📚 A-Level Chemistry: Synthesis and Applications of Polyesters | A-Level 化学：聚酯的合成与应用
Polyesters are one of the most important classes of synthetic polymers, formed through condensation polymerisation between dicarboxylic acids and diols. This article explores their synthesis, key structural features, properties, and real-world applications, aligned with the CIE A-Level Chemistry syllabus.
聚酯是合成聚合物中最重要的类别之一，由二元羧酸和二元醇通过缩聚反应形成。本文围绕 CIE A-Level 化学考纲，深入探讨聚酯的合成、关键结构特征、性质及实际应用。
1. What Are Polyesters? | 什么是聚酯？
A polyester is a polymer whose repeating units are joined by ester linkages (–COO–). The ester bond forms between a carboxylic acid group (–COOH) on one monomer and an alcohol group (–OH) on another monomer, releasing a small molecule, usually water, in the process.
聚酯是一种重复单元通过酯键（–COO–）连接的聚合物。酯键在一个单体的羧基（–COOH）与另一个单体的羟基（–OH）之间形成，同时释放一个小分子，通常是水。
Because the monomers each contain two functional groups, the polymer chain can grow in both directions indefinitely, producing long, high-molecular-weight chains. This difunctional requirement is essential for linear polymer formation.
由于每个单体都含有两个官能团，聚合物链可以向两端无限延伸，从而形成长链的高分子量聚合物。这种双官能团要求是形成线型聚合物的必要条件。
- Key functional group: ester (–COO–) | 关键官能团：酯基（–COO–）
- Monomers: dicarboxylic acid + diol | 单体：二元羧酸 + 二元醇
- Process: condensation polymerisation | 过程：缩聚反应
2. The Mechanism of Condensation Polymerisation | 缩聚反应的机理
In condensation polymerisation, each step involves the reaction between two functional groups, forming a covalent bond and eliminating a small molecule. For polyesters, the small molecule eliminated is water (H₂O). The process is a step-growth mechanism, meaning that monomers first form dimers, then trimers, and gradually build up to long chains.
在缩聚反应中，每一步都涉及两个官能团之间的反应，形成共价键并消去一个小分子。对聚酯而言，消去的小分子是水（H₂O）。该过程属于逐步增长机理，单体先形成二聚体，再形成三聚体，逐渐增长为长链。
Unlike addition polymerisation, condensation polymerisation does not require an initiator. However, it often requires heat and a catalyst to drive the reaction to completion. The removal of water (or other small molecules) shifts the equilibrium toward the product, increasing the average chain length.
与加成聚合不同，缩聚反应不需要引发剂。然而，通常需要加热和催化剂以促进反应完全。去除水（或其他小分子）可使平衡向产物方向移动，从而提高平均链长。
n HOOC–R–COOH + n HO–R'–OH → [–OC–R–COO–R'–O–]ₙ + 2n H₂O
3. Key Monomers: Dicarboxylic Acids and Diols | 关键单体：二元羧酸与二元醇
Common dicarboxylic acids used in polyester synthesis include benzene-1,4-dicarboxylic acid (terephthalic acid) and ethanedioic acid (oxalic acid). The acid provides the –COOH groups that react with alcohol groups on both ends.
常用于合成聚酯的二元羧酸包括苯-1,4-二甲酸（对苯二甲酸）和乙二酸（草酸）。酸提供能与两端的醇羟基反应的 –COOH 基团。
Common diols include ethane-1,2-diol (ethylene glycol) and propane-1,3-diol. The diol provides two –OH groups, one at each end of the molecule, allowing chain extension in both directions.
常见的二元醇包括乙烷-1,2-二醇（乙二醇）和丙烷-1,3-二醇。二元醇在其分子两端各提供一个 –OH 基团，允许链向两个方向延展。
- Benzene-1,4-dicarboxylic acid (terephthalic acid) | 苯-1,4-二甲酸（对苯二甲酸）
- Ethane-1,2-diol (ethylene glycol) | 乙烷-1,2-二醇（乙二醇）
- Propane-1,3-diol | 丙烷-1,3-二醇
- Ethanedioic acid (oxalic acid) | 乙二酸（草酸）
4. Synthesis of PET: A Classic Example | PET 的合成：经典实例
Polyethylene terephthalate (PET) is the most common polyester, produced from benzene-1,4-dicarboxylic acid (terephthalic acid) and ethane-1,2-diol (ethylene glycol). PET is widely used in textile fibres, beverage bottles, and food packaging.
