📚 Addition Polymerisation | 加成聚合
Addition polymerisation is a fundamental reaction in organic chemistry where small unsaturated molecules, known as monomers, link together to form very long chain macromolecules called polymers. This process is characterised by the absence of any small molecule by‑product; the polymer has the same empirical formula as the monomer. Understanding this topic is essential for A‑Level Cambridge Chemistry, as it bridges organic reaction mechanisms with important real‑world materials such as plastics, fibres and coatings.
加成聚合是有机化学中的一个基本反应，由不饱和的小分子（称为单体）连接在一起形成很长的链状大分子，即聚合物。这一过程的特点是不生成任何小分子副产物；聚合物的最简式与单体相同。理解本主题对剑桥 A‑Level 化学至关重要，因为它将有机反应机理与重要的现实材料（如塑料、纤维和涂层）联系起来。
1. Introduction to Addition Polymerisation | 加成聚合简介
Addition polymerisation can be defined as a chain‑ monomer molecules, typically alkenes or substituted alkenes, repeatedly add to a reactive site at the end of a growing polymer chain. The carbon– he functional group that enables this transformation. A general equation can be written as n CH₂=CHX → –(CH₂–CHX)–ₙ, where X represents a side group such as H, CH₃, Cl, or CN, and n is a large integer representing the degree of polymerisation.
加成聚合可定义为一种链增长过程，其中不饱和单体分子（通常为烯烃或取代烯烃）反复加到增长中的聚合物链末端的活性位点上。碳碳双键（C=C）是实现这一转化的官能团。通用方程式可写为 n CH₂=CHX → –(CH₂–CHX)–ₙ，其中 X 代表侧基，如 H、CH₃、Cl 或 CN，n 是一个很大的整数，表示聚合度。
Unlike condensation polymerisation, addition polymerisation does not involve the loss of small molecules such as water or hydrogen chloride. The polymer contains exactly the same atoms in the same proportions as the original monomer, which means that the repeating unit of the polymer has the same elemental composition as the monomer itself. For example, the repeating unit of poly(ethene) is –CH₂–CH₂–, identical in composition to ethene (C₂H₄).
与缩合聚合不同，加成聚合不失去水或氯化氢等小分子。聚合物所含原子及其比例与初始单体完全相同，这意味着聚合物的重复单元与单体本身具有相同的元素组成。例如，聚乙烯的重复单元为 –CH₂–CH₂–，其组成与乙烯（C₂H₄）相同。
2. The Monomers: Alkenes and Common Derivatives | 单体：烯烃及其常见衍生物
monomers that undergo addition polymerisation are molecules containing at least one carbon– ene monomers include propene (CH₂=CHCH₃) for poly(propene), chloroethene (vinyl chloride, CH₂=CHCl) for polyvinyl chloride (PVC), and tetrafluoroethene (CF₂=CF₂) for polytetrafluoroethylene (PTFE).
能进行加成聚合的单体是那些含有至少一个碳碳双键的分子。最简单且工业上最重要的单体是乙烯（CH₂=CH₂），用于生产聚乙烯。其他重要的烯烃单体包括丙烯（CH₂=CHCH₃）用于聚丙烯，氯乙烯（CH₂=CHCl）用于聚氯乙烯（PVC），以及四氟乙烯（CF₂=CF₂）用于聚四氟乙烯（PTFE）。
are widely used to make polystyrene and poly(methyl methacrylate) (PMMA), respectively. The presence of different side groups X in the monomer introduces structural variety, leading to polymers with distinct physical and chemical properties, from flexibility and transparency to chemical resistance and high melting points.
此外，苯乙烯（C₆H₅CH=CH₂）和甲基丙烯酸甲酯（CH₂=C(CH₃)COOCH₃）等取代烯烃被广泛用于生产聚苯乙烯和聚甲基丙烯酸甲酯（PMMA）。单体中不同侧基 X 的存在引入了结构多样性，使聚合物具备各异的物理和化学性质，从柔韧性和透明性到耐化学性和高熔点。
3. Free‑Radical Mechanism: Overview | 自由基聚合机理概述
In A‑Level studies, addition polymerisation is usually explained via a free‑radical chain mechanism. The reaction occurs under conditions of high pressure (or sometimes moderate pressure with a catalyst) and elevated temperature, with the help of an initiator—often an organic peroxide such as benzoyl peroxide. The overall process is divided into three distinct stages: initiation, propagation, and termination.
