What is initiation polymerization
Manufacture of plastics:
Free radical polymerization
In radical polymerization, as the name suggests, radicals react, these are particles that have unpaired electrons and are therefore usually very reactive.
And as a reminder: A polymerization reaction can be divided into the following four steps:
To initiation A so-called "starter", which delivers radicals, serves the reaction; frequently used starters are benzoyl peroxide or 2,2'-azo-bis-isobutyronitrile (AIBN): These molecules break down easily and form radicals (the curved arrows show the migration of individual electrons ):
The resulting radicals with their unpaired electrons now try to find electrons that they can pair with their individual electron in order to be "complete" again. Good electron suppliers are C = C double bonds, such as those found in ethene, for example, because these are easily attacked by radicals. If our starter radical comes close to an ethene molecule, it grabs an electron from the double bond and thus forms a bond with ethene. If you count the electrons, you notice that even now an unpaired electron remains, so we still have a radical (albeit a larger one) that can react with another ethene molecule, i.e. the chain reaction has started.
With that we are already at the Growth response (scientific "propagation") arrived. Here, as the name suggests, the molecules grow, i.e. in the case of radical polymerization, the radical reacts again and again with ethene molecules and the molecular chain becomes longer and longer.
A reaction like this, which by itself goes on as long as there are still reactive particles present, is called a "chain reaction". Theoretically it could go on and on, but besides the growth reaction there are also other reactions that prevent further growth of the chain by "destroying" radicals and so on Chain termination to lead.
The most common way of breaking off a radical chain reaction is through Recombination or through Disproportionation, but you can also add "radical scavengers" to the reaction mixture, which - as their name suggests - trap the radicals so that they cannot react any further.
Chain termination by recombination
During recombination, two radicals form a bonding pair of electrons from their two unpaired electrons, so two particles result in one that is not a radical and is therefore no longer reactive.
Chain termination through disproportionation
This type of chain termination is somewhat more complex than recombination, it is based on the fact that instead of the unpaired electron of another radical, a radical can also grab an electron and hydrogen atom from a CH bond of another radical, with its unpaired electron and the hydrogen atom can form a CH bond itself and is therefore no longer a radical. More precisely, it takes away the hydrogen atom and electron from another radical that is adjacent to the carbon atom with the unpaired electron. The other radical now has two unpaired electrons, which are, however, located on neighboring carbon atoms, so that they can easily combine to form a bonding pair of electrons, creating a double bond.
Conclusion: An alkane and an alkene are formed, neither of them has unpaired electrons, the radical chain reaction is over here.
Those who have thought about it have probably already asked themselves why I initially skipped the chain transfer, even though it was mentioned above before the chain was broken. This is simply due to the fact that chain transfer and chain termination by disproportion work according to the same principle, the only difference is that the first radical does not take the hydrogen atom and electron that is adjacent to the carbon atom with the unpaired electron, but any hydrogen atom in the middle off the chain. This leaves a carbon atom with an unpaired electron in the middle of the chain.
However, since there is not an unpaired electron on the neighboring atom as above, no electron pair bond can be formed; the molecule now has a new way of reacting with ethene molecules. In doing so, a new chain begins to grow somewhere in the middle of the original chain, the molecular chain branches out.
But why is it called chain transfer?
Good question, because after all, not a piece of chain is transferred from a molecule, as the name might initially suggest. In fact, if you look closely, "only" a single hydrogen atom is transferred, but this means that the unpaired electron and thus the possibility of reacting with other monomers is no longer on the original chain, but on the other chain that has lost the hydrogen atom, so that the The growth reaction can continue here, i.e. indirectly ultimately a chain or the prerequisite for the formation of a chain passes from one chain to the other.
Copyright © mk 2000
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