IB Chemistry: Polymers Key Points Review | IB 化学：聚合物考点精讲

📚 IB Chemistry: Polymers Key Points Review | IB 化学：聚合物考点精讲

Polymers are giant molecules made from many repeating units called monomers. In IB Chemistry, you need to understand how they form, classify them, relate their properties to their structures, and consider their environmental impact. This guide covers every essential topic from addition and condensation polymerisation to thermoplastics, thermosets, biodegradable polymers, and common examination pitfalls.

聚合物是由许多称为单体的重复单元构成的巨型分子。在 IB 化学中，你需要理解它们如何形成、如何分类、将性质与结构联系起来，并思考它们对环境的影响。本指南涵盖从加成聚合和缩合聚合到热塑性塑料、热固性塑料、可生物降解聚合物以及常见考试陷阱的每一个核心主题。

1. Introduction to Polymers | 聚合物简介

A polymer (from Greek poly- “many” and meros “part”) is a long-chain molecule with a high relative molecular mass. The individual small molecules that link together to form the polymer are called monomers. The process of joining monomers is polymerisation. Polymers can be natural, like cellulose and proteins, or synthetic, like polyethylene and nylon.

聚合物（源自希腊语 poly- “多” 和 meros “部分”）是相对分子质量很高的长链分子。连接在一起形成聚合物的小分子称为单体。单体的连接过程称为聚合。聚合物可以是天然的，如纤维素和蛋白质，也可以是合成的，如聚乙烯和尼龙。

Most synthetic polymers are carbon-based; the monomer usually contains a carbon-carbon double bond or two functional groups that can react with each other. In IB, we focus on the structure-property relationship: why low-density polyethene is flexible while high-density polyethene is rigid, or why Kevlar is bullet-resistant.

大多数合成聚合物是以碳为基础的；单体通常含有一个碳碳双键或两个可以相互反应的官能团。在 IB 中，我们重点考察结构与性质的关系：为什么低密度聚乙烯柔韧而高密度聚乙烯刚硬，为什么凯夫拉可以防弹。

2. Addition Polymerisation | 加成聚合

Addition polymerisation occurs when unsaturated monomers (containing C=C bonds) join together without the loss of any atoms. The double bond opens up, and monomers add to the growing chain. A typical example is the polymerisation of ethene to form poly(ethene), commonly known as polythene.

加成聚合发生在含有 C=C 双键的不饱和单体连接在一起而没有原子损失的情况下。双键打开，单体不断加成到增长的链上。典型的例子是乙烯聚合生成聚（乙烯），通常称为聚乙烯。

The general equation can be represented as: n CH₂=CH₂ → [─CH₂─CH₂─]ₙ. The repeating unit is shown inside square brackets with extending bonds. It is essential to draw the repeating unit correctly in exams: show the carbon backbone, the side groups (like H, Cl, CH₃), and the continuation bonds at both ends.

一般方程式可表示为：n CH₂=CH₂ → [─CH₂─CH₂─]ₙ。重复单元在方括号内标出，两端各有一个延伸键。在考试中正确画出重复单元至关重要：显示碳骨架、侧基（如 H、Cl、CH₃），以及两端的连接键。

Addition polymers are usually thermoplastics because the chains are held together by weak intermolecular forces, allowing them to soften when heated and harden when cooled. This cycle can be repeated.

加成聚合物通常是热塑性塑料，因为链与链之间通过弱分子间作用力结合，受热软化、冷却硬化，这一循环可重复进行。

3. Condensation Polymerisation | 缩合聚合

Condensation polymerisation involves the joining of monomers with two functional groups, accompanied by the elimination of a small molecule, often water or hydrogen chloride. Each monomer must have two reactive sites; for example, a dioic acid and a diol form a polyester by eliminating water.

缩合聚合是指带有两个官能团的单体进行连接，同时脱去一个小分子（通常是水或氯化氢）的过程。每个单体必须有两个反应位点；例如，二元酸和二元醇通过脱去水形成聚酯。

The formation of polyesters and polyamides is a central topic. Ethane-1,2-diol and benzene-1,4-dicarboxylic acid react to form the polyester PET. 1,6-diaminohexane and hexanedioic acid form nylon 6,6 through amide linkages, with water released. The repeating unit includes the ester or amide linkage.

聚酯和聚酰胺的形成是核心主题。乙二醇和对苯二甲酸反应生成聚酯 PET。1,6-己二胺和己二酸通过酰胺键形成尼龙 66，同时脱去水分子。重复单元中包含酯键或酰胺键。

Condensation polymers often have stronger intermolecular forces (hydrogen bonding in polyamides) than addition polymers, giving them higher melting points and greater tensile strength. They can be thermoplastics or thermosets depending on the degree of cross-linking.

