📚 Polymerisation | 聚合反应
Polymerisation is a fundamental process in organic chemistry where small repeating units called monomers join together to form long-chain macromolecules known as polymers. These reactions are essential in both natural and synthetic contexts, giving rise to materials ranging from DNA and proteins to plastics, fibres, and resins. Understanding the two principal types – addition and condensation polymerisation – is a core requirement of the Cambridge A-Level Chemistry syllabus, as it links functional group reactivity with the properties and environmental impact of polymeric materials.
聚合反应是有机化学中的一个基本过程，指称为单体的小分子重复单元相互连接，形成称为聚合物的长链大分子。这些反应在天然和合成领域都至关重要，产生的材料涵盖从DNA、蛋白质到塑料、纤维和树脂等。理解加成聚合与缩合聚合这两种主要类型，是剑桥A-Level化学大纲的核心要求，因为这一主题将官能团反应性与聚合材料的性质及其环境影响紧密联系起来。
1. Introduction to Polymerisation | 聚合反应简介
A polymer is a large molecule composed of many identical or similar repeating subunits. The term originates from the Greek poly (many) and meros (part). Monomers are typically alkenes, diols, diamines, dicarboxylic acids or amino acids. Polymerisation can proceed via two mechanistically distinct pathways: addition polymerisation, which involves the opening of π-bonds without the loss of small molecules, and condensation polymerisation, where each linking step eliminates a small molecule such as water or hydrogen chloride.
聚合物是由许多相同或相似的重复亚基组成的大分子。该术语源自希腊语poly（多）和meros（部分）。单体通常是烯烃、二醇、二胺、二羧酸或氨基酸。聚合可经由两种机理不同的途径进行：加成聚合涉及打开 π 键而不失去小分子；缩合聚合则在每一步连接中都脱去一个小分子，如水或氯化氢。
The degree of polymerisation (n) represents the number of monomer units in a polymer chain. It directly influences physical properties such as tensile strength, melting point and flexibility. Polymers can be homopolymers (made from one type of monomer) or copolymers (made from two or more different monomers), giving rise to a vast diversity of materials.
聚合度（n）表示聚合物链中单体单元的数目，直接影响其拉伸强度、熔点和柔韧性等物理性质。聚合物可以是均聚物（由一种单体制成）或共聚物（由两种或更多不同单体制成），从而产生种类繁多的材料。
2. Addition Polymerisation | 加成聚合
Addition polymerisation occurs when monomers containing carbon– peated addition reactions. The π-bond in each alkene opens up, allowing monomers to link covalently into a saturated carbon backbone. No atoms are lost during this process, so the empirical formula of the polymer is identical to that of the monomer. Common examples include poly(ethene), poly(propene) and poly(phenylethene) (polystyrene).
加成聚合发生在含碳碳双键（C=C）的单体进行反复加成反应时。每个烯烃中的 π 键打开，使单体通过共价键连接，形成饱和的碳主链。此过程不丢失任何原子，因此聚合物的经验式与单体完全相同。常见实例有聚（乙烯）、聚（丙烯）和聚（苯乙烯）。
er. For instance, poly(ethene), derived from ethene (CH₂=CH₂), has a simple repeating unit –(CH₂–CH₂)–. In poly(propene), from propene (CH₂=CHCH₃), the repeating unit is –(CH₂–CH(CH₃))– by free radicals, cations or anions, though A-Level focuses on the free-radical mechanism.
加成聚合物的关键结构特征是一条长的饱和链，其侧基取决于原始单体。例如，由乙烯（CH₂=CH₂）制得的聚（乙烯）具有简单的重复单元 –(CH₂–CH₂)–；由丙烯（CH₂=CHCH₃）制得的聚（丙烯）的重复单元为 –(CH₂–CH(CH₃))–，带有一个甲基侧基。该反应通常由自由基、阳离子或阴离子引发，但A-Level着重于自由基机理。
3. Mechanism of Radical Addition Polymerisation | 自由基加成聚合机理
the homolytic cleavage of an initiator molecule, commonly an organic peroxide or azo compound, by heat or UV radiation to produce reactive free radicals. For example, dibenzoyl peroxide decomposes to form benzoyloxyl radicals, which then add across the C=C bond of an alkene monomer, generating a new radical centre on the carbon chain.