在 A‑Level 的学习中，加成聚合通常通过自由基链式机理解释。反应在高压（或有时在中压下使用催化剂）和升温条件下进行，并借助引发剂——通常是有机过氧化物，如过氧化苯甲酰。整个过程分为三个明确的阶段：引发、增长和终止。
During initiation, the peroxide O–O bond undergoes homolytic fission to produce two alkoxy radicals (RO•). These highly reactive species attack a monomer molecule, forming a new carbon‑centred radical that begins the chain. Propagation consists of a rapid sequence of addition steps in which the radical at the end of the growing chain opens another monomer double bond, adding to the chain and regenerating a terminal radical. Termination occurs when two radical‑containing chains meet and quench each other.
在引发阶段，过氧化物中的 O–O 键发生均裂，生成两个烷氧自由基（RO•）。这些高活性物种进攻单体分子，形成一个新的碳中心自由基，从而引发链。增长阶段由一连串快速的加成步骤组成，增长链末端的自由基打开另一个单体的双键，加到链上并再生末端自由基。当两个带有自由基的链相遇并互相猝灭时，发生终止。
4. Initiation Step: Generating Free Radicals | 引发步骤：生成自由基
The initiation step begins with the thermal or photochemical homolysis of an initiator molecule. A common example is benzoyl peroxide, (C₆H₅COO)₂, which undergoes O–O bond cleavage to generate two benzoyloxy radicals:
引发阶段始于引发剂分子的热或光化学均裂。一个常见的例子是过氧化苯甲酰 (C₆H₅COO)₂，其发生 O–O 键断裂，生成两个苯甲酰氧自由基：
(C₆H₅COO)₂ → 2 C₆H₅COO•
The alkoxy radical then reacts with the π‑bond of an alkene monomer. The radical adds to one carbon atom of the double bond, leaving the other carbon atom as the new radical centre. Using ethene as the monomer, this can be expressed as:
随后，烷氧自由基与烯烃单体的 π 键反应。自由基加到双键的一个碳原子上，另一个碳原子则成为新的自由基中心。以乙烯为单体，可表示为：
C₆H₅COO• + CH₂=CH₂ → C₆H₅COO–CH₂–CH₂•
This newly formed carbon‑centred radical is now able to add further monomer units, marking the end of the initiation stage and the beginning of chain propagation. The concentration of radicals is kept low throughout the reaction to minimise premature termination.
这个新形成的碳中心自由基现在能够添加更多的单体单元，标志着引发阶段的结束和链增长的开始。整个反应过程中，自由基浓度保持在较低水平，以尽量减少过早终止。
5. Propagation Step: Chain Growth | 增长步骤：链增长
During propagation, the carbon‑centred radical at the end of the growing chain adds rapidly to another monomer molecule, opening its π‑bond and creating a new, longer radical chain. The process is repeated many thousands of times in quick succession. For poly(ethene), the propagation steps can be summarised as:
在增长阶段，增长链末端的碳中心自由基迅速地加到另一个单体分子上，打开其 π 键，形成一个新的、更长的自由基链。这一过程重复成千上万次。对聚乙烯来说，增长步骤可概括为：
~CH₂–CH₂• + CH₂=CH₂ → ~CH₂–CH₂–CH₂–CH₂•
Because each addition regenerates a terminal radical, the chain continues to grow as long as monomer is available. The rate of propagation is very high, and the molecular weight of the polymer increases dramatically within seconds. The head‑to‑tail addition is favoured for steric and electronic reasons, meaning that the substituent X typically ends up on every other carbon atom along the backbone.
由于每次加成都会再生一个末端自由基，只要单体存在，链就会持续增长。增长速率非常高，聚合物的分子量在几秒钟内急剧增大。出于空间位阻和电子效应，倾向于头‑尾加成，这意味着取代基 X 通常会在主链上每隔一个碳原子出现。
If the monomer has a side group larger than a hydrogen atom, stereo‑regularity can arise. For instance, in the polymerisation of propene using specific Ziegler–Natta catalysts, isotactic or syndiotactic poly(propene) can be obtained, but the free‑radical mechanism covered at A‑Level typically gives atactic (random) arrangements.
