📚 Condensation Polymerisation | 缩聚反应
Condensation polymerisation is a key topic in Cambridge A-Level Chemistry, forming the basis for materials such as polyesters and polyamides. This process involves difunctional monomers that react together, eliminating small molecules like water. Understanding the mechanisms, repeating units, properties, and environmental impact of these polymers is essential for the examination.
缩聚反应是剑桥A-Level化学的关键主题，为聚酯和聚酰胺等材料奠定了基础。该过程涉及双官能团单体相互反应，消除水等小分子。理解这些聚合物的机理、重复单元、性质及环境影响对考试至关重要。
1. Definition of Condensation Polymerisation | 缩聚聚合的定义
Condensation polymerisation is a step-growth process in which monomers with at least two functional groups react to form a long polymer chain, accompanied by the elimination of a small molecule – most commonly water, but sometimes hydrogen chloride or methanol. The monomers must be difunctional, meaning each end of the molecule carries a reactive group. For example, a dicarboxylic acid (HOOC–R–COOH) and a diol (HO–R'–OH) can link together, releasing H₂O at every ester bond formed.
缩聚反应是一种逐步增长的过程，带有至少两个官能团的单体反应形成长聚合物链，同时消除小分子——最常见的是水，有时是氯化氢或甲醇。单体必须是双官能团的，即分子的每一端都带有一个反应性基团。例如，二元羧酸（HOOC–R–COOH）和二元醇（HO–R'–OH）可以连接在一起，每形成一个酯键就释放一分子H₂O。
- Monomers must be difunctional (e.g. diacid, diol, diamine, diacyl chloride).
- Small molecule eliminated: H₂O, HCl, CH₃OH, etc.
- Polymer chain grows in steps; any two species can react at any time.
- 单体必须为双官能团（如二元酸、二元醇、二元胺、二酰氯）。
- 消除的小分子：H₂O、HCl、CH₃OH等。
- 聚合物链逐步增长；任意两个物种可在任意时刻反应。
2. Comparison with Addition Polymerisation | 与加聚反应的比较
Condensation polymerisation differs fundamentally from addition polymerisation. Addition polymerisation requires alkene monomers with a carbon–carbon double bond, proceeds via a chain reaction (usually free radical), and generates no by-products. In contrast, condensation polymerisation uses monomers with two reactive ends, grows stepwise, and always eliminates a small molecule. The table below summarises key differences.
缩聚反应与加聚反应有着根本的不同。加聚反应需要带有碳碳双键的烯烃单体，通过连锁反应（通常为自由基）进行，并且不产生副产物。相比之下，缩聚反应使用带有两个反应端的单体，逐步增长，并总是消除一个小分子。下表总结了主要区别。
| Feature | Condensation Polymerisation | Addition Polymerisation |
|---|---|---|
| Monomers | Difunctional (e.g. HO–R–OH + HOOC–R'–COOH) | Alkene (contains C=C bond) |
| Mechanism | Step‑growth; any two molecules react | Chain‑growth; free‑radical or ionic |
| By‑product | Small molecule eliminated (H₂O, HCl) | No elimination |
| Empirical formula of polymer | Sum of monomers minus small molecule | Identical to monomer |
| Examples | Polyesters, polyamides (nylon, Kevlar) | Polyethene, polypropene, PVC |
特征 | 缩聚反应 | 加聚反应
单体 | 双官能团（如 HO–R–OH + HOOC–R'–COOH） | 烯烃（含 C=C 键）
机理 | 逐步增长，任意两分子可反应 | 连锁增长，自由基或离子型
副产物 | 消除小分子 (H₂O, HCl) | 无
聚合物经验式 | 单体之和减去小分子 | 与单体相同
示例 | 聚酯、聚酰胺（尼龙、凯夫拉） | 聚乙烯、聚丙烯、PVC
3. General Requirements for Monomers | 单体的一般要求
For condensation polymerisation to occur, each monomer must possess two functional groups. The most common combinations are diacid + diol, diacid + diamine, or diacyl chloride + diamine. The functional groups react with each other, forming a link (e.g. ester or amide) and ejecting a small molecule. The polymer chain can continue growing because each end still carries a reactive group until termination.
要发生缩聚反应，每个单体必须带有两个官能团。最常见的组合是二元酸+二元醇、二元酸+二元胺或二酰氯+二元胺。官能团相互反应，形成一个键（如酯键或酰胺键），并释放一个小分子。因为链的每一端仍带有反应性基团，所以聚合物链可持续增长，直到终止。
- Diol + dicarboxylic acid → polyester + water
- Diamine + dicarboxylic acid → polyamide + water
- Diamine + diacyl chloride → polyamide + hydrogen chloride
- 二醇 + 二元羧酸 → 聚酯 + 水
- 二元胺 + 二元羧酸 → 聚酰胺 + 水
- 二元胺 + 二酰氯 → 聚酰胺 + 氯化氢
When a diacyl chloride is used instead of a carboxylic acid, the condensation is faster and does not require a catalyst, but corrosive HCl gas is evolved.
