Inorganic — insertion polymerization
Ziegler-Natta polymerization
How the Ziegler-Natta 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: TiCl₄ or TiCl₃ + Et₃Al (cocatalyst); propene or ethylene. Conditions: Heterogeneous (MgCl₂ support), 1–30 bar, 50–80 °C. Bonds formed: C–C (chain growth); bonds broken: Ti–C (migrates); alkene π. Stereochemistry: Isotactic PP from stereocontrol at chiral Ti site. Rate and selectivity: Chain growth stops by β-H elimination or chain transfer (H₂, AlEt₃).
At a glance
Step 1: Ti–alkyl with an open site
A Ti(III) or Ti(IV) active site holds a growing polymer chain as an alkyl ligand, with an open coordination site nearby.
Step 2: Coordination, then migratory insertion
An incoming alkene monomer coordinates to that open site, then undergoes migratory insertion into the Ti–C bond — the exact same insertion step seen in hydroformylation and Wilkinson's cycle, just repeated thousands of times.
Step 3: Chain one unit longer, site regenerated
Each insertion regenerates an open coordination site with the chain now one monomer longer, so the cycle repeats — chain growth stops only when β-hydride elimination or chain transfer intervenes.
Abbreviations and terms
- R
- R group. Any carbon substituent (alkyl, aryl…) whose identity does not matter for the mechanism.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- TiCl4
- Titanium tetrachloride. Strong, oxophilic Lewis acid that chelates carbonyls.
- Wilkinson
- Wilkinson’s catalyst, RhCl(PPh₃)₃. Homogeneous hydrogenation catalyst.
- TS
- Transition state. The highest-energy point on the pathway between two minima; it cannot be isolated.
How it compares with related reactions
| Ziegler-Natta polymerization | ROMP | Associative ligand substitution at a square-planar Pt(II) complex | |
|---|---|---|---|
| Reagents | TiCl₄ or TiCl₃ + Et₃Al (cocatalyst); propene or ethylene | Strained cyclic alkene (norbornene), Ru=CHR (Grubbs) or Mo/W alkylidene (Schrock) | K₂[PtCl₄] + NH₃ (aq) |
| Conditions | Heterogeneous (MgCl₂ support), 1–30 bar, 50–80 °C | CH₂Cl₂ or toluene, 25 °C, inert atmosphere | H₂O, 25 °C |
| Stereochemistry | Isotactic PP from stereocontrol at chiral Ti site | Cis/trans and tacticity depend on catalyst | Trigonal-bipyramidal intermediate; retention of geometry |
| Rate / selectivity | Chain growth stops by β-H elimination or chain transfer (H₂, AlEt₃) | Driven by ring strain (≈ 27 kcal/mol for norbornene); reversible for low-strain rings | 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 Ziegler-Natta polymerization?
TiCl₄ or TiCl₃ + Et₃Al (cocatalyst); propene or ethylene. Conditions: Heterogeneous (MgCl₂ support), 1–30 bar, 50–80 °C.
Which bonds are formed and which are broken?
Formed: C–C (chain growth). Broken: Ti–C (migrates); alkene π.
What is the stereochemical outcome?
Isotactic PP from stereocontrol at chiral Ti site
What controls the rate or the selectivity?
Chain growth stops by β-H elimination or chain transfer (H₂, AlEt₃)
Study guides that help with this reaction
- How to learn reaction mechanisms (without memorising everything)
- How to answer mechanism questions in an exam