如果单体具有比氢原子更大的侧基，就可能产生立构规整性。例如，在使用特定齐格勒‑纳塔催化剂聚合丙烯时，可获得等规或间规聚丙烯，但在 A‑Level 所涵盖的自由基机理中，通常得到无规（随机的）排列。
6. Termination Step: Ending the Chain | 终止步骤：链终止
Termination occurs when two growing polymer radicals come into contact and react, ending the chain growth. There are two principal mechanisms: combination (coupling) and disproportionation. In combination, two radical ends simply join together to form a single, longer polymer chain:
当两个正在增长的聚合物自由基相遇并反应时，链增长终止，这便是终止阶段。主要有两种机理：偶合（耦合）和歧化。在偶合终止中，两个自由基末端直接连接在一起，形成一条更长的聚合物链：
~CH₂–CH₂• + •CH₂–CH₂~ → ~CH₂–CH₂–CH₂–CH₂~
In disproportionation, a hydrogen atom is transferred from one radical centre to the other, resulting in one saturated chain end and one unsaturated chain end without an increase in chain length. For ethene‑derived chains this produces a terminal alkene and an alkane end group:
在歧化终止中，一个氢原子从一个自由基中心转移到另一个自由基中心，生成一个饱和链端和一个不饱和链端，链长不发生增加。对于乙烯衍生链，这会生成一个末端烯烃基团和一个烷烃端基：
~CH₂–CH₂• + •CH₂–CH₂~ → ~CH=CH₂ + CH₃–CH₂~
Both processes convert reactive radical species into stable, dead polymer. The relative importance of combination versus disproportionation depends on the nature of the monomer and reaction conditions. After termination, the polymer can be isolated and processed.
两种过程都将反应性自由基物种转化为稳定的“死”聚合物。偶合与歧化这两种终止方式的相对重要性取决于单体的性质和反应条件。终止后，聚合物可被分离和加工。
7. Common Addition Polymers and Their Repeating Units | 常见加成聚合物的重复单元
The repeating unit of an addition polymer is written by enclosing the structure of the monomer showing the opened double bond in square brackets, with a subscript n to denote repetition. The table below summarises several key polymers, their monomers, repeating units, and typical uses.
加成聚合物的重复单元是将展示打开双键的单体结构括在方括号中，并加下标 n 表示重复。下表总结了几种重要聚合物的单体、重复单元和典型用途。
| Polymer | Monomer | Repeating unit | Common uses |
|---|---|---|---|
| Poly(ethene) (PE) | CH₂=CH₂ | –(CH₂–CH₂)ₙ– | Plastic bags, bottles, films |
| Poly(propene) (PP) | CH₂=CHCH₃ | –(CH(CH₃)–CH₂)ₙ– | Crates, ropes, automotive parts |
| Poly(chloroethene) (PVC) | CH₂=CHCl | –(CHCl–CH₂)ₙ– | Pipes, window frames, flooring |
| Poly(tetrafluoroethene) (PTFE) | CF₂=CF₂ | –(CF₂–CF₂)ₙ– | Non‑stick coatings, electrical insulation |
| Polystyrene (PS) | CH₂=CHC₆H₅ | –(CH(C₆H₅)–CH₂)ₙ– | Packaging, disposable cups, insulation |
s the same atoms as the monomer but with the C=C bond replaced by a single bond, and the continuity of the chain is indicated by extending bonds on both sides of the unit.
每种聚合物都直接由烯烃单体通过自由基或催化加成聚合制得。注意，重复单元与单体含有相同的原子，但 C=C 双键被单键取代，单元两侧的延长键表示链的连续性。
8. Structure–Property Relationships: Crystallinity and Density | 结构与性质关系：结晶度和密度
The macroscopic properties of addition polymers are profoundly influenced by the microstructure of the polymer chains. Branched poly(ethene), known as low‑density poly(ethene) (LDPE), contains numerous short and long branches along the main chain. These branches prevent the chains from packing closely together, lowering the degree of crystallinity and density. As a result, LDPE is a flexible, translucent material with a relatively low melting point, suitable for films and bags.
加成聚合物的宏观性质深受聚合物链微观结构的影响。支化聚乙烯，即低密度聚乙烯（LDPE），在主链上含有许多短支链和长支链。这些支链阻止链紧密堆积，降低了结晶度和密度。因此，LDPE 是一种柔韧、半透明的材料，熔点较低，适用于制造薄膜和袋子。
In contrast, high‑density poly(ethene) (HDPE) is produced under conditions that favour linear chains with minimal branching. These straight chains align readily, yielding a highly crystalline, dense, and tough material with a higher softening point, used for rigid containers and pipes. The difference in crystallinity between LDPE and HDPE demonstrates how subtle variations in synthesis conditions influence polymer performance.
与此相反，高密度聚乙烯（HDPE）是在有利于形成线性且极少支链的条件下生产的。这些直链容易排列，形成高度结晶、致密且坚韧的材料，其软化点更高，用于硬质容器和管道。LDPE 与 HDPE 之间结晶度的差异说明了合成条件的微小变化如何影响聚合物的性能。
For atactic poly(propene) produced by free‑radical polymerisation, the random orientation of methyl groups leads to an amorphous, low‑strength product of limited commercial value. However, stereoregular isotactic poly(propene), made using Ziegler–Natta catalysts, attains high crystallinity, strength, and thermal resistance, making it suitable for engineering applications. Similarly, the highly symmetric –CF₂–CF₂– repeat unit in PTFE results in an extremely high melting point and a very low coefficient of friction, explaining its role in non‑stick surfaces and high‑temperature insulation.
