Addition Polymerisation | 加成聚合反应

📚 Addition Polymerisation | 加成聚合反应

Addition polymerisation is a process in which unsaturated monomers, containing carbon-carbon double bonds, join together to form long-chain polymers without the elimination of any small molecules. It is one of the most important industrial reactions in modern chemistry, producing materials such as poly(ethene), poly(propene), PVC and polystyrene.

加成聚合是含有碳碳双键的不饱和单体相互连接形成长链聚合物，且不消除任何小分子的过程。这是现代化学中最重要的工业反应之一，可生产聚乙烯、聚丙烯、聚氯乙烯及聚苯乙烯等材料。

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1. Monomers and Polymers | 单体与聚合物

Monomers are small molecules that can join together to form polymers. In addition polymerisation, each monomer must contain at least one carbon-carbon double bond (C=C), making it unsaturated.

单体是能够相互连接形成聚合物的小分子。在加成聚合中，每个单体必须含有至少一个碳碳双键（C=C），因此为不饱和化合物。

Polymers are large molecules made up of many repeating structural units derived from the monomer. The process of forming a polymer from monomers is called polymerisation.

聚合物是由源自单体的许多重复结构单元组成的大分子。由单体形成聚合物的过程称为聚合反应。

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2. Key Monomers | 关键单体

The most common monomers for addition polymerisation are alkenes and their derivatives. The table below shows key examples:

加成聚合最常见的单体是烯烃及其衍生物。下表展示了关键示例：

Monomer | 单体	Formula | 化学式	Polymer | 聚合物
Ethene | 乙烯	CH₂=CH₂	Poly(ethene) | 聚乙烯
Propene | 丙烯	CH₃CH=CH₂	Poly(propene) | 聚丙烯
Chloroethene | 氯乙烯	CH₂=CHCl	Poly(chloroethene) (PVC) | 聚氯乙烯
Phenylethene | 苯乙烯	C₆H₅CH=CH₂	Poly(phenylethene) | 聚苯乙烯
Tetrafluoroethene | 四氟乙烯	CF₂=CF₂	Poly(tetrafluoroethene) (PTFE) | 聚四氟乙烯

Note that the polymer name is derived from the monomer name by adding the prefix ‘poly’. If the monomer name contains more than one word or requires clarity, the monomer name is placed in brackets.

请注意，聚合物名称的命名规则是在单体名称前添加前缀”poly”（聚）。若单体名称含多个单词或为求清晰，将单体名称置于括号中。

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3. What is Addition Polymerisation? | 什么是加成聚合？

Addition polymerisation involves the repeated addition of unsaturated monomer molecules to a growing polymer chain. The carbon-carbon double bond opens up, allowing monomers to link together. Crucially, no small molecules such as water or HCl are eliminated during this process.

加成聚合是不饱和单体分子反复加成到生长中的聚合物链上的过程。碳碳双键打开，使单体连接在一起。关键在于，在此过程中不会消除水或HCl等任何小分子。

The general equation for addition polymerisation can be written as:

加成聚合的通式可写为：

n CH₂=CHR → (-CH₂-CHR-)ₙ

Where R can be H, CH₃, Cl, C₆H₅, etc. The letter n represents a large number, typically thousands or even millions.

其中R可以是H、CH₃、Cl、C₆H₅等。字母n代表大数目，通常为数千甚至数百万。

Since the double bond is converted into single bonds during polymerisation, the polymer is saturated. This is why addition polymers are generally chemically unreactive.

由于聚合过程中双键转化为单键，聚合物为饱和化合物。这就是加成聚合物通常化学性质不活泼的原因。

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4. The Free-Radical Mechanism | 自由基机理

Under the Cambridge A-Level specification, the mechanism of addition polymerisation is studied through free-radical polymerisation. This involves three key stages: initiation, propagation and termination.

在剑桥A-Level考试大纲中，加成聚合的机理通过自由基聚合来学习。这涉及三个关键阶段：引发、增长和终止。

An initiator molecule, such as an organic peroxide (ROOR), undergoes homolytic fission when heated or exposed to UV light, producing free radicals.

引发剂分子（如有机过氧化物ROOR）在加热或紫外光照射下发生均裂，产生自由基。

ROOR → 2RO•

Free radicals are species with an unpaired electron, making them highly reactive. The dot (•) represents the unpaired electron.

自由基是含有未配对电子的物种，具有极高的反应活性。圆点（•）表示未配对电子。

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5. Initiation | 引发阶段

In the initiation step, the free radical RO• attacks the carbon-carbon double bond of the alkene monomer. The π bond breaks, and a new radical species is formed:

在引发步骤中，自由基RO•攻击烯烃单体的碳碳双键。π键断裂，形成新的自由基物种：

RO• + CH₂=CH₂ → ROCH₂CH₂•

This new radical is still highly reactive and can go on to react with more monomer molecules. Each radical produces one growing chain in the initiation step.

这个新的自由基仍然非常活跃，可以继续与更多单体分子反应。在引发步骤中，每个自由基仅产生一条增长链。

It is important to note that the initiator is consumed in this step, and the concentration of free radicals determines the overall rate of polymerisation.

值得注意的是，引发剂在此步骤中被消耗，自由基的浓度决定了聚合的总体速率。

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6. Propagation | 增长阶段

During propagation, the growing polymer chain continuously reacts with monomer molecules. Each addition extends the chain by one monomer unit:

在增长阶段，生长中的聚合物链持续与单体分子反应。每加成一次，链就延长一个单体单元：

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