Polymers
Polymers are large molecules (macromolecules) built up from many smaller molecules called monomers. The word comes from Greek: poly meaning 'many' and meros meaning 'parts'.
The process of joining monomers together to form a polymer is called polymerisation.
There are two main types of polymerisation:
- Addition polymerisation: Monomers join without losing any atoms. This typically involves alkenes (molecules with C=C double bonds).
- Condensation polymerisation: Monomers join together, releasing a small molecule (usually water, H_2O) as a by-product. This typically involves monomers with two functional groups.
The Mechanism:
Ethene has a carbon-carbon double bond (C=C). During polymerisation, this double bond 'opens up'. The electrons that formed the second bond become available to form new single bonds with adjacent ethene molecules.
Visualising the Change:
Monomer: CH_2=CH_2
Polymer: -[CH_2-CH_2]_n-
Notice that the double bond in the monomer becomes a single bond in the polymer chain. No atoms are lost; the empirical formula of the repeat unit is the same as the molecular formula of the monomer.
General Rule for Addition Polymers:
To draw the repeat unit of an addition polymer from an alkene:
- Break the double bond (C=C) into a single bond (C-C).
- Draw continuation bonds (lines extending outwards) from each carbon atom to show it connects to other units.
- Keep all side groups (substituents) attached to their original carbons.
Structure: CH_2=CH_2
Repeat Unit of Poly(ethene):
<br>\begin{array}{c}<br>H \quad H \<br>| \quad | \<br>-C - C- \<br>| \quad | \<br>H \quad H<br>\end{array}<br>
(Note: The vertical lines extending from the top and bottom carbons are continuation bonds, indicating the chain continues.)
Why is this addition polymerisation?
Because the monomer contains a double bond that opens up, and no small molecules (like water) are eliminated during the reaction.
Correction: The double bond in the monomer is broken to form the polymer chain. The repeat unit must show single bonds between the carbon atoms of the backbone.
Mistake: Forgetting the continuation bonds (the lines sticking out of the brackets).
Correction: A repeat unit represents a segment of a long chain. You must draw lines extending from the ends of the repeat unit to show it connects to other identical units.
Examiner Acceptance: Examiners look for specific keywords. For addition, state: "The monomer contains a double bond (C=C) which opens up." For condensation, state: "A small molecule (such as water) is eliminated/lost during the reaction."
Reasoning: This directly addresses the fundamental chemical difference. Addition polymerisation conserves all atoms from the monomer in the polymer, whereas condensation polymerisation involves a loss of mass via a by-product.
Example Answer: "Poly(ethene) is formed by addition polymerisation because the ethene monomers join together without losing any atoms. Nylon is formed by condensation polymerisation because water molecules are eliminated when the monomers join."
Condensation polymerisation occurs when monomers with two different functional groups (or two of the same group) react.
1. Polyamides (e.g., Nylon):
Formed from a dicarboxylic acid and a diamine.
- The carboxyl group (-COOH) of the acid reacts with the amine group (-NH_2) of the diamine.
- They form an amide linkage: -CO-NH- (also written as -C(=O)-NH-).
- Water (H_2O) is eliminated.
2. Polyesters (e.g., PET):
Formed from a dicarboxylic acid and a diol (a molecule with two alcohol groups, -OH).
- The carboxyl group (-COOH) reacts with the hydroxyl group (-OH).
- They form an ester linkage: -CO-O- (also written as -C(=O)-O-).
- Water (H_2O) is eliminated.
- Hexanedioic acid (Adipic acid): HOOC-(CH_2)_4-COOH
- Hexane-1,6-diamine: H_2N-(CH_2)_6-NH_2
Reaction:
The -OH from the acid and an -H from the amine combine to form water (H_2O). The remaining parts join via an amide bond.
Repeat Unit Structure of Nylon-6,6:
<br>\begin{array}{c}<br>O \quad H \<br>|| \quad | \<br>-C - NH - (CH_2)_6 - NH - CO - (CH_2)_4 - \<br>| \quad | \<br>H \quad O<br>\end{array}<br>
(Note: The continuation bonds are on the Nitrogen and the Carbonyl Carbon.)
Key Feature: Look for the amide linkage -CO-NH- in the chain.
- Benzene-1,4-dicarboxylic acid (Terephthalic acid): A benzene ring with two -COOH groups opposite each other.
- Ethane-1,2-diol: HO-CH_2-CH_2-OH
Repeat Unit Structure of PET:
<br>\begin{array}{c}<br>O \quad O \<br>|| \quad || \<br>-C - (C_6H_4) - C - O - CH_2 - CH_2 - O - \<br>| \quad | \<br>H \quad H<br>\end{array}<br>
(Note: The benzene ring is often drawn as a hexagon with a circle or alternating double bonds.)