自由基加成聚合通过三个明显的阶段进行：引发、增长和终止。引发涉及引发剂分子（通常为有机过氧化物或偶氮化合物）在加热或紫外线照射下发生均裂，产生活性自由基。例如，过氧化二苯甲酰分解生成苯甲氧自由基，随后该自由基加成到烯烃单体的C=C键上，在碳链上产生一个新的自由基中心。
. This step repeats thousands of times in rapid succession, building a long polymer chain. The propagation step can be represented as: R–CH₂–ĊHX + CH₂=CHX → R–CH₂–CHX–CH₂–ĊHX. The dot denotes the unpaired electron; X represents a generic side group or hydrogen.
在增长阶段，自由基中心进攻另一个单体分子，使链增长一个单元，同时再生一个末端自由基。该步骤快速重复数千次，构建起长聚合物链。增长步骤可表示为：R–CH₂–ĊHX + CH₂=CHX → R–CH₂–CHX–CH₂–ĊHX。圆点表示未成对电子；X代表一个通用侧基或氢。
Termination stops chain growth when two radicals combine (combination) or a radical abstracts a hydrogen atom from another chain end (disproportionation). Combination gives a longer saturated chain, while disproportionation yields one saturated end and one unsaturated end. The final polymer is a mixture of chain lengths, leading to a distribution of molar masses.
终止反应使链增长停止，当两个自由基结合（偶合终止）或一个自由基从另一条链端夺走一个氢原子（歧化终止）时发生。偶合生成更长的饱和链，而歧化则产生一个饱和端基和一个不饱和端基。最终聚合物是不同链长的混合物，导致摩尔质量有一个分布。
4. Common Addition Polymers | 常见加成聚合物
ow each monomer contains a C=C double bond that ultimately becomes a single-bonded backbone.
下表总结了剑桥A-Level要求的四种关键加成聚合物、其单体、重复单元及典型用途。请注意，每种单体都含有一个C=C双键，最终形成单键连接的主链。
| Polymer | Monomer | Repeating unit | Uses |
|---|---|---|---|
| Poly(ethene) (PE) | CH₂=CH₂ | –CH₂–CH₂– | Plastic bags, bottles, films |
| Poly(propene) (PP) | CH₂=CHCH₃ | –CH₂–CH(CH₃)– | Crates, ropes, carpets, syringes |
| Poly(chloroethene) (PVC) | CH₂=CHCl | –CH₂–CHCl– | Window frames, pipes, flooring |
| Poly(phenylethene) (PS) | CH₂=CHC₆H₅ | –CH₂–CH(C₆H₅)– | Packaging, insulation, disposable cups |
The properties vary with the side group: poly(ethene) is flexible and waxy; poly(chloroethene) is rigid due to the polar C–Cl bond; polystyrene with its bulky phenyl ring can be rigid or foamed. These thermoplastics soften on heating and can be reshaped, making them ideal for recycling processes.
这些聚合物的性质随侧基而异：聚（乙烯）柔软、呈蜡状；聚（氯乙烯）因极性的C–Cl键而呈刚性；带有大体积苯环的聚苯乙烯可呈刚性或发泡状态。这些热塑性塑料受热软化，可以重新塑形，因此适合回收处理。
5. Condensation Polymerisation | 缩合聚合
Condensation polymerisation involves monomers with two complementary functional groups that react together, eliminating a small molecule (often water or HCl) at each linkage. Unlike addition polymerisation, the repeating unit has fewer atoms than the monomer(s) due to the loss of the small molecule. Two main classes are polyesters and polyamides, formed respectively from dicarboxylic acids with diols, and from dicarboxylic acids with diamines or from amino acids.
缩合聚合涉及带有两个互补官能团的单体，它们相互反应，在每形成一个链接时脱去一个小分子（通常是水或HCl）。与加成聚合不同，由于失去小分子，重复单元所含的原子数少于单体。主要有两类：聚酯（由二羧酸与二醇形成）和聚酰胺（由二羧酸与二胺或由氨基酸形成）。
A typical condensation polymerisation proceeds stepwise; any two functional groups can react, so high molar mass is only achieved at very high conversion. The reaction is often driven by removal of the small molecule. Polymers like poly(ethylene terephthalate) (PET) and nylon-6,6 exemplify this class and are essential to the Cambridge syllabus.
