Polymerisation | 聚合反应

📚 Polymerisation | 聚合反应

Polymerisation is the chemical process in which many small, relatively simple molecules called monomers are joined together by covalent bonds to form a very large molecule called a polymer. The number of repeating units in the polymer chain is represented by n, the degree of polymerisation, and a single polymer molecule may contain thousands of monomer units.

聚合反应是将许多称为单体的小分子通过共价键连接起来，形成一种非常大分子（称为聚合物）的化学过程。聚合物链中重复单元的数目用 n 表示，即聚合度；一个聚合物分子中可能含有成千上万个单体单元。

At A Level, polymerisation is divided into two major types: addition polymerisation, which is typical of alkenes, and condensation polymerisation, which produces polyesters and polyamides. This article explains both mechanisms, common examples, how to identify monomers from repeat units, and the environmental problems associated with synthetic polymers.

在 A Level 课程中，聚合反应分为两大类：加成聚合（烯烃的典型反应）和缩合聚合（生成聚酯和聚酰胺）。本文介绍两种机理、常见实例、如何从重复单元识别单体，以及合成聚合物带来的环境问题。

1. What Is a Polymer? | 什么是聚合物？

A polymer is a giant covalent or macromolecular structure built from repeating structural units called monomers. The monomer must contain at least one reactive functional group, or in the case of addition polymerisation a carbon-carbon double bond, so that it can form at least two new bonds and extend a chain.

聚合物是由称为单体的重复结构单元构成的巨型共价结构或大分子。单体必须至少含有一个活泼官能团，或在加成聚合中含有一个碳碳双键，以便能形成至少两个新键并延伸分子链。

Polymers can be natural, such as cellulose, starch, DNA and proteins, or synthetic, such as poly(ethene), PVC, nylon and PET. The physical properties of a polymer depend on chain length, branching, side groups, and the strength of intermolecular forces between chains.

聚合物可以是天然的，如纤维素、淀粉、DNA 和蛋白质；也可以是合成的，如聚乙烯、PVC、尼龙和 PET。聚合物的物理性质取决于链长、支化程度、侧基以及链间分子间作用力的强弱。

2. Addition Polymerisation | 加成聚合

Addition polymerisation occurs when unsaturated monomers, usually alkenes, join together without the elimination of any small molecule. The carbon-carbon double bond opens, and monomer units add to one another to form a saturated carbon backbone.

加成聚合发生在不饱和单体（通常是烯烃）之间，单体相互连接时不会脱去任何小分子。碳碳双键打开，单体单元彼此加成，形成饱和碳骨架。

The general equation for the polymerisation of ethene is:

乙烯聚合的总方程式为：

n CH₂=CH₂ → –[–CH₂–CH₂–]ₙ–

The product is poly(ethene), commonly called polythene. The double bond is the key functional group: each monomer must have a C=C bond because this bond provides two new single bonds for chain extension.

产物是聚乙烯，通常称为聚乙烯塑料。双键是关键官能团：每个单体必须具有 C=C 键，因为该键提供两个新的单键用于链的增长。

Addition polymerisation is a chain reaction. It can be initiated by free radicals, and it produces only the polymer as the reaction product. No atoms are lost, so the empirical formula of the polymer repeat unit is exactly the same as that of the monomer.

加成聚合是一种链式反应。它可以由自由基引发，反应产物只有聚合物。由于没有原子损失，聚合物重复单元的经验式与单体完全相同。

3. Free-Radical Mechanism of Addition Polymerisation | 加成聚合的自由基机理

The polymerisation of ethene is often described as a free-radical chain reaction. It proceeds in three stages: initiation, propagation and termination. An organic peroxide, written as ROOR, is commonly used as the initiator because the O–O bond is weak and breaks homolytically under heat or UV light.

乙烯的聚合通常用自由基链式反应来描述。反应分为三个阶段：引发、增长和终止。常用有机过氧化物 ROOR 作为引发剂，因为其 O–O 键较弱，在加热或紫外光下可发生均裂。

Initiation: the peroxide forms two alkoxy radicals.

