Polymerisation

All three subtopics here are printed under "A Level subject content" in the 9701 syllabus and every objective carries the tier "A Level". None of it is AS, and it is a separate topic from the AS topic 20 of the same name, which covers addition polymerisation. It is examined on Paper 4. Paper 1 is the AS multiple-choice paper and the whole 9701 bank on this site comes from it, so no practice here is tagged to this topic.

Syllabus points

35.1 Condensation polymerisation

• Describe the formation of polyesters: the reaction between a diol and a dicarboxylic acid or dioyl chloride; the reaction of a hydroxycarboxylic acid.
• Describe the formation of polyamides: the reaction between a diamine and a dicarboxylic acid or dioyl chloride; the reaction of an aminocarboxylic acid; the reaction between amino acids.
• Deduce the repeat unit of a condensation polymer obtained from a given monomer or pair of monomers.
• Identify the monomer or monomers present in a given section of a condensation polymer molecule.

35.2 Predicting the type of polymerisation

• Predict the type of polymerisation reaction for a given monomer or pair of monomers.
• Deduce the type of polymerisation reaction which produces a given section of a polymer molecule.

35.3 Degradable polymers

• Recognise that poly(alkenes) are chemically inert and can therefore be difficult to biodegrade.
• Recognise that some polymers can be degraded by the action of light.
• Recognise that polyesters and polyamides are biodegradable by acidic and alkaline hydrolysis.

The two kinds of polymerisation

Addition polymerisation is the AS topic 20 material. Everything new here is condensation.

The single test that decides which is which: is a small molecule lost? If the repeat unit contains exactly the atoms of the monomer, it is addition. If atoms are missing, it is condensation.

Predicting the type from the monomer

The routine is short.

1. Does the monomer contain a C=C? If so, and it has no other functional groups that could react with one another, the answer is addition.
2. Does the monomer, or the pair of monomers, provide two reactive groups that can join? Look for two of the same group on one molecule, or a molecule with one of each.

• diol + dicarboxylic acid or diol + dioyl chloride: condensation, giving a polyester.
• diamine + dicarboxylic acid or diamine + dioyl chloride: condensation, giving a polyamide.
• a hydroxycarboxylic acid, with an OH at one end and a COOH at the other: condensation with itself, giving a polyester.
• an aminocarboxylic acid, or an amino acid: condensation with itself, giving a polyamide.

The requirement of two functional groups per monomer is what makes a polymer rather than a single small product. A molecule with one OH and nothing else can esterify once and then the chain stops.

Polyesters

The link is the ester group, -COO-, and it is formed by an OH reacting with a COOH, losing water, or with an acyl chloride, losing HCl.

From two monomers. A diol and a dicarboxylic acid:

HO-CH₂CH₂-OH + HOOC-C₆H₄-COOH

give Terylene, also called PET, with the repeat unit

-O-CH₂CH₂-O-CO-C₆H₄-CO-

From one monomer. A hydroxycarboxylic acid such as 2-hydroxypropanoic acid, HO-CH(CH₃)-COOH, polymerises with itself to give poly(lactic acid), with the repeat unit

-O-CH(CH₃)-CO-

Polyamides

The link is the amide group, -CONH-, formed by an NH₂ reacting with a COOH, losing water, or with an acyl chloride, losing HCl.

From two monomers. A diamine and a dicarboxylic acid:

H₂N-(CH₂)₆-NH₂ + HOOC-(CH₂)₄-COOH

give nylon-6,6, with the repeat unit

-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-

The numbering says how many carbons are in each monomer: six in the diamine, six in the diacid, counting the two carboxyl carbons.

Using the dioyl chloride instead of the diacid, ClOC-(CH₂)₄-COCl, gives the same polymer faster and at room temperature, losing HCl instead of water, and this is the version used in the "nylon rope trick" demonstration.

Kevlar is the aromatic equivalent, made from a benzene-1,4-dicarboxylic acid and a benzene-1,4-diamine, and the flat aromatic rings and extensive hydrogen bonding between chains are what make it so strong.

From one monomer. An aminocarboxylic acid such as 6-aminohexanoic acid polymerises with itself to give nylon-6.

From amino acids. The same chemistry gives proteins, where the amide link is called a peptide bond, as in topic 34.

Drawing the repeat unit

Two rules and a check.

To draw the repeat unit from the monomers:

1. Write the two monomers side by side, facing each other.
2. Remove the elements of water (or HCl) from between them: an OH from the acid and an H from the alcohol or amine.
3. Join what remains, and draw bonds extending out of both ends of the unit, in brackets with an n outside.

The bonds sticking out at each end are what make it a repeat unit rather than a molecule, and leaving them off is the commonest way to lose the mark.

To identify the monomers from a section of polymer:

1. Find the ester or amide links in the chain.
2. Cut each link, between the C and the O of an ester, or between the C and the N of an amide.
3. Add back the elements of water: an OH to the carbonyl carbon to remake COOH, and an H to the oxygen or nitrogen to remake OH or NH₂.

Then check the count: if you cut two links you should recover two monomers, or one monomer twice if the polymer came from a single bifunctional molecule.

Degradable polymers

Poly(alkenes) are difficult to biodegrade. Their backbone is a chain of strong, non-polar C-C and C-H bonds, with no polar sites for a nucleophile or an enzyme to attack, so they are chemically inert. Poly(ethene) persists in landfill and in the sea for a very long time.

Some polymers can be degraded by light. A polymer manufactured with carbonyl groups built into the chain absorbs ultraviolet radiation at those groups, and the absorbed energy breaks bonds in the backbone, so the chain fragments and the pieces become small enough to be attacked by microorganisms. Such polymers are described as photodegradable. Note that this requires exposure to light, so a photodegradable bag buried in a landfill does not degrade.

Polyesters and polyamides are biodegradable, because their chains contain ester and amide links, which are polar and can be hydrolysed by aqueous acid or aqueous alkali, and by enzymes in microorganisms.

• Acid hydrolysis of a polyester gives the dicarboxylic acid and the diol; of a polyamide, the dicarboxylic acid and the salt of the diamine, since the amine is protonated in acid.
• Alkaline hydrolysis of a polyester gives the salt of the dicarboxylic acid and the diol; of a polyamide, the carboxylate salt and the free diamine.

That is the same rule as for hydrolysing a simple ester or amide in topics 33 and 34, applied along a chain: work out which reagent is in excess and protonate or deprotonate the products accordingly.

The environmental point follows directly from the chemistry. Condensation polymers break down because the links that make them are the same links nature already knows how to hydrolyse, and addition polymers do not because their backbone is nothing but alkane.

Common mistakes

• Drawing a repeat unit without bonds extending from both ends.
• Including the lost water molecule in the repeat unit, so the atom count is wrong.
• Cutting an ester link in the wrong place when working back to the monomers, so an OH ends up on the wrong fragment.
• Forgetting to add back the elements of water when deducing monomers from a polymer chain.
• Calling a polymer made from a diol and a diacid a polyamide, or one from a diamine an ester.
• Saying condensation polymerisation from an acyl chloride releases water. It releases HCl.
• Predicting addition polymerisation for a monomer with two functional groups and no C=C.
• Saying poly(ethene) is non-biodegradable because it is a long molecule. It is because its backbone has no polar, hydrolysable links.
• Saying a photodegradable polymer will break down in a landfill.
• Giving the free carboxylic acid as the product of alkaline hydrolysis, or the free amine as the product of acid hydrolysis.