Nylon is found in many everyday products. When you read the word «nylon» you immediately think of stockings. However, you can also find nylon fiber in sportswear, underwear, and carpets. All you have to do is look at the label of your sports pants. In addition, nylon is used in the automotive industry and is also used in household appliances. But how does nylon relate to the subject of polycondensation?
Definition of polycondensation
One polycondensation is a condensation reaction in which monomers are joined together to form a macromolecule. The monomers have at least two functional groups that are capable of reacting.
In a condensation reaction, two molecules are linked together. be there small, low-molecular compounds split off. Water, alcohols or hydrogen chloride are examples of this. Such by-products do not arise during polyaddition and polymerization.
As you have already learned, the monomers must contain two functional groups. You are probably wondering which functional groups are involved. The hydroxy group (-OH), the carboxyl group (-COOH), the aldehyde group (-CHO) and the amino group (-NH2) occur, for example, in the polycondensation monomers.
Monomers that have two functional groups are bifunctional. When these molecules react with each other, linear polymers without branches are formed, also called thermoplastics. An example is the reaction of diols and dicarboxylic acids to form esters. An ester group is formed by the reaction of a hydroxy group with a carbyl group. This releases water.
Monomers with more than two functional groups are called polyfunctional. Such molecules react to form three-dimensional polymers that have branching. Thermosets, such as phenoplasts, are formed.
Thermoplastics consist of linear polymer chains without branches. There are only weak physical forces between the polymer chains. As a result, these plastics can be deformed by heating and cooling.
Thermosets, on the other hand, are three-dimensionally crosslinked polymers. Covalent bonds create a network that is close-meshed. This structure means that these plastics are not deformable.
If you want to know more about the two types of plastic, take a look at the explanations.
The condensation reaction produces polycondensates, such as polyesters and polyamides.
Reaction conditions of the polycondensation
As you have already learned, polycondensation produces low-molecular compounds as a by-product. They must be constantly removed from the reaction. If this does not happen, the polycondensation will stop if the degree of polymerisation is too low. Thermodynamic reasons and Le Chatelier’s principle play a role here.
Le Chatelier’s principle is also called the principle of least compulsion. It states: If an external constraint acts on a material system that is in equilibrium, the balance shifts in such a way that the effects of the constraint are minimized.
Thermodynamics deals with systems in equilibrium and the effects of changes in temperature, pressure and volume.
Furthermore, the conversion of the reaction should be at least 99%, so that polycondensates with a sufficiently large molar mass are formed. The amount of monomers used should also be matched to the stoichiometric factors of the reaction equation. This means that attention should be paid to the ratio of the amounts of substance of the monomers. Otherwise it is possible that all intermediates end up with the same functional groups. A link is no longer possible and the polycondensation breaks off.
Polycondensation is one step growth reaction. All condensation reactions take place independently and represent one stage. The macromolecules are thus formed in stages or step by step. The resulting intermediate products are called oligomers. They are stable and capable of further reaction.
You can imagine it like this: First, two monomers react with each other. A dimer is formed. It can react further with either a third monomer or a second dimer. This is how a trimer or tetramer is formed. The scheme continues until finally polymers are formed.
Examples of polycondensation
Now that you already know a bit about polycondensation, you will get to know some examples in this section. The mechanism is somewhat different depending on the polymer formed.
Representation of polyethylene terephthalate
Polyethylene terephthalate is a polyester. Polyesters are formed by the reaction of a dicarboxylic acid, a compound with two carboxy groups, and a diol, a molecule with two hydroxy groups. In the case of polyethylene terephthalate, terephthalic acid, a dicarboxylic acid, and ethanediol, a diol, react with each other. Water is formed as a by-product during polycondensation.
Basically, the formation of a polyester is an esterification that takes place multiple times. An esterification is an acid catalyzed reaction of a carboxylic acid and an alcohol. In the process, water is split off.
A further possibility for preparing a polyester is the use of a hydroxycarboxylic acid. In this compound, both the hydroxy group and the carboxyl group are in the same molecule. Thus, not two different types of monomers react with each other, but the reaction proceeds with only one type of monomer.
You want to know more about polyester? Then take a look at the explanation.
Representation of nylon
As you read in the introduction, nylon can be found in many everyday products. Nylon is a polyamide. Polyamides are formed by the reaction of diamines, compounds with two amino groups, and dicarboxylic acids. Macromolecules are formed whose monomers are linked via an amide bond. Water is also formed as a by-product.