Key Feature: Look for the ester linkage -CO-O- in the chain.
Mistake: When asked to deduce the monomers of a condensation polymer, students often draw the monomers with the continuation bonds still attached or forget to add back the eliminated atoms.
Correction: To find the monomers from a repeat unit:
- Identify the linkage (amide -CO-NH- or ester -CO-O-).
- Break the bond in the linkage.
- Add an -OH group to the carbonyl carbon (C=O) side.
- Add an -H atom to the nitrogen or oxygen side.
- Crucially: The final monomer structures must NOT have continuation bonds. They are complete, stable molecules.
Examiner Acceptance: Examiners accept the linkage drawn explicitly as -C(=O)-NH- for amides and -C(=O)-O- for esters. Do not write -CO-NH- without showing the double bond to oxygen if a structural formula is required, as this can be ambiguous.
Reasoning: Showing the C=O double bond clearly distinguishes the carbonyl group from other carbon chains and confirms you understand the functional groups involved in the condensation reaction.
Example Answer: "The repeat unit contains an amide linkage formed between the carboxyl group of the dicarboxylic acid and the amine group of the diamine, with water eliminated."
General Structure of an Amino Acid:
H_2N-CH(R)-COOH
Where R is a variable side chain (e.g., -H for glycine, -CH_3 for alanine).
Each amino acid has two key functional groups:
- An amine group (-NH_2)
- A carboxyl group (-COOH)
Polymerisation:
Just like synthetic polyamides, amino acids join via condensation polymerisation. The -COOH of one amino acid reacts with the -NH_2 of another.
Linkage: They form a peptide bond (which is an amide linkage): -CO-NH-.
Repeat Unit of a Protein:
<br>\begin{array}{c}<br>O \quad H \<br>|| \quad | \<br>-C - NH - CH(R) - \<br>| \quad | \<br>H \quad R<br>\end{array}<br>
(Note: The backbone is -N-C-C- repeating.)
Monomer: H_2N-CH_2-COOH
When two glycine molecules join:
- One loses -OH from COOH.
- The other loses -H from NH_2.
- Water (H_2O) is formed.
- They link via -CO-NH-.
Structure of the Dipeptide:
H_2N-CH_2-CO-NH-CH_2-COOH
Key Feature: The central part -CO-NH- is the peptide bond. The ends remain as an amine group (H_2N-) and a carboxyl group (-COOH), allowing the chain to continue growing.
Correction:
- Nylon is made from a dicarboxylic acid AND a diamine (two separate molecules).
- Proteins are made from amino acids (one type of molecule that contains BOTH the amine and carboxyl groups).
Always check if the monomer has both functional groups on the same carbon chain (protein) or if two different monomers are required (nylon).
Properties and Disposal:
Most synthetic polymers (like poly(ethene)) are chemically inert and non-biodegradable. This means they do not react easily with other chemicals and cannot be broken down by microorganisms.
Implications for Disposal:
- Landfill Sites: Because they do not biodegrade, plastics persist in landfill sites for hundreds of years, taking up space permanently.
- Accumulation in Oceans: Plastics float or sink but do not rot. They accumulate in ocean gyres (e.g., the Great Pacific Garbage Patch), harming marine life through ingestion and entanglement.
- Burning (Incineration): Burning plastics releases toxic gases. For example, burning PVC releases hydrogen chloride (HCl) gas, which causes acid rain and respiratory problems. Other plastics may release dioxins.
Examiner Acceptance: Use the phrase "non-biodegradable". Examiners specifically look for this term to explain persistence in the environment. Also, mention "toxic gases" when discussing burning.
Reasoning: "Non-biodegradable" directly explains why plastics accumulate in landfills and oceans. Mentioning specific toxic outcomes (like acid rain from HCl) shows detailed knowledge of the combustion products.
Example Answer: "Plastics are non-biodegradable, so they do not break down in landfill sites or oceans, leading to long-term pollution. Burning them can release toxic gases such as hydrogen chloride, which contributes to acid rain."
Process:
Poly(ethene terephthalate) (PET) can be hydrolysed (reacted with water) to reverse the polymerisation process.
Result:
PET breaks down into its monomers:
- Benzene-1,4-dicarboxylic acid
- Ethane-1,2-diol
These monomers can then be re-polymerised to form new PET. This makes polyester recycling chemically possible, unlike most addition polymers which cannot be easily depolymerised.