Condensation Polymerisation and Polymer Degradability

This guide covers subtopics 35.1, Condensation polymerisation, 35.2, Predicting the type of polymerisation, and 35.3, Degradable polymers, from Topic 35, Polymerisation, of Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. This is A Level content, extending the AS-level treatment of addition polymers to a second major polymer class.

Before studying this

This resource assumes addition polymerisation and repeat-unit deduction from Addition Polymerisation, and benefits from the peptide-bond and amide chemistry in Amides and Amino Acids, since a polyamide’s repeat linkage is the same amide bond amino acids form when they link into proteins.

Syllabus coverage

CAMBRIDGE INTERNATIONAL AS & A LEVEL CHEMISTRY 9701 — A Level, Topic 35

35.1 Condensation polymerisation — describing polyester formation (a diol + a dicarboxylic acid or dioyl chloride; or a hydroxycarboxylic acid alone); describing polyamide formation (a diamine + a dicarboxylic acid or dioyl chloride; an aminocarboxylic acid alone; or amino acids linking together); deducing the repeat unit from given monomer(s); identifying the monomer(s) present in a given polymer section.

35.2 Predicting the type of polymerisation — predicting the type of polymerisation for a given monomer or pair of monomers; deducing the type of polymerisation that produced a given polymer section.

35.3 Degradable polymers — recognising that poly(alkenes) are chemically inert and difficult to biodegrade; recognising that some polymers degrade under light; recognising that polyesters and polyamides are biodegradable by acidic and alkaline hydrolysis.

Condensation polymerisation

Unlike addition polymerisation (where monomers simply join, atom-for-atom, across a broken double bond), condensation polymerisation builds the polymer chain by repeatedly forming a new bond between two functional groups with the loss of a small molecule (usually H₂O, or HCl if a dioyl/acyl chloride is used instead of the free acid) at every linkage.

Polyesters form by repeated ester-bond formation, in two possible ways:

• A diol (two –OH groups) reacting with a dicarboxylic acid (two –COOH groups) or a dioyl chloride (two –COCl groups) — each new bond is an ester linkage, releasing H₂O (from the diacid route) or HCl (from the dioyl chloride route) at each step.
• A single hydroxycarboxylic acid (one molecule with both an –OH and a –COOH group) reacting with itself, each molecule’s –OH reacting with the next molecule’s –COOH, releasing H₂O each time.

Worked example. Deduce the repeat unit formed from HOCH₂CH₂OH (ethane-1,2-diol) and HOOCC₆H₄COOH (benzene-1,4-dicarboxylic acid).

Each ester linkage forms between one –OH of the diol and one –COOH of the diacid, losing H₂O each time. The repeat unit is:

–OCH₂CH₂OOCC₆H₄CO–

(this is the repeat unit of PET, poly(ethylene terephthalate), the polymer used in plastic drinks bottles and polyester fabric).

Polyamides form the same way, but linking through amide bonds instead of ester bonds:

• A diamine (two –NH₂ groups) reacting with a dicarboxylic acid or dioyl chloride — each new bond is an amide linkage.
• A single aminocarboxylic acid (one molecule with both –NH₂ and –COOH) reacting with itself.
• Amino acids reacting together — exactly the peptide-bond-forming reaction from Amides and Amino Acids, making a polyamide that is, biologically, a protein.

Deducing monomers and repeat units

From a pair of monomers to a repeat unit: identify the two functional groups that react (–OH/–COOH for a polyester, –NH₂/–COOH for a polyamide), remove the small molecule lost at each linkage (H₂O, or HCl if a chloride was used), and join what remains.

From a polymer section back to its monomer(s): look for the repeating linkage (ester, –COO–, or amide, –CONH–) in the backbone, then work outward from each linkage to identify the two ends of what must have been a diol/diamine and a diacid (or a single hydroxy-/amino-acid, if the same unit repeats identically on both sides of every linkage) — effectively running the condensation reaction in reverse, adding back an H₂O (or HCl) at each linkage broken.

