Polymers and chain extension
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Addition polymerisation is the joining of a large number of alkene monomers to form an addition polymer.
The -bond of the of the alkene breaks and new -bonds are formed between monomer units.
To show one repeating unit of an addition polymer from a given monomer:
- break the -bond from the
- replace it with two trailing bonds going through brackets.
To identify the monomer from a single repeating unit:
- remove the trailing bonds and brackets
- add a -bond between the carbons.
In addition polymerisation, repeating units will always have two carbons – there are two carbons in the alkene functional group.
Additional carbons may feature as side chains.
IUPAC states that addition polymers are named after the monomers that they are made from.
- To name an addition polymer, you add the prefix ‘poly’ to the bracketed name of the monomer.
- In the case of the monomer being an alkene, the suffix will still be -ene despite the pi bond having broken during polymerisation.
For example, the addition polymer of ethene would have the IUPAC name poly(ethene), although alternatives such as polythene and polyethylene are frequently used.
The general formula of addition polymers is shown by identifying the repeating unit and putting brackets around it followed by the subscript ‘n’ indicating the number of repeat units in the chain.
Addition polymers are unreactive.
During polymerisation the double bonds are broken, resulting in a saturated carbon chain.
Sigma covalent bonds are very stable and not easily broken, resulting in the polymer backbone being highly inert. The main polymer backbone is non-polar, further reducing the reactivity.
The side chains on addition polymers can influence reactivity but these are often hydrocarbon based and therefore possess the same resistance to reaction as the polymer backbone.
Polymers are commonly viscous liquids or solids and are never gases.
The intermolecular forces between molecules of polyalkenes are primarily London dispersion forces due to the non-polar nature of the polymer chains. These forces arise due to the temporary dipoles induced in the electron cloud of the molecules.
Compared to simple covalent molecules, polymers are big and London dispersion forces can be significantly stronger. Longer polymer chains lead to more electrons, and therefore stronger dispersion forces.
Branched polymer chains are less able to make surface contact than straight chains, reducing the strength of the London dispersion forces. This also reduces the rigidity of the polymer and its melting point.
Condensation polymerisation occurs when many monomers combine with the elimination of a small molecule, such as , or .
where = a whole number.
Condensation polymerisation can occur with one monomer or a pair of monomers.
The two classes of polymer formed by condensation polymerisation are polyesters and polyamides.
Polyesters have the ester group, , between monomers.
The group between monomers in a polyamide is the amide group,
Dicarboxylic acid monomers and diol monomers undergo condensation polymerisation to form polyesters, and water as a by-product.
This is an example of two monomers synthesising a polymer product. The polyester formed from this reaction features an ester bond within the repeat unit.
Two molecules of water are produced for every repeat unit in the chain; there is an ester link formed within the repeat unit, as well as one connecting to the main chain.
Diacyl chloride monomers and diol monomers undergo condensation polymerisation to form polyesters, and hydrochloric acid as a byproduct.
The polyester formed from this reaction features an ester bond within the repeat unit.
The use of a diacyl chloride will produce the same polyester as with a dioic acid, but the reaction will be faster.
This comes with the disadvantage of giving off toxic hydrochloric acid gas, (g), rather than water as a by-product.
Polyesters can be formed from a single monomer with both the carboxylic acid and alcohol group on the same carbon skeleton.
These monomers are known as hydroxycarboxylic acids.
It is important to note that in this reaction the repeat unit does not contain an ester group.
Polyamides have the secondary amide group when the two functional groups react together.
Dicarboxylic acid monomers react with diamine monomers via condensation polymerisation, forming polyamides and water.
The dicarboxylic acid and the diamine have functional groups at both ends, so each monomer can form two amide links.
There is an amide group within the repeat unit, and two water molecules are produced per repeat unit formed.
Polyesters and polyamides are formed via condensation reactions, where two functional groups add together with the elimination of a small molecule.
The reverse reaction is to break up the polymer by adding back in the small molecule lost. If this is water, the reaction is called a hydrolysis (a reaction that involves the splitting of water molecules).
When polyesters are hydrolysed the ester bond splits to give a carboxylic acid and an alcohol.
In acidic conditions the products are protonated, whereas in alkaline conditions the carboxylic acid is deprotonated and the product is a carboxylate salt.
When stating the products of polymer hydrolysis, remember to consider the impact of on the functional groups.
When polyamides are hydrolysed the amide bond splits to give a carboxylic acid and an amine.
In acidic conditions the products are protonated and the amine is converted to an ammonium salt, whereas in alkaline conditions the carboxylic acid is deprotonated and the product is a carboxylate salt.
