Ch 1 · Condensation polymerisation · AS & A Level Chemistry 9701
Topic 1 of 3 in Polymerisation — AS & A Level Chemistry 9701 — 48 lessons.
Introduction
Condensation polymerisation joins monomers that each carry two reactive functional groups, forming a new covalent linkage at every step and releasing a small molecule such as water or hydrogen chloride. Unlike addition polymerisation, the repeat unit does not have the same formula as the monomer(s) that formed it.
Key ideas — Forming polyesters
A diol (two -OH groups) condenses with a dicarboxylic acid (two -COOH groups); each -OH reacts with a -COOH to form an ester linkage, -mathrmCO-O-, releasing a molecule of water: n mathrmHOOC-R-COOH + n mathrmHO-R'-OH arrow mathrm(-OC-R-CO-O-R'-O-)_n + 2n H_2O The same polyester forms faster, and without an equilibrium to shift, if the diol instead reacts with a dioyl chloride, releasing hydrogen chloride instead of water: n mathrmClOC-R-COCl + n mathrmHO-R'-OH arrow mathrm(-OC-R-CO-O-R'-O-)_n + 2n HCl A single hydroxycarboxylic acid, carrying both an -OH and a -COOH group, can also self-condense: each molecule's -OH reacts with the next molecule's -COOH, building the same type of ester-linked chain from one monomer and releasing water at every step.
🧑🏫 Teacher's explanation
Think of condensation polymerisation like building a long chain of paperclips. In addition polymerisation, you just snap pieces together. In condensation polymerisation, every time you snap two pieces together, a tiny bit of 'waste' (like a small bead or a drop of water) falls off. Because a small piece is lost every single time a link is made, the monomers must be 'bifunctional'—they need two 'hooks' to keep the chain growing in both directions.
📖 Definitions
🔍 Worked example — Easy
A monomer is hydroxypropanoic acid, HO-CH_2-CH_2-COOH. Predict the repeat unit of the polymer formed by the self-condensation of this monomer.
🔍 Worked example — Standard
Calculate the molar mass of the repeat unit of the polyester formed from ethane-1,2-diol (HO-CH_2-CH_2-OH) and propanedioic acid (HOOC-CH_2-COOH).
🔍 Worked example — Exam-style
A polymer is formed from propanedioic acid and propane-1,3-diol. Write the balanced equation for the formation of the polymer, including state symbols and the byproduct.
⚠️ Common errors
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🎯 Exam question
Key ideas — Forming polyamides
A diamine (-NH_2 at both ends) condenses with a dicarboxylic acid or a dioyl chloride in the same way, but each new linkage is an amide (peptide) linkage, -mathrmCO-NH-: n mathrmHOOC-R-COOH + n mathrmH_2N-R'-NH_2 arrow mathrm(-OC-R-CO-NH-R'-NH-)_n + 2n H_2O using HCl instead of H_2O if a dioyl chloride is used. An aminocarboxylic acid, carrying both groups on one monomer, self-condenses in the same way as a hydroxycarboxylic acid, and amino acids, mathrmH_2N-CHR-COOH, condense with each other by exactly this amine–acid reaction, forming the peptide-linked chains of polypeptides and proteins.
🧑🏫 Teacher's explanation
Think of condensation polymerisation like building a long chain of paperclips. To make a chain, you don't just stick them together; you have to snap the loops. In chemistry, when two monomers join, they 'snap' off a small molecule—usually water—to create a bridge. This bridge is the functional group that defines the polymer.
📖 Definitions
🔍 Worked example — Easy
Identify the repeat unit of a polymer formed from hexamethylenediamine (NH_2(CH_2)_6NH_2) and adipic acid (HOOC(CH_2)_4COOH).
🔍 Worked example — Standard
A polymer is formed from the self-condensation of 3-aminopropanoic acid (H_2N-CH_2-CH_2-COOH). Write the structure of the repeat unit.
🔍 Worked example — Exam-style
Calculate the molar mass of a repeat unit of Nylon 6,6, formed from hexamethylenediamine (mathrmC_6H_16N_2) and adipic acid (mathrmC_6H_10O_4), assuming the loss of two H_2O molecules per repeat unit.
