Anionic and Cationic Polymerisations
The other types of chain polymerisations, besides free radical polymerisations, are anionic and cationic polymerisation. In this case, the active side is anionic or cationic polymerisation. This section, we will discuss anionic polymerisation then followed by cationic polymerisation.
As its name suggests, anionic polymerisations use anionic active site in the polymerisation reaction. With this anionic active site, it gives more selectivity as 2 anions cannot react together due coulombic repulsion forces; hence, no intrinsic termination. When a suitable vinyl monomer is polymerised in non-reacting solvent and in the absence of impurities it forms so-called "living polymer". Further monomer addition to this living polymer when p = 1.00, it will increase MW because no active centre is produced. Therefore, there are several requirements to form living polymers:
In this polymerisation, the reaction is iniated by transfer electron from iniator to monomer to form anionic adduct and typical initiators in this polymerisation are BuLi, t-BuOK or NaNH2. Another possible initiator in this type is bifunctional initiator. This bifunctional can be achieved by addition of sodium to styrene which leads to dimerisation of styril radical anion to give living dianion polymer.
Then, the propagation reaction only occurs if monomer-initiator adduct is capable for further electron transfer to neutral monomer.
This propagation reaction is exclusively head to tail coupling due to anionic active centre.
In termination step, this step would not happen in the absence of impurities which means an external terminator is required. This means, a desired capping group can be added to control termination reaction.
This type of polymerisation is known to have narrow MWD (Mw/Mn < 1.20) and this can be obtained if the following criteria are followed. To achieve narrown MWD, kI >> kp which means essentially no propagation occurs unless initiation step is finish. This is different with free radical polymerisations where propagation can occurs along with initiation process. Secondly, monomer and temperature are needed to be uniform throughout reaction. Thirdly, no side reaction or branching occurs during propagation. However, this is a bit of problem with methacrylate and acrylate polymers.
Last criteria is the reaction must be free from protic impurities. In contrast with free radical polymerisations, anionic polymerisation cannot be done in water but it works well in the presence of oxygen.
This narrow MWD is important for molecular weight-dependant studies, GPC calibration, etc. Besides that, there is an increase of interest of this polymerisation in industry but it is still little used due to expensive operating cost.
If all conditions above are fulfilled, p = 1.00 and every initiator molecule reacts with monomer to produce living polymer (f = 1.00) then degree of polymerisation and can be calculated as follow.
There are several characteristics of living polymerisation in anionic polymerisation.
Linear evolution of MW with monomer conversion which is disticntly different with FR polymerisation where sudden increase in evolution of MW and high MW at higher monomer conversion for step polymerisation. Besides that, it also has narrow MWD as mentioned above.
The last characteristic of living polymerisation is the ability to form diblock copolymers via sequential addition of monomers.
Besides that, this characteristic can also be employed to give wide range polymer architectures such as ring or star polymers. The synthesis of ring polymer is mainly theoretical interest and this can be achieved by making living anionic polymer with two anionic chain-ends using bifunctional initiator. Then, to close the ring a bifunctional terminator is needed and the reaction has to be done in low concentration to avoid the formation of high MW polymers.
Furthermore, the study of near monodisperse ring polymer might be useful for adsorption studies.
Meanwhile, there are two possibility to synthesis star polymers. Firstly, using multifunctional initiator but this is less likely due to high charge of initiator which makes it insoluble in aprotic solvent. The second possibility is to use multifunctional terminator such as SiCl4.
Unlike linear polymers of the same MW, the solution viscosity of star polymer has only a weak temperature dependance. Therefore, star polymers can be used as an additive for engine oil formulation. This makes the performance of hot engine oil and cold engine oil is similar which is important in certain climates.
Another type of anionic polymerisations is the ring-opening polymerisation which is useful for the synthesis of poly(ethylene oxide) or PEO.
This living polymer has the negative charge on O atom which means more stable anionic living polymer. Besides that, PS-PEO diblock copolymer can be synthesised using anionic polymerisation but styrene need to be polymerised first.
From the discussion above, the advantages of using anionic polymerisation are:
However, there are several drawbacks of this polymerisations which are synthetically demanding and inappropriate monomers with labile proton can terminate the polymerisation prematurely. The second problem can be solved by masking or protecting the functional group which can be deprotected easily after polymerisation.
Now, we can move to cationic polymerisation where it employs cationic active centre. This can be promoted by reacting the monomers with Lewis acid (BF3, AlCl3, FeCl3, or SnCl4) or H2SO4. The positive charge on the living polymer is stabilised by electron-donating group. Unlike anionic polymerisation, cationic polymerisation suffers from intrinsic termination and chain transfer reaction. Thus, cationic polymerisation offers little control over either target molecular weight or MWD; hence, it is much less studied and more limited synthetic utility.
Examples of some important cationic polymerisations are polymerisation of polyisobutylene and polytetraydrofuran (PTHF). Polyisobutylene is used for butyl rubber in car tyres while PTHF is used to make elastic fibres and precursor of thermoplastics.