对于自由基聚合产生的无规聚丙烯，甲基的无规则取向导致产物为非晶态、强度低，商业价值有限。然而，使用齐格勒‑纳塔催化剂制备的具有立体规整性的等规聚丙烯，可获得高结晶度、高强度与耐热性，适合工程应用。类似地，PTFE 中高度对称的 –CF₂–CF₂– 重复单元使其拥有极高的熔点和极低摩擦系数，这解释了它在不粘表面和高温绝缘中的应用。
9. Environmental Impact and Recycling | 环境影响与回收
Addition polymers, particularly polyalkenes such as PE and PP, are highly resistant to chemical and biological degradation. This durability, while useful during their service life, creates significant environmental challenges once products become waste. Disposal in landfills leads to persistent litter, while incineration can release CO₂ and, for halogenated polymers like PVC, toxic gases such as HCl and dioxins.
加成聚合物，尤其是聚烯烃如 PE 和 PP，对化学降解和生物降解具有很高的抵抗力。这种耐用性在使用年限内很有用，但一旦产品成为废物，就带来了显著的环境挑战。填埋处理会造成持久性垃圾，而焚烧可能释放 CO₂，对于 PVC 等含卤聚合物，还会放出 HCl 和二噁英等有毒气体。
Recycling offers a more sustainable route. Mechanical recycling involves sorting, shredding, melting and remoulding plastic waste, though contamination and degradation during reprocessing can limit the number of cycles. Feedstock recycling breaks polymers down into monomers or small molecules, which can be used to make new polymers or fuels. For example, poly(ethene) can be pyrolysed to reclaim ethene. Additionally, biodegradable plastics derived from renewable resources are being developed as alternatives, although they currently represent only a small fraction of the market.
回收利用提供了更具可持续性的途径。机械回收包括分拣、破碎、熔融和重新成型塑料废物，但加工过程中的污染和降解会限制循环次数。原料回收将聚合物分解成单体或小分子，可用于制造新的聚合物或燃料。例如，聚乙烯可以热解回收乙烯。此外，源自可再生资源的生物可降解塑料正在作为替代品开发，尽管目前它们仅占市场的一小部分。
In the context of A‑Level chemistry, understanding these environmental aspects links the chemistry of polymerisation to wider societal issues, including the need for circular economies, improved waste management, and the development of greener polymers.
在 A‑Level 化学的语境中，理解这些环境问题将聚合反应的化学与更广泛的社会问题联系起来，包括对循环经济、改善废物管理和开发更绿色聚合物的需求。
10. Summary and Exam Focus | 总结与备考重点
Addition polymerisation is a high‑yield topic in Cambridge A‑ nition and general equation; the requirement for monomers to contain a C=C bond; the three‑step free‑ nship between chain structure (linear vs branched, stereoregularity) and polymer properties; and environmental considerations surrounding disposal and recycling.
加成聚合是剑桥 A‑Level 化学中的高频考点。必须掌握的知识包括：定义和通用方程式；单体必须含有 C=C 双键；三步自由基机理解（引发、增长、终止）；能够根据给定单体画出重复单元，反之亦然；链结构（线性 vs 支化、立体规整性）与聚合物性质的关系；以及围绕处置与回收的环境考量。
Common exam questions ask students to identify the repeating unit of a polymer, complete equations for initiation or propagation steps, explain why addition polymers are chemically unreactive but thermally processable, or compare the structures and properties of LDPE and HDPE. Be prepared to write radical mechanisms using curly arrows in the initiation step and to discuss why isotactic poly(propene) is stronger than atactic poly(propene).
常见考题要求学生识别聚合物的重复单元，完成引发或增长步骤的方程式，解释为何加成聚合物化学性质不活泼但可热加工，或者比较 LDPE 与 HDPE 的结构与性质。应准备好用弯箭头书写引发步骤的自由基机理，并讨论为何等规聚丙烯比无规聚丙烯强度更高。
When drawing polymer structures, always show the repeating unit with extended bonds through the brackets and indicate the variable n. Emphasise that addition polymerisation does not alter the elemental composition of the monomer, a key distinction from condensation polymerisation. These core ideas, coupled with careful vocabulary and precise chemical notation, will ensure strong exam performance.
在绘制聚合物结构时，始终展示带延长键穿过方括号的重复单元，并标注变量 n。强调加成聚合不改变单体的元素组成，这是与缩合聚合的关键区别。掌握这些核心概念，配合准确的术语和精准的化学符号，将确保优异的考试表现。
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