Chain-growth polymerization — cationic
Cationic polymerization
How the Cationic 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: Isobutylene (monomer), BF₃ (co-initiator), trace H₂O (initiator). Conditions: CH₂Cl₂, –80 to 0 °C. Bonds formed: C–C (chain growth); C–H (initiation); bonds broken: C=C π of each monomer. Stereochemistry: Atactic/none (isobutylene is symmetrical). Rate and selectivity: Very fast; chain transfer and termination limit the molar mass.
At a glance
Step 1: Initiation: strong acid protonates the monomer
A Lewis acid (like BF₃) combines with a trace of water or another proton source to generate a strong Brønsted acid, which protonates an electron-rich monomer (like isobutylene) at the terminal carbon — Markovnikov selectivity, forming the more stable tertiary carbocation.
Step 2: Propagation: cation adds to the next monomer
That carbocation is attacked by the next monomer's alkene π electrons, exactly like any acid-catalyzed alkene addition, extending the chain by one unit and regenerating a new terminal carbocation.
Step 3: Termination by proton loss
This repeats until the chain terminates — usually by loss of a proton (regenerating the acid catalyst) rather than living indefinitely, since carbocations are prone to chain-transfer and elimination side reactions that anionic or living radical methods largely avoid.
Abbreviations and terms
- DCM
- Dichloromethane. Common non-polar-ish solvent for reactions that need no nucleophilic or protic solvent.
- H2
- Hydrogen gas. Reductant for catalytic hydrogenation.
- Lewis acid
- Lewis acid. Electron-pair acceptor (AlCl₃, BF₃, TiCl₄…) that activates carbonyls and halides.
- BF3
- Boron trifluoride. Lewis acid (usually as the Et₂O complex).
- Markovnikov
- Markovnikov’s rule. In HX addition, H goes to the carbon with more H’s so the more stable cation forms.
How it compares with related reactions
| Cationic polymerization | Living anionic polymerization | Atom transfer radical polymerization (ATRP) | |
|---|---|---|---|
| Reagents | Isobutylene (monomer), BF₃ (co-initiator), trace H₂O (initiator) | n-BuLi (initiator), styrene (monomer) | Alkyl halide initiator (R–Br), Cu(I)Br/ligand (bipyridine, PMDETA), monomer |
| Conditions | CH₂Cl₂, –80 to 0 °C | THF or cyclohexane, rigorously dry, O₂-free, −78 to 25 °C | 60–110 °C, degassed |
| Stereochemistry | Atactic/none (isobutylene is symmetrical) | Atactic in THF; narrow dispersity (Đ ≈ 1.05) | Low [R•] (~10⁻⁸ M); controlled growth |
| Rate / selectivity | Very fast; chain transfer and termination limit the molar mass | Fast initiation then steady propagation; no termination | 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 Cationic polymerization?
Isobutylene (monomer), BF₃ (co-initiator), trace H₂O (initiator). Conditions: CH₂Cl₂, –80 to 0 °C.
Which bonds are formed and which are broken?
Formed: C–C (chain growth); C–H (initiation). Broken: C=C π of each monomer.
What is the stereochemical outcome?
Atactic/none (isobutylene is symmetrical)
What controls the rate or the selectivity?
Very fast; chain transfer and termination limit the molar mass
Study guides that help with this reaction
- Radical chain reactions: initiation, propagation and termination
- Curved-arrow notation: rules and the mistakes students make