当使用二酰氯代替羧酸时，缩聚更快且不需要催化剂，但会产生腐蚀性HCl气体。
4. Polyesters: Formation from Dicarboxylic Acids and Diols | 聚酯：由二元羧酸和二醇形成
Polyesters are produced when a dicarboxylic acid reacts with a diol. Each carboxylic acid group reacts with a hydroxyl group to form an ester linkage (–COO–) and a water molecule. Because each monomer is difunctional, the chains can extend in two directions. The simplest example is the condensation of ethane-1,2-diol with benzene-1,4-dicarboxylic acid (terephthalic acid) to give the well‑known polyester PET, used in drinks bottles and synthetic fibres.
当二元羧酸与二醇反应时，生成聚酯。每个羧酸基团与一个羟基反应，形成一个酯键（–COO–）和一个水分子。由于每个单体是双官能团的，链可以向两个方向延伸。最简单的例子是乙二醇（ethane-1,2-diol）与对苯二甲酸（benzene-1,4-dicarboxylic acid）缩聚，生成著名的聚酯 PET，用于饮料瓶和合成纤维。
n HO–CH₂CH₂–OH + n HOOC–C₆H₄–COOH → –[–O–CH₂CH₂–O–OC–C₆H₄–CO–]ₙ– + 2n H₂O
In the repeating unit, the ester group is shown inside the brackets with the monomer residues linked together. The subscript n denotes the large number of repeat units.
在重复单元中，酯基显示在方括号内，单体残基连接在一起。下标 n 表示重复单元的大量数目。
5. Naming and Repeating Units of Polyesters | 聚酯的命名和重复单元
When naming the repeating unit, we identify the diol part and the diacid part. For instance, if the diol is ethane-1,2-diol and the diacid is terephthalic acid, the repeat unit is –O–CH₂CH₂–O–CO–C₆H₄–CO–. The polymer is named poly(ethylene terephthalate) or PET. In Cambridge exams, you must be able to draw the repeat unit from given monomers and vice versa.
命名重复单元时，我们要确定二醇部分和二酸部分。例如，若二醇为乙二醇，二酸为对苯二甲酸，则重复单元为 –O–CH₂CH₂–O–CO–C₆H₄–CO–。该聚合物命名为聚对苯二甲酸乙二醇酯，即 PET。在剑桥考试中，你必须能从给定的单体画出重复单元，反之亦然。
- The ester linkage is highlighted as –COO– (write it as –OC– or –CO– depending on direction, but the full group is –O–CO–). In repeat units the ester oxygen is usually written next to the diol carbon.
- If the diol is propane-1,3-diol, the repeat unit becomes –O–CH₂CH₂CH₂–O–CO–R–CO–.
- If the acid is a simple aliphatic diacid such as hexanedioic acid (HOOC–(CH₂)₄–COOH), the repeat unit includes an alkylene chain.
- 酯键突出显示为 –COO–（根据方向写为 –OC– 或 –CO–，但完整基团是 –O–CO–）。在重复单元中，酯氧通常写在二醇碳的旁边。
- 如果二醇是1,3-丙二醇，重复单元变为 –O–CH₂CH₂CH₂–O–CO–R–CO–。
- 若酸是简单的脂肪族二酸，如己二酸 (HOOC–(CH₂)₄–COOH)，重复单元包含一个亚烷基链。
6. Polyamides: Formation from Dicarboxylic Acids and Diamines | 聚酰胺：由二元羧酸和二元胺形成
Polyamides are formed when a dicarboxylic acid (or its diacyl chloride) reacts with a diamine. The amine group (–NH₂) attacks the carboxyl group (–COOH) to form an amide (peptide) linkage –CONH– with the elimination of water. These polymers are known for their strength and are used in textiles, ropes and engineering plastics.
当二元羧酸（或其二酰氯）与二元胺反应时形成聚酰胺。胺基（–NH₂）进攻羧基（–COOH）形成酰胺（肽）键 –CONH–，同时消除水。这些聚合物以强度著称，用于纺织品、绳索和工程塑料。
n H₂N–(CH₂)₆–NH₂ + n HOOC–(CH₂)₄–COOH → –[–NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–]ₙ– + 2n H₂O
This equation shows the synthesis of nylon‑6,6 from hexane-1,6-diamine and hexanedioic acid. The name '6,6' refers to the number of carbon atoms in each monomer (6 in the diamine, 6 in the diacid).