Predicting the type of polymerisation

Given an unfamiliar monomer or pair of monomers, the functional groups present determine which type of polymerisation applies:

• A C=C double bond present (with no other reactive functional groups needed) → addition polymerisation, joining monomers across the broken double bond, no small molecule lost.
• Two matching reactive functional groups per monomer (or one of each on two different monomers) — specifically –OH/–COOH or –COCl pairs, or –NH₂/–COOH or –COCl pairs → condensation polymerisation, losing H₂O or HCl at each linkage.

The same logic runs in reverse for a given section of polymer: an unbroken, saturated carbon backbone (no ester or amide linkages within it) indicates addition polymerisation; a backbone containing repeating ester (–COO–) or amide (–CONH–) linkages indicates condensation polymerisation, and which one tells you whether the original monomers were diol/diacid-type or diamine/diacid-type.

Degradable polymers

Poly(alkenes) are chemically inert and difficult to biodegrade. Their backbone is a long chain of C–C and C–H single bonds — strong, non-polar, and unrecognisable to the enzymes that break down natural materials, so poly(alkenes) like poly(ethene) and poly(propene) can persist in the environment for a very long time (already touched on for PVC’s disposal problems in Addition Polymerisation).

Some polymers degrade under light (photodegradable polymers) — certain bonds or additives within the polymer absorb UV radiation and break, causing the material to fragment, though this doesn’t necessarily mean full biodegradation into harmless small molecules.

Polyesters and polyamides are biodegradable by acidic and alkaline hydrolysis. The ester and amide linkages holding the polymer chain together are exactly the same functional groups that hydrolyse under acidic or alkaline conditions in small-molecule chemistry (as in Carboxylic Acids and Acyl Chlorides and Amides and Amino Acids) — the same chemistry simply applies repeatedly along the polymer backbone, breaking it back down into its original small-molecule monomers (or something close to them). This is precisely why polyesters and polyamides are generally more biodegradable than poly(alkenes): the backbone itself contains a chemically reactive linkage that water (with acid or alkali) can attack, rather than an inert carbon chain.

Common mistakes

Forgetting to lose H₂O (or HCl) when deducing a condensation repeat unit. Simply joining the monomers end-to-end without removing the small molecule is the single most common error in this topic — always check the repeat unit’s formula accounts for the loss.

Mixing up which small molecule is lost. A free diacid (–COOH) loses H₂O at each linkage; a dioyl chloride (–COCl) loses HCl instead — read the monomer given carefully before deciding.

Assuming any polymer with nitrogen in it is automatically biodegradable. It’s specifically the amide (or ester) linkage in the backbone that hydrolyses — a polymer needs that reactive linkage repeating through its main chain, not just nitrogen present somewhere in a side group.

Confusing “biodegradable” with “photodegradable.” These are different degradation mechanisms (hydrolysis by water/enzymes vs bond-breaking by absorbed UV light) — a polymer can be one, both, or neither, and a question naming the specific mechanism expects the matching type of degradation.

Quick revision checklist

• Polyester: diol + diacid/dioyl chloride, or a hydroxycarboxylic acid alone; ester linkages, loses H₂O (or HCl)
• Polyamide: diamine + diacid/dioyl chloride, aminocarboxylic acid alone, or amino acids together; amide linkages, loses H₂O (or HCl)
• C=C present → addition polymerisation; matching –OH/–NH₂ and –COOH pairs → condensation polymerisation
• Poly(alkenes): inert, hard to biodegrade (unreactive C–C/C–H backbone)
• Some polymers photodegrade (UV-triggered bond breaking)
• Polyesters/polyamides biodegrade by acidic/alkaline hydrolysis of the ester/amide linkage in the backbone

Related resources

• Addition Polymerisation — the AS-level polymer class this topic contrasts with
• Amides and Amino Acids — the amide (peptide) bond that links a polyamide’s repeat units
• Carboxylic Acids and Acyl Chlorides — ester formation and hydrolysis, the chemistry behind polyester degradability
• Cambridge AS & A Level Chemistry hub

Written against Cambridge International AS & A Level Chemistry 9701, 2025–2027 series. Always check the current syllabus for your examination year.