📚 AS Chemistry: Polymers Revision Guide | AS 化学：聚合物 考点精讲
Polymers are large molecules made up of many repeating units called monomers. In AS Chemistry, you need to understand two main types of polymerisation: addition and condensation, how to draw polymer structures from monomers and vice versa, and the environmental impact of polymers. This revision guide will walk you through all the key concepts, common exam questions, and essential tips to ace your exams.
聚合物是由许多称为单体的重复单元组成的大分子。在 AS 化学中，你需要掌握两种主要的聚合类型：加成聚合和缩合聚合，如何从单体画出聚合物结构以及从聚合物推断单体，以及聚合物的环境影响。这份考点精讲将带领你逐一回顾核心概念、常见考题和应考重点。
1. What are Polymers? | 什么是聚合物？
A polymer is a long-chain molecule built from many small, identical units called monomers. The process of linking monomers together is called polymerisation. The repeating unit is the smallest part of the polymer that, when repeated, gives the whole structure. The number of repeating units is denoted by the subscript ‘n’, known as the degree of polymerization. Polymers can be natural (starch, proteins) or synthetic (plastics, nylon).
聚合物是由许多称为单体的相同小单元组成的长链分子。单体连接起来的过程称为聚合反应。重复单元是聚合物的最小部分，不断重复即构成整个结构。重复单元的数量用下标 n 表示，称为聚合度。聚合物可以是天然的（淀粉、蛋白质）或合成的（塑料、尼龙）。
Monomers must be able to form at least two bonds — for addition polymers, the monomer contains a C=C double bond, while for condensation polymers, monomers have two functional groups (e.g. dicarboxylic acid, diol).
单体必须能够形成至少两个键——对加成聚合物而言，单体含有 C=C 双键；对缩合聚合物而言，单体有两个官能团（例如二元羧酸、二元醇）。
2. Addition Polymerisation | 加成聚合
Addition polymerisation occurs when alkene monomers (containing C=C) join together without the loss of any atoms. The double bond opens up, and the monomers add to each other to form a saturated carbon backbone. Common alkene monomers include ethene (CH₂=CH₂), chloroethene (CH₂=CHCl), and phenylethene (styrene, CH₂=CHC₆H₅).
加成聚合发生时，含有 C=C 的烯烃单体相互加成，不损失任何原子。双键打开，单体彼此加成，形成饱和碳链主链。常见的烯烃单体包括乙烯 (CH₂=CH₂)、氯乙烯 (CH₂=CHCl) 和苯乙烯 (CH₂=CHC₆H₅)。
The general addition polymerisation equation for ethene is:
n CH₂=CH₂ → -[-CH₂-CH₂-]-ₙ
The product is poly(ethene) (polythene), used for plastic bags and bottles. For chloroethene, the polymer is poly(chloroethene) (PVC), used in pipes and window frames. The repeat unit is drawn with the side group still attached, and the bonds through the brackets must pass through the brackets to show the continuation of the chain.
产物是聚（乙烯）（聚乙烯），用于塑料袋和瓶子。氯乙烯的聚合物是聚（氯乙烯）(PVC)，用于管道和窗框。绘制重复单元时侧基仍然连接，括号间的键必须穿过括号以显示链的延续。
3. Drawing Addition Polymers | 绘制加成聚合物
When drawing the repeat unit of an addition polymer, you must show the opened double bond as a single bond in the backbone, with any side groups in their original positions. The bracket around the repeat unit must be followed by the subscript ‘n’. Two straight lines should extend from the brackets to indicate the connections to the rest of the chain. Do not draw end groups or dangling bonds without brackets — always encapsulate one repeat unit.
绘制加成聚合物的重复单元时，必须在主链中将已打开的双键表示为单键，侧基保持在原位。重复单元周围的括号后必须跟以下标 n。从括号向外伸出两条直线，表示与链其余部分的连接。不要画出端基或没有括号的悬挂键——始终只包含一个重复单元。
For example, poly(propene) from CH₃-CH=CH₂:
-[-CH(CH₃)-CH₂-]-ₙ
The methyl group is attached to a carbon in the backbone. Make sure all atoms have the correct number of bonds and that the repeat unit matches the monomer structure in a ‘head-to-tail’ fashion unless otherwise specified.
例如，由 CH₃-CH=CH₂ 得到的聚（丙烯）重复单元为 -[-CH(CH₃)-CH₂-]-ₙ。甲基连接到主链的一个碳上。确保所有原子具有正确的键数，且重复单元按“头-尾”方式连接，除非题目另有说明。
4. Condensation Polymerisation | 缩合聚合
Condensation polymerisation involves monomers with two functional groups that react together, eliminating a small molecule such as water or hydrogen chloride for each new bond formed. The two monomers can be the same (if one monomer carries both types of functional group) or, more commonly, two different monomers are used. The reaction forms a long chain with ester or amide linkages, along with the small by-product.
