0620 · 11.8Polymers

Polymer basics and addition polymerisation

0:00A plastic bag, a nylon thread and a protein in your body have something important in common. Each contains very large molecules assembled from smaller building blocks. Their properties differ because their building blocks and connecting bonds are different. By the end of this video, you will understand how these chains form and how to work backwards to their starting molecules. Polymers are large molecules built up from many smaller molecules called monomers.

0:24The word monomer describes a starting molecule that can join others during polymerisation. A polymer is the large molecule produced by that process. Imagine connecting many individual pieces into a chain, with chemical bonds holding the pieces together. That analogy has one important limitation. Monomers do not simply sit beside each other like beads threaded onto a string. Their bonding changes as the polymer forms. To understand polymerisation properly, we need to follow which bonds change, which atoms remain, and whether any small molecules are released.

0:56We need only a few ideas from earlier organic chemistry. Carbon usually forms four bonds, and a double bond counts as two. A functional group is the part of an organic molecule responsible for its characteristic reactions. Here, carbon to carbon double bonds, carboxylic acid groups, alcohol groups and amino groups provide the joining points. Ethene provides our simplest example. Its molecule contains a carbon to carbon double bond, with two hydrogen atoms attached to each carbon.

1:23The double bond makes ethene an alkene. During addition polymerisation, this bond changes so that neighbouring molecules can become connected along a much longer chain. Think of the double bond as providing the possibility of new connections. During polymerisation, the carbon to carbon double bond becomes a single bond. New single bonds connect those carbon atoms to neighbouring units. Each carbon still has four bonds overall, so the resulting structure obeys the same bonding rules as the starting molecule.

1:53Many ethene molecules join to form polyethene. The name connects the product to its starting monomer, ethene. All the atoms from those starting molecules remain within the growing polymer. Addition polymerisation therefore produces only the polymer, rather than a polymer together with another small molecular product. Notice the difference between the original molecule and the section inside the chain. Ethene has a double bond, but the polymer backbone has single bonds.

2:20That backbone is the continuous sequence of atoms running along the chain. Leaving the original double bond between each neighbouring carbon pair would give those atoms too many bonds. Instead of drawing an entire enormous molecule, chemists show a repeat unit inside brackets. The repeat unit is the section repeated along the chain. Bonds pass through the brackets to show connections to neighbouring sections. The letter n outside means that this unit repeats many times, rather than showing a molecule of fixed length.

2:49Plastics are made from polymers. A plastic object contains huge numbers of polymer molecules, not just one enormous chain filling the whole object. Manufacturers may also include other substances to adjust its properties. Keep the terms separate: polymer describes the large molecules, while plastic describes the material made using them.

Drawing addition polymer repeat units

3:08This next part is for the Extended paper. You need to identify repeat units and deduce an addition polymer from a given alkene. The essential rule is to change the carbon to carbon double bond into a single bond. Keep all the groups attached to their original carbon atoms, then add connections into the continuing chain. Let us work through propene rather than merely copy a finished answer. First, locate the two carbon atoms joined by the double bond.

3:35These become the backbone atoms in the repeat unit. The third carbon, together with its hydrogen atoms, forms a methyl group that stays attached to the same backbone carbon. Next, change the double bond between the backbone atoms to a single bond. Keep two hydrogen atoms attached to the first carbon. The second retains one hydrogen atom and its methyl group. Finally, extend a bond outwards from each backbone carbon and enclose the complete repeating section in brackets.

4:00The methyl group does not disappear, and it does not move into the backbone. Only the bonding associated with the original double bond changes in this example. Check the finished drawing by counting the bonds around each carbon. Every carbon must have four, including the connections leading into neighbouring repeat units. Working backwards uses exactly the same reasoning in reverse. Identify the two backbone carbon atoms in the addition polymer repeat unit.

4:26Remove the outward connections representing the rest of the chain. Restore a double bond between those two carbons, while keeping their attached atoms or groups in their original positions. For the structure shown, the first carbon keeps its two hydrogen atoms, while the second has one hydrogen atom and the methyl group. Restoring the double bond gives propene again. This method works because addition polymerisation retains the monomer atoms, so the repeat unit contains the information needed to reconstruct the starting alkene.

4:53A frequent examination mistake is drawing brackets without bonds passing through them. That makes the structure look disconnected from the rest of the chain. Another mistake is removing a side group when opening the double bond. Avoid both by checking the continuation bonds and accounting for every atom in the original monomer. Try the same method with chloroethene, an alkene in which one hydrogen atom of ethene is replaced by a chlorine atom.

5:18The two carbon atoms of the double bond again become the backbone. One carbon keeps its two hydrogen atoms. The other carbon keeps one hydrogen atom and the chlorine atom, which stays below the backbone exactly as in the monomer. Change the double bond to a single bond and draw a continuation bond from each carbon through the brackets. The product is poly chloroethene, and its repeat unit contains two carbons, three hydrogens and one chlorine.