该方程式显示了由己二胺（hexane-1,6-diamine）和己二酸（hexanedioic acid）合成尼龙‑6,6 的过程。‘6,6’ 这一名称指的是每种单体中的碳原子数（二元胺中6个，二元酸中6个）。
7. Naming and Repeating Units of Polyamides | 聚酰胺的命名和重复单元
In polyamides, the repeat unit always contains the amide group –CONH–. For nylon‑6,6 the repeat unit is written as –NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–. Notice that the direction matters: one end is the amine nitrogen, the other the carbonyl carbon. When drawing the structure, make sure the amide group links the diamine residue to the diacid residue without breaking the chain.
在聚酰胺中，重复单元总是包含酰胺基团 –CONH–。对于尼龙‑6,6，重复单元写为 –NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–。注意方向很重要：一端是胺基氮，另一端是羰基碳。绘制结构时，确保酰胺基团将二元胺残基与二元酸残基连接起来而不打断主链。
- If a diacyl chloride is used, the by‑product is HCl, not H₂O. Example: n ClOC–(CH₂)₄–COCl + n H₂N–(CH₂)₆–NH₂ → –[–NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–]ₙ– + 2n HCl
- Other common polyamides include nylon‑6 (made from caprolactam, a ring‑opening polymerisation), but this is not strictly condensation; Cambridge focuses on nylon‑6,6 and Kevlar.
- 如果使用二酰氯，副产物是 HCl 而非 H₂O。示例：n ClOC–(CH₂)₄–COCl + n H₂N–(CH₂)₆–NH₂ → –[–NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–]ₙ– + 2n HCl
- 其他常见的聚酰胺包括尼龙‑6（由己内酰胺开环聚合制成），但这并非严格的缩聚；剑桥课程侧重于尼龙‑6,6 和凯夫拉。
8. Kevlar and Nylon-6,6 as Examples | 以凯夫拉和尼龙-6,6为例
Kevlar is an aromatic polyamide synthesised from benzene-1,4-diamine and benzene-1,4-dicarbonyl dichloride. The rigid aromatic rings and extensive hydrogen bonding between polymer chains give Kevlar exceptional tensile strength, making it suitable for bullet‑proof vests and composite materials.
凯夫拉是一种芳香族聚酰胺，由对苯二胺和对苯二甲酰氯合成。刚性的芳香环和聚合物链间广泛的氢键赋予凯夫拉卓越的抗拉强度，使其适用于防弹背心和复合材料。
n H₂N–C₆H₄–NH₂ + n ClOC–C₆H₄–COCl → –[–NH–C₆H₄–NH–CO–C₆H₄–CO–]ₙ– + 2n HCl
Nylon‑6,6, on the other hand, contains aliphatic chains and is more flexible. It is used widely in fabrics, carpets and mechanical parts. Both polymers form fibres because the linear chains can align and interact via intermolecular forces.
另一方面，尼龙‑6,6 含有脂肪链，柔韧性更好。它广泛用于织物、地毯和机械部件。这两种聚合物都能形成纤维，因为线性链可以通过分子间作用力排列并相互作用。
- Kevlar chains pack very tightly, with hydrogen bonds between adjacent amide groups.
- Nylon‑6,6 also forms hydrogen bonds, but the longer aliphatic segments provide elasticity.
- 凯夫拉链堆积非常紧密，相邻酰胺基团之间存在氢键。
- 尼龙‑6,6 也能形成氢键，但较长的脂肪链段提供了弹性。
9. Properties of Condensation Polymers | 缩聚物的性质
The properties of condensation polymers depend on the functional groups, chain length, and degree of crystallinity. Polyesters are generally tough, resistant to chemicals, and can be transparent (e.g. PET bottles). Polyamides have higher melting points and greater strength due to hydrogen bonding between the chains. Aramid fibres like Kevlar are flame‑resistant and have extremely high tensile strength.
缩聚物的性质取决于官能团、链长和结晶度。聚酯通常坚韧、耐化学腐蚀，并且可以是透明的（如 PET 瓶）。聚酰胺由于链间氢键而具有更高的熔点和更高的强度。像凯夫拉这样的芳纶纤维具有阻燃性和极高的抗拉强度。
| Polymer | Repeating Unit | Key Properties | Uses |
|---|---|---|---|
| PET | –O–CH₂CH₂–O–CO–C₆H₄–CO– | Clear, tough, good barrier to O₂ and CO₂ | Bottles, fibres (polyester fabric) |
| Nylon‑6,6 | –NH–(CH₂)₆–NH–CO–(CH₂)₄–CO– | High tensile strength, elastic, wear‑resistant | Ropes, carpets, clothing |
| Kevlar | –NH–C₆H₄–NH–CO–C₆H₄–CO– | Very high strength, heat resistant | Bullet‑proof vests, composites |
聚合物 | 重复单元 | 关键性质 | 用途
PET | –O–CH₂CH₂–O–CO–C₆H₄–CO– | 透明、坚韧、阻隔氧气和二氧化碳 | 瓶子、纤维（聚酯织物）
尼龙‑6,6 | –NH–(CH₂)₆–NH–CO–(CH₂)₄–CO– | 高抗拉强度、弹性好、耐磨 | 绳索、地毯、服装
凯夫拉 | –NH–C₆H₄–NH–CO–C₆H₄–CO– | 极高强度、耐热 | 防弹背心、复合材料
10. Hydrolysis of Condensation Polymers | 缩聚物的水解
Condensation polymers can be broken down by hydrolysis – the reverse of their formation. Polyesters are hydrolysed more easily than polyamides. Hot aqueous acid or alkali (NaOH) can cleave the ester linkages, regenerating the dicarboxylic acid (or its salt) and the diol. Polyamides require more vigorous conditions (prolonged heating with concentrated acid or base) to break the amide bonds.
