Living cationic polymerization
Encyclopedia
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.
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 monomer
s are vinyl ether
s, 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 a 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
/HI
system the electrophile is again a proton and the carbocation is stabilized by the triodide
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.
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
polymerization typically takes place in chloroform
or dichloromethane
as a solvent
at temperatures below 0°C. With more polar solvents polyisobutylene solubility becomes a problem. Initiators can be esters, alcohols, halides and ethers. Co-initiators 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.
, 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
.
, THF
, an oxazoline
or an aziridine
such as t-butylaziridine. The propagating species is not a carbocation but an onium 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 alkylating agents such as triflic acid. Triflic anhydride
is an initiator for bifunctional polymer.
Living polymerization
In polymer chemistry, living polymerization is a form of addition polymerization where the ability of a growing polymer chain to terminate has been removed. This can be accomplished in a variety of ways. Chain termination and chain transfer reactions are absent and the rate of chain initiation is...
technique involving cationic propagating species. It enables the synthesis of very well defined polymers (low molar mass distribution
Molar mass distribution
In linear polymers the individual polymer chains rarely have exactly the same degree of polymerization and molar mass, and there is always a distribution around an average value. The molar mass distribution in a polymer describes the relationship between the number of moles of each polymer species...
) 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
The basic reaction steps are: iodine. In carbocationic polymerization the active site is a carbocation with a counterion in close proximity.- A+B- + H2C=CHR → A-CH2-RHC+----B-
- Chain propagationChain propagationChain propagation is a process in which a reactive intermediate is continuously regenerated during the course of a chemical reaction. In polymerization reaction, the reactive end-groups of a polymer chain react in each propagation step with a new monomer molecule transferring the reactive group to...
:
- Chain propagation
- A-CH2-RHC+----B- + H2C=CHR → A-(CH2-RHC)n-CH2-RHC+----B-
- Chain terminationChain terminationChain termination is any chemical reaction that ceases the formation of reactive intermediates in a chain propagation step in the course of a polymerization, effectively bringing it to a halt.- Mechanisms of Termination :...
:
- Chain termination
- A-(CH2-RHC)n-CH2-RHC+----B- → A-(CH2-RHC)n-CH2-RHC-B
- chain transferChain transferChain transfer is a polymerization reaction by which the activity of a growing polymer chain is transferred to another molecule.Chain transfer reactions reduce the average molecular weight of the final polymer...
:
- 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
Chemical equilibrium
In a chemical reaction, chemical equilibrium is the state in which the concentrations of the reactants and products have not yet changed with time. It occurs only in reversible reactions, and not in irreversible reactions. Usually, this state results when the forward reaction proceeds at the same...
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 monomer
Monomer
A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
s are vinyl ether
Vinyl ether
Vinyl ether, also known as divinyl ether, divinyl oxide, Vinethene and ethenoxyethene , is a clear, nearly colorless, volatile liquid which was briefly used as an inhalation anesthetic...
s, alpha-methyl vinyl ethers, isobutene, styrene, methylstyrene and N-vinylcarbazole
N-Vinylcarbazole
N-Vinylcarbazole is an organic compound used as a monomer in the production of poly, a photoconductive polymer used in the photoreceptors of photocopiers. Upon exposure to γ-irradiation, N-vinylcarbazole undergoes solid-state polymerisation....
. 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 a initiation/coinitiation binary system, for example an alcohol and a Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
. The active electrophile is then a proton and the counter ion the remaining alkoxide
Alkoxide
An alkoxide is the conjugate base of an alcohol and therefore consists of an organic group bonded to a negatively charged oxygen atom. They can be written as RO−, where R is the organic substituent. Alkoxides are strong bases and, when R is not bulky, good nucleophiles and good ligands...
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
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
/HI
Hydrogen iodide
Hydrogen iodide is a diatomic molecule. Aqueous solutions of HI are known as iohydroic acid or hydroiodic acid, a strong acid. Gas and aqueous solution are interconvertible...
system the electrophile is again a proton and the carbocation is stabilized by the triodide
Triodide
Triodide can be:* a common misspelling for triiodide, the I3− ion in chemistry.* Orion Safety and Pacific Dynamics trademarked brand name for Trifluoroiodomethane....
ion. Polymerizations with Diethylaluminium chloride rely on trace amounts of water. A proton is then accompanied by the counterion Et2AlClOH-. With tert-butyl chloride
Tert-Butyl chloride
tert-Butyl chloride is a colorless, liquid organic compound at room temperature. It is sparingly soluble in water, with a tendency to undergo spontaneous solvolysis when dissolved into it...
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 iodineIodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
or acetyl perchlorate, on the polymerization of isobutyl vinyl ether by iodine and with Mitsuo Sawamoto by iodine/HI
Hydrogen iodide
Hydrogen iodide is a diatomic molecule. Aqueous solutions of HI are known as iohydroic acid or hydroiodic acid, a strong acid. Gas and aqueous solution are interconvertible...
and on the formation of p-methoxystyrene - isobutyl vinyl ether block copolymers.