5:44Those are exactly the atoms of one chloroethene molecule. Counting atoms like this is a quick check that nothing was lost during addition.

Condensation polymers: nylon and PET

5:52This next part is for the Extended paper. Condensation polymerisation uses a different way of connecting molecules. Functional groups react with one another, forming new links and releasing a small molecule. For the examples in this syllabus, that small molecule is water, so some starting atoms leave the growing chain. To build a long chain, each monomer must have two suitable functional groups. One group allows a connection in one direction, and the other allows another connection.

6:19A molecule with only one suitable joining group could form a connection but could not continue the chain at that end. Two joining points make repeated growth possible. A dicarboxylic acid has a carboxylic acid group at each end, and a diamine carries an amino group at both ends. The boxes represent the molecular sections between those functional groups. They are not extra atoms, and their exact contents depend on which starting molecules have been chosen.

6:45Consider one carboxylic acid group reacting with one amino group. The acid supplies an oxygen atom and a hydrogen atom from its hydroxyl group. The amino group supplies another hydrogen atom. Together, these form water, while a new bond connects the acid carbon to the amino nitrogen. The new connection is called an amide linkage. It contains a carbon to oxygen double bond beside a carbon to nitrogen single bond.

7:08In these examples, the nitrogen also retains one hydrogen atom. A polymer containing repeated amide linkages is called a polyamide, and nylon is an example. The other ends of the monomers can undergo the same type of reaction. Repeated connections produce a chain containing alternating acid and diamine sections. Water is released whenever a new amide connection forms by this reaction. The boxes make the chain easier to read, but the joining groups must still be drawn correctly.

7:35Read the nylon diagram from left to right. The shaded acid section lies between two carbonyl groups, each containing a carbon to oxygen double bond. The plain diamine section lies between nitrogen atoms, each carrying one hydrogen. The amide linkages connect these alternating sections into a continuous polymer chain. For a condensation polymer repeat unit, include a complete contribution from each type of monomer. Choosing only the acid section would leave out the alternating diamine section.

8:03The unit must reproduce the correct chain when copied repeatedly. Its boundary can be placed differently, provided the connections and the full repeating pattern remain correct. To deduce the polymer from two given monomers, work at each joining end in turn. Remove the hydroxyl group from every carboxylic acid end of the acid. Then take one hydrogen atom from each amino group of the diamine. Join each exposed carbon to a nitrogen, and draw continuation bonds at the two outer ends of the repeat unit.

8:31A polyester forms when a dicarboxylic acid reacts with a diol. A diol has two alcohol groups, providing a joining point at each end. Here, an acid hydroxyl group and a hydrogen from an alcohol group form water. The alcohol oxygen remains in the chain and bonds to the acid carbon. This connection is an ester linkage. It contains a carbon to oxygen double bond beside a carbon to oxygen single bond.

8:56Repeated ester connections produce a polyester. The important distinction is the atom beside the carbonyl carbon: nitrogen gives the amide connection here, while oxygen gives the ester connection. The polyester you need to know is called PET, and it is used in products such as drinks bottles. Its full chemical name is not required. The shaded boxes represent its dicarboxylic acid sections, while the plain boxes stand for its diol sections.

9:21Ester linkages connect the alternating parts. The nylon and PET diagrams look similar because both contain alternating contributions from two monomer types. However, their connecting atoms are different. Nylon contains nitrogen in its amide connections, whereas PET contains oxygen in its ester connections. Those small structural differences identify the polymer family and reveal the functional groups of its monomers.

Deducing monomers and comparing processes

9:45We can now work backwards from a condensation polymer. First, identify whether the connections are amide or ester linkages. Then separate the chain at the appropriate bonds and restore the groups lost during water formation. This gives the original functional groups rather than fragments with incomplete bonding at their ends. For this worked example, the polymer has ester linkages, so its monomers are a dicarboxylic acid and a diol. Start with the shaded section and its two carbonyl groups.

10:14Restore a hydroxyl group on each carbonyl carbon to make the acid monomer. Each end now contains a complete carboxylic acid group. Next, follow the plain section and its oxygen atoms. Restore a hydrogen atom onto each oxygen to recover the diol. The oxygen atoms belong to the alcohol ends of that starting molecule. Checking both recovered structures confirms that each has two functional groups and can therefore participate in forming a long chain.

10:40For nylon, use the same reverse reasoning with amide linkages. Restore hydroxyl groups onto the acid carbonyl carbons. Add a hydrogen atom to each terminal nitrogen of the diamine section, recovering the original amino groups. Do not turn those nitrogen atoms into oxygen atoms, because that would incorrectly change a polyamide into a polyester. PET can be converted back into monomers and re-polymerised. Breaking its ester linkages allows the starting building blocks to be recovered.