⚠️ Common errors
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🎯 Exam question
Key ideas — Deducing the repeat unit
The repeat unit of a condensation polymer is found by joining one molecule of each monomer through the new linkage(s) formed, removing the atoms lost as the small molecule, and enclosing the result in brackets with subscript n; because each monomer contributes two reactive ends, the repeat unit always contains a whole number of both monomers used. For terephthalic acid, mathrmHOOC-C_6H_4-COOH, and ethane-1,2-diol, HOCH_2CH_2OH, forming Terylene (PET), the repeat unit is mathrm(-OC-C_6H_4-CO-O-CH_2CH_2-O-)_n.
🧑🏫 Teacher's explanation
Think of condensation polymerisation like building a long chain of paperclips. To make a chain, you need two loops to interlock. In chemistry, we use 'bifunctional' monomers—molecules that have a reactive group at both ends. When they react, they don't just stick together; they 'spit out' a tiny molecule, usually water (H_2O), to create a chemical bond.
📖 Definitions
🔍 Worked example — Easy
A monomer is HO-CH_2CH_2-OH and another is HOOC-CH_2CH_2-COOH. Deduce the repeat unit of the resulting polyester.
🔍 Worked example — Standard
Identify the monomers for the following repeat unit: -NH-CH(CH_3)-CO-NH-CH(CH_3)-CO-
🔍 Worked example — Exam-style
A polymer is formed from HOOC-C_6H_4-COOH and NH_2NH_2. Write the repeat unit of the resulting polyamide.
⚠️ Common errors
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🎯 Exam question
(a) Deduce the repeat unit of the polymer formed. [2] (b) Identify the type of polymer formed. [1] [3 marks]
Key ideas — Identifying monomers from a polymer section
Given a section of chain, the monomer(s) are found by locating each repeating ester or amide linkage, breaking the chain at every such linkage, and restoring the atoms removed during condensation: an -mathrmO-CO- ester linkage is split back into -OH and mathrmHOOC-, and a -mathrmNH-CO- amide linkage is split back into -NH_2 and mathrmHOOC-. A section mathrm-OC-(CH_2)_4-CO-NH-(CH_2)_6-NH- therefore comes from hexanedioic acid, HOOC(CH_2)_4COOH, and 1,6-diaminohexane, H_2N(CH_2)_6NH_2 — the monomers of nylon-6,6.
🧑🏫 Teacher's explanation
Think of condensation polymerisation like building a long necklace using two different types of beads. To make the necklace, you have to snap the beads together, but every time you snap two beads together, a tiny little bead (a water molecule) is popped off and discarded.
📖 Definitions
🔍 Worked example — Easy
Identify the two monomers used to form the following polymer section: -[O-CO-(CH_2)_2-CO-O-(CH_2)_2]-
🔍 Worked example — Standard
A polymer section is given as -[NH-CO-(CH_2)_4-CO-NH-(CH_2)_4]-. Identify the monomers.
🔍 Worked example — Exam-style
A polymer is formed from a single type of monomer: 3-hydroxypropanoic acid (HOCH_2CH_2COOH). Deduce the repeat unit of the resulting polyester.
⚠️ Common errors
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🎯 Exam question
(a) Draw the structure of the repeat unit of the resulting polyamide. [2] (b) Identify the small molecule eliminated during this reaction. [1] [3 marks]
Worked example
Question. Lactic acid, CH_3CH(OH)COOH, can be condensation-polymerised to form poly(lactic acid), a biodegradable polyester. (a) Identify the two functional groups in lactic acid that react together during polymerisation. [1] (b) Deduce the repeat unit of poly(lactic acid). [2] (c) State the small molecule released during this polymerisation, and identify the type of bond formed. [2]
🔍 Worked example — Easy
Identify the functional groups in hexanedioic acid, HOOC(CH_2)_4COOH, and ethan-1,2-diol, HOCH_2CH_2OH, that react to form a polyester.
🔍 Worked example — Standard
Deduce the repeat unit of a polymer formed from the condensation polymerisation of HOOC(CH_2)_4COOH and HOCH_2CH_2OH.
🔍 Worked example — Exam-style
A polymer is formed from H_2N(CH_2)_6NH_2 and HOOC(CH_2)_4COOH. Deduce the repeat unit and the small molecule released.
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