Free Radical Polymerisations
Chain polymerisation is probably the most common polymerisation that is widely used, either for lab or industrial scale and one the example is free radical polymerisation. There is an obvious reason why this is callen free radical polymerisation and it is simply due to using radical species as the active species. In the same way with step polymerisation, this section will discuss mainly about ita kinetics.
Chain polymerisation is much faster polymerisation relative to step polymerisation. The growth of polymer chain is very rapid and once the growth stops, the chain is no longer reactive. This type of polymerisation requires an external iniator to initiate the reaction. Besides that, the growth of polymer chain is caused by kinetic chain of reactions and the chain reaction proceeds via monomer addition to an active centre. The active centre can be radical (as in this case), anionic, cationic or polymer-catalyst bond. Chain polymerisation is mainly useful for vinyl monomer such as styrene (vinyl benzene), vinyl chloride and methacrylate.
The main advantages of free radical polymerisation are it is a robust technique which means applicable to a wide range of vinyl monomers, and also it has wide range range of operating conditions; free radical polymerisation can be done in water solvent. However, this type of polymerisation has certain drawbacks such as poor selectivity due to radical species and relatively poor control of the product such as its MWD, architecture and stereochemistry.
As mentioned earlier, this type of polymerisation requires an external initiator such as chemical initiators (which will be our focus), UV radiation (initiated via photolysis reaction) and gamma radiation. The common chemical initiator for that is used this polymerisation is based on peroxides or azo compounds.
The driving force of peroxide-based initiator is the dissociation of the weak O-O bond while in azo-based compound is the formation of stable nitrogen gas the byproduct. Furthermore, both initiators undergo unimolecular decomposition via first-order kinetic.
One of the important parameters of chemical initiator is the initiator efficiency, f, which is the fraction of radicals available for polymerisation of monomer. The typical f of chemical initiator is between 0.6 to 1.0, initiator efficiency of 1.0 simply means that all radicals formed from the initiator lead to the polymer chain growth.
This correction factor is needed due to the recombination with free radical rather than the monomer, this effect is known as Cage effect.
Hence, Cage effect would reduce the efficiency of initiator which means factor f is needed to correct the rate of initiation to give the actual rate of the formation of radicals which lead to polymer chain growth.
In general, the kinetics of free radical polymerisation can be devided into 4 steps: decomposition, initiation, propagation and termination.
In free radical polymerisation, the rate limiting step (r.l.s.) is decomposition reaction (typical kd is 10-4 - 10-6 s-1), and as in step polymerisation, the rate of propagation is assumed to be independent from the chain length. Then, termination can happen in two ways, termination by combination where 2 active polymer chains join together and termination by disproportionation where 2 active polymer chains produce 2 dead polymers.
The dominant mechanism depends on both monomer type and temperature, for example termination by combination generally happens for styrene, acrylates while termination by disproportionation occurs with methacrylates (up to 80% at 80 °C).
The rate law of polymerisation can be derived using steady-state approximation, and considering the rate of initiation (rate of radical production) Ri and rate of termination (rate of radical annihilation) Rt.
Now, by rearranging equation above, it gives
then, considering rate of polymerisation as the depletion rate of monomer and assuming the number of monomers consumed during initiation is negligible compared to the number of monomers consumed during propagation,
so the rate of polymerisation can be written as shown below.
From the equation above, it can be deduced that the rate of polymerisation is proportional to concentration of monomer [M] and square-root of initiator concentration [I]1/2.
From the kinetics above, a new parameter can be introduced to describe FR polymerisation which is kinetic chain length Dk. This parameter is defined as the average number of monomer units consumed per radical acitve centre; hence it can be defined as the ratio between Rp and Ri (or Rt).
This implies for the formation of high MW polymer requires high concentration of monomer but lower concentration of initiator. However, lowering the concentration of initiator means lowering the rate of polymerisation so an optimum condition between the concentration of initiator and monomer is necessary.
The real rate of polymerisation can slightly deviate from the theoretical rate law and one of the factors is intrinsic side-reactions such chain transfer reactions. Firstly, chain transfer reaction can happen between polymer radical to solvent and this one generally occurs if the solvent has high chain transfer constant.
The second possibility of chain transfer reaction is polymer radical to monomer and it is mainly important for polymerisation of vinyl acetate at low conversion. This chain transfer reaction has two possibilities as shown below.
Both possibilities increase the number of polymer chains produced per number of initiated chains which loeads to a lower Mn. Besides that, an appropriate solvent can be chosen or deliberately adding thiols of high chain transfer constant to control (lower) Mn.
In high conversion, the third possibility of chain tranfer reaction can happen which is polymer radical to polymer. This type of chain transfer reaction also has two possibilities, intermolecular or intramolecular. Intermolecular transfer to polymer leads to long-chain branching and this is particular problem for poly(vinyl acetate) since the pendant methyl protons of acetyl group are readily abstracted.
Such chain branching leads to a large increase in Mw and Mw/Mn.
Intramolecular transfer to polymer can also occur such as in polyethene or poly (vinyl chloride) and it gives short-branching which reduces crystallinity and lowers polymer density.
In practice, transfer to polymer is more important than transfer to monomer.
In FR polymerisation, it requires oxygen-free condition because oxygen cna react with active polymer to form peroxy radical which can inhibit the addition of monomer to active site.
Hence, oxygen can act as retarder in FR polymerisation. Besides that, if the inhibitor is so effective, it is called inhibitor. Mainly, inihibitor is added into monomers to prevent spontaneous polymerisation during long-term storage. This inhibitor can be easily removed, e.g. using distillation, when the monomer is going to be used. Furthermore, examples of common inhibitor are p-benzoquinone and nitrobenzene.
