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What is the role of a solvent in cationic polymerization?
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Karl Jansson
What is the role of a solvent in cationic polymerization?
Very subtle question.
Unlike free radical polymerisation[1], the reactions of ionic polymerisation[2] need counterions[3] to keep electrical neutrality. This, in turn, may affect the mechanism of polymerisation itself and thus make ionic polymerisations trickier than the free radical ones.
A solvent, in particular, can affect the degree in which ion pairs associate to one another: this association (or lack thereof) therefore depends on solvation effects[4], i.e. the degree with which solvent molecules interact with said ions.
In cationic polymerisation, an electrophile E⁺ triggers the initiation step[5] on a monomer solution: if a, say, double bond is in it,
E⁺ + CH₂=CR₂ → E-CH₂-CR₂⁺
Since the positive charge is localised on a C atom, a carbocation[6] forms; this is the species that propagates the reaction. Electrophiles, on the other hand, are mostly formed by reaction between initiators and co-initiators (e.g. Lewis acids[7]: AlCl₃, BF₃, and so on).
In the propagation step[8] , the more stable the carbocation, the more favourable the reaction (keep in mind: in free radical reactions it is the other way round).
The polarity of the solvent affects the rate of the polymerisation: the more polar a solvent, the more favoured the initiation step (because a charged species is formed); however, the same polar solvent can hamper the propagation step, because the charge on the as-formed carbocation is dispersed in the transition state.
Another trouble that comes from the solvent: the degree of association between the growing, cationic end of the polymer chain and the counterion that goes with it. In general, the more intimate this association, the lower the propagation rate. From this viewpoint, a poorly solvating solvent (e.g. an ether) can improve it; however, as the polarity increases, so does its solvating power and, thus, propagation may slow down.
Unlike free radical polymerisation[1], the reactions of ionic polymerisation[2] need counterions[3] to keep electrical neutrality. This, in turn, may affect the mechanism of polymerisation itself and thus make ionic polymerisations trickier than the free radical ones.
A solvent, in particular, can affect the degree in which ion pairs associate to one another: this association (or lack thereof) therefore depends on solvation effects[4], i.e. the degree with which solvent molecules interact with said ions.
In cationic polymerisation, an electrophile E⁺ triggers the initiation step[5] on a monomer solution: if a, say, double bond is in it,
E⁺ + CH₂=CR₂ → E-CH₂-CR₂⁺
Since the positive charge is localised on a C atom, a carbocation[6] forms; this is the species that propagates the reaction. Electrophiles, on the other hand, are mostly formed by reaction between initiators and co-initiators (e.g. Lewis acids[7]: AlCl₃, BF₃, and so on).
In the propagation step[8] , the more stable the carbocation, the more favourable the reaction (keep in mind: in free radical reactions it is the other way round).
The polarity of the solvent affects the rate of the polymerisation: the more polar a solvent, the more favoured the initiation step (because a charged species is formed); however, the same polar solvent can hamper the propagation step, because the charge on the as-formed carbocation is dispersed in the transition state.
Another trouble that comes from the solvent: the degree of association between the growing, cationic end of the polymer chain and the counterion that goes with it. In general, the more intimate this association, the lower the propagation rate. From this viewpoint, a poorly solvating solvent (e.g. an ether) can improve it; however, as the polarity increases, so does its solvating power and, thus, propagation may slow down.
Chain growth polymerization is also called as living polymerization. As growth was considered to be a factor of life's definition and it was kind of chain growth that made large polymeric structures in cell at the start of life.But the process involves termination of the chain by anion or cation by addition and removal respectively.
So the process has no relation with the name but the chemists regarded chain growth in polymers was the fundamental of life. The process was discovered by Karl Zeigler and later improvised by Michael Szwarc.
Chain growth polymerization is also called as living polymerization. As growth was considered to be a factor of life's definition and it was kind of chain growth that made large polymeric structures in cell at the start of life.But the process involves termination of the chain by anion or cation by addition and removal respectively.
So the process has no relation with the name but the chemists regarded chain growth in polymers was the fundamental of life. The process was discovered by Karl Zeigler and later improvised by Michael Szwarc.
