Radicals and Us I: the Fundamentals and Polymer Manufacture
In this section we will explore the chemistry behind the radicals and how radicals relate closely in our world. This part we will explore the fundamentals of radical which include how to generate radical and their stability. After that, we will see how radicals can be used in daily life, mainly in polymer synthesis.
Radical is a single-electron species, the opposite of radical is non-radical which has spin-paired electrons, and commonly radical does not have charge. Radicals can be generated in many ways that employs many variations. The first variation is using heat to generate radicals which commonly called thermolysis. The example of thermolysis is the radical generation from AIBN (azobisisobutylonitrile).
The second process to generate radicals is by employing the photon energy or UV light and this process is also common to be used, mainly in polymer synthesis. A precursor compound absorbs photon and there are three possibilities that might happens after the photon absorption.
The first possibility is the excited species just returns to its initial energy state without a reaction (1). Meanwhile, when the excited molecules react with each other the energy of the molecules will lose and it will return to lower energy state than their initial level (2). However, if the excited molecules only collide with each other without a reaction, it will lose their energy as a heat which lower their energy, then they just return to its initial energy level (3). All these reactions are known as Norish I type reaction. The example of photolysis in commercial is shown below.
The photolysis reaction is also used in polymer synthesis to give a branch polymer as shown below.
Besides that, in some cases the photolysis reaction requires a photosensitizer to ensure the reaction is happened. The typical photosensitizer is tertiary amine such as triethylamine.
The other generation process is by bombarding the compound or precursor molecule with radiation such as X-rays, β or γ radiation. Basically, this process in injecting an electron into the substrate as the radiation is fired onto the surface which then kick out an electron from the surface of the compound. This process requires a surface modification to give efficient process. The fourth process to generate radicals is by using redox reactions where an electron transfer occurred.
For your information, Ce(IV) is specific radical generation agent for generation radical at α-C to hydroxyl functional group. Lastly, Radicals can also be generated from mechanical process where high molecular weight polymer is broken to form homolytic fission.
The stability of radical is affected by both electron withdrawing and donating groups, while carbocation is stabilised by electron-donating group and carbanion is stabilised by electron-withdrawing group. The stability of radical means it decrease the reactivity of radical which means has longer half-life time. Besides that, the radical can also be stabilised by resonance as it stabilised through delocalised structure. This means the electrons are more spread out through the system as more resonance structure will lower the energy which enhance the stability. The resonance structure involves of π-bonding system.
Another similar model with resonance structure but using σ-bond is known as hyperconjugation and this is also important to predict the reactive species such as radicals. Hyperconjugation involves the β-H from the radical. In this hyperconjugation effect, more resonance forms as the number of β-H increases; hence increasing the stability.
Besides that, not all stabilities or reactivities are reconcilled by electron distribution models; sterics is also very important to predict the reactivity of radicals. Steric factor is associated with the ability to access the reactive site. Therefore, if the reactive site of radical is hindered, the stability is increased.
Furthermore, in the radical below the stability is not from the resonance structure but the steric factor.
The radical as mentioned earlier on is a single electron species that occupies an orbital and this orbital is called Singlet Occupied Molecular Orbital (SOMO) and it has high energy. Therefore, the reaction of radicals commonly requires to have spin pair formation to lower its energy. Generally there are 4 main types of radical reactions:
Besides that, combination and addition reaction is highly exothermic and this happens to compensate the loss of entropy. In addition-fragmentation reaction, the leaving group should be more stable than the group that replace it.
The radical chemistry is basically a real-state organic chemistry because it is used commonly in a lot of industry. The first reaction in radical chemistry is the cascade reaction which is a stepwise reaction and this multiple reactions happen in one reaction.
The first step of this reaction happens so fast because the radical and the double-bond are in juxta position and the reaction is intramolecular reaction. Based on Baldwin's rule the product 5-exo has radical outside the ring and this is the dominant; meanwhile 6-endo has radical inside the ring. Another radical reaction is the radical-analogue reaction of Michael addition where Bu3SnH is used to provide H radical.
In polymerisation reaction, the radicals are commonly used in this process and this process is divided into 3 different stages. The first step is initiation where the initiator radical react with a monomer through addition process. Then, the product of this reaction react with the other monomers to propagate the polymeric chain through addition process, and this reaction is called propagation. This type of polymerisation is called chain growth propagation and in conventional process the chain does not grow. The growth of polymeric chain is killed by reacting with another radical and this stage is called termination.
Besides that, in termination stages the radical can also be killed by H-transfer. The first possibility is with H-transfer it would form a saturated and an unsaturated chain. In the other sides, H-transfer could also forms another radical.
This transfer reaction makes polymerisation process produce no unique molecular weight as a distribution of molecular weight is produced. The highest molecular weight can be produced is up to 5 x 106 g mol-1 with the lowest possible molecular weight is 500 g mol-1.
Furthermore, in figures below is the common monomer that is used in industries.
Isoprene (2-methyl-1,3-butadiene) is the monomer of natural rubber which is the derivates of 1,3-butadiene. The polymerisation of butadiene has 2 possibilities due to its resonance structure as demonstrated below.
Both products are co-exist with each other, meanwhile in rubber tree an enzymatic process could produce only cis-1,4 addition product.