缩合聚合涉及带有两个官能团的单体，它们相互反应，每形成一个新键会脱去一个如 H₂O 或 HCl 的小分子。两种单体可以相同（若一种单体同时携带两种类型的官能团），但更常见的是使用两种不同的单体。反应形成带有酯键或酰胺键的长链，同时产生小分子副产物。
The two most important types of condensation polymers in AS Chemistry are polyesters and polyamides. Polyesters form from dicarboxylic acids (or diacyl chlorides) and diols; polyamides form from dicarboxylic acids and diamines. Both require monomers with two identical functional groups at each end.
AS 化学中最重要的两种缩聚物是聚酯和聚酰胺。聚酯由二元羧酸（或二元酰氯）和二元醇形成；聚酰胺由二元羧酸和二元胺形成。两者都需要两端各带有一个相同官能团的单体。
5. Polyesters | 聚酯
Polyesters are formed by the reaction between a dicarboxylic acid and a diol. Each ester link (-COO-) is formed when the -OH from the carboxylic acid group and the -H from the alcohol group are eliminated as water. A well-known polyester is poly(ethylene terephthalate) (PET), made from benzene-1,4-dicarboxylic acid (terephthalic acid) and ethane-1,2-diol.
聚酯由二元羧酸和二元醇反应生成。每个酯键 (-COO-) 由羧基的 -OH 和醇羟基的 -H 脱去一分子水而形成。著名的聚酯是聚对苯二甲酸乙二醇酯 (PET)，由对苯二甲酸和乙二醇制得。
n HOOC-C₆H₄-COOH + n HO-CH₂-CH₂-OH → -[-OC-C₆H₄-COO-CH₂-CH₂-O-]-ₙ + 2n H₂O
The repeat unit of PET contains an ester linkage. When identifying the monomers from PET, cut the C-O bond on the ester group and add H to the O of the alcohol part and OH to the C=O part to regenerate the acid and the diol.
PET 的重复单元含有一个酯键。从 PET 中辨认单体时，切断酯基中 C-O 键，在醇部分的 O 上加 H，在 C=O 部分的 C 上加 OH，即可重新生成酸和二醇。
6. Polyamides | 聚酰胺
Polyamides contain the amide (peptide) linkage -CONH-. They are produced from a dicarboxylic acid and a diamine, with the elimination of water. The most common example is Nylon-6,6, formed from hexanedioic acid (adipic acid) and 1,6-diaminohexane (hexamethylenediamine). The ‘6,6’ refers to the number of carbon atoms in each monomer.
聚酰胺含有酰胺（肽）键 -CONH-。它们由二元羧酸和二元胺反应生成，同时脱去水。最常见的例子是 Nylon-6,6，由己二酸和 1,6-己二胺制成。“6,6” 指每种单体中的碳原子数。
n HOOC-(CH₂)₄-COOH + n H₂N-(CH₂)₆-NH₂ → -[-OC-(CH₂)₄-CONH-(CH₂)₆-NH-]-ₙ + 2n H₂O
Another well-known polyamide is Kevlar, made from benzene-1,4-dicarboxylic acid and 1,4-diaminobenzene, which gives very strong fibres used in bulletproof vests. The structure of polyamides resembles that of protein chains, which are natural polyamides.
另一种著名的聚酰胺是凯夫拉 (Kevlar)，由对苯二甲酸和对苯二胺制成，其纤维强度极高，用于防弹背心。聚酰胺的结构类似于天然聚酰胺——蛋白质链。
7. From Polymer to Monomer | 从聚合物推断单体
Exam questions frequently ask you to deduce the monomers from a given section of a polymer chain. For addition polymers, identify the repeating unit, then re-insert the double bond between the two carbon atoms that form the backbone. Make sure all side groups and hydrogen atoms are added correctly to give the original alkene.
考试中经常会要求你从给定的聚合物片段推断出单体。对于加成聚合物，先辨认重复单元，然后在构成主链的两个碳原子之间重新插入双键。确保所有侧基和氢原子正确添加，得到原来的烯烃。
For condensation polymers, look for the ester or amide links. Break the bond between the C-O (for ester) or C-N (for amide) in the linkage. Then, add OH to the C=O carbon to form the carboxylic acid group, and add H to the O (or N) to form the alcohol (or amine) group. If the two monomers are different, you will obtain a diacid and a diol (or diamine). Always check that each monomer you derive contains two functional groups.
对于缩聚物，寻找酯键或酰胺键。切断连接中的 C-O（酯）或 C-N（酰胺）键。接着，在 C=O 碳上加 OH 以形成羧基，在 O（或 N）上加 H 以形成醇（或胺）基。如果两种单体不同，你会得到一种二元酸和一种二元醇（或二元胺）。务必检查你推导出的每种单体都含有两个官能团。
8. Thermoplastic vs Thermosetting | 热塑性 vs 热固性
Polymers can be classified as thermoplastics or thermosetting plastics based on their response to heat. Thermoplastics consist of long, linear chains held together by weak intermolecular forces. When heated, these forces are overcome, the chains can slide past each other, and the plastic softens and can be remoulded. Common thermoplastics include poly(ethene), poly(propene) and PVC.