引发：过氧化物生成两个烷氧基自由基。

ROOR → 2 RO•

Propagation: a radical attacks the C=C bond of ethene, producing a new carbon radical. This radical can attack another ethene molecule, so the chain grows rapidly.

增长：自由基进攻乙烯的 C=C 键，生成新的碳自由基。该自由基可以继续进攻另一个乙烯分子，因此链迅速增长。

RO• + CH₂=CH₂ → RO–CH₂–CH₂•

RO–CH₂–CH₂• + CH₂=CH₂ → RO–CH₂–CH₂–CH₂–CH₂•

Termination: two radicals combine to form a stable covalent bond, ending that particular chain.

终止：两个自由基结合形成稳定的共价键，使该链终止。

RO–(CH₂–CH₂)ₘ• + •(CH₂–CH₂)ₙ–OR → RO–(CH₂–CH₂)ₘ₊ₙ–OR

The free-radical method is industrially important because it allows poly(ethene) to be produced under high pressure and at moderate temperatures. However, the resulting polymer can have some branching due to side reactions.

自由基法在工业上非常重要，因为它可以在高压和中等温度下生产聚乙烯。但所得聚合物由于副反应可能带有一定支链。

4. Common Addition Polymers | 常见加成聚合物

Many familiar plastics are addition polymers. Their names are derived from the monomer by placing ‘poly’ before the monomer name, and the monomer name is put in brackets if it has more than one word.

许多常见塑料都是加成聚合物。它们的名称是在单体名称前加上 poly，若单体名称超过一个单词则使用括号。

The table below summarises the monomers, polymers and typical uses of several addition polymers.

下表总结了若干加成聚合物的单体、聚合物和典型用途。

Monomer \| 单体	Polymer \| 聚合物	Common uses \| 常见用途
Ethene, CH₂=CH₂	Poly(ethene), –[–CH₂–CH₂–]ₙ–	Plastic bags, bottles, packaging \| 塑料袋、瓶子、包装
Propene, CH₂=CHCH₃	Poly(propene), –[–CH₂–CH(CH₃)–]ₙ–	Crates, ropes, carpets \| 周转箱、绳索、地毯
Chloroethene, CH₂=CHCl	Poly(chloroethene), –[–CH₂–CHCl–]ₙ–	PVC pipes, window frames, cable insulation \| PVC 管材、窗框、电缆绝缘层
Tetrafluoroethene, CF₂=CF₂	Poly(tetrafluoroethene), –[–CF₂–CF₂–]ₙ–	Non-stick coatings, PTFE tape \| 不粘涂层、PTFE 密封带
Phenylethene, CH₂=CHC₆H₅	Poly(phenylethene), –[–CH₂–CH(C₆H₅)–]ₙ–	Packaging, insulation, disposable cups \| 包装、绝缘材料、一次性杯子

When drawing the repeat unit of an addition polymer, you must show the extended carbon chain with the side groups attached, and place the whole repeat unit inside square brackets with the subscript n outside. There should be no double bond in the repeat unit of a polyalkene.

绘制加成聚合物的重复单元时，必须画出带有侧基的延伸碳链，并将整个重复单元置于方括号内，外标下标 n。聚烯烃的重复单元中不应出现双键。

5. Condensation Polymerisation | 缩合聚合

Condensation polymerisation occurs when monomers with two functional groups react together, and a small molecule such as water or hydrogen chloride is eliminated each time a new link is formed. The two most important classes are polyesters and polyamides.

缩合聚合发生在具有两个官能团的单体之间，每形成一个新的连接键就会脱去一个小分子，如水或氯化氢。最重要的两类缩聚物是聚酯和聚酰胺。

For condensation polymerisation to occur, each monomer must contain two reactive functional groups, either two of the same group or two different groups. For example, a diol and a dicarboxylic acid can react to form a polyester, while a diamine and a dicarboxylic acid can form a polyamide.