典型的缩合聚合逐步进行；任意两个官能团均可反应，因此只有在转化率很高时才能达到高摩尔质量。通常通过移除小分子来驱动反应。诸如聚对苯二甲酸乙二醇酯（PET）和尼龙-6,6等聚合物属于此类，也是剑桥大纲的重点。
6. Polyesters | 聚酯
Polyesters are produced when a dicarboxylic acid reacts with a diol. Each ester linkage (–COO–) releases a water molecule. A classic example is Terylene (PET), formed from benzene-1,4-dicarboxylic acid (terephthalic acid) and ethane-1,2-diol. The overall equation can be written as:
聚酯由二羧酸与二醇反应生成。每个酯键（–COO–）释放一个水分子。一个经典例子是特纶（Terylene，即PET），由1,4-苯二甲酸（对苯二甲酸）与1,2-乙二醇形成。总方程式可写为：
n HOOC–C₆H₄–COOH + n HO–CH₂–CH₂–OH → –[OC–C₆H₄–COO–CH₂–CH₂–O]n– + (2n−1) H₂O
The repeating unit contains an ester group linking the aromatic and aliphatic portions. In practice, a slight excess of one monomer may be used, and the reaction is catalysed by acids. Polyesters are widely used in synthetic fabrics, plastic bottles and films.
重复单元含有连接芳香部分与脂肪部分的酯基。实践中可能使用某一种单体稍过量，反应由酸催化。聚酯广泛用于合成织物、塑料瓶和薄膜中。
ylactic acid), is derived from lactic acid, a hydroxycarboxylic acid. Its monomer has both –OH and –COOH groups, so it undergoes self-condensation: n HO–CH(CH₃)–COOH → –[O–CH(CH₃)–CO]n– + n H₂O. PLA is biodegradable and used in medical sutures and compostable packaging.
另一种重要的聚酯是聚乳酸（PLA），由羟基羧酸——乳酸衍生而来。其单体同时含–OH和–COOH基团，因此能发生自缩合：n HO–CH(CH₃)–COOH → –[O–CH(CH₃)–CO]n– + n H₂O。PLA可生物降解，用于医用缝合线和可堆肥包装。
7. Polyamides | 聚酰胺
Polyamides are formed by the reaction between a dicarboxylic acid and a diamine, or by the polymerisation of amino acids. Each amide (or peptide) linkage (–CONH–) is accompanied by the loss of water. The most cited industrial polyamide is nylon-6,6, synthesised from hexane-1,6-dioic acid (adipic acid) and hexane-1,6-diamine. The numbers 6,6 refer to the carbon atoms in each monomer.
聚酰胺由二羧酸与二胺反应形成，或由氨基酸聚合形成。每形成一个酰胺（或肽）键（–CONH–），伴随失去一分子水。最常被引用的工业聚酰胺是尼龙-6,6，由己二酸和1,6-己二胺合成。数字6,6指每种单体中的碳原子数。
n H₂N–(CH₂)₆–NH₂ + n HOOC–(CH₂)₄–COOH → –[HN–(CH₂)₆–NHCO–(CH₂)₄–CO]n– + (2n−1) H₂O
Nylon-6 is produced by ring‑opening polymerisation of caprolactam, not by condensation, but it is still a polyamide with repeating unit –[HN–(CH₂)₅–CO]–. Both nylons are strong, elastic fibres used in clothing, ropes and engineering plastics.
尼龙-6是由己内酰胺开环聚合制得，并非缩合，但它仍是聚酰胺，重复单元为–[HN–(CH₂)₅–CO]–。两种尼龙都是强韧、富有弹性的纤维，用于服装、绳索和工程塑料。
Natural polyamides include proteins, which are condensation polymers of α-amino acids. Here the monomers have both amine and carboxylic acid groups; the resulting peptide bond is chemically an amide link. Silk and wool are protein fibres, sharing the same amide backbone as synthetic nylon.