根据对热的反应，聚合物可分为热塑性塑料和热固性塑料。热塑性塑料由弱分子间力结合的长线型链组成。加热时，这些力被克服，链可以相互滑动，塑料软化并可重新塑形。常见的热塑性塑料包括聚乙烯、聚丙烯和 PVC。
Thermosetting plastics have strong covalent cross-links between polymer chains, forming a rigid three-dimensional network. Once set during the initial moulding process, they cannot be softened by heating; further heating causes decomposition. Examples include Bakelite (used in electrical sockets) and epoxy resins. In summary:
热固性塑料在聚合物链之间有强共价交联，形成刚性的三维网络。一旦在初始成型过程中定型，就不能通过加热软化；继续加热会导致分解。例如电木（用于电插座）和环氧树脂。概括对比如下：
| Property / 性质 | Thermoplastic / 热塑性 | Thermosetting / 热固性 |
|---|---|---|
| Chain structure / 链结构 | Linear, little/no cross-linking / 线型，极少或无交联 | Heavily cross-linked covalent network / 高度交联的共价网络 |
| Effect of heat / 加热效果 | Softens, can be remoulded / 软化，可重塑 | Does not soften, chars or burns / 不软化，碳化或燃烧 |
| Examples / 例子 | Poly(ethene), PVC, nylon (as fibre) / 聚乙烯，PVC，尼龙纤维 | Bakelite, melamine, epoxy resins / 电木，三聚氰胺，环氧树脂 |
9. Properties and Uses | 性质与用途
The physical properties of a polymer depend on its chain length, the presence of side groups, and the degree of crystallinity. Short branches lead to low-density poly(ethene) (LDPE) which is flexible and used for carrier bags, while unbranched chains give high-density poly(ethene) (HDPE) which is stronger and used for containers. The polar C-Cl bonds in PVC make it rigid and flame-retardant, ideal for pipes and window frames.
聚合物的物理性质取决于链长、侧基的存在以及结晶度。短支链产生低密度聚乙烯 (LDPE)，它柔韧，用于塑料袋；无支链则得到高密度聚乙烯 (HDPE)，它强度更高，用于容器。PVC 中的极性 C-Cl 键使其刚硬且阻燃，是管道和窗框的理想材料。
Polyesters like PET are strong, lightweight and impermeable to gases, which makes them perfect for drinks bottles and textile fibres (Terylene). Nylon has high tensile strength and abrasion resistance, so it is used in ropes, clothing and engineering plastics. The intermolecular forces (van der Waals, dipole-dipole, and hydrogen bonds in polyamides) determine the melting point and mechanical strength.
像 PET 这样的聚酯强度高、重量轻且不透气，使其成为饮料瓶和纺织纤维（涤纶）的理想选择。尼龙具有高拉伸强度和耐磨性，因此用于绳索、服装和工程塑料。分子间力（范德华力、偶极-偶极力以及聚酰胺中的氢键）决定了熔点和机械强度。
10. Environmental Issues and Biodegradability | 环境问题与生物降解性
Most synthetic polymers are non-biodegradable because the long carbon-carbon chains are resistant to microbial breakdown. Discarded plastics persist in the environment, leading to pollution of land and oceans. Incineration of certain polymers produces toxic gases — for instance, burning PVC releases hydrogen chloride (HCl), and incomplete combustion can give dioxins. Recycling is one solution, but mixed polymers are difficult to separate and reprocess.
大多数合成聚合物不可生物降解，因为长碳碳链能抵抗微生物分解。废弃的塑料持久存在于环境中，导致陆地和海洋污染。焚烧某些聚合物会产生有毒气体——例如，燃烧 PVC 会释放氯化氢 (HCl)，而不完全燃烧可能产生二噁英。回收是一种解决方案，但混合聚合物难以分离和再加工。
Biodegradable polymers have been developed to address these problems. Poly(lactic acid) (PLA) is made from renewable resources such as corn starch; it contains ester linkages that can be hydrolysed by microorganisms. Other biodegradable polyesters and starch-based blends are used for packaging and medical sutures. However, they often have inferior mechanical properties and require specific conditions (e.g. industrial composting) to degrade efficiently.
为解决这些问题，人们开发了可生物降解聚合物。聚乳酸 (PLA) 由玉米淀粉等可再生资源制成，它含有可被微生物水解的酯键。其他可生物降解聚酯和淀粉基共混物被用于包装和医用缝合线。然而，它们的力学性能通常较差，并且需要特定条件（如工业堆肥）才能有效降解。
Exam questions often link polymer structures to environmental impact: you should be able to argue why addition polymers with pure C-C backbones persist, while polyester or polyamide chains can undergo hydrolysis, making them more biodegradable. Also be ready to discuss the pros and cons of landfill, incineration with energy recovery, and feedstock recycling.
考试题目常将聚合物结构与环境影响联系起来：你应该能够阐述为何纯 C-C 主链的加成聚合物能长期存留，而聚酯或聚酰胺链可以发生水解，使其更易生物降解。同时准备好讨论填埋、焚烧回收能量以及原料回收的利弊。
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