📚 Polyesters: Drawing Structural and Displayed Formulae | 聚酯：绘制结构式与显示式
Polyesters are an important class of condensation polymers formed from diols and dicarboxylic acids. In Edexcel IGCSE Chemistry (specifically point 4.49), you need to be able to draw both the structural and displayed formulae of a polyester, following the pattern of the given repeating unit. This skill relies on understanding how monomers join through ester linkages and how to represent the long-chain molecule clearly in diagrams.
聚酯是一类重要的缩聚物，由二醇与二羧酸反应制得。在 Edexcel IGCSE 化学（考点 4.49）中，你需要能够按照给定重复单元的模式，画出聚酯的结构式和显示式。这项技能取决于你是否理解单体如何通过酯键连接，以及如何在图示中清晰地表示长链分子。
1. What Are Polyesters? | 什么是聚酯？
Polyesters are polymers made by condensation polymerisation. Each time two monomers join, a small molecule, usually water, is eliminated. The backbone of a polyester contains the ester functional group -COO- that repeats along the chain. The monomers are a diol (a molecule with two -OH groups) and a dicarboxylic acid (a molecule with two -COOH groups).
聚酯是通过缩聚反应制成的聚合物。每两个单体连接时，会脱去一个小分子（通常是水）。聚酯的主链上含有重复出现的酯官能团 -COO-。单体是一种二醇（含两个 -OH 基团的分子）和一种二羧酸（含两个 -COOH 基团的分子）。
2. Monomers: Diols and Dicarboxylic Acids | 单体：二醇与二羧酸
A diol has the general formula HO-R-OH, where R is a hydrocarbon chain. The simplest example is ethane-1,2-diol, HO-CH₂-CH₂-OH. A dicarboxylic acid has the formula HOOC-R'-COOH. A common aromatic diacid is benzene-1,4-dicarboxylic acid, HOOC-C₆H₄-COOH, where the two carboxyl groups are opposite each other on a benzene ring.
二醇的通式为 HO-R-OH，其中 R 是一个烃链。最简单的例子是乙二醇 (ethane-1,2-diol)，HO-CH₂-CH₂-OH。二羧酸的通式为 HOOC-R'-COOH。常见的芳香族二酸是对苯二甲酸 (benzene-1,4-dicarboxylic acid)，HOOC-C₆H₄-COOH，两个羧基在苯环上处于对位。
3. The Ester Linkage and Condensation Reaction | 酯键与缩合反应
When an -OH group from the diol reacts with a -COOH group from the diacid, an ester linkage (-COO-) forms and a water molecule is released. Because each monomer has two functional groups, the reaction can continue at both ends, building a long chain. This is the principle of condensation polymerisation for polyesters.
当二醇的一个 -OH 基团与二酸的一个 -COOH 基团反应时，会形成酯键 (-COO-)，并释放出一分子水。由于每个单体都有两个官能团，反应可以在两端持续进行，从而形成长链。这就是聚酯缩聚反应的基本原理。
4. Repeating Units in Polyester Chains | 聚酯链中的重复单元
The repeating unit of a polyester is the smallest segment that reflects the full structure of the polymer. It must contain one ester linkage derived from the diol and one from the diacid. For PET, the repeating unit can be written as: ―O―CH₂―CH₂―O―CO―C₆H₄―CO― . Notice how the ester group appears twice: once as -CO- from the acid and one -O- from the alcohol, linked to form -COO-.
聚酯的重复单元是能够反映聚合物完整结构的最小链段。它必须包含一个来自二醇的酯键部分和一个来自二酸的部分。对于 PET，重复单元可以写作：―O―CH₂―CH₂―O―CO―C₆H₄―CO― 。注意酯基出现了两次：一个来自酸的 -CO- 和一个来自醇的 -O-，连接成 -COO-。
5. How to Draw a Displayed Formula | 如何绘制显示式
ining carbon skeleton. Make sure to indicate that the unit continues on both sides, usually by showing bonds extending beyond brackets.
显示式要画出所有的原子和共价键。要绘制聚酯的重复单元，可以先画出二醇的部分，标出每一个 C-H 和 C-O 键，然后连接二酸部分，标出 C=O 键以及剩余的碳骨架。一定要在两端标出延伸的键，通常用方括号外的短横线表示链的延续。
6. Step-by-Step Example: PET | 步骤示例：PET
PET is made from ethane-1,2-diol and benzene-1,4-dicarboxylic acid. The displayed formula of the repeating unit is often drawn as: ―O―CH₂―CH₂―O―C(=O)―C₆H₄―C(=O)― nzene ring is para-substituted, meaning the connecting bonds are at opposite vertices. Don't forget the extension bonds on oxygen at the left and on the carbon at the right.
