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Example · Example 3

Q.Explain condensation polymerization using the formation of nylon-6,6 from hexamethylenediamine and adipic acid as an example, and write the repeat unit of the polymer.

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✓ Free question

Nylon-6,6 is made by heating hexamethylenediamine, H2N–(CH2)6–NH2\text{H}_2\text{N--}(\text{CH}_2)_6\text{--NH}_2, with adipic acid, HOOC–(CH2)4–COOH\text{HOOC--}(\text{CH}_2)_4\text{--COOH}, under pressure. At each linking step, one −NH2-\text{NH}_2 of the diamine reacts with one −COOH-\text{COOH} of the diacid to form an amide linkage, −CO–NH−-\text{CO--NH}-, and release one molecule of water; because BOTH monomers are bifunctional (two reactive ends each), this condensation can repeat at both ends of every growing chain, in many independent places across the reaction mixture at once (step-growth), building up the long chain:

n H2N–(CH2)6–NH2+n HOOC–(CH2)4–COOH→[−NH–(CH2)6–NH–CO–(CH2)4–CO–]n+2n H2On\,\text{H}_2\text{N--}(\text{CH}_2)_6\text{--NH}_2 + n\,\text{HOOC--}(\text{CH}_2)_4\text{--COOH} \rightarrow \big[-\text{NH--}(\text{CH}_2)_6\text{--NH--CO--}(\text{CH}_2)_4\text{--CO--}\big]_n + 2n\,\text{H}_2\text{O}

✓Final answer

Repeated condensation of the diamine's −NH2-\text{NH}_2 groups with the diacid's −COOH-\text{COOH} groups, each step releasing water and forming an amide linkage, gives the repeat unit [−NH–(CH2)6–NH–CO–(CH2)4–CO–]n\big[-\text{NH--}(\text{CH}_2)_6\text{--NH--CO--}(\text{CH}_2)_4\text{--CO--}\big]_n.

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