary olefin metathesis forms a metallacyclobutane fused to the strained ring.
Step 2: Ring opens the productive way
The ring reopens the other way, relieving the substrate's ring strain and generating a new, longer-chain metal carbene with the former ring now incorporated linearly into the growing polymer backbone.
Step 3: Chain grows
This carbene reacts with another monomer, repeating the cycle — because the driving force is relief of ring strain rather than a thermodynamic sink, ROMP can also run in reverse, making it useful for both polymer synthesis and controlled depolymerization.
Abbreviations and terms
- L
- Ligand. A Lewis-base group (often a phosphine) bound to a metal centre.
- M
- Metal. A generic metal centre.
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- Grubbs
- Grubbs catalyst. Ruthenium carbene catalyst for olefin metathesis.
- ROMP
- Ring-opening metathesis polymerisation. Polymerisation of strained cycloalkenes with a metal carbene.
How it compares with related reactions
| ROMP | Ziegler-Natta polymerization | Associative ligand substitution at a square-planar Pt(II) complex | |
|---|---|---|---|
| Reagents | Strained cyclic alkene (norbornene), Ru=CHR (Grubbs) or Mo/W alkylidene (Schrock) | TiCl₄ or TiCl₃ + Et₃Al (cocatalyst); propene or ethylene | K₂[PtCl₄] + NH₃ (aq) |
| Conditions | CH₂Cl₂ or toluene, 25 °C, inert atmosphere | Heterogeneous (MgCl₂ support), 1–30 bar, 50–80 °C | H₂O, 25 °C |
| Stereochemistry | Cis/trans and tacticity depend on catalyst | Isotactic PP from stereocontrol at chiral Ti site | Trigonal-bipyramidal intermediate; retention of geometry |
| Rate / selectivity | Driven by ring strain (≈ 27 kcal/mol for norbornene); reversible for low-strain rings | Chain growth stops by β-H elimination or chain transfer (H₂, AlEt₃) | Rate = k₁[Pt] + k₂[Pt][NH₃] (associative; entering-ligand dependent) |
Test yourself
Try to answer each question before opening it.
Which reagents and conditions are typical for the ROMP?
Strained cyclic alkene (norbornene), Ru=CHR (Grubbs) or Mo/W alkylidene (Schrock). Conditions: CH₂Cl₂ or toluene, 25 °C, inert atmosphere.
Which bonds are formed and which are broken?
Formed: C=C (new, in chain); M=C (new carbene). Broken: ring C=C π; M=C π.
What is the stereochemical outcome?
Cis/trans and tacticity depend on catalyst
What controls the rate or the selectivity?
Driven by ring strain (≈ 27 kcal/mol for norbornene); reversible for low-strain rings
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam