Synthetic Polyamides | 合成聚酰胺

📚 Synthetic Polyamides | 合成聚酰胺

Polyamides are a class of step-growth polymers that contain the amide linkage as a repeating functional group. The amide bond, -CONH-, forms the backbone of these versatile materials. While proteins are nature’s polyamides, this article focuses on synthetic polyamides, especially nylon and Kevlar, which are central to the Cambridge A-Level Chemistry specifications. We will explore their synthesis via condensation polymerisation, the relationship between structure and properties, key applications, and chemical behaviour such as hydrolysis.

聚酰胺是一类逐步增长聚合物，以酰胺键（-CONH-）作为重复官能团连接主链。蛋白质是天然的聚酰胺，而本文聚焦于合成聚酰胺，尤其是尼龙和Kevlar，它们是剑桥A-Level化学大纲的核心内容。我们将探讨通过缩合聚合反应的合成方法、结构与性质的关系、主要应用以及水解等化学行为。

1. What Are Polyamides? | 什么是聚酰胺？

Polyamides are polymers whose repeating units are joined by amide (also called peptide) linkages. The functional group -CO-NH- is formed from a condensation reaction between a carboxylic acid and an amine. In synthetic polyamides, the monomers are typically diamines and dicarboxylic acids, or amino acids that self-condense.

聚酰胺是重复单元通过酰胺键（又称肽键）连接而成的聚合物。官能团 -CO-NH- 由羧酸和胺之间的缩合反应生成。在合成聚酰胺中，单体通常是二胺和二羧酸，或者自缩合的氨基酸。

The term ‘polyamide’ tells us that the polymer contains many amide groups. Each amide link is polar due to the electronegativity difference between nitrogen, carbon, and oxygen, and it can participate in hydrogen bonding. This hydrogen bonding is key to the high melting points, strength, and crystallinity observed in many nylons.

“聚酰胺”一词表明聚合物含有大量酰胺基团。由于氮、碳和氧之间的电负性差异，每个酰胺键都具有极性，并能参与氢键作用。这种氢键是许多尼龙具有高熔点、高强度和高结晶度的关键。

2. Condensation Polymerisation | 缩合聚合反应

Condensation polymerisation is a step-growth process where monomers join together with the elimination of a small molecule, often water or hydrogen chloride. For polyamides, each amide bond formation releases a small molecule. The most common route for nylon synthesis uses a diamine and a dicarboxylic acid, eliminating water.

缩合聚合是一种逐步增长过程，单体连接在一起同时脱去一个小分子（通常是水或氯化氢）。对于聚酰胺，每形成一个酰胺键都会释放一个小分子。最常见的尼龙合成路线使用二胺和二羧酸，脱去水分子。

A general equation for the reaction of a diamine H₂N-R-NH₂ with a dicarboxylic acid HOOC-R’-COOH is:

二胺 H₂N-R-NH₂ 与二羧酸 HOOC-R’-COOH 反应的一般方程式为：

n H₂N−R−NH₂ + n HOOC−R’−COOH → [−HN−R−NHCO−R’−CO−]ₙ + (2n−1) H₂O

Note that each chain requires (2n-1) molecules of water for n monomer pairs. The repeat unit contains the amide linkage flanked by the R groups. For a balanced equation with n moles of each monomer, we often write 2n H₂O for simplicity, as each polymer chain needs 2n-1 waters and one end group; in practice, the stoichiometry is approximately 2n H₂O for high molecular weight.

需注意每一条链对于 n 对单体需要脱去 (2n-1) 个水分子。重复单元中含有酰胺键以及两侧的 R 基团。对于 n 摩尔每种单体的配平方程式，为简便常写作 2n H₂O，实际高分子量时化学计量比接近 2n H₂O。

3. Nylon 6,6: Monomers and Polymerisation | 尼龙 6,6：单体与聚合

Nylon 6,6 is the most iconic synthetic polyamide. It is produced from two monomers each containing six carbon atoms: 1,6-diaminohexane (hexamethylenediamine) and hexanedioic acid (adipic acid). The ‘6,6’ refers to the number of carbons in the diamine and in the diacid, respectively.

尼龙 6,6 是最具代表性的合成聚酰胺。它由两种各含六个碳原子的单体制成：1,6-己二胺（己二胺）和己二酸（己二酸）。‘6,6’ 分别指二胺和二酸中的碳原子数。

The monomers have the following structures:

单体的结构如下：

• 1,6-diaminohexane (hexane-1,6-diamine): H₂N−(CH₂)₆−NH₂
• Hexanedioic acid (adipic acid): HOOC−(CH₂)₄−COOH

The polymerisation reaction is:

聚合反应为：

n H₂N(CH₂)₆NH₂ + n HOOC(CH₂)₄COOH → [−NH(CH₂)₆NHCO(CH₂)₄CO−]ₙ + 2n H₂O

The repeat unit for nylon 6,6 is thus −NH(CH₂)₆NHCO(CH₂)₄CO−. The amide groups can form strong inter-chain hydrogen bonds, giving the polymer high tensile strength and a relatively high melting point (≈265 °C).

因此尼龙 6,6 的重复单元为 −NH(CH₂)₆NHCO(CH₂)₄CO−。酰胺基团可形成牢固的链间氢键，赋予聚合物高拉伸强度和较高熔点（约 265 °C）。

In industry, nylon 6,6 is prepared by the melt polymerisation of the salt formed from the diamine and the diacid, often called nylon salt, at elevated temperatures. The reaction is carried out under a nitrogen atmosphere to prevent oxidation.

工业上，尼龙 6,6 通过二胺和二酸形成的盐（常称尼龙盐）在高温下熔融聚合制得。反应在氮气氛围下进行以防止氧化。

4. Structure and Intermolecular Forces in Nylon | 尼龙的结构与分子间作用力

The strength and thermal stability of nylon 6,6 arise mainly from hydrogen bonding between the N−H group of one amide link and the C=O group of another on an adjacent chain. These hydrogen bonds are regularly spaced, allowing the formation of crystalline sheet-like structures when the polymer is drawn into fibres.

尼龙 6,6 的强度和热稳定性主要源于酰胺键的 N−H 基团与相邻链上另一个酰胺键的 C=O 基团之间的氢键。这些氢键分布规则，当聚合物被牵引成纤维时，能形成晶体片状结构。

The long hydrocarbon segments (−CH₂−)₆ and (−CH₂−)₄ provide flexibility and hydrophobic character, but the amide groups dominate the intermolecular forces. The degree of crystallinity can be controlled by the drawing process: stretching aligns the polymer chains, maximising hydrogen bonding and thus increasing strength and stiffness.

长碳氢链段 (−CH₂−)₆ 和 (−CH₂−)₄ 提供柔顺性和疏水性，但酰胺基团主导分子间作用力。结晶度可通过牵引工艺控制：拉伸使聚合物链排列整齐，最大化氢键，从而提高强度和刚度。

In addition to hydrogen bonding, van der Waals forces between the methylene groups contribute to the overall cohesion. The balance between these forces makes nylon 6,6 an excellent fibre-forming polymer with good elasticity and abrasion resistance.

除氢键外，亚甲基之间的范德华力也对整体内聚力有贡献。这些力的平衡使尼龙 6,6 成为优良的成纤聚合物，富有弹性和耐磨性。

5. Properties and Applications of Nylon | 尼龙的性质与应用

Nylon 6,6 is a semi-crystalline thermoplastic with high mechanical strength, toughness, and resistance to wear and chemicals. It is lightweight and can be melt-spun into fibres or injection-moulded into plastic parts.

尼龙 6,6 是一种半结晶热塑性塑料，具有高机械强度、韧性以及耐磨性和耐化学性。它质轻，可熔融纺丝成纤维或注塑成型为塑料部件。

Common applications include: textiles (stockings, sportswear, parachutes, ropes), engineering plastics (gears, bearings, automotive components), and thin films (food packaging). Nylon is often blended with other fibres, like cotton or elastane, to improve comfort and elasticity.

常见应用包括：纺织品（丝袜、运动服、降落伞、绳索）、工程塑料（齿轮、轴承、汽车部件）及薄膜（食品包装）。尼龙常与棉或氨纶等其他纤维混纺，以提升舒适度和弹性。

The melting point (Tm) of nylon 6,6 is about 255–265 °C, while its glass transition temperature (Tg) is around 50 °C in dry conditions. Absorbed moisture plasticises the polymer, lowering Tg and increasing impact resistance but reducing stiffness.

尼龙 6,6 的熔点 (Tm) 约 255–265 °C，玻璃化转变温度 (Tg) 在干燥条件下约为 50 °C。吸收水分会增塑聚合物，降低 Tg 并提高抗冲击性，但刚度下降。

6. Kevlar: Aromatic Polyamide | Kevlar：芳香族聚酰胺

Kevlar is the trade name for a class of aromatic polyamides (aramids) where the amide bond is directly attached to aromatic rings. The monomers are benzene-1,4-diamine (para-phenylenediamine) and benzene-1,4-dicarbonyl chloride (terephthaloyl chloride).

Kevlar 是一类芳香族聚酰胺（芳纶）的商品名，其酰胺键直接连接在芳环上。单体为 1,4-苯二胺（对苯二胺）和 1,4-苯二甲酰氯（对苯二甲酰氯）。

The polymerisation is a condensation reaction that eliminates HCl instead of water:

聚合反应是一缩合反应，脱去的是 HCl 而非水：

n H₂N−C₆H₄−NH₂ + n ClOC−C₆H₄−COCl → [−HN−C₆H₄−NHCO−C₆H₄−CO−]ₙ + 2n HCl

The use of the more reactive acid chloride allows the reaction to occur at lower temperatures and with higher efficiency. The polymer is generally produced by interfacial polymerisation, where the diamine is dissolved in water and the diacid chloride in an organic solvent; the polymer forms at the interface and is drawn as a continuous fibre.

使用活性更高的酰氯可使反应在较低温度下高效进行。聚合物通常通过界面聚合制备：将二胺溶于水，二酰氯溶于有机溶剂，聚合物在界面形成并牵拉成连续纤维。

The repeat unit of Kevlar is −HN−C₆H₄−NHCO−C₆H₄−CO−, featuring para-substituted benzene rings linked by amide groups. These rigid, rod-like segments stack into highly ordered arrays stabilised by hydrogen bonds and π-π interactions between aromatic rings.

Kevlar 的重复单元为 −HN−C₆H₄−NHCO−C₆H₄−CO−，特征是酰胺基团连接的对位取代苯环。这些刚性棒状链段堆砌成高度有序的阵列，由氢键和芳环之间的 π-π 相互作用稳定。

7. Properties and Applications of Kevlar | Kevlar 的性质与应用

Kevlar possesses exceptional tensile strength (about five times that of steel on an equal weight basis), high modulus, and excellent thermal stability. It decomposes above 450 °C without melting. Its density is low (~1.44 g cm⁻³), making it ideal for lightweight protective equipment.

Kevlar 具有卓越的拉伸强度（等重下约为钢的五倍）、高模量以及出色的热稳定性。它在 450 °C 以上分解而不熔融。密度低（约 1.44 g cm⁻³），是轻量防护装备的理想材料。

Primary uses include: ballistic body armour, cut-resistant gloves, helmets, composite reinforcement (e.g. in boat hulls and aircraft parts), and ropes/cables. The high stiffness and impact resistance arise from the extended-chain crystalline structure, where polymer chains are almost fully aligned along the fibre axis.

主要用途包括：防弹衣、防割手套、头盔、复合材料增强（如船体和飞机部件）以及绳索/缆线。高刚度和抗冲击性源自伸直链晶体结构，聚合物链几乎完全沿纤维轴排列。

Kevlar is sensitive to UV radiation and absorbs moisture, which can gradually degrade its mechanical performance. However, proper coating and design mitigate these issues.

Kevlar 对紫外光敏感并会吸湿，这可能逐渐降低其力学性能。但通过适当的涂层和设计可以缓解这些问题。

8. Nylon 6: An Alternative Polyamide | 尼龙 6：另一种聚酰胺

Although not as prominent in the Cambridge specification, Nylon 6 is another important synthetic polyamide. It is produced by the ring-opening polymerisation of caprolactam, a cyclic amide with six carbon atoms. The repeat unit is −NH(CH₂)₅CO−, identical to nylon 6,6 in terms of amide density but with a slightly different arrangement of methylene groups.

尼龙 6 虽在剑桥大纲中没有尼龙 6,6 突出，但也是重要的合成聚酰胺。它由含六个碳原子的环状酰胺——己内酰胺开环聚合而成。重复单元为 −NH(CH₂)₅CO−，酰胺密度与尼龙 6,6 相同，但亚甲基排列略有不同。

The polymerisation is catalysed by water and heat, opening the ring and forming a linear chain without elimination of a small molecule; however, it is still classified as a condensation-like polymerisation because the amide bond forms via a condensation step in the mechanism.

聚合反应在水和热催化下进行，开环形成线性链，不脱去小分子；但由于其机理中包含缩合步骤形成酰胺键，仍归为类缩合聚合。

Nylon 6 has a slightly lower melting point (~220 °C) and good impact strength. It finds applications similar to nylon 6,6, notably in fibres and engineering plastics.

尼龙 6 的熔点略低（约 220 °C），冲击强度良好。应用与尼龙 6,6 相似，尤以纤维和工程塑料见长。

9. Hydrolysis of Polyamides | 聚酰胺的水解

Polyamides can be broken down by hydrolysis, the reverse of condensation polymerisation. Both acidic and alkaline hydrolysis cleave the amide link, regenerating the monomer or its salt.

聚酰胺可通过水解降解，即缩合聚合的逆反应。酸性和碱性水解均能断裂酰胺键，重新生成单体或其盐。

Acid hydrolysis uses hot dilute hydrochloric acid and yields the diamine (as the ammonium salt) and the dicarboxylic acid. For nylon 6,6:

酸性水解使用热的稀盐酸，生成二胺（以铵盐形式）和二羧酸。对于尼龙 6,6：

[−NH(CH₂)₆NHCO(CH₂)₄CO−]ₙ + 2n H₂O + 2n H⁺ → n ⁺H₃N(CH₂)₆NH₃⁺ + n HOOC(CH₂)₄COOH

The diamine appears as a protonated salt, reflecting the acidic medium. Alkaline hydrolysis with hot aqueous sodium hydroxide produces the diamine and the dicarboxylate salt:

二胺以质子化铵盐形式存在，反映酸性介质。用热的氢氧化钠水溶液进行碱性水解，生成二胺和二羧酸盐：

[−NH(CH₂)₆NHCO(CH₂)₄CO−]ₙ + 2n NaOH → n H₂N(CH₂)₆NH₂ + n NaOOC(CH₂)₄COONa + n H₂O

Kevlar also undergoes hydrolysis, but its aromatic structure makes it much more resistant to chemical attack than aliphatic nylons. The extreme conditions needed (concentrated acids or alkalis at high temperatures) limit its degradation to specific recycling processes.

Kevlar 同样可水解，但其芳香结构使其比脂肪族尼龙更耐化学侵蚀。所需极端条件（高温浓酸或浓碱）限制了其降解，特定回收工艺才可分解。

10. Environmental Considerations | 环境考量

Synthetic polyamides are derived from petrochemical feedstocks and are not readily biodegradable due to the stability of the amide bond. Discarded nylon products persist in the environment as microplastics, contributing to pollution. Efforts are ongoing to develop bio-based polyamides and improve recycling techniques.

合成聚酰胺来自石化原料，且因酰胺键稳定性不易生物降解。废弃的尼龙制品以微塑料形式持久存在于环境中，造成污染。目前正努力开发生物基聚酰胺并改进回收技术。

Chemical recycling can depolymerise nylon 6 back to caprolactam, and nylon 6,6 can be broken down to recover adipic acid and hexamethylenediamine, though energy costs are significant. Mechanical recycling and repurposing into lower-grade products are also practiced.

化学回收可将尼龙 6 解聚回己内酰胺，尼龙 6,6 也可降解回收己二酸和己二胺，但能耗较高。尚可进行机械回收及降级再利用。

Students should understand that condensation polymers can, in principle, be hydrolysed back to monomers, which offers a pathway to a circular economy, but practical and economic challenges remain.

学生应理解，缩合聚合物原则上可水解回单体，为循环经济提供了途径，但仍面临实际和经济挑战。

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