Since the nature of active polymer, the monomer can be added (coupled) in two ways, head-to-tail (HtT) or head-to-head (HtH) coupling.
HtT coupling is much favoured than HtH (around 99:1)on both steric and electronic grounds for chain polymerisations, regardless of the nature of the active centre. In the worst case, HtH can get up to 7% in poly(vinyl alcohol) and this coupling can be selectively cleaved.
Chain polymerisation is much faster polymerisation relative to step polymerisation. The growth of polymer chain is very rapid and once the growth stops, the chain is no longer reactive. This type of polymerisation requires an external iniator to initiate the reaction. Besides that, the growth of polymer chain is caused by kinetic chain of reactions and the chain reaction proceeds via monomer addition to an active centre. The active centre can be radical (as in this case), anionic, cationic or polymer-catalyst bond. Chain polymerisation is mainly useful for vinyl monomer such as styrene (vinyl benzene), vinyl chloride and methacrylate.
The main advantages of free radical polymerisation are it is a robust technique which means applicable to a wide range of vinyl monomers, and also it has wide range range of operating conditions; free radical polymerisation can be done in water solvent. However, this type of polymerisation has certain drawbacks such as poor selectivity due to radical species and relatively poor control of the product such as its MWD, architecture and stereochemistry.
As mentioned earlier, this type of polymerisation requires an external initiator such as chemical initiators (which will be our focus), UV radiation (initiated via photolysis reaction) and gamma radiation. The common chemical initiator for that is used this polymerisation is based on peroxides or azo compounds.
The driving force of peroxide-based initiator is the dissociation of the weak O-O bond while in azo-based compound is the formation of stable nitrogen gas the byproduct. Furthermore, both initiators undergo unimolecular decomposition via first-order kinetic.
One of the important parameters of chemical initiator is the initiator efficiency, f, which is the fraction of radicals available for polymerisation of monomer. The typical f of chemical initiator is between 0.6 to 1.0, initiator efficiency of 1.0 simply means that all radicals formed from the initiator lead to the polymer chain growth.
This correction factor is needed due to the recombination with free radical rather than the monomer, this effect is known as Cage effect.
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| Cage effect |
In general, the kinetics of free radical polymerisation can be devided into 4 steps: decomposition, initiation, propagation and termination.
The dominant mechanism depends on both monomer type and temperature, for example termination by combination generally happens for styrene, acrylates while termination by disproportionation occurs with methacrylates (up to 80% at 80 °C).
The rate law of polymerisation can be derived using steady-state approximation, and considering the rate of initiation (rate of radical production) Ri and rate of termination (rate of radical annihilation) Rt.
Now, by rearranging equation above, it gives
then, considering rate of polymerisation as the depletion rate of monomer and assuming the number of monomers consumed during initiation is negligible compared to the number of monomers consumed during propagation,
From the equation above, it can be deduced that the rate of polymerisation is proportional to concentration of monomer [M] and square-root of initiator concentration [I]1/2.
From the kinetics above, a new parameter can be introduced to describe FR polymerisation which is kinetic chain length Dk. This parameter is defined as the average number of monomer units consumed per radical acitve centre; hence it can be defined as the ratio between Rp and Ri (or Rt).
The real rate of polymerisation can slightly deviate from the theoretical rate law and one of the factors is intrinsic side-reactions such chain transfer reactions. Firstly, chain transfer reaction can happen between polymer radical to solvent and this one generally occurs if the solvent has high chain transfer constant.
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| Polymer to solvent transfer |
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| Polymer to monomer transfer |
Both possibilities increase the number of polymer chains produced per number of initiated chains which loeads to a lower Mn. Besides that, an appropriate solvent can be chosen or deliberately adding thiols of high chain transfer constant to control (lower) Mn.
In high conversion, the third possibility of chain tranfer reaction can happen which is polymer radical to polymer. This type of chain transfer reaction also has two possibilities, intermolecular or intramolecular. Intermolecular transfer to polymer leads to long-chain branching and this is particular problem for poly(vinyl acetate) since the pendant methyl protons of acetyl group are readily abstracted.
 |
| intermolecular polymer radical to polymer transfer |
Such chain branching leads to a large increase in Mw and Mw/Mn.
Intramolecular transfer to polymer can also occur such as in polyethene or poly (vinyl chloride) and it gives short-branching which reduces crystallinity and lowers polymer density.
 |
| intramolecular polymer radical to polymer transfer |
In practice, transfer to polymer is more important than transfer to monomer.
In FR polymerisation, it requires oxygen-free condition because oxygen cna react with active polymer to form peroxy radical which can inhibit the addition of monomer to active site.
Hence, oxygen can act as retarder in FR polymerisation. Besides that, if the inhibitor is so effective, it is called inhibitor. Mainly, inihibitor is added into monomers to prevent spontaneous polymerisation during long-term storage. This inhibitor can be easily removed, e.g. using distillation, when the monomer is going to be used. Furthermore, examples of common inhibitor are p-benzoquinone and nitrobenzene.
Since the nature of active polymer, the monomer can be added (coupled) in two ways, head-to-tail (HtT) or head-to-head (HtH) coupling.
HtT coupling is much favoured than HtH (around 99:1)on both steric and electronic grounds for chain polymerisations, regardless of the nature of the active centre. In the worst case, HtH can get up to 7% in poly(vinyl alcohol) and this coupling can be selectively cleaved.
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