The chemical name for nylon is polyhexamethylene adipamide, but you don’t need to remember that. It forms from the reaction of 1,6-diaminohexane, a diamine, and hexanedioic acid, a dicarboxylic acid.
Curious? You can find more information about polyamides in the corresponding declaration.
Presentation of polycarbonates
as polycarbonates are called the polyesters of carbonic acid (H2CO3). Phosgene, a dichloride of carbonic acid, and diols are used for polycondensation. Another possibility is the polymerization of cyclic carbonates. However, this reaction will not be discussed further. Today you are dealing with the representation of polycarbonates using polycondensation.
You can derive the name polycarbonate from the phosgene used. Because carbonates are salts of carbonic acid and phosgene is also a derivative of carbonic acid.
A polycarbonate that is widely used is made from phosgene and bisphenol-A. In purely formal terms, the chlorine atoms in phosgene are replaced by bisphenol A. The eliminated chlorine atom and the donated proton of the hydroxy group of bisphenol A combine so that HCl is released during the polycondensation.
Polycarbonates are plastics with high mechanical stability. It is also possible to produce transparent polymers. That’s why you often find polycarbonate in CDs, glasses and safety helmets, for example.
More information about polycarbonates can be found in a separate statement.
Depiction of phenolics
Finally, you are still learning phenolics know. Phenoplasts are formed by the reaction of phenols and aldehydes. The polycondensation forms thermosets that are hard and brittle.
Using an example, you will learn the reaction mechanism. Bakelite, a phenoplast made from the educts phenol and formaldehyde, serves as an example. Bakelite is the first phenolic resin to be synthetically manufactured and eventually commercialized.
The first step is the formation of a precursor. Depending on how much formaldehyde, also known as methanal, is used, precursors with 1 to 3 hydroxymethyl groups are formed.
The reaction taking place here is an electrophilic substitution on phenol. Acids such as hydrochloric acid and oxalic acid serve as a catalyst. If you want to know what electrophilic substitution is, feel free to check out the explanation about it.
In the second step, the polycondensation takes place. The phenol derivatives formed in the first step gradually form intermediates which are linked to one another via methyl groups. These intermediates condense further to eventually form a crosslinked polymer. The product is a phenol-formaldehyde resin, also known as bakelite.
Application of polycondensation
You now know the chemical background of polycondensation. However, you are now wondering where polycondensation or where polycondensates play a role.
Polycondensation is a central process in the chemical industry to produce polymers. You can find many of the polycondensates almost everywhere in everyday life. You only have to look at your plastic bottle, because it is made of polyethylene terephthalate (PET). PET is also used in packaging and foils as well as in textile fibers.
In addition to nylon, there is Perlon as a competing product. Perlon is also a polyamide that has almost the same properties as nylon. Perlon is used as an artificial fiber. It can also be found in gears and screws in machines that are only subjected to low loads.
Bakelite, which you learned about in the phenolic section, is used in telephones, pan and saucepan handles, socket housings, and numerous other household and kitchen products.
Finally, you should know that polycondensation also plays a role in the manufacture of adhesives and brake pads for cars.
Polycondensation – The most important thing
- Polycondensation is a condensation reaction in which monomers are combined to form a macromolecule and a low-molecular by-product is split off.
- The monomers must contain two functional groups, the hydroxy group, the carboxyl group, the aldehyde group and the amino group often occurring.
- Polycondensation is a step growth reaction. In addition, the by-product must be constantly removed from the reaction, since otherwise the condensation reaction ends when the degree of polymerization is too low.
- Known polycondensates are:
- Polyesters, such as polyethylene terephthalate, are formed by the reaction of a dicarboxylic acid and a diol.
- Polyamides are formed through the polycondensation of diamines and dicarboxylic acids. Nylon is an example of a polyamide.
- Polyesters of carbonic acid are called polycarbonates. They are formed, for example, through the reaction of phosgene and bisphenol A.
- Phenol and aldehydes react to form phenoplasts such as Bakelite.
proof
- faszinationchemie.de: What actually is… nylon?. (08/22/2022)
- MD Lechner et al. (2010). macromolecular chemistry. Birkhauser Verlag.
- A. Wollrab (1999).