Interestingly, polymerisation of isobutylene is very fast, it is complete within a second even at -100 °C.
As its name suggests, anionic polymerisations use anionic active site in the polymerisation reaction. With this anionic active site, it gives more selectivity as 2 anions cannot react together due coulombic repulsion forces; hence, no intrinsic termination. When a suitable vinyl monomer is polymerised in non-reacting solvent and in the absence of impurities it forms so-called "living polymer". Further monomer addition to this living polymer when p = 1.00, it will increase MW because no active centre is produced. Therefore, there are several requirements to form living polymers:
- rigorous purification of monomer, initiator and solvent to remove water. Any source of proton, even from water, can terminate the polymerisation. This purification can be done by repeated distillation and degassing cycles.
- Dry vessel reaction is needed and it can be prepared by overnight baking-out under high vacuum (10-4 torr at 200 °C)
- Need an electron withdrawing group to stabilise the negative charge.
In this polymerisation, the reaction is iniated by transfer electron from iniator to monomer to form anionic adduct and typical initiators in this polymerisation are BuLi, t-BuOK or NaNH2. Another possible initiator in this type is bifunctional initiator. This bifunctional can be achieved by addition of sodium to styrene which leads to dimerisation of styril radical anion to give living dianion polymer.
Then, the propagation reaction only occurs if monomer-initiator adduct is capable for further electron transfer to neutral monomer.
In termination step, this step would not happen in the absence of impurities which means an external terminator is required. This means, a desired capping group can be added to control termination reaction.
This type of polymerisation is known to have narrow MWD (Mw/Mn < 1.20) and this can be obtained if the following criteria are followed. To achieve narrown MWD, kI >> kp which means essentially no propagation occurs unless initiation step is finish. This is different with free radical polymerisations where propagation can occurs along with initiation process. Secondly, monomer and temperature are needed to be uniform throughout reaction. Thirdly, no side reaction or branching occurs during propagation. However, this is a bit of problem with methacrylate and acrylate polymers.
Last criteria is the reaction must be free from protic impurities. In contrast with free radical polymerisations, anionic polymerisation cannot be done in water but it works well in the presence of oxygen.
This narrow MWD is important for molecular weight-dependant studies, GPC calibration, etc. Besides that, there is an increase of interest of this polymerisation in industry but it is still little used due to expensive operating cost.
If all conditions above are fulfilled, p = 1.00 and every initiator molecule reacts with monomer to produce living polymer (f = 1.00) then degree of polymerisation and can be calculated as follow.
Linear evolution of MW with monomer conversion which is disticntly different with FR polymerisation where sudden increase in evolution of MW and high MW at higher monomer conversion for step polymerisation. Besides that, it also has narrow MWD as mentioned above.
Besides that, this characteristic can also be employed to give wide range polymer architectures such as ring or star polymers. The synthesis of ring polymer is mainly theoretical interest and this can be achieved by making living anionic polymer with two anionic chain-ends using bifunctional initiator. Then, to close the ring a bifunctional terminator is needed and the reaction has to be done in low concentration to avoid the formation of high MW polymers.
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| Roover and Toporowski (1983) Ring polymer synthesis |
Furthermore, the study of near monodisperse ring polymer might be useful for adsorption studies.
Meanwhile, there are two possibility to synthesis star polymers. Firstly, using multifunctional initiator but this is less likely due to high charge of initiator which makes it insoluble in aprotic solvent. The second possibility is to use multifunctional terminator such as SiCl4.
Unlike linear polymers of the same MW, the solution viscosity of star polymer has only a weak temperature dependance. Therefore, star polymers can be used as an additive for engine oil formulation. This makes the performance of hot engine oil and cold engine oil is similar which is important in certain climates.
Another type of anionic polymerisations is the ring-opening polymerisation which is useful for the synthesis of poly(ethylene oxide) or PEO.
This living polymer has the negative charge on O atom which means more stable anionic living polymer. Besides that, PS-PEO diblock copolymer can be synthesised using anionic polymerisation but styrene need to be polymerised first.
From the discussion above, the advantages of using anionic polymerisation are:
- homopolymers with narrow MWD can be prepared over a wide range of MW by varying the molar ratio of monomer and initiator,
- can cap the living polymer with specific ebd-groups,
- a well defined diblock copolymer (e.g. AB) can be synthesised by adding monomer B to living polymer A.
However, there are several drawbacks of this polymerisations which are synthetically demanding and inappropriate monomers with labile proton can terminate the polymerisation prematurely. The second problem can be solved by masking or protecting the functional group which can be deprotected easily after polymerisation.
Now, we can move to cationic polymerisation where it employs cationic active centre. This can be promoted by reacting the monomers with Lewis acid (BF3, AlCl3, FeCl3, or SnCl4) or H2SO4. The positive charge on the living polymer is stabilised by electron-donating group. Unlike anionic polymerisation, cationic polymerisation suffers from intrinsic termination and chain transfer reaction. Thus, cationic polymerisation offers little control over either target molecular weight or MWD; hence, it is much less studied and more limited synthetic utility.
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