缩合聚合物通常比加成聚合物具有更强的分子间作用力（聚酰胺中有氢键），因此熔点更高、抗拉强度更大。根据交联程度的不同，它们可以是热塑性塑料或热固性塑料。

4. Homopolymers vs. Copolymers | 均聚物与共聚物

A homopolymer is made from only one type of monomer. For example, poly(propene) comes entirely from propene monomers. Most simple addition polymers are homopolymers. The structure and properties are determined by the nature of that single monomer and the stereochemistry of the chain.

均聚物由一种单体构成。例如，聚丙烯完全由丙烯单体合成。大多数简单的加成聚合物都是均聚物。其结构和性质取决于单体的性质和链的立体化学。

A copolymer is formed when two or more different monomers are polymerised together. An important example is ABS (acrylonitrile-butadiene-styrene), which combines the toughness of polybutadiene with the hardness of styrene-acrylonitrile. Nylon can also be made as a copolymer by mixing different diamines or diacids.

共聚物由两种或多种不同单体共同聚合而成。一个重要的例子是 ABS（丙烯腈-丁二烯-苯乙烯共聚物），它结合了聚丁二烯的韧性和苯乙烯-丙烯腈的硬度。尼龙也可通过混合不同的二胺或二酸制成共聚物。

Copolymers can be classified as random, alternating, block, or graft, based on how the monomers are arranged along the chain. In IB, you mainly need to recognise that varying monomers alters the material’s properties widely, which is useful for designing plastics with specific flexibility, transparency, or heat resistance.

根据单体在链上的排列方式，共聚物可分为无规、交替、嵌段或接枝共聚物。在 IB 中，你主要需要认识到改变单体会极大地改变材料的性质，这对于设计具有特定柔韧性、透明度或耐热性的塑料非常有用。

5. Thermoplastics and Thermosets | 热塑性塑料与热固性塑料

Thermoplastics consist of linear or slightly branched polymer chains held by weak intermolecular forces (van der Waals, dipole-dipole, or hydrogen bonds). Upon heating, these forces are overcome, the chains can slide past each other, and the material softens. Cooling allows the forces to re-form, returning hardness. This melting-resolidifying cycle enables recycling.

热塑性塑料由线性或轻度支化的聚合物链通过弱分子间作用力（范德华力、偶极-偶极或氢键）结合在一起。加热时，这些作用力被克服，链可相互滑动，材料软化。冷却时作用力重新形成，恢复硬度。这种熔融-再凝固的循环使其能够回收利用。

Thermosets have extensive cross-links (covalent bonds) between polymer chains, forming a three-dimensional rigid network. Once set, they cannot be remelted; heating causes decomposition rather than softening. Examples include Bakelite (phenol-formaldehyde resin) and epoxy resins. They are hard, infusible, and used in high-heat applications.

热固性塑料在聚合物链之间具有大量的交联（共价键），形成三维刚性网络。一旦固化，它们不能重新熔融；加热会导致分解而非软化。例子包括电木（酚醛树脂）和环氧树脂。它们坚硬、不熔，用于高温应用。

In the exam, you must link structure to thermal behaviour: linear chains = thermoplastic; network cross-links = thermoset. The difference determines whether a polymer can be recycled mechanically, which is an environmental concern addressed in Option B or the core curriculum.

在考试中，你必须将结构与受热行为联系起来：线性链 = 热塑性；网络交联 = 热固性。这种差异决定了聚合物是否能被机械回收，这是选修部分B或核心课程中涉及的环境问题。

6. Addition Polymers: Key Examples | 加成聚合物：关键例子

The IB syllabus expects you to know the monomers, repeating units, and typical uses of common addition polymers. The table below summarises the most important ones. Remember that the monomer name always starts with the alkene (ethene, propene, etc.) and the polymer name uses “poly(alkene)”.