When stating the products of polymer hydrolysis, remember to consider the impact of on the functional groups.
A single repeat unit of a polymer contains all the detail needed to understand the structure of the chain. It is the building block within the polymer which repeats.
Repeat units can be deduced from the monomers used, or the polymer chain, in condensation polymerisation.
If a polyester is made from two different monomers, a dicarboxylic acid (or diacyl chloride) and a diol, the order of the ester connectivity will alternate across the polymer and both monomers must appear in the repeating unit.
To draw the repeat unit:
- Draw the ester group in the middle of the structure.
- Add the dicarboxylic acid (or diacyl chloride) carbon skeleton to the left of the group, and terminate the left side with the last group, a trailing bond, and a repeat unit bracket.
- Draw the diol carbon skeleton on the right of the central from the ester and terminate on the right with the group, a trailing bond, and a repeat unit bracket.
When amino acids join together they form polyamides. As there is a single monomer, the amide group will not be seen in the repeat unit and the order of connectivity of the amide group remains the same along the chain.
Repeat units can be used to identify monomers.
First, identify any ester or amide link within the repeat unit and break this bond at the hydrolysis site.
Next, complete the functional groups to obtain the structure of your monomer(s).
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Polyamides can be formed from a single monomer with both the carboxylic acid and the amine group on the same carbon skeleton.
These monomers are known as amino acids.
During polymerisation, amino acids link together to form a type of polyamide called a polypeptide through condensation reactions.
All polypeptides are polyamides but not all polyamides are polypeptides.
Be cautious when reactions involve amino acids; they can contain additional functional groups which will impact their reactivity.
Polyesters are condensation polymers that feature polar ester groups along their chain backbone.
The intermolecular forces in polyesters always include permanent dipole-dipole interactions, arising from the polarity of the ester groups.
Hydrogen bonding may occur in a polyester, depending on the structure. For hydrogen bonding to occur an or must feature within the chain.
These forces contribute to polyesters having higher melting and boiling points, as well as increased strength and durability, compared to addition polymers.
Polyesters are versatile and widely used in applications such as textiles and packaging.
Polyesters contain ester bonds within their polymer chains, which are also polar and susceptible to hydrolysis.
Polyesters are potentially biodegradable, but their degradation is typically slow.
Degradation of condensation polymers is commonly facilitated by microbes.
Polyalkenes are formed from the polymerisation of alkene monomers through addition polymerisation.
Polyalkenes are chemically inert and non-biodegradable due to their structural composition.
Their polymer chains consist of non-polar, saturated hydrocarbon backbones. This non-polar nature makes polyalkenes resistant to interaction with polar substances, including enzymes and microbial activity, which contributes to their inability to biodegrade.
The strong sigma bonds in the saturated polyalkenes enhance their chemical stability, making them resistant to hydrolysis, oxidation, and other chemical reactions.
Materials formed from polyalkenes are often referred to as persistent plastics as they do not decompose in landfill.
Combustion of waste polymers for energy production reduces the volume of landfill and contributes to the energy demand.
The heat produced is used in the production of electricity.
Halogenated plastics, such as polyvinyl chloride (PVC), produce toxic hydrogen halides during combustion, which must not be released to the environment.
Sodium hydrogen carbonate in the processor neutralises the hydrogen halides in flue gas.
Waste polymers can be used as chemical feedstocks.
Waste polymers are physically and chemically broken down before being separated into a combination of monomers, gases and oils. These can be used in production of new polymers and organic materials.
Use of waste polymers as chemical feedstocks has an advantage over traditional recycling as mixed and unwashed materials can be used. It also reduces the reliance on crude oil for the supply of monomers.
Reusing and recycling of waste polymers has many advantages;
- Using a polymer based product multiple times reduces the number of products that need to be made and disposed of. For example, refillable water bottles and ‘bags for life’.
- Once their useful life is over, polymers can often be reprocessed and the material used again in the manufacture of new products.
- This reduces the demand on raw materials as well as reducing the amount of polymer sent to landfill.
Since the first simple addition polymers technology has significantly expanded the functionality and properties of polymers.
Biodegradable and photodegradable polymers are being developed to reduce the environmental impact of persistent plastic waste.
Biodegradable polymers are designed to decompose in the presence of bacteria or other living organisms.
Photodegradable polymers are designed to decompose in the presence of UV light.
Starting materials for the synthesis of these novel polymers are increasingly biobased, reducing the dependency on the finite resource of crude oil and natural gases.