聚对苯二甲酸乙二醇酯（PET）是最常见的聚酯，由苯-1,4-二甲酸（对苯二甲酸）和乙烷-1,2-二醇（乙二醇）制得。PET 广泛用于纺织纤维、饮料瓶和食品包装。
The repeating unit of PET contains an ester linkage derived from the acid and the diol. The structure can be represented as:
PET 的重复单元包含由酸和醇形成的酯键。其结构可表示为：
[–OC–C₆H₄–COO–CH₂–CH₂–O–]ₙ
Under industrial conditions, the reaction is carried out at high temperature (around 250–280 °C) under vacuum, with antimony trioxide (Sb₂O₃) or titanium dioxide (TiO₂) as catalysts. The removal of ethylene glycol (by-product) drives the equilibrium forward.
在工业条件下，反应在高温（约 250–280 °C）和真空条件下进行，使用三氧化二锑（Sb₂O₃）或二氧化钛（TiO₂）作为催化剂。副产物乙二醇的移除推动平衡向正方向进行。
5. Structural Features of Polyesters | 聚酯的结构特征
The presence of polar ester groups (–COO–) along the polymer backbone significantly influences the physical properties of polyesters. The ester groups can form dipole–dipole interactions between adjacent chains, leading to moderate intermolecular forces.
聚合物主链上极性酯基（–COO–）的存在显著影响聚酯的物理性质。酯基可以在相邻链之间形成偶极–偶极相互作用，产生中等强度的分子间作用力。
In addition, the aromatic rings in PET contribute to chain rigidity, which increases the glass transition temperature (Tg) and improves mechanical strength. This is why PET is strong and dimensionally stable at room temperature.
此外，PET 中的芳香环增加了链的刚性，从而提高了玻璃化转变温度（Tg）并改善了机械强度。这就是 PET 在室温下强度高、尺寸稳定的原因。
- Polar ester groups → dipole–dipole forces | 极性酯基 → 偶极–偶极作用力
- Aromatic rings → chain rigidity and strength | 芳香环 → 链刚性与强度
- No hydrogen bonding (unless end groups) | 无氢键（除非端基）
- Thermoplastic behaviour: can be melted and reshaped | 热塑性行为：可熔融再塑
6. Chemical Properties: Hydrolysis of Polyesters | 化学性质：聚酯的水解
One of the key chemical properties of polyesters is their susceptibility to hydrolysis. In the presence of water and an acid or base catalyst, the ester linkages can be cleaved, breaking the polymer back into its monomer units or smaller fragments.
聚酯的关键化学性质之一是其对水解的敏感性。在水和酸或碱催化剂存在下，酯键可以被断裂，将聚合物还原为单体单元或较小的碎片。
The hydrolysis of PET under alkaline conditions is particularly important in recycling. When PET bottles are treated with aqueous sodium hydroxide at elevated temperatures, they undergo saponification to produce terephthalic acid and ethylene glycol, which can be recovered and reused.
在碱性条件下，PET 的水解对回收利用尤为重要。当 PET 瓶在高温下用氢氧化钠水溶液处理时，会发生皂化反应，生成对苯二甲酸和乙二醇，这两种物质可被回收再利用。
[–OC–C₆H₄–COO–CH₂–CH₂–O–]ₙ + 2n NaOH → n NaOOC–C₆H₄–COONa + n HO–CH₂–CH₂–OH
This depolymerisation process is chemically reversible in principle, but in practice requires harsh conditions to proceed at a useful rate.
这种解聚过程原则上可逆，但在实际应用中需要苛刻条件才能以有用速率进行。
7. Comparison: Polyesters vs Polyamides | 聚酯与聚酰胺的对比
The table below compares polyesters with polyamides, another major class of condensation polymers, highlighting differences relevant to the A-Level syllabus.
下表将聚酯与另一类重要的缩聚聚合物——聚酰胺——进行比较，突出与 A-Level 考纲相关的差异。
| Property | Polyester | Polyamide |
| Linkage | Ester (–COO–) | Amide (–CONH–) |
| Monomers | Dicarboxylic acid + diol | Dicarboxylic acid + diamine |
| By-product | Water | Water |
| Intermolecular forces | Dipole–dipole | Hydrogen bonding (stronger) |
| Example | PET | Nylon-6,6 |
Because polyamides can form hydrogen bonds between the N– f another, they tend to have higher melting points and greater tensile strength than comparable polyesters.
