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The creation of plastics from polymers is now an old industry. So, there are many methods, sub-methods, and specific synthetic variations now used by academics and industry scientists to create plastics. While there are far too many single methods to document individually, there are some common method classes which are used to make polymers—and it is those methods which will be discussed here.
Plastics are made from polymers. Polymers are long-chain organic molecules which utilize a carbon-based backbone (much like hydrocarbons). These chains can either be linear, or they can be branched with different functional or sterically hindering groups, and this is how different polymers are created. Polymers are built up from (one or more) monomers—which are essentially small molecular fragments—that act as building blocks and are built modularly next to each other. The generation of these building blocks next to each other creates a long carbon chain which is the polymer. However, there are various methods which can be used to produce polymers. The production of long polymer chains creates amorphous solids, which are the plastics we know (although some can be crystalline or semi-crystalline in nature). Here, we’re going to take a look at the most common polymerization methods used to create the polymers which are used to make plastics.
Addition polymerization, also known as chain-growth polymerization, uses a series of different steps termed the initiation, propagation and termination steps. This is a polymerization reaction that involves a double-bonded (alkene) carbon molecule alongside a free-radical catalyst (which acts as an initiator molecule for the reaction). In theory, only one initiation is required, as the process should continue indefinitely. But in practice, multiple initiations are required to yield the desired polymer. It is a common method that can be used to create both linear and branched alkenes depending on the monomer(s) used.
The polymerization reaction starts at the initiation stage, where the free radical catalyst reacts with the double bond of the alkene. This causes the double bond to break, whereupon the radical species bonds to one side of the now-alkene molecule and the free radical is transferred to the other side of the molecule. This happens because the free-radical catalyst donates its unpaired electron to monomer, which makes the monomer reactive. From here, the monomer acts as the reactive species for other monomers to join to.
Once the monomer is reactive, the reaction goes through a repetitive process of adding monomers together to form a polymer chain. This is the propagation stage. The monomers react with the reactive side of the monomer, in which the double bond is opened upon each molecular addition. When the double bond breaks, the monomer bonds to the other monomers in the polymer chain to form a long polymer chain, and the reactive center is transferred to the other side of the monomer unit (the side which has just become attached). This means that the reactive center is always at the end of the chain, which allows a (theoretically) infinite number of monomers to join on to the polymer chain. The shifting of the reactive free-radical center happens because the energy of the chemical system is lowered as the polymer chain grows, so thermodynamically speaking, the single bonds in the polymer chain are more stable than the double bonds of the monomer.
While the process could continue indefinitely, most polymer/plastic manufactures want the polymer to have a certain weight, as that governs its properties. So, a termination stage is employed which ends the propagation of the polymer chain. To terminate the reaction, another free-radical molecule is used. The free radical molecule reacts with the last CH2 in the polymer chain and this then ‘caps’ the end of the chain so no more reactions can commence. Termination can also occur by two growing polymer chains coming together at their reactive ends, where the reactive ends form a bond, ending the reaction.
Condensation polymerization, also referred to condensation-elimination polymerizations or step-reaction polymerization, uses an elimination reaction to create the polymer. In this polymerization reaction, the monomers need to have two functional groups which will react with each other to form covalent bonds, e.g. a C=O and N-H group to form an amide linkage—one common example is the synthesis of nylon 6,6. In many cases, two different monomers will be used in condensation polymerization reactions and lower density polymers are often the result.
In condensation polymerization, the monomers react together to form dimers, and in this reaction process, water is released as a by-product—i.e. a hydrogen ion and a hydroxyl ion are lost in the bond formation process. Because the monomers used have two functional groups, the chains can grow in either direction to form oligomers (short polymer chains). This process continues (in either growing direction) until there are no reactant monomers left, and often produces linear non-cross-linked polymer chains. Branched chains are possible, but this requires a third monomer to be used.
It should be noted that whilst the classes of addition and condensation polymerization are seen as chain growth and step-growth reactions, respectively, there are cases where the creation of a polymer can be considered both types of method for different reasons. So, it is not always straightforward to classify polymerization approaches, and this stems from the high amount of monomer combinations and monomer compositions which are possible, as well as the many possible variations in the reaction parameters. But the methods detailed here are seen as the most common classes of methods from which other methods stem from.
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