天然聚酰胺包括蛋白质，后者是α-氨基酸的缩合聚合物。此时单体同时具有氨基和羧基；所形成的肽键在化学本质上就是酰胺键。丝绸和羊毛是蛋白质纤维，与合成尼龙共享相同的酰胺主链。
8. Comparing Addition and Condensation Polymers | 加成与缩合聚合物比较
Addition polymers are generally formed from a single type of unsaturated monomer, and the backbone is a continuous carbon chain. No small molecules are eliminated, so the atom economy is theoretically 100%. These polymers are typically non-biodegradable synthetic thermoplastics, and their main chain is purely hydrocarbon-based (except for PVC, etc.).
加成聚合物通常由单一类型的不饱和单体制得，主链为连续的碳链。不脱去小分子，因此原子经济性在理论上达100%。这些聚合物通常是非生物降解的合成热塑性塑料，其主链纯粹由碳氢骨架构成（PVC等除外）。
Condensation polymers, by contrast, require two distinct functional groups (which may be on the same or different monomers) and produce a small by-product. Their repeating units contain ester or amide links, which are susceptible to hydrolysis, giving them some potential for biodegradation. The backbone frequently contains heteroatoms (O, N), and the atom economy is less than 100% due to the loss of water or HCl.
相比之下，缩合聚合物需要两个不同的官能团（可位于同一或不同单体上），并产生小分子副产物。其重复单元含有酯键或酰胺键，这些键易水解，赋予它们一定的生物降解潜力。主链常含有杂原子（O、N），且由于失去水或HCl，原子经济性低于100%。
The table below highlights key differences:
下表强调了关键区别：
| Feature | Addition polymerisation | Condensation polymerisation |
|---|---|---|
| Monomer type | Alkenes (C=C) | Two functional groups (e.g., –OH, –COOH, –NH₂) |
| Small molecule lost? | No | Yes (H₂O, HCl) |
| Backbone composition | Carbon only (usually saturated) | Contains O or N (ester/amide links) |
| Atom economy | 100% | <100% |
| Biodegradability | Generally low | Hydrolysable; some are biodegradable (PLA, proteins) |
9. Thermoplastic and Thermosetting Polymers | 热塑性与热固性聚合物
Polymers are often classified by their response to heat. Thermoplastic polymers (e.g., PE, PP, PS, PVC, PET, nylon) consist of linear or branched chains with no cross-links between them. When heated, the weak intermolecular forces (van der Waals, dipole–dipole interactions or hydrogen bonds) are overcome, allowing chains to slide past each other. This softening–hardening cycle can be repeated many times, making thermoplastics economical to mould and recycle.
聚合物常按对热的响应分类。热塑性聚合物（如PE、PP、PS、PVC、PET、尼龙）由线型或支链型分子链组成，链间无交联。加热时，较弱的分子间力（范德华力、偶极-偶极作用或氢键）被克服，使得链可以彼此滑移。这种软化-硬化循环可多次重复，使热塑性塑料易于模塑和回收。
Thermosetting polymers (e.g., Bakelite, melamine, epoxy resins) contain extensive covalent cross-links forming a rigid three-dimensional network. Once set, they do not soften on reheating; excessive heat causes decomposition. They are hard, infusible, and ideal for high-heat applications such as electrical fittings, adhesives and kitchen worktops. The initial curing process is irreversible.
热固性聚合物（如电木、三聚氰胺树脂、环氧树脂）含有大量的共价交联，形成刚性的三维网络。一经固化，再加热不会软化；过度加热导致分解。它们坚硬、不熔，非常适用于高温场合，如电器配件、黏合剂和厨房台面。初始固化过程是不可逆的。
In A-Level questions, students must relate structure to properties. The absence of cross-links in thermoplastics explains their flexibility and recyclability, while extensive cross-linking in thermosets accounts for their rigidity and resistance to solvents.
在A-Level考题中，学生需要将结构与性质关联。热塑性塑料缺乏交联，解释了其柔韧性和可回收性；而热固性塑料广泛交联，则解释了其刚性和耐溶剂性。
10. Environmental Issues and Recycling | 环境问题与回收
Many synthetic addition polymers (polyalkenes) are non-biodegradable because their saturated hydrocarbon backbones resist enzymatic attack. This leads to persistent environmental waste, with plastics accumulating in landfills and oceans. Burning waste plastics can generate toxic gases – for example, PVC produces hydrogen chloride when incinerated. Additionally, the monomers for addition polymers are derived from crude oil, a non-renewable resource.
