Condensation Polymers, Polyesters & Polyamides

Condensation polymerisation requires bifunctional monomers, two reactive groups per monomer. At each bond formed, a small molecule (H₂O, HCl) is released. Option A describes addition polymerisation. Option C is partially correct about the single bifunctional monomer but wrong about no by-product, condensation ALWAYS produces a small molecule. Option D is completely wrong, the two types are fundamentally different.

Linkage: -CO-NH- = amide bond → polyamide. Break each amide bond: add H to N → H₂N-; add OH to C=O → -COOH. Fragment 1: HOOC-(CH₂)₄-COOH = hexanedioic acid (6 carbons total). Fragment 2: H₂N-(CH₂)₆-NH₂ = hexane-1,6-diamine (6 carbons). This is Nylon 6,6. Option B gives diol → polyester (wrong linkage identified). Option D: the diacid fragment has 6 carbons total (hexanedioic acid), not 4 (butanedioic acid).

PET's ester (-COO-) linkages can be cleaved by water under acidic or alkaline conditions, or by esterase/PETase enzymes, the C-O bond of the ester is susceptible to nucleophilic attack by water. Polystyrene has only C-C and C-H bonds, no ester or amide groups, and is not attacked by water or hydrolase enzymes. Option D is wrong, neither hydrolyses rapidly in ocean water at ambient conditions.

The repeat unit [-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-]ₙ contains TWO amide linkages per repeat unit, one -CO-NH- on the left side (between (CH₂)₄ and (CH₂)₆) and one -CO-NH- on the right side. The diacid contributes two carbonyl groups, each forming one amide bond with the two ends of the diamine. One amide linkage per repeat unit would require only one -COOH end reacting, which would not produce a linear chain.

Hexane (CH₃(CH₂)₄CH₃) is an alkane, it has no reactive functional groups (-NH₂, -OH, -COOH). Without functional groups at each end, hexane cannot form covalent bonds with hexanedioic acid by any condensation mechanism. It cannot form a polyamide (or any polymer). Option A: correct diamine + diacid pairing → Nylon 6,6. Option C: amino acid with -NH₂ + -COOH can self-condense → Nylon 6 (or protein). Option D: diamine + acid chloride → polyamide via HCl by-product (industrial route).

(a) PET repeat unit: [-O-CH₂CH₂-O-CO-C₆H₄-CO-]ₙ, square brackets, subscript n, open bonds at each end. [1 mark]
(b) Monomer 1: ethylene glycol (ethane-1,2-diol, HOCH₂CH₂OH). Monomer 2: terephthalic acid (benzene-1,4-dicarboxylic acid, HOOC-C₆H₄-COOH). Linkage type: ester (-COO-). [1 mark]
(c) By-product: H₂O. One H₂O forms per ester linkage. A finite chain from n diol and n diacid molecules has 2n - 1 linkages, so the amount approaches two H₂O per repeat unit only for a very long chain. [1 mark]

(a) Linkage: -COO- (ester bond). Polymer X is a polyester, specifically PET (polyethylene terephthalate). [0.5 mark]
(b) Break ester bonds (insert H₂O): Monomer 1: HO-CH₂CH₂-OH = ethylene glycol (ethane-1,2-diol). Monomer 2: HOOC-C₆H₄-COOH = terephthalic acid (benzene-1,4-dicarboxylic acid). [1 mark]
(c) PET's ester bonds (-COO-) contain a C-O single bond that can be attacked by water (hydrolysis) under acidic or alkaline conditions, the lone pairs on water's oxygen attack the electrophilic carbonyl carbon, regenerating the diol and diacid. Polyethylene has only C-C and C-H bonds, no electrophilic carbonyl carbon for water to attack. C-C bonds are not hydrolysed under any normal conditions. [1.5 marks]
(d) Application: plastic drink bottles (or polyester fabric/Dacron). Benzene ring contribution: the rigid planar benzene ring restricts free rotation of the polymer backbone, promoting crystalline ordering and increasing the energy required to disrupt the chain, resulting in a high melting point (~260°C). [1 mark]

(i) Monomer requirements: PP (addition), propene has ONE functional group: C=C double bond per monomer. Nylon 6,6 (condensation), requires TWO bifunctional monomers: hexane-1,6-diamine (two -NH₂ groups) and hexanedioic acid (two -COOH groups). [1 mark]
(ii) By-product: PP, no by-product. Nylon 6,6 releases one H₂O per amide linkage, approaching two per repeat unit for a long chain. [1 mark]
(iii) Linkage formation: PP, C=C pi bond opens; two new C-C sigma bonds form between monomers. Nylon 6,6, N from -NH₂ bonds to carbonyl C of -COOH; H₂O leaves. Linkage: amide (-CO-NH-). [1 mark]
(iv) Hydrolysability: PP is not hydrolysable because its C-C backbone has no hydrolysable linkage. Nylon 6,6 amide bonds require harsh acidic or alkaline hydrolysis; natural proteases do not readily recognise the synthetic chain, so it remains highly persistent. [1 mark]
(v) Environmental persistence: PP, very high; no hydrolysable linkage; degrades only by UV photodegradation and mechanical fragmentation into microplastics. Nylon 6,6, also high persistence in ocean water, but amide bonds offer a potential enzymatic degradation pathway not available to PP. In practice, both persist for decades to centuries in ocean environments. [1 mark]