由于聚酰胺可以在一条链的 N–H 基团与另一条链的 C=O 基团之间形成氢键，它们通常比同类聚酯具有更高的熔点和更大的抗拉强度。
8. Applications of Polyesters | 聚酯的应用
Polyesters are incredibly versatile materials. PET is used to manufacture synthetic fibres such as Dacron and Terylene, which are woven into wrinkle-resistant fabrics for clothing, curtains, and upholstery. These fibres are strong, lightweight, and quick-drying.
聚酯是用途极广的材料。PET 被用于制造合成纤维，如大可纶（Dacron）和涤纶（Terylene），可织成抗皱织物，用于服装、窗帘和家具面料。这些纤维强度高、重量轻且快干。
PET is also the primary material for plastic beverage bottles. Its low gas permeability, transparency, and shatter resistance make it ideal for carbonated drinks and mineral water packaging. In addition, PET films are used in food packaging, electrical insulation, and photographic films.
PET 也是塑料饮料瓶的主要材料。其低透气性、透明性和抗碎性使其成为碳酸饮料和矿泉水包装的理想选择。此外，PET 薄膜用于食品包装、电绝缘和摄影胶卷。
- Textile fibres (clothing, carpets) | 纺织纤维（服装、地毯）
- Beverage bottles and food containers | 饮料瓶和食品容器
- Packaging films and sheets | 包装薄膜和片材
- Engineering plastics and medical devices | 工程塑料和医疗器械
9. Environmental Impact and Recycling | 环境影响与回收利用
The widespread use of polyesters, especially PET bottles, has raised significant environmental concerns. PET is not biodegradable, meaning it persists in the environment for hundreds of years. This has driven the development of recycling technologies.
聚酯，尤其是 PET 瓶的广泛使用，引发了重大的环境问题。PET 不可生物降解，这意味着它会在环境中存留数百年。这推动了回收技术的发展。
Two main recycling routes exist: mechanical recycling and chemical recycling. In mechanical recycling, PET is cleaned, melted, and reformed into new products. In chemical recycling, the polyester is depolymerised into its monomers, which are then purified and repolymerised, offering a closed-loop solution.
存在两条主要回收途径：机械回收和化学回收。在机械回收中，PET 被清洗、熔融并重新成型为新制品。在化学回收中，聚酯被解聚为单体，然后经过纯化并重新聚合，提供了一种闭环解决方案。
Biodegradable polyesters, such as polylactic acid (PLA), have also been developed as environmentally friendlier alternatives. PLA is produced from renewable resources like corn starch and can degrade under industrial composting conditions.
可生物降解聚酯，如聚乳酸（PLA），也被开发为更环保的替代品。PLA 由玉米淀粉等可再生资源生产，可在工业堆肥条件下降解。
10. Exam Tips and Common Pitfalls | 考试技巧与常见误区
When writing equations for condensation polymerisation in exams, students frequently forget to include the by-product molecule. For polyester synthesis, always write the correct stoichiometric number of water molecules – two per repeating unit – and balance both sides of the equation.
在考试中书写缩聚反应方程式时，学生经常忘记包括副产物小分子。对于聚酯合成，务必写出正确的化学计量数——每个重复单元对应两个水分子——并平衡方程两边。
Another common mistake is confusing condensation polymerisation with addition polymerisation. Remember that condensation involves monomers with two functional groups and produces a small molecule by-product; addition involves double bonds and produces no by-product.
另一个常见错误是混淆缩聚反应与加成聚合反应。请记住，缩聚涉及具有双官能团的单体并产生小分子副产物；加成聚合涉及双键且不产生副产物。
Finally, when asked to identify the monomers from a given polyester structure, locate the ester linkage (–COO–) and imagine "cleaving" it. The C–O bond breaks; the carbon side belongs to the acid monomer, and the oxygen side belongs to the alcohol monomer.
最后，当题目要求从给定的聚酯结构识别单体时，找到酯键（–COO–）并想象将其"切断"。C–O 键断裂；碳一侧属于酸单体，氧一侧属于醇单体。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图