Another interesting polymerisation is the polymerisation of ethene (ethylene) to produce polyethylene. Ethene itself can produce polyethylene where propene (propylene) cannot polymerise to produce polypropylene. The polymerisation of propylene does not occur because of the transfer H and then it is stabilised by resonance structure.
Redox reaction is also common to be used in industry by using commonly transition metal. One of the example is the generation of benzylic radical by using CuBr.
In industry, it is important to control of radical addition reactions as the conventional radicals have short life time, usually less than 1 s, so living polymerisation seeks to lengthen these lifetimes. This lifetimes can be achieved by keeping the radicals in low concentration so termination can be minimised. The step to lengthen the lifetimes is shown in the diagram below.
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| Radical, single-electron species |
Radical is a single-electron species, the opposite of radical is non-radical which has spin-paired electrons, and commonly radical does not have charge. Radicals can be generated in many ways that employs many variations. The first variation is using heat to generate radicals which commonly called thermolysis. The example of thermolysis is the radical generation from AIBN (azobisisobutylonitrile).
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| AIBN radical generation |
The second process to generate radicals is by employing the photon energy or UV light and this process is also common to be used, mainly in polymer synthesis. A precursor compound absorbs photon and there are three possibilities that might happens after the photon absorption.
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| Norish I reaction of energy level diagram |
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| Example of photolysis |
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| Photolysis in polymer synthesis |
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| Photosensitizer (R) in action |
The other generation process is by bombarding the compound or precursor molecule with radiation such as X-rays, β or γ radiation. Basically, this process in injecting an electron into the substrate as the radiation is fired onto the surface which then kick out an electron from the surface of the compound. This process requires a surface modification to give efficient process. The fourth process to generate radicals is by using redox reactions where an electron transfer occurred.
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| Redox radical generation |
The stability of radical is affected by both electron withdrawing and donating groups, while carbocation is stabilised by electron-donating group and carbanion is stabilised by electron-withdrawing group. The stability of radical means it decrease the reactivity of radical which means has longer half-life time. Besides that, the radical can also be stabilised by resonance as it stabilised through delocalised structure. This means the electrons are more spread out through the system as more resonance structure will lower the energy which enhance the stability. The resonance structure involves of π-bonding system.
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| Delocalised structure of benzylic radical |
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| Hyperconjugation effect |
Besides that, not all stabilities or reactivities are reconcilled by electron distribution models; sterics is also very important to predict the reactivity of radicals. Steric factor is associated with the ability to access the reactive site. Therefore, if the reactive site of radical is hindered, the stability is increased.
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| Steric effect to radical stability |
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| Triphenylmethyl radical, steric hindrance effect |
The radical as mentioned earlier on is a single electron species that occupies an orbital and this orbital is called Singlet Occupied Molecular Orbital (SOMO) and it has high energy. Therefore, the reaction of radicals commonly requires to have spin pair formation to lower its energy. Generally there are 4 main types of radical reactions:
- combination reaction where 2 radicals come together to match the spin;
- abstraction reaction involves photosensitizer and another radical species to react;
- addition where radical react with alkene and the driving force of this reaction is highly exothermic reaction;
- addition-fragmentation is similar as addition and it reforms by kicking out another group.
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| Reactions of radical |
The radical chemistry is basically a real-state organic chemistry because it is used commonly in a lot of industry. The first reaction in radical chemistry is the cascade reaction which is a stepwise reaction and this multiple reactions happen in one reaction.
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| Cascade reaction |
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| Radical-analogue Michael addition |
In polymerisation reaction, the radicals are commonly used in this process and this process is divided into 3 different stages. The first step is initiation where the initiator radical react with a monomer through addition process. Then, the product of this reaction react with the other monomers to propagate the polymeric chain through addition process, and this reaction is called propagation. This type of polymerisation is called chain growth propagation and in conventional process the chain does not grow. The growth of polymeric chain is killed by reacting with another radical and this stage is called termination.
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| Three stages in polymerisation |
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| Hydrogen transfer |
This transfer reaction makes polymerisation process produce no unique molecular weight as a distribution of molecular weight is produced. The highest molecular weight can be produced is up to 5 x 106 g mol-1 with the lowest possible molecular weight is 500 g mol-1.
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| Polymer's molecular weight distribution in polymerisation |
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| Common monomers in industries |
Isoprene (2-methyl-1,3-butadiene) is the monomer of natural rubber which is the derivates of 1,3-butadiene. The polymerisation of butadiene has 2 possibilities due to its resonance structure as demonstrated below.
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| Polymerisation of 1,3-butadiene |
Another interesting polymerisation is the polymerisation of ethene (ethylene) to produce polyethylene. Ethene itself can produce polyethylene where propene (propylene) cannot polymerise to produce polypropylene. The polymerisation of propylene does not occur because of the transfer H and then it is stabilised by resonance structure.
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| Propylene radical stabilisation |
Redox reaction is also common to be used in industry by using commonly transition metal. One of the example is the generation of benzylic radical by using CuBr.
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| Benzylic radical generation |
In industry, it is important to control of radical addition reactions as the conventional radicals have short life time, usually less than 1 s, so living polymerisation seeks to lengthen these lifetimes. This lifetimes can be achieved by keeping the radicals in low concentration so termination can be minimised. The step to lengthen the lifetimes is shown in the diagram below.
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| Control of radicals' lifetime |
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