Ch 2 · Predicting the type of polymerisation · AS & A Level Chemistry 9701

Topic 2 of 3 in Polymerisation — AS & A Level Chemistry 9701 — 39 lessons.

Introduction

Recognising whether a polymer forms by addition or condensation, from either the monomer(s) supplied or a section of finished chain, draws on both polymerisation topics together and is a common examiner test of understanding rather than recall.

Key ideas — Predicting the type from the monomer(s)

A single monomer containing a reactive mathrmC=C double bond and no other reactive functional group undergoes addition polymerisation, opening the double bond with no other product formed. A monomer carrying two reactive functional groups capable of condensing — such as a hydroxycarboxylic acid or an amino acid — undergoes condensation polymerisation with itself; two different monomers, each carrying two of the same or complementary reactive groups (a diol with a dicarboxylic acid, or a diamine with a dicarboxylic acid or dioyl chloride), also undergo condensation polymerisation, releasing water or hydrogen chloride.

Key ideas — Deducing the type from a section of polymer

A section of polymer chain reveals its polymerisation type from its backbone: an unbroken chain of carbon atoms joined only by single mathrmC-C bonds, with no other atoms interrupting the backbone, indicates addition polymerisation. A backbone interrupted at regular intervals by an ester linkage, -mathrmO-CO-, or an amide linkage, -mathrmNH-CO-, indicates condensation polymerisation, and the identity of the linkage further shows whether the polymer is a polyester or a polyamide.

🧑‍🏫 Teacher's explanation

Think of polymerisation like building a long chain. In addition polymerisation, you are taking small, identical links (monomers) and simply snapping them together. Because you aren't losing any pieces during the process, the backbone of the chain remains a continuous, unbroken string of the same atoms—usually carbon. It is like a train where every carriage is identical and they are just coupled together.

📖 Definitions

🔍 Worked example — Easy

Identify the type of polymerisation for a polymer with the repeat unit: -[CH_2-CH(Cl)]_n-

🔍 Worked example — Standard

A polymer has a repeat unit containing the sequence -[CO-NH-CH(R)-]_n-. Identify the polymer type and the class of polymer.

🔍 Worked example — Exam-style

A monomer has the formula CH_2=CH-COOH. Predict the type of polymerisation and the functional group in the polymer backbone.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

(a) State the type of polymerisation used to form this polymer. [1] (b) Identify the functional group present in the polymer backbone. [1] (c) Predict whether the monomer used was a single type of molecule or a pair of different molecules. [1] [3 marks]

Key ideas — Comparing the two types of repeat unit

An addition polymer's repeat unit contains exactly the same atoms as its single monomer, since no atoms are lost and it comes from one monomer only; a condensation polymer's repeat unit is smaller than the sum of the monomer(s) that formed it, since a small molecule (water or hydrogen chloride) is lost at every linkage, and it may combine two different monomers, each supplying one part of the repeat unit.

🧑‍🏫 Teacher's explanation

Think of polymerisation like building a long chain. Addition polymerisation is like taking identical LEGO bricks and snapping them together; you end up with a chain that is just a long repetition of the exact same brick. Nothing is lost, and the chain is just a 'stretched out' version of the original piece.

📖 Definitions

🔍 Worked example — Easy

A monomer has the molecular formula C_2H_4. Predict the molecular formula of the repeat unit of its addition polymer.

🔍 Worked example — Standard

A condensation polymer is formed from a dicarboxylic acid (HOOC-R-COOH) and a diamine (H_2N-R'-NH_2). If the small molecule lost is H_2O, write the formula for the repeat unit.

🔍 Worked example — Exam-style

A polymer has a repeat unit with the formula [O-CH_2-CH_2-C(=O)-]_n. The monomers used were HO-CH_2-CH_2-OH and HO-C(=O)-CH_2-CH_2-COOH. Calculate the difference in molar mass between the sum of the two monomers and the repeat unit.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Key ideas — Functional-group clues in the finished polymer

Even without seeing the original monomers, specific functional groups in the polymer backbone identify it as a condensation polymer: an ester group, -mathrmCOO-, appearing repeatedly signals a polyester, and an amide group, -mathrmCONH-, appearing repeatedly signals a polyamide. The complete absence of any such linking group, with the backbone made only of carbon atoms, rules out condensation and confirms addition polymerisation.

🧑‍🏫 Teacher's explanation

Think of polymerisation like building a long chain. In addition polymerisation, you are just linking identical Lego bricks together. The bricks are already complete, so you just snap them together, and the chain is just a long row of the same plastic material. There is no 'waste' or extra pieces left over; the chain is just a repetition of the original monomer structure.

📖 Definitions

🔍 Worked example — Easy

Identify the type of polymerisation for a polymer with the repeat unit -[CH_2-CH(Cl)]_n.

🔍 Worked example — Standard

A polymer is formed from a dicarboxylic acid and a diamine. The repeat unit contains the group -CONH-. Identify the polymer type.

🔍 Worked example — Exam-style

A polymer has the repeat unit -[O-CH_2-CH_2-COO]_n. Determine the type of polymerisation and the small molecule likely eliminated during formation.

⚠️ Common errors

🧠 Quick check 1

🧠 Quick check 2

🎯 Exam question

Worked example

Question. A polymer has the repeating section mathrm-CH_2-CH(C_6H_5)-CH_2-CH(C_6H_5)-, and a second, different polymer has the repeating section mathrm-O-(CH_2)_2-O-CO-C_6H_4-CO-. (a) State the type of polymerisation that formed each polymer, giving a reason from the structure of each section. [4] (b) Suggest the monomer(s) that formed the second polymer. [2]

Ch 2 · Predicting the type of polymerisation · AS & A Level Chemistry 9701

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