📚 A-Level Chemistry: Preparation and Properties of Synthetic Polyamides | A-Level 化学：合成聚酰胺的制备与性能
Synthetic polyamides are high-molecular-weight polymers formed by condensation polymerisation, where monomer units are joined by amide linkages. These materials, including nylon and Kevlar, combine strength with flexibility and are central to A-level chemistry studies.
合成聚酰胺是通过缩聚反应形成的高分子量聚合物，单体单元通过酰胺键连接。这类材料包括尼龙和凯夫拉，兼具强度与柔韧性，是 A-level 化学学习的核心内容。
1. Monomers for Polyamide Synthesis | 合成聚酰胺的单体
Synthetic polyamides are typically made from two bifunctional monomers: a diamine (contains two -NH2 groups) and a dicarboxylic acid (contains two -COOH groups). An alternative is an amino acid, which provides both functional groups in one molecule.
合成聚酰胺通常由两种双官能团单体制备：二胺（含有两个 -NH2 基团）和二元羧酸（含有两个 -COOH 基团）。另一种选择是氨基酸，它在同一分子中同时提供两种官能团。
For nylon-6,6, the monomers are hexane-1,6-diamine, H2N(CH2)6NH2, and hexanedioic acid, HOOC(CH2)4COOH. The “6,6” refers to six carbon atoms between each pair of functional groups in both monomers.
对于尼龙-66，单体是己-1,6-二胺 H2N(CH2)6NH2 和己二酸 HOOC(CH2)4COOH。名称中的“66”表示两种单体中每对官能团之间都有六个碳原子。
For Kevlar, the monomers are benzene-1,4-diamine and terephthaloyl chloride. These aromatic monomers introduce rigid benzene rings, giving Kevlar its high tensile strength.
对于凯夫拉，单体是对苯二胺和对苯二甲酰氯。这些芳香族单体引入刚性的苯环，使凯夫拉具有高抗拉强度。
2. Condensation Polymerisation Mechanism | 缩聚反应机理
Condensation polymerisation is a step-growth process in which each step forms an amide bond and releases a small molecule, usually water or hydrogen chloride. The polymer chain grows slowly, and the degree of polymerisation increases with reaction time.
缩聚反应是逐步增长的过程，每步形成酰胺键并释放小分子，通常是水或氯化氢。聚合物链缓慢增长，聚合度随反应时间增加。
The general equation for producing a polyamide from a diamine and a dicarboxylic acid is:
由二胺与二元羧酸制备聚酰胺的通式为：
n H2N-R-NH2 + n HOOC-R’-COOH → [-NH-R-NH-CO-R’-CO-]n + 2n H2O
Here, n is the number of repeating units. The end groups remain active, so chain growth continues until one monomer is fully consumed or the system becomes too viscous.
式中 n 为重复单元数。末端基团保持活性，因此链增长持续到某一单体完全耗尽或体系过于粘稠为止。
Compare this with addition polymerisation: condensation polymers lose a small molecule, and no C=C bond is required for the monomer. This is a key distinction for exam answers.
与加成聚合反应相比：缩聚反应会失去一个小分子，且单体不需要 C=C 双键。这是答题中的关键区别。
3. Making Nylon-6,6 | 尼龙-66 的制备
In the laboratory, nylon-6,6 can be made by interfacial polymerisation. A solution of hexanedioyl dichloride in an organic solvent is carefully layered on top of an aqueous solution of hexane-1,6-diamine. The polymer forms at the interface.
在实验室中，可通过界面缩聚制备尼龙-66。将己二酰氯的有机溶液小心地铺在水溶性己-1,6-二胺溶液上，聚合物在界面处生成。
Industrially, the acid is used instead of the acyl chloride. Hexanedioic acid and hexane-1,6-diamine are heated under reflux in an inert atmosphere; the water produced is distilled off to drive the equilibrium to the right.
工业上使用羧酸而非酰氯。己二酸和己-1,6-二胺在惰性气氛下加热回流；生成的水被蒸馏除去，以推动平衡向右移动。
If you are writing a reaction scheme, always include n on both monomer sides, write the amide bond -CO-NH-, and balance the small molecule product. For nylon-6,6, the repeat unit is -NH(CH2)6NHCO(CH2)4CO-.