Very subtle question.
Unlike free radical polymerisation[1], the reactions of ionic polymerisation[2] need counterions[3] to keep electrical neutrality. This, in turn, may affect the mechanism of polymerisation itself and thus make ionic polymerisations trickier than the free radical ones.
A solvent, in particular, can affect the degree in which ion pairs associate to one another: this association (or lack thereof) therefore depends on solvation effects[4], i.e. the degree with which solvent molecules interact with said ions.
In cationic polymerisation, an electrophile E⁺ triggers the initiation step[5] on a monomer solution: if a, say, double bond is in it,
E⁺ + CH₂=CR₂ → E-CH₂-CR₂⁺
Since the positive charge is localised on a C atom, a carbocation[6] forms; this is the species that propagates the reaction. Electrophiles, on the other hand, are mostly formed by reaction between initiators and co-initiators (e.g. Lewis acids[7]: AlCl₃, BF₃, and so on).
In the propagation step[8] , the more stable the carbocation, the more favourable the reaction (keep in mind: in free radical reactions it is the other way round).
The polarity of the solvent affects the rate of the polymerisation: the more polar a solvent, the more favoured the initiation step (because a charged species is formed); however, the same polar solvent can hamper the propagation step, because the charge on the as-formed carbocation is dispersed in the transition state.
Another trouble that comes from the solvent: the degree of association between the growing, cationic end of the polymer chain and the counterion that goes with it. In general, the more intimate this association, the lower the propagation rate. From this viewpoint, a poorly solvating solvent (e.g. an ether) can improve it; however, as the polarity increases, so does its solvating power and, thus, propagation may slow down.
Footnotes
[1] Radical polymerization - WikipediaVery subtle question.
Unlike free radical polymerisation[1], the reactions of ionic polymerisation[2] need counterions[3] to keep electrical neutrality. This, in turn, may affect the mechanism of polymerisation itself and thus make ionic polymerisations trickier than the free radical ones.
A solvent, in particular, can affect the degree in which ion pairs associate to one another: this association (or lack thereof) therefore depends on solvation effects[4], i.e. the degree with which solvent molecules interact with said ions.
In cationic polymerisation, an electrophile E⁺ triggers the initiation step[5] on a monomer solution: if a, say, double bond is in it,
E⁺ + CH₂=CR₂ → E-CH₂-CR₂⁺
Since the positive charge is localised on a C atom, a carbocation[6] forms; this is the species that propagates the reaction. Electrophiles, on the other hand, are mostly formed by reaction between initiators and co-initiators (e.g. Lewis acids[7]: AlCl₃, BF₃, and so on).
In the propagation step[8] , the more stable the carbocation, the more favourable the reaction (keep in mind: in free radical reactions it is the other way round).
The polarity of the solvent affects the rate of the polymerisation: the more polar a solvent, the more favoured the initiation step (because a charged species is formed); however, the same polar solvent can hamper the propagation step, because the charge on the as-formed carbocation is dispersed in the transition state.
Another trouble that comes from the solvent: the degree of association between the growing, cationic end of the polymer chain and the counterion that goes with it. In general, the more intimate this association, the lower the propagation rate. From this viewpoint, a poorly solvating solvent (e.g. an ether) can improve it; however, as the polarity increases, so does its solvating power and, thus, propagation may slow down.
Footnotes
[1] Radical polymerization - WikipediaMore
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Chain growth polymerization is also called as living polymerization. As growth was considered to be a factor of life's definition and it was kind of chain growth that made large polymeric structures in cell at the start of life.But the process involves termination of the chain by anion or cation by addition and removal respectively.
So the process has no relation with the name but the chemists regarded chain growth in polymers was the fundamental of life. The process was discovered by Karl Zeigler and later improvised by Michael Szwarc.
Chain growth polymerization is also called as living polymerization. As growth was considered to be a factor of life's definition and it was kind of chain growth that made large polymeric structures in cell at the start of life.But the process involves termination of the chain by anion or cation by addition and removal respectively.
So the process has no relation with the name but the chemists regarded chain growth in polymers was the fundamental of life. The process was discovered by Karl Zeigler and later improvised by Michael Szwarc.
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