许多合成加成聚合物（聚烯烃）不可生物降解，因为其饱和碳氢主链能抵抗酶的攻击。这就导致持久性环境废物，塑料在填埋场和海洋中不断累积。焚烧废弃塑料会产生有毒气体——例如，PVC焚烧时产生氯化氢。此外，加成聚合物的单体源于原油，是一种不可再生资源。
Strategies to mitigate these issues include mechanical recycling, where plastics are sorted, cleaned, melted and remoulded; feedstock recycling, which breaks polymers back into monomers or oils; and incineration with energy recovery. However, mixed polymer waste and additives complicate recycling. Condensation polyesters and polyamides, with hydrolysable links, can be chemically recycled or depolymerised under controlled conditions.
缓解这些问题的策略包括机械回收——将塑料分类、清洗、熔融并再成型；原料回收——将聚合物分解回单体或油类；以及带能量回收的焚烧。然而，混合聚合物废料和添加剂使回收变得复杂。含有可水解键的缩合聚酯和聚酰胺可在受控条件下化学回收或解聚。
Biodegradable polymers such as PLA and polyhydroxyalkanoates (PHAs) offer a compostable alternative. They degrade through hydrolysis of ester bonds catalysed by microbial enzymes. Nonetheless, industrial composting conditions are often needed for complete breakdown, and their disposal in conventional landfills may still persist for years.
可生物降解聚合物如PLA和聚羟基脂肪酸酯（PHAs）提供了一种可堆肥的替代方案。它们通过微生物酶催化的酯键水解而降解。然而，完全降解常常需要工业堆肥条件，若在常规填埋场中处置，仍可能存留多年。
11. Biopolymers (Natural Condensation Polymers) | 生物聚合物（天然缩合聚合物）
Nature produces condensation polymers that are vital for life. Proteins are linear polyamides built from up to 20 different α-amino acids. Each protein chain folds into a specific conformation, dictated by the sequence of its monomers. The primary structure is simply the sequence of amino acids held by peptide bonds (–CONH–). Biodegradation of proteins is mediated by proteases that cleave these amide links.
自然界产生的缩合聚合物对生命至关重要。蛋白质是由多达20种不同的α-氨基酸构成的线型聚酰胺。每条蛋白质链根据其单体序列折叠成特定的构象。一级结构即氨基酸通过肽键（–CONH–）连接的序列。蛋白质的生物降解由蛋白酶介导，这些酶切割酰胺键。
Polysaccharides such as starch and cellulose are condensation polymers of glucose. Each glycosidic bond (–O–) forms with the elimination of water. Cellulose, a structural polymer in plant cell walls, consists of β-glucose units linked by 1,4-glycosidic bonds; starch (amylose) uses α-glucose. Despite the same monomer, the difference in stereochemistry yields profoundly different properties and digestibility.
多糖如淀粉和纤维素是葡萄糖的缩合聚合物。每个糖苷键（–O–）的形成脱去一分子水。纤维素是植物细胞壁中的结构聚合物，由β-葡萄糖单元通过1,4-糖苷键连接；淀粉（直链淀粉）则使用α-葡萄糖。尽管单体相同，立体化学的差异带来了全然不同的性质和可消化性。
DNA is a condensation polymer of nucleotides, each consisting of a phosphate group, a deoxyribose sugar and a nitrogenous base. The phosphate–sugar backbone is linked by phosphodiester bonds, also formed via condensation. The sequence of bases encodes genetic information. Understanding these natural polymers reinforces the principle that condensation linkages (amide, ester, glycosidic and phosphodiester) are ubiquitous in biology.
DNA是核苷酸的缩合聚合物，每个核苷酸由磷酸基团、脱氧核糖和含氮碱基组成。磷酸-糖骨架通过磷酸二酯键连接，同样由缩合形成。碱基序列编码遗传信息。理解这些天然聚合物进一步巩固了一个原理：缩合连接（酰胺、酯、糖苷和磷酸二酯键）在生物学中无处不在。
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