IB 课程大纲要求你掌握常见加成聚合物的单体、重复单元和典型用途。下表总结了最重要的几种。请记住，单体名称总是以烯烃（乙烯、丙烯等）开头，聚合物名称使用“聚（烯烃）”。

Polymer	Monomer	Repeating Unit	Uses
Poly(ethene) LDPE	CH₂=CH₂	─CH₂─CH₂─	Plastic bags, films
Poly(ethene) HDPE	CH₂=CH₂	─CH₂─CH₂─ (linear)	Bottles, pipes
Poly(propene)	CH₂=CHCH₃	─CH₂─CH(CH₃)─	Ropes, carpets, containers
Poly(chloroethene) PVC	CH₂=CHCl	─CH₂─CHCl─	Pipes, window frames
Poly(tetrafluoroethene) PTFE	CF₂=CF₂	─CF₂─CF₂─	Non-stick coatings
Poly(styrene) PS	CH₂=CHC₆H₅	─CH₂─CH(C₆H₅)─	Packaging, insulation

A common mistake is drawing the repeating unit with a double bond still present. Polymerisation saturates the backbone, so the repeating unit must show single C–C bonds and side groups correctly attached. Also, when asked for an equation, always use “n” before the monomer and the product with brackets and subscript n, e.g., n CH₂=CHCl → [─CH₂─CHCl─]ₙ.

一个常见错误是画重复单元时仍保留了双键。聚合使主链饱和，因此重复单元必须显示单键 C–C，侧基正确连接。此外，当要求写方程式时，总是在单体前使用“n”，产物使用方括号和下标 n，例如 n CH₂=CHCl → [─CH₂─CHCl─]ₙ。

7. Condensation Polymers: Polyesters and Polyamides | 缩合聚合物：聚酯与聚酰胺

The formation of a polyester requires a diol (two –OH groups) and a dioic acid (two –COOH groups) or a diacyl dichloride (two –COCl groups). Each ester linkage is formed with the elimination of H₂O (or HCl when using acid chlorides). PET, poly(ethylene terephthalate), is the most famous example, used in drink bottles and clothing fibres.

聚酯的形成需要一种二元醇（两个 –OH 基团）和一种二元酸（两个 –COOH 基团）或一种二酰氯（两个 –COCl 基团）。每形成一个酯键，会脱去一个 H₂O（如果使用酰氯则脱去 HCl）。PET（聚对苯二甲酸乙二酯）是最著名的例子，用于饮料瓶和服装纤维。

Polyamides are formed from a diamine (two –NH₂ groups) and a dioic acid or diacyl dichloride. The link is an amide group –CONH–. The most common are nylon 6,6 (from hexanedioic acid and 1,6-diaminohexane) and Kevlar (from benzene-1,4-diamine and benzene-1,4-dicarboxylic acid). Water is the by-product; using acid chlorides gives faster reactions and HCl.

聚酰胺由二元胺（两个 –NH₂ 基团）与二元酸或二酰氯形成。连接键是酰胺基团 –CONH–。最常见的是尼龙 66（由己二酸和 1,6-己二胺制得）和凯夫拉（由对苯二胺和对苯二甲酸制得）。副产物是水；使用酰氯可获得更快的反应速率并生成 HCl。

The equation for nylon 6,6 formation: n H₂N–(CH₂)₆–NH₂ + n HOOC–(CH₂)₄–COOH → [–NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–]ₙ + 2n H₂O. Notice the amide linkage and the loss of water. When drawing repeating units, show the amide or ester group in the backbone, not as a side chain.

尼龙 66 的形成方程式：n H₂N–(CH₂)₆–NH₂ + n HOOC–(CH₂)₄–COOH → [–NH–(CH₂)₆–NH–CO–(CH₂)₄–CO–]ₙ + 2n H₂O。注意酰胺键和水分子的脱去。画重复单元时，应将酰胺或酯基团画在主链上，而不是作为侧链。

Compare the properties: polyesters often have dipole-dipole interactions, while polyamides exhibit strong hydrogen bonding between chains (due to –NH– and C=O). Hence polyamides like nylon have high melting points, good tensile strength, and are widely used as engineering plastics and fibres.

性质比较：聚酯通常具有偶极-偶极相互作用，而聚酰胺在链间存在强氢键（由于 –NH– 和 C=O）。因此尼龙这类聚酰胺具有高熔点、良好的抗拉强度，被广泛用作工程塑料和纤维。

8. Biodegradable Polymers | 可生物降解聚合物

The accumulation of non-biodegradable synthetic polymers in the environment is a major concern. Biodegradable polymers can be broken down by microorganisms into water, CO₂, and biomass. IB discusses two main types: biopolyesters like poly(lactic acid) (PLA) and polyhydroxyalkanoates (PHA), and starch-based polymers.

不可生物降解的合成聚合物在环境中的积累是一个重大问题。可生物降解聚合物能被微生物分解成水、CO₂ 和生物质。IB 讨论两类主要类型：生物聚酯，如聚乳酸 (PLA) 和聚羟基链烷酸酯 (PHA)，以及基于淀粉的聚合物。

PLA is derived from renewable resources such as corn starch. The monomer is lactic acid, which has both –OH and –COOH groups, enabling condensation polymerisation. PLA is used for compostable cutlery, medical implants, and packaging films. The ester linkages in the backbone are susceptible to hydrolysis and enzymatic degradation.