Kennedy and Faust studied methylstyrene / boron trichloride
Boron trichloride
Boron trichloride is a chemical compound with the formula BCl3. This colorless gas is a valuable reagent in organic synthesis. It is also dangerously reactive.-Production and properties:Boron reacts with halogens to give the corresponding trihalides...
polymerization (then called quasi-living) in 1982 and that of isobutylene
Isobutylene
Isobutylene is a hydrocarbon of significant industrial importance. It is a four-carbon branched alkene , one of the four isomers of butylene. At standard temperature and pressure it is a colorless flammable gas.-Uses:...
(system with cumyl acetate, 2,4,4-trimethylpentane-2-acetate and BCl3) in 1984
Isobutylene polymerization
Living isobutyleneIsobutylene
Isobutylene is a hydrocarbon of significant industrial importance. It is a four-carbon branched alkene , one of the four isomers of butylene. At standard temperature and pressure it is a colorless flammable gas.-Uses:...
polymerization typically takes place in chloroform
Chloroform
Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...
or dichloromethane
Dichloromethane
Dichloromethane is an organic compound with the formula CH2Cl2. This colorless, volatile liquid with a moderately sweet aroma is widely used as a solvent. Although it is not miscible with water, it is miscible with many organic solvents...
as a solvent
Solvent
A solvent is a liquid, solid, or gas that dissolves another solid, liquid, or gaseous solute, resulting in a solution that is soluble in a certain volume of solvent at a specified temperature...
at temperatures below 0°C. With more polar solvents polyisobutylene solubility becomes a problem. Initiators can be esters, alcohols, halides and ethers. Co-initiators are boron trichloride
Boron trichloride
Boron trichloride is a chemical compound with the formula BCl3. This colorless gas is a valuable reagent in organic synthesis. It is also dangerously reactive.-Production and properties:Boron reacts with halogens to give the corresponding trihalides...
, tin tetrachloride and organoaluminum halides. With ethers and alcohols the true initiator is the chlorinated product. Polymer with molar mass
Molar mass
Molar mass, symbol M, is a physical property of a given substance , namely its mass per amount of substance. The base SI unit for mass is the kilogram and that for amount of substance is the mole. Thus, the derived unit for molar mass is kg/mol...
of 160,000 g/mole and polydispersity index
Polydispersity index
In physical and organic chemistry, the polydispersity index , is a measure of the distribution of molecular mass in a given polymer sample. The PDI calculated is the weight average molecular weight divided by the number average molecular weight. It indicates the distribution of individual...
1.02 can be obtained.
Vinyl ether polymerization
Vinyl ethers (CH2=CHOR, R = methyl, ethylEthyl
Ethyl may refer to:* Ethyl group, a functional group in organic chemistry* Ethyl Corporation, a fuel additive company* Ethanol, also known as ethyl alcohol* Cold Ethyl, a rock band founded by Nicke Andersson...
, isobutyl, benzyl
Benzyl
In organic chemistry, benzyl is the term used to describe the substituent or molecular fragment possessing the structure C6H5CH2-. Benzyl features a benzene ring attached to a CH2 group.-Nomenclature:...
) are very reactive vinyl monomers. Studied systems are based on I2/HI and on zinc halides zinc chloride
Zinc chloride
Zinc chloride is the name of chemical compound with the formula ZnCl2 and its hydrates. Zinc chlorides, of which nine crystalline forms are known, are colorless or white, and are highly soluble in water. ZnCl2 itself is hygroscopic and even deliquescent. Samples should therefore be protected from...
, zinc bromide
Zinc bromide
Zinc bromide is a inorganic compound with the chemical formula ZnBr2. It is a colourless salt that shares many properties with zinc chloride , namely a high solubility in water forming acidic solutions, and solubility in organic solvents...
and zinc iodide
Zinc iodide
Zinc iodide is a chemical compound of zinc and iodine, ZnI2. The anhydrous form is white and readily absorbs water from the atmosphere. It can be prepared by the direct reaction of zinc and iodine in refluxing ether...
.
Living cationic ring-opening polymerization
In Living cationic ring-opening polymerization the monomer is a heterocycle such as an epoxideEpoxide
An epoxide is a cyclic ether with three ring atoms. This ring approximately defines an equilateral triangle, which makes it highly strained. The strained ring makes epoxides more reactive than other ethers. Simple epoxides are named from the parent compound ethylene oxide or oxirane, such as in...
, THF
ThF
Follicular B helper T cells , are antigen-experienced CD4+ T cells found in the B cell follicles of secondary lymphoid organs such as lymph nodes, spleens and Peyer's patches, and are identified by their constitutive expression of the B cell follicle homing receptor CXCR5...
, an oxazoline
Oxazoline
Oxazoline is both the five-membered ring heterocyclic chemical compound with the formula C3H5NO and the class of compounds containing this ring.- See also :* Desoxazoline * Oxazole* Oxazolidine* Oxazolidinedione...
or an aziridine
Aziridine
Aziridines are organic compounds containing the aziridine functional group, a three-membered heterocycle with one amine group and two methylene groups...
such as t-butylaziridine. The propagating species is not a carbocation but an onium 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 alkylating agents such as triflic acid. Triflic anhydride
Triflic anhydride
Triflic anhydride is the chemical compound with the formula 2O. This compound is a particularly strong electrophile, useful for introducing the triflyl group, CF3SO2. Triflic anhydride is the acid anhydride of the strong acid triflic acid, CF3SO2OH....
is an initiator for bifunctional polymer.