Compostable Cutlery Contaminated with Nylon-6,6
A recycling plant receives bales of 'compostable' plastic cutlery. The supplier claims the cutlery is polylactic acid (PLA), a polyester made from lactic acid (2-hydroxypropanoic acid). A sample is contaminated with a nylon-6,6 stirrer. The plant's test heats a small chip with 2 mol L−1 sodium hydroxide for 20 minutes, then neutralises the mixture and identifies the hydrolysis products. PLA and nylon-6,6 both hydrolyse, but they give different products. The plant must decide whether a 500 kg batch is pure enough to compost. If nylon-6,6 is present above 2% by mass, the batch is rejected because nylon does not compost and releases hexane-1,6-diamine and adipic acid. A 12.0 g sample from the batch is hydrolysed and 0.210 g of hexane-1,6-diamine is isolated. M(hexane-1,6-diamine) = 116 g mol−1; M(repeating unit of nylon-6,6) = 226 g mol−1.
1. Identify the linkage in each polymer and state the monomers produced by hydrolysis of PLA and nylon-6,6. Explain why both are condensation polymers.
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PLA contains ester linkages, −COO−. Its monomer is lactic acid, CH3CH(OH)COOH (2-hydroxypropanoic acid). Hydrolysis of the ester bonds gives lactic acid units. Nylon-6,6 contains amide linkages, −CONH−. Its monomers are hexane-1,6-diamine, H2N(CH2)6NH2, and hexanedioic acid (adipic acid), HOOC(CH2)4COOH. Hydrolysis gives those two monomers (as the diamine and the dicarboxylic acid, or their salts in base).
Both are condensation polymers because each monomer has two functional groups, and polymerisation joins them with loss of a small molecule - water in both cases. The ester linkage forms between the −OH of lactic acid and the −COOH of another lactic acid; the amide linkage forms between an −NH2 of the diamine and a −COOH of the diacid.
2. Explain why PLA softens near 60 °C and dissolves in dichloromethane, while nylon-6,6 softens near 265 °C and is insoluble in dichloromethane.
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PLA chains are held together mainly by dipole–dipole attractions and dispersion forces. It has ester groups but no N−H bond, so it cannot form extensive hydrogen bonds between chains. These moderate intermolecular forces are disrupted at about 60 °C, and dichloromethane can solvate the chains, so PLA softens and dissolves.
Nylon-6,6 has amide groups with N−H and C=O. Adjacent chains form strong N−H···O=C hydrogen bonds, giving a tightly held, partly crystalline structure. Much more thermal energy is needed to break these interactions, so it softens near 265 °C. The same strong chain–chain hydrogen bonding makes it insoluble in dichloromethane; the solvent cannot easily separate the chains.
3. Use the 0.210 g of hexane-1,6-diamine to calculate the mass percentage of nylon-6,6 in the sample. Should the batch be rejected? Show your reasoning.
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Each repeat unit of nylon-6,6 contains one hexane-1,6-diamine unit, so moles of nylon repeat units = moles of diamine isolated. n(diamine) = m/M = 0.210 g / 116 g mol−1 = 1.81 × 10−3 mol. Mass of nylon-6,6 = n × M(repeat unit) = 1.81 × 10−3 mol × 226 g mol−1 = 0.409 g. Mass percentage = (0.409 g / 12.0 g) × 100 = 3.41%. This is above the 2% limit, so the batch should be rejected. The calculation assumes all the isolated diamine comes from nylon-6,6 and that hydrolysis is complete; if hydrolysis were incomplete, the true nylon content would be even higher, so the rejection decision is still safe.