缩聚物可以通过水解分解——即其形成的逆过程。聚酯比聚酰胺更容易水解。热的酸溶液或碱溶液（NaOH）可以切断酯键，再生成二元羧酸（或其盐）和二醇。聚酰胺需要更剧烈的条件（用浓酸或浓碱长时间加热）才能断裂酰胺键。
- Acid hydrolysis of a polyester yields the original dicarboxylic acid and diol.
- Alkaline hydrolysis yields the dicarboxylate salt and the diol.
- Hydrolysis is one reason why condensation polymers may be susceptible to environmental degradation.
- 聚酯的酸性水解生成原始的二元羧酸和二醇。
- 碱性水解生成二元羧酸盐和二醇。
- 水解是缩聚物易受环境降解的原因之一。
–[–O–R–O–CO–R'–CO–]ₙ– + 2n H₂O → n HO–R–OH + n HOOC–R'–COOH
11. Environmental Considerations and Biodegradability | 环境考虑与生物降解性
The stability of condensation polymers makes them durable but also contributes to plastic pollution. Polyesters and polyamides do not biodegrade readily because the ester and amide bonds are protected within crystalline regions and hydrophobic chains. However, aliphatic polyesters such as polylactic acid (PLA, derived from renewable resources) are designed to be compostable. Kevlar and nylon are highly resistant to degradation, which is both a strength and an environmental challenge.
缩聚物的稳定性使其经久耐用，但也加剧了塑料污染。聚酯和聚酰胺不易生物降解，因为酯键和酰胺键被包裹在结晶区和疏水链中。然而，脂肪族聚酯如聚乳酸（PLA，来源于可再生资源）被设计为可堆肥。凯夫拉和尼龙高度耐降解，这既是优点，也是环境挑战。
- PET can be recycled mechanically or chemically (glycolysis, methanolysis) to recover monomers.
- Green chemistry aims to develop condensation polymers from bio‑based monomers and those with built‑in degradable links.
- PET 可通过机械或化学方法（糖酵解、甲醇醇解）回收以回收单体。
- 绿色化学旨在开发来自生物基单体和含有内置可降解键的缩聚物。
12. Summary and Key Points | 总结与关键点
Condensation polymerisation is a powerful synthetic tool that couples difunctional monomers with the loss of a small molecule. Polyesters and polyamides dominate industrial polymers, offering a wide range of properties. In the examination, you must be able to write equations, draw repeat units, identify monomers from a polymer section, discuss properties in terms of intermolecular forces, and evaluate environmental aspects. Remember: difunctional monomers, step‑growth, elimination of H₂O or HCl, and the characteristic linkages –COO– (ester) and –CONH– (amide).
缩聚反应是一种强大的合成手段，它将双官能团单体偶联在一起并失去一个小分子。聚酯和聚酰胺主导着工业聚合物，提供了广泛的性能。在考试中，你必须能够书写方程式、画出重复单元、根据聚合物片段识别单体、用分子间作用力讨论性质，并评估环境方面。记住：双官能团单体、逐步增长、消除 H₂O 或 HCl，以及特征键 –COO–（酯）和 –CONH–（酰胺）。
- Condensation polymers eliminate a small molecule; addition polymers do not.
- Polyesters use diol + dicarboxylic acid (or diacyl chloride).
- Polyamides use diamine + dicarboxylic acid (or diacyl chloride).
- Kevlar and nylon‑6,6 are key examples.
- Hydrolysis breaks the polymer back into monomers.
- 缩聚物消除小分子；加聚物不消除。
- 聚酯由二醇 + 二元羧酸（或二酰氯）合成。
- 聚酰胺由二元胺 + 二元羧酸（或二酰氯）合成。
- 凯夫拉和尼龙‑6,6 是关键示例。
- 水解将聚合物分解回单体。
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