PET 由乙二醇和对苯二甲酸制成。其重复单元的显示式通常画成：―O―CH₂―CH₂―O―C(=O)―C₆H₄―C(=O)― 。展开时：画出 O-CH₂-CH₂-O 及所有 H 原子，然后画羰基 C=O，再画苯环（六边形内加圆圈或交替双键均可），接着再画一个 C=O。苯环为对位取代，即两个连接键位于相对的顶点。别忘了在左侧氧原子和右侧碳原子上画出延伸键。
7. Drawing Structural Formulae: Omitting Some Bonds | 绘制结构式：省略部分键
A structural formula simplifies the drawing by omitting bonds between carbon and hydrogen atoms, grouping them as -CH₂- or -C₆H₄-. For the PET repeating unit, the structural formula can be written as -OCH₂CH₂OOC-C₆H₄-COOCH₂CH₂O- (if showing two units) or simply -[OCH₂CH₂OCOC₆H₄CO]-. This is the style often required in IGCSE answers: brackets for the repeating unit, with the structure written horizontally.
结构式通过省略碳氢键来简化绘图，将 C 和 H 表示为 -CH₂- 或 -C₆H₄-。对于 PET 重复单元，结构式可写作 -OCH₂CH₂OOC-C₆H₄-COOCH₂CH₂O-（如果表示两个单元）或简单地写作 -[OCH₂CH₂OCOC₆H₄CO]- 。这正是 IGCSE 答案中常要求的式样：用方括号括起重复单元，结构沿水平方向书写。
8. Common Mistakes to Avoid | 常见错误要避免
One frequent mistake is forgetting that the ester group has two oxygen atoms: one from the acid and one from the alcohol. Always show -COO- or -OOC- with three consecutive atoms. Another error is drawing only one -COOH and one -OH reacting, resulting in a simple ester rather than a polymer. Also, check that the number of carbon atoms in the repeating unit matches the sum of carbons from both monomers minus the atoms lost in condensation (none). Finally, ensure the benzene ring, if present, is correctly oriented with para substitution.
一个常见错误是忘记酯基含有两个氧原子：一个来自酸，一个来自醇。务必将 -COO- 或 -OOC- 的三个原子画在一起。另一个错误是只画出一个 -COOH 和一个 -OH 反应，导致生成了普通酯而非聚合物。还要检查重复单元中的碳原子数是否等于两个单体碳原子数之和（缩聚中碳没有丢失）。如果存在苯环，要确保对位取代方向正确。
9. Identifying the Repeating Unit from a Given Polymer Section | 从聚合物片段中识别重复单元
The exam may present a short section of a polyester chain and ask you to draw the repeating unit. Look for the pattern that repeats: find the ester linkage -COO- and then trace one complete diol–diacid sequence. The repeating unit should start at the oxygen atom attached to the diol carbon and end just before the next identical oxygen atom in the chain.
考试可能会给出聚酯链的一个短片段，要求你画出重复单元。找出重复的规律：先找到酯键 -COO-，然后回溯一个完整的“二醇–二酸”序列。重复单元的起始点通常是连接在二醇碳上的氧原子，终点是链中下一个相同氧原子之前。
10. Practice with Other Diols and Diacids | 用其他二醇和二酸练习
To master this skill, practise using different monomers, such as propane-1,3-diol (HOCH₂CH₂CH₂OH) with ethanedioic acid (HOOCCOOH). The repeating unit will be -[OCH₂CH₂CH₂OCOCO]-. Notice how the pattern – diol segment then diacid segment – stays the same. For aromatic polyesters, you might meet benzene-1,3-dicarboxylic acid, but IGCSE normally uses the 1,4- isomer. Drawing several examples will build your confidence.
要掌握这项技能，可以练习使用不同单体，比如 1,3-丙二醇 (HOCH₂CH₂CH₂OH) 与乙二酸 (HOOCCOOH)。其重复单元为 -[OCH₂CH₂CH₂OCOCO]-。请注意“二醇链段–二酸链段”的固有规律仍然不变。对于芳香族聚酯，你可能会遇到间苯二甲酸，但 IGCSE 通常使用对位异构体。多画几个例子有助于增强信心。
11. Summary and Exam Tips | 总结与考试技巧
Polyesters are condensation polymers with ester linkages in the backbone. The repeating unit must contain one diol and one diacid segment. When drawing displayed formulae, show all atoms and bonds clearly, with extension bonds at each end. Structural formulae can be written as -[O-R-O-CO-R'-CO]- inside brackets. In the exam, always label your brackets and explicitly draw the water molecules lost in the equation if asked for the polymerisation reaction.
聚酯是主链上含有酯键的缩聚物，重复单元必须包含一个二醇链段和一个二酸链段。画显示式时，要清晰地标出所有原子和键，并在两端画出延伸键。结构式可在方括号内写成 -[O-R-O-CO-R'-CO]- 的形式。考试中如果要求写出聚合反应，一定要在方程式中明确画出脱去的水分子，并标注方括号。
Published by TutorHao | Chemistry Revision Series | aleveler.com
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