若需书写反应式，务必在两侧单体的系数中写上 n，写出酰胺键 -CO-NH-，并配平小分子产物。尼龙-66 的重复单元为 -NH(CH2)6NHCO(CH2)4CO-。
4. Making Kevlar | 凯夫拉的制备
Kevlar is produced by condensation polymerisation of benzene-1,4-diamine and terephthaloyl chloride. The reaction releases hydrogen chloride, not water, because an acyl chloride is used.
凯夫拉由对苯二胺和对苯二甲酰氯缩聚而成。由于使用酰氯，反应释放氯化氢而非水。
The monomers are rigid and para-substituted, so Kevlar chains are highly linear. This linearity allows polymer chains to pack closely and form strong intermolecular hydrogen bonds.
单体为刚性且对位取代，因此凯夫拉链高度线性。这种线性使聚合物链紧密堆积并形成强分子间氢键。
The reaction is carried out in cold sulfuric acid solution because the product is insoluble in most solvents; this also prevents degradation of the polymer chain during synthesis.
反应在冷硫酸溶液中进行，因为产物在大多数溶剂中不溶；同时可防止合成过程中聚合物链降解。
5. Intermolecular Forces and Hydrogen Bonding | 分子间作用力与氢键
The amide group contains a carbonyl C=O and an N-H group. The carbonyl oxygen is a hydrogen-bond acceptor, while the N-H hydrogen is a hydrogen-bond donor. Therefore strong N-H···O=C hydrogen bonds form between adjacent polymer chains.
酰胺基团含有羰基 C=O 和 N-H 基团。羰基氧是氢键受体，N-H 氢是氢键供体，因此在相邻聚合物链之间形成强 N-H···O=C 氢键。
In nylon-6,6, hydrogen bonding occurs between aligned chains. This gives the material good tensile strength and elasticity, as the chains can slip slightly under stress.
在尼龙-66 中，排列整齐的链之间形成氢键。这使材料具有良好抗拉强度和弹性，因为链在应力下可轻微滑动。
In Kevlar, hydrogen bonds act within and between layers, like a sheet. The aromatic rings also provide hydrophobic and rigid regions, leading to higher melting point and modulus than nylon.
在凯夫拉中，氢键在链内和层间起作用，类似片层结构。苯环还提供疏水和刚性区域，使其熔点与模量高于尼龙。
6. Crystallinity and Thermal Properties | 结晶度与热性能
Regular polymer chains can crystallise partially. Crystallinity increases density, stiffness, and resistance to solvents. Nylon-6,6 can crystallise because its chains are symmetric and flexible enough to align.
规整的聚合物链可部分结晶。结晶度会增加密度、刚性和耐溶剂性。尼龙-66 的链对称且足够柔韧，因此能够结晶。
Kevlar’s rigid aromatic chains lead to high crystallinity and a high glass transition temperature. It does not melt but decomposes near 500 °C.
凯夫拉的刚性芳香链导致高结晶度和高玻璃化转变温度。它不熔化，而是在接近 500 °C 时分解。
The table below contrasts the two polyamides:
下表对比两种聚酰胺：
| Property | Nylon-6,6 | Kevlar |
|---|---|---|
| Monomer structure | Aliphatic chains | Aromatic rings |
| Hydrogen bonding | Moderate | Strong and extensive |
| Melting behaviour | Melts near 265 °C | Decomposes near 500 °C |
| Tensile strength | High | Very high |
| Stiffness | Flexible and tough | Rigid and high-modulus |
| Typical applications | Textiles, ropes, gears | Bulletproof vests, composites |
7. Chemical Resistance and Hydrolysis | 耐化学性与水解
The amide bond is susceptible to acid- and base-catalysed hydrolysis. When polyamides are heated with strong acid or alkali, the chain breaks into smaller fragments and eventually returns to monomers.
酰胺键易受酸催化或碱催化的水解。当聚酰胺与强酸或强碱加热时，链断裂成较小片段，最终回到单体。
Nylon-6,6 is attacked by concentrated acids and strong bases, but resists dilute weak acids and most organic solvents.
尼龙-66 会被浓酸和强碱侵蚀，但耐稀弱酸和大多数有机溶剂。
Kevlar is resistant to many chemicals but is damaged by strong acids and strong bases, which hydrolyse the amide bond. This limits its use in harsh chemical environments.
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