11:07These can then undergo polymerisation again to make new polymer molecules. This is different from simply melting a plastic and reshaping it, because the chemical connections are broken and later re-formed. Keep the two polymerisation processes clearly separate. Addition polymerisation joins alkene monomers through changes involving their double bonds and produces only the polymer. Condensation polymerisation joins suitable functional groups and also produces a small molecule. In the polyamide and polyester examples studied here, that additional product is water.

11:40The monomers also differ. An addition polymer usually forms from one type of alkene, and every monomer must contain a carbon to carbon double bond. A condensation monomer needs two reactive functional groups instead. Nylon and PET each form from two different monomers, which then alternate along the chain. An exam question may ask you to describe the differences between addition and condensation polymerisation. A full answer gives separate creditworthy points.

12:09Addition polymerisation uses monomers with a carbon to carbon double bond. It forms the polymer as the only product. Condensation polymerisation joins monomers with two functional groups, and a small molecule such as water is also formed.

Proteins and plastic disposal

12:22This next part is for the Extended paper. Proteins are natural polyamides, formed from amino acid monomers. Each amino acid contains both an amino group and a carboxylic acid group. Unlike the nylon example, these two different joining groups are already present within the same type of starting molecule. In the general amino acid structure, the central carbon also carries a hydrogen atom and a side-chain. The letter above that carbon represents the side-chain, which differs between amino acids.

12:51The amino and carboxylic acid groups provide the joining sites. The variable side-chain remains attached when the amino acid becomes part of a protein. The carboxylic acid group of one amino acid reacts with the amino group of another. Water is released, and an amide linkage forms between them. Further amino acids can join through the remaining groups at the chain ends. Repeating this process produces the polyamide chain that makes up a protein.

13:17The different boxes represent parts from different amino acids. Read the repeating backbone sequence as nitrogen, amino acid section, then carbonyl carbon. Each connection to the next section contains an amide linkage. The side-chains may vary along a protein, so its entire structure need not consist of identical small sections. That variation does not stop proteins being polymers. The definition requires many smaller monomers to build a large molecule, not that every starting molecule must be identical.

13:46Proteins also show why polymer and plastic are not interchangeable terms. Natural polyamides in living organisms are polymers, but they are not ordinarily described as plastics. The same durability that makes many plastics useful creates problems when we throw them away. Common plastics resist chemical attack and do not biodegrade. This means microorganisms do not break them down into simpler substances. A discarded object may remain in the environment long after its useful life has ended.

14:14In landfill sites, discarded plastics occupy space and persist because they do not biodegrade. Their resistance to breakdown means disposal does not quickly remove the material from the environment. As further waste is added, landfill capacity is used up. The useful property of durability therefore becomes a disadvantage after disposal. Plastics can also accumulate in oceans. Animals may swallow pieces or become trapped in discarded objects. Breaking a plastic object into smaller fragments does not mean that it has biodegraded.

14:45The fragments can remain as plastic, allowing the material to persist even when the original object is no longer recognisable. Burning plastics can form toxic gases, creating a different environmental challenge. These gases can harm people and other organisms. Do not assume every plastic releases exactly the same substances, because its composition matters. The required point is that burning plastic waste can release toxic gases rather than providing a harmless disposal route.

Exam recap and key methods

15:12An examination may ask you to describe the formation of polyethene using ethene monomers. A clear answer states that many ethene molecules join by addition polymerisation. Their carbon to carbon double bonds become single bonds as connections form between neighbouring units. Only the polymer is produced, and its chain contains repeating sections derived from ethene. For an Extended question, you may need to deduce a repeat unit, identify a linkage, or recover the monomers.

15:41Follow the bonding rather than guessing from the polymer name. An amide linkage points to a polyamide, while an ester linkage identifies a polyester. Restore the missing atoms when reconstructing condensation monomers, but restore the double bond for an addition monomer. For this exam-style task, connect each challenge to a relevant property or consequence. Plastics do not biodegrade, so they persist and occupy space in landfill sites. Burning plastics can produce toxic gases that harm living organisms.

16:10These sentences explain distinct problems, rather than merely saying that plastic waste is bad for the environment. The central idea is that polymer properties begin with molecular structure. Addition polymerisation turns alkene monomers into chains without producing another small molecule. Condensation polymerisation connects functional groups and releases water in the examples studied here. Amide linkages identify nylon and proteins, while ester linkages identify polyesters. To draw or decode a polymer, track the backbone, the attached groups and the joining bonds.

16:42Use brackets and continuation bonds to show a repeat unit correctly. Remember that plastics are made from polymers, and their durability complicates disposal. Understanding those connections lets you explain the chemistry instead of memorising a collection of unrelated diagrams.