Unit 11: Polymers
Understanding the structure, formation, and properties of large macromolecules.
11.19 Introduction to Polymers (Monomers, Polymerisation)
Polymers are very large molecules (macromolecules) made up of many small, repeating structural units called monomers. The process of joining these monomers together to form a polymer is called polymerisation.
Polymers can be natural (like starch, protein, and rubber) or synthetic (like poly(ethene) and nylon). Synthetic polymers are often referred to as plastics.
There are two main types of polymerisation:
- Addition Polymerisation: Monomers add to one another in such a way that the polymer contains all the atoms of the monomer unit. No other product is formed.
- Condensation Polymerisation: Monomers join together with the loss of a small molecule, usually water. The repeating unit in the polymer is therefore different from the monomers.
Solved Examples:
- What is a polymer?
Solution: A large molecule (macromolecule) composed of many repeating subunits called monomers. - What is a monomer?
Solution: A small molecule that can be chemically bonded to other monomers to form a polymer. - What is the name of the process that forms
polymers?
Solution: Polymerisation. - Name a natural polymer.
Solution: Starch, cellulose, or protein. - What is the key difference between addition and condensation
polymerisation?
Solution: In addition polymerisation, no atoms are lost. In condensation polymerisation, a small molecule (like water) is eliminated. - What is the monomer for the polymer
poly(ethene)?
Solution: Ethene. - Is the formation of a protein from amino acids an example of addition or
condensation polymerisation?
Solution: Condensation polymerisation, because a water molecule is lost for each peptide bond formed. - What does the "poly-" prefix in a polymer name
mean?
Solution: It means "many," referring to the many repeating monomer units. - What is another term for a synthetic
polymer?
Solution: A plastic. - What is a co-polymer?
Solution: A polymer made from two or more different types of monomers.
11.20 Addition Polymers & Plastics
Addition polymerisation is the reaction of many unsaturated monomers (usually alkenes) joining together. The double bond in each monomer breaks, and the molecules link end-to-end to form a long, saturated polymer chain.
Example: Polymerisation of ethene to form poly(ethene).
$n(CH_2=CH_2) \rightarrow -[CH_2-CH_2]_n-$
Addition polymers are non-biodegradable because their long, non-polar alkane-like chains are very unreactive.
Types of Plastics:
Plastics made from addition polymers can be classified by how they respond to heat:
- Thermoplastics: These plastics soften when heated and can be remoulded into new shapes. They consist of long polymer chains with only weak intermolecular forces (van der Waals forces) between them. Poly(ethene) and PVC are examples.
- Thermosetting plastics (Thermosets): These plastics do not soften when heated. They are hard and rigid because the polymer chains are held together by strong covalent bonds called cross-links. Once formed, their shape is permanent. Bakelite is an example.
Solved Examples:
- What type of monomer undergoes addition
polymerisation?
Solution: An unsaturated monomer, typically an alkene. - Draw the repeating unit of
poly(propene).
Solution: $-[CH_2-CH(CH_3)]_n-$. - What is the difference between a thermoplastic and a thermosetting
plastic?
Solution: A thermoplastic can be repeatedly softened by heat and remoulded, while a thermosetting plastic becomes permanently hard once heated. - What structural feature is responsible for the properties of thermosetting
plastics?
Solution: Strong covalent cross-links between the polymer chains. - What is the monomer for PVC (polyvinyl
chloride)?
Solution: Chloroethene (vinyl chloride), $CH_2=CHCl$. - Why are addition polymers generally
unreactive?
Solution: Because they are long-chain saturated alkanes with strong, non-polar C-C and C-H bonds. - What type of intermolecular forces exist between chains in a
thermoplastic?
Solution: Weak van der Waals forces. - What happens to the double bond of the monomers during addition
polymerisation?
Solution: It breaks and forms single bonds that link the monomer units together. - Is poly(ethene) a thermoplastic or a
thermoset?
Solution: A thermoplastic. - What is a major environmental problem associated with addition
polymers?
Solution: They are non-biodegradable and persist in the environment for a very long time.
11.21 Condensation Polymers (Polyesters, Polyamides)
Condensation polymerisation involves the reaction between monomers that each have at least two functional groups. When the monomers join, a small molecule (usually water) is eliminated. The resulting polymers are often biodegradable because the linking group (e.g., an ester or amide) can be broken by hydrolysis.
Polyesters
Polyesters are formed from the reaction between a dicarboxylic acid (a
molecule with two -COOH groups) and a diol (a molecule with two -OH
groups). The monomers are linked by ester bonds.
Example: Terylene is formed from benzene-1,4-dicarboxylic acid and
ethane-1,2-diol.
Polyamides
Polyamides are formed from the reaction between a dicarboxylic acid and a
diamine (a molecule with two -$NH_2$ groups). The monomers are linked by
amide (or peptide) bonds.
Example: Nylon-6,6 is formed from hexanedioic acid and
1,6-diaminohexane.
Both polyesters and polyamides are important synthetic fibres used in clothing and textiles.
Solved Examples:
- What is the key feature of a condensation polymerisation
reaction?
Solution: The elimination of a small molecule (like water) for each bond formed between monomers. - What two types of monomers are needed to form a
polyester?
Solution: A dicarboxylic acid and a diol. - What is the name of the linkage in a
polyamide?
Solution: An amide or peptide link. - What is the name of the linkage in a
polyester?
Solution: An ester link. - What is the small molecule that is typically eliminated in the formation of
nylon?
Solution: Water. - Are condensation polymers generally biodegradable?
Why?
Solution: Yes, because the ester or amide links can be broken down by hydrolysis. - What are the two monomers used to make the polyamide
Nylon-6,6?
Solution: Hexanedioic acid and 1,6-diaminohexane. - Proteins are natural polymers. Are they addition or condensation
polymers?
Solution: Condensation polymers, as they are polyamides formed from amino acid monomers. - What property of polyamides like nylon makes them suitable for use as
fibres?
Solution: They are very strong due to the hydrogen bonds that can form between the polymer chains. - Draw the structure of the ester link.
Solution: -COO-.