PLA 来源于玉米淀粉等可再生资源。单体为乳酸，它同时含有 –OH 和 –COOH 基团，可进行缩合聚合。PLA 用于可堆肥餐具、医疗植入物和包装膜。主链中的酯键易于水解和被酶降解。

PHA is produced naturally by certain bacteria. Poly-3-hydroxybutyrate (PHB) is the most common, with the repeating unit –O–CH(CH₃)–CH₂–CO–. Its brittleness can be improved by copolymerising with valerate to form PHBV. These polymers break down in soil or marine environments, reducing plastic pollution.

PHA 由某些细菌天然产生。最常见的是聚-3-羟基丁酸酯 (PHB)，重复单元为 –O–CH(CH₃)–CH₂–CO–。通过与戊酸酯共聚形成 PHBV 可改善其脆性。这些聚合物在土壤或海洋环境中降解，减少塑料污染。

Exam questions often ask you to identify the structural feature that allows biodegradability: hydrolysable ester or amide groups in the main chain. You may also be asked to compare PLA with poly(ethene), noting that PLA has polar ester groups making it susceptible to nucleophilic attack, while poly(ethene) has only C–C and C–H bonds that are chemically inert.

考试题目常常要求你识别使材料可生物降解的结构特征：主链中可水解的酯基或酰胺基团。你还可能被要求比较 PLA 与聚乙烯，指出 PLA 具有极性的酯基，易受亲核攻击，而聚乙烯只有 C–C 和 C–H 键，化学惰性。

9. Recycling and Environmental Impact | 回收与环境影响

Thermoplastics can be recycled by melting and remoulding, but the quality often degrades due to chain shortening from thermal stress. Thermosets cannot be remelted and are therefore more challenging to recycle; they are often ground into filler material or incinerated with energy recovery.

热塑性塑料可通过熔融和重塑进行回收，但由于热应力导致链断裂，质量通常会下降。热固性塑料无法再熔化，因此更难回收；它们通常被磨碎用作填料或通过焚烧回收能量。

Plastic waste poses a threat to ecosystems, particularly marine life. Microplastics result from fragmentation and can adsorb toxic pollutants. The IB expects you to discuss the advantages and disadvantages of landfill, incineration, and chemical recycling (feedstock recycling, e.g., pyrolysis to monomers).

塑料废弃物对生态系统构成威胁，尤其是对海洋生物。微塑料来自碎裂，并能吸附有毒污染物。IB 期望你讨论填埋、焚烧和化学回收（原料回收，如热解为单体）的优缺点。

One effective strategy is designing for sustainability: using bioplastics (PLA, PHA) where appropriate, minimising packaging, and improving sorting technologies. You may be asked to evaluate the life cycle of a polymer from raw material extraction to disposal.

一种有效的策略是进行可持续性设计：在合适场合使用生物塑料（PLA、PHA），减少包装，改进分拣技术。你可能会被要求评估一种聚合物从原料提取到废弃的全生命周期。

10. Key Equations and Reaction Mechanisms | 关键方程与反应机理

While IB does not require a detailed radical mechanism for addition polymerisation, you should understand the concept of an initiator (e.g., organic peroxides) providing free radicals that open the π-bond of the monomer. The chain reaction involves initiation, propagation, and termination steps. The overall equation is what matters for the exam.

虽然 IB 不要求详细描述加成聚合的自由基机理，但你应理解引发剂（如有机过氧化物）提供自由基使单体的 π 键打开的概念。链反应包括引发、增长和终止步骤。考试中重要的是总和方程式。

For condensation polymerisation, you need to write stoichiometric equations showing the elimination of water or HCl. Always balance the amount of small molecules produced: two water molecules per repeat unit when linking a diamine and dioic acid via amide bonds, because each amide link releases one water.

对于缩合聚合，你需要写出计量方程式，标明脱去水或 HCl。始终要平衡产生的小分子数量：当通过酰胺键连接二元胺和二元酸时，每个重复单元产生两个水分子，因为每个酰胺键释放一分子水。

n H₂N─R─NH₂ + n HOOC─R’─COOH → [─NH─R─NH─CO─R’─CO─]ₙ + 2n H₂O

Be precise with brackets and continuation bonds when drawing repeating units. Examiners look for extended lines at both ends and correct positioning of functional groups.

