chemistry

Living cationic polymerization

Living cationic polymerization is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Living cationic polymerization rather than just read about it. In short: Living cationic polymerization is a living polymerization technique involving cationic propagating species. It enables the synthesis of very well defined polymers (low molar mass distribution) and of polymers with unusual architecture such as star polymers and block copolymers and living cationic polymerization is therefore as such of commercial and academic interest.

Key takeaways

Living cationic polymerization belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Living cationic polymerization to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Living cationic polymerization from memory before moving on to harder problems.

Reference excerpt

Living cationic polymerization is a living polymerization technique involving cationic propagating species. It enables the synthesis of very well defined polymers (low molar mass distribution) and of polymers with unusual architecture such as star polymers and block copolymers and living cationic polymerization is therefore as such of commercial and academic interest.

Basics In carbocationic polymerization the active site is a carbocation with a counterion in close proximity. The basic reaction steps are:

A+B− + H2C=CHR → A-CH2-RHC+----B− Chain propagation: A-CH2-RHC+----B− + H2C=CHR → A-(CH2-RHC)n-CH2-RHC+----B− Chain termination: A-(CH2-RHC)n-CH2-RHC+----B− → A-(CH2-RHC)n-CH2-RHC-B chain transfer: A-(CH2-RHC)n-CH2-RHC+----B− → A-(CH2-RHC)n-CH2=CR H+B− Living cationic polymerization is characterised by defined and controlled initiation and propagation while minimizing side-reactions termination and chain transfer. Transfer and termination do occur but in ideal living systems the active ionic propagating species are in chemical equilibrium with the dormant covalent species with an exchange rate much faster than the propagation rate. Solution methods require rigorous purification of monomer and solvent although conditions are not as strict as in anionic polymerization. Common monomers are vinyl ethers, alpha-methyl vinyl ethers, isobutene, styrene, methylstyrene and N-vinylcarbazole. The monomer is nucleophilic and substituents should be able to stabilize a positive carbocationic charge. For example, para-methoxystyrene is more reactive than styrene itself. Initiation takes place by an initiation/coinitiation binary system, for example an alcohol and a Lewis acid. The active electrophile is then a proton and the counter ion the remaining alkoxide which is stabilized by the Lewis acid. With organic acetates such as cumyl acetate the initiating species is the carbocation R+ and the counterion is the acetate anion. In the iodine/hydrogen iodide system the electrophile is again a proton and the carbocation is stabilized by the triiodide ion. Polymerizations with diethylaluminium chloride rely on trace amounts of water. A proton is then accompanied by the counterion Et2AlClOH−. With tert-butyl chloride Et2AlCl abstracts a chlorine atom to form the tert-butyl carbocation as the electrophile. Efficient initiators that resemble the monomer are called cationogens. Termination and chain transfer are minimized when the initiator counterion is both non-nucleophilic and non-basic. More polar solvents promote ion dissociation and hence increase molar mass. Common additives are electron donors, salts and proton traps. Electron donors (e.g. nucleophiles, Lewis bases) for example dimethylsulfide and dimethylsulfoxide are believed to stabilize the carbocation. The addition of salt for example a tetraalkylammonium salt, prevents dissociation of the ion pair that is the propagating reactive site. Ion dissociation into free ions lead to non-living polymerization. Proton traps scavenge protons originating from protic impurities.

History The method was developed starting in the 1970s and 1980s with contributions from Higashimura on the polymerization of p-methoxystyrene using iodine or acetyl perchlorate, on the polymerization of isobutyl vinyl ether by iodine and with Mitsuo Sawamoto by iodine/HI and on the formation of p-methoxystyrene - isobutyl vinyl ether block copolymers. Kennedy and Faust studied methylstyrene / boron trichloride polymerization (then called quasi-living) in 1982 and that of isobutylene (system with cumyl acetate, 2,4,4-trimethylpentane-2-acetate and BCl3) in 1984 Around same time Kennedy and Mishra discovered very efficient living polymerization of isobutylene (system with Tertiary Alkyl (or Aryl) Methyl Ether and BCl3)[ that paved the way for rapid development of macromolecularly engineered polymers.

Isobutylene polymerization Living isobutylene polymerization typically takes place in a mixed solvent system comprising a non-polar solvent, such as hexane, and a polar solvent, such as chloroform or dichloromethane, at temperatures below 0 °C. With more polar solvents polyisobutylene solubility becomes a problem. Initiators can be alcohols, halides and ethers. Coinitiators are boron trichloride, tin tetrachloride and organoaluminum halides. With ethers and alcohols the true initiator is the chlorinated product. Polymer with molar mass of 160,000 g/mole and polydispersity index 1.02 can be obtained.

Vinyl ether polymerization Vinyl ethers (CH2=CHOR, R = methyl, ethyl, isobutyl, benzyl) are very reactive vinyl monomers. Studied systems are based on I2/HI and on zinc halides zinc chloride, zinc bromide and zinc iodide.

Living cationic ring-opening polymerization

In Living cationic ring-opening polymerization the monomer is a heterocycle such as an epoxide, THF, an oxazoline or an aziridine such as t-butylaziridine. The propagating species is not a carbocation but an oxonium ion. Living polymerization is more difficult to achieve because of the ease of termination by nucleophilic attack of a heteroatom in the growing polymer chain. Intramolecular termination is called backbiting and results in the formation of cyclic oligomers. Initiators are strong electrophiles such as triflic acid. Triflic anhydride is an initiator for bifunctional polymer.

References

Worked examples

Example 1 — a first encounter with Living cationic polymerization

Start with the simplest possible case. Write down what Living cationic polymerization claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Living cationic polymerization before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Living cationic polymerization ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Living cationic polymerization

In research: Living cationic polymerization appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Living cationic polymerization in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Living cationic polymerization is common in secondary-school and first-year university syllabi. It links to neighbouring topics Polymerization reactions, so understanding it makes those chapters shorter.
In everyday life: Look for Living cationic polymerization outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Living cationic polymerization in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Living cationic polymerization means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Living cationic polymerization out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Living cationic polymerization in simple terms?

Why does Living cationic polymerization matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Living cationic polymerization?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Living cationic polymerization.