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7.3 Synthetic and naturally occurring polymers

7.3.1 Addition polymerisation

- One of the most important ways that chemicals from crude oil are used is to make polymers.
- Polymers are substances made from very large molecules made up of many smaller repeating units (monomers).
- Polymers can be natural (e.g. starch, cellulose, proteins) or synthetic (e.g. poly(ethene), poly(propene)).
- Most synthetic polymers are made from monomers that contain carbon-carbon double bonds (alkenes).
- Alkenes can be used to make polymers such as poly(ethene) and poly(propene) by addition polymerisation.
- In addition polymerisation reactions, many small molecules (monomers) join together to form very large molecules (polymers), and nothing else.
- In addition polymers the repeating unit has the same atoms as the monomer because no other molecule is formed in the reaction.

Equations

Ethene → Poly(ethene)
n(CH₂=CH₂) → (–CH₂–CH₂–)_n Propene → Poly(propene)
n(CH₂=CHCH₃) → (–CH₂–CH(CH₃)–)_n

How we use polymers

- Ethene monomers can be used to make poly(ethene), which is used to make plastic bags and bottles.
- It is cheap, flexible, and strong.
- Propene monomers can be used to make poly(propene), which is used to make ropes, carpets, and crates.
- It is strong, durable, and has a high melting point.

7.3.2 Condensation polymerisation

- Condensation polymerisation involves monomers with two functional groups.
- This also results in formation of two products: the polymer and a small molecule (often water).
- Therefore, condensation polymerisation is many monomers with two functional groups joined together to make a very long repeating molecule and another small molecule such as H₂O or HCl.

An example of a condensation reaction

methanol + butanoic acid → methyl butanoate + water
CH₃OH + CH₃CH₂CH₂COOH → CH₃CH₂CH₂COOCH₃ + H₂O
An H₂SO₄ catalyst is required.

Polyesters

- Polyesters are formed from dicarboxylic acids and diols (alcohols with two -OH groups).
- The -COOH group of the dicarboxylic acid reacts with the -OH group of the diol to form an ester link (-COO-) and a molecule of water.
- The repeating unit in the polyester contains atoms from both monomers, unlike in addition polymerisation where the repeating unit contains only atoms from the monomer.
- An example of a polyester is poly(ethene terephthalate) (PET), which is made from the dicarboxylic acid benzene-1,4-dicarboxylic acid and the diol ethane-1,2-diol.
- Polyesters are used to make fibres for clothing and plastic bottles.
- Here is how to make an example polyester: (methanediol + ethanedioic acid → poly(methanediol ethanedioate) + water) - And another: (ethanediol + propanedioic acid → poly(ethanediol propanedioate) + water)

Polyamides

- Polyamides are formed from dicarboxylic acids and diamines (amines with two -NH₂ groups).
- The -COOH group of the dicarboxylic acid reacts with the -NH₂ group of the diamine to form an amide link (-CONH-) and a molecule of water.
- An example of a polyamide is nylon-6,6, which is made from the dicarboxylic acid hexanedioic acid and the diamine hexane-1,6-diamine.
- Polyamides are used to make fibres for clothing and carpets.
- Here is how to make an example polyamide: (and H₂O)

7.3.3 Amino acids

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- Amino acids have two different functional groups in a molecule.
- Amino acids react by condensation polymerisation to produce polypeptides.
- For example: glycine is H₂NCH₂COOH and polymerises to produce the polypeptide: - Different amino acids can be combined in the same chain to produce proteins.

7.3.4 DNA (deoxyribonucleic acid) and other naturally occurring polymer

Carbohydrates

- Glucose and fructose are examples of sugar monomers, called monosaccharides.
- Monosaccharides bond together to produce polymers, called polysaccharides.
- Starch and cellulose are both made from glucose monomers joined by condensation polymerisation.

Proteins

- Amino acids have two different functional groups.
- One group is basic (the amine group -NH₂) and one is acidic (the carboxylic acid group -COOH).
- Amino acids react by condensation polymerisation to produce polypeptides.
- E.g. glycine (H₂NCH₂COOH) polymerises to produce the polypeptide (-HNCH₂CO-)n and n H₂O.

Polypeptides

- Different amino acids can be combined in the same chain to produce polypeptide chains, which form proteins.
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DNA

- DNA is a polymer that stores genetic information in cells.
- DNA is made from two polymer chains, made from four different monomers called nucleotides, arranged in a double helix structure.
- Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base.
- The sugar and phosphate groups form the backbone of the DNA strand, while the nitrogenous bases pair up to form the rungs of the DNA ladder.
- The sequence of nitrogenous bases in DNA encodes genetic information.
- Other naturally occurring polymers include carbohydrates (e.g. starch, cellulose; both made from sugar) and proteins (polypeptides).