绘制重复单元时，要准确使用方括号和延伸键。考官会检查两端的延伸线和官能团的正确位置。

11. Practical Identification and Testing | 实验鉴别与检验

IB practical work may involve identifying unknown plastics. Common methods include density tests (float/sink in water), flame tests (colour of flame, smell of fumes, self-extinguishing nature), and solubility tests. For example, poly(ethene) floats, burns with a blue-yellow flame and drips, smelling of candle wax. PVC sinks, is self-extinguishing, and produces acidic fumes (HCl) that turn blue litmus red.

IB 实验可能涉及鉴别未知塑料。常用方法包括密度测试（水中沉浮）、燃烧测试（火焰颜色、烟雾气味、自熄性质）和溶解性测试。例如，聚乙烯浮于水面，燃烧时有蓝黄色火焰并滴落，有蜡烛味。PVC 下沉，能自熄，产生使蓝色石蕊试纸变红的酸性烟雾 (HCl)。

Another test is the copper wire test: heat a copper wire in a Bunsen flame, then touch the plastic; put it back in the flame. A green flame indicates the presence of halogens (like chlorine in PVC). This helps distinguish halogenated plastics from non-halogenated ones.

另一种测试是铜丝测试：将铜丝在本生灯火焰中加热，然后接触塑料，再放回火焰中。绿色火焰表明存在卤素（如 PVC 中的氯）。这有助于区分含卤塑料与不含卤塑料。

You should also be able to hydrolyse a polyester or polyamide with hot sodium hydroxide, then identify the products. The ester or amide bond breaks, releasing the diol/diamine and the acid salt. The acid salt can be acidified to regenerate the original dioic acid, which can be filtered and tested.

你还应能用热的氢氧化钠溶液水解聚酯或聚酰胺，然后鉴定产物。酯键或酰胺键断裂，释放出二元醇/二元胺和酸性盐。酸性盐经酸化可重新生成原始的二元酸，可过滤并进行测试。

12. Exam Tips and Common Mistakes | 考试技巧与常见错误

Repeating unit drawing: Show the backbone with single bonds only; do not leave a double bond. Use brackets and indicate continuation bonds at both ends. For condensation polymers, the functional group (ester/amide) must be in the main chain.
重复单元绘制：只显示单键的主链；不要留下双键。使用方括号，并标出两端的延伸键。对于缩合聚合物，官能团（酯/酰胺）必须在主链上。
Equation balancing: For addition, just n monomer → [repeating unit]ₙ. For condensation, remember to add the small molecules (usually 2n H₂O or 2n HCl).
方程式配平：加成聚合：n 单体 → [重复单元]ₙ。缩合聚合：记得加上小分子（通常是 2n H₂O 或 2n HCl）。
Naming: Poly(propene) not polypropylene; poly(chloroethene) not polyvinyl chloride (PVC is the common name, but IUPAC is preferred in some questions). Check the question context.
命名：Poly(propene) 而非 polypropylene；poly(chloroethene) 而非 polyvinyl chloride（PVC 是通用名，但某些问题中首选 IUPAC 名称）。注意题目语境。
Thermoplastic vs. thermoset: Do not confuse melting behaviour. Thermosets do not melt; they char or burn. The ability to melt is directly related to a lack of cross-links.
热塑性 vs. 热固性：不要混淆熔化行为。热固性塑料不熔化；它们会焦化或燃烧。熔化能力直接关联到缺乏交联。
Biodegradability: Cite specific functional groups. Just saying “can be broken down” is insufficient. Mention hydrolysable ester/amide linkages and the role of microorganisms.
可生物降解性：引用具体的官能团。只说“能被分解”是不够的。要提及可水解的酯/酰胺键和微生物的作用。
Intermolecular forces: When comparing properties, always link to the strongest intermolecular forces present: addition polymers usually van der Waals; polyamides have hydrogen bonds; polyesters may have dipole-dipole and some H-bonding.
分子间作用力：比较性质时，始终联系到存在的最强分子间作用力：加成聚合物通常为范德华力；聚酰胺有氢键；聚酯可能有偶极-偶极作用和部分氢键。

Practise past IB paper questions: draw the repeating unit of a given monomer or identify the monomer from a repeating unit. Be comfortable reversing the process for condensation polymers, which is trickier than for addition polymers.

练习过去的 IB 试题：根据给定单体画出重复单元，或从重复单元识别单体。能熟练地逆向推导缩合聚合物的单体，这比加成聚合物更复杂。

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IB Chemistry: Polymers Key Points Review | IB 化学：聚合物 考点精讲