要发生缩合聚合，每个单体必须含有两个活性官能团，可以是两个相同基团，也可以是两个不同基团。例如，二元醇和二元羧酸可反应生成聚酯，而二元胺和二元羧酸可生成聚酰胺。

Unlike addition polymerisation, condensation polymerisation produces two products: the polymer and a small molecule. The empirical formula of the repeat unit is therefore no longer identical to the monomers because atoms have been removed.

与加成聚合不同，缩合聚合生成两种产物：聚合物和小分子。因此重复单元的经验式不再与单体相同，因为部分原子被脱除。

6. Polyesters | 聚酯

A polyester is formed from a diol and a dicarboxylic acid, or from a single monomer containing both an alcohol group and a carboxylic acid group. The ester linkage, –COO–, joins the monomer units together, and water is eliminated during the reaction.

聚酯由二元醇和二元羧酸形成，或由同时含有醇基和羧酸基的单体形成。酯键 –COO– 将单体单元连接起来，反应过程中脱除水。

A common A Level example is the formation of poly(ethylene terephthalate), PET, from ethane-1,2-diol and benzene-1,4-dicarboxylic acid, also called terephthalic acid.

A Level 中常见的例子是用乙二醇（乙烷-1,2-二醇）和对苯二甲酸（苯-1,4-二甲酸）形成聚对苯二甲酸乙二醇酯，即 PET。

n HOCH₂CH₂OH + n HOOC–C₆H₄–COOH → –[–OCH₂CH₂OOC–C₆H₄–CO–]ₙ– + 2n H₂O

The repeat unit contains two ester linkages: one from each end of the ethane-1,2-diol and one from each end of the dicarboxylic acid. PET is used widely for drink bottles, clothing fibres and food containers.

重复单元含有两个酯键：一个来自乙二醇的两端，另一个来自对苯二甲酸的两端。PET 广泛用于饮料瓶、服装纤维和食品容器。

When drawing a polyester repeat unit, you should show the ester group clearly as –O–C(=O)–, or in a simplified form as –OOC– or –COO–, and ensure the chain extends on both sides of the bracket.

绘制聚酯重复单元时，应清晰地标示酯基 –O–C(=O)–，也可简写为 –OOC– 或 –COO–，并确保链在括号两侧继续延伸。

7. Polyamides and Proteins | 聚酰胺与蛋白质

A polyamide is formed from a diamine and a dicarboxylic acid, or from an amino acid. The amide linkage, –CONH–, is formed between the monomers, and water is eliminated. The most common synthetic polyamide is nylon-6,6, made from hexane-1,6-diamine and hexanedioic acid.

聚酰胺由二元胺和二元羧酸或氨基酸形成。单体之间形成酰胺键 –CONH–，同时脱去水。最常见的合成聚酰胺是尼龙-6,6，由己烷-1,6-二胺和己二酸制成。

n H₂N–(CH₂)₆–NH₂ + n HOOC–(CH₂)₄–COOH → –[–NH–(CH₂)₆–NHCO–(CH₂)₄–CO–]ₙ– + 2n H₂O

Each nylon-6,6 repeat unit has two amide links. The ‘6,6’ name refers to the fact that both monomers have six carbon atoms. Nylon is used in textiles, ropes, carpets and engineering components because of its high tensile strength and durability.

每个尼龙-6,6 重复单元含有两个酰胺键。名称中的 6,6 表示两种单体都含 6 个碳原子。尼龙因具有高拉伸强度和耐用性，被广泛用于纺织品、绳索、地毯和工程部件。

Proteins are natural polyamides, also called polypeptides. They are formed by the condensation of amino acids, in which the amine group of one amino acid reacts with the carboxylic acid group of another, forming a peptide bond and releasing water.

蛋白质是天然聚酰胺，也称为多肽。它们由氨基酸缩合而成，一个氨基酸的氨基与另一个氨基酸的羧基反应，形成肽键并释放水。

H₂N–CHR–COOH + H₂N–CHR’–COOH → H₂N–CHR–CONH–CHR’–COOH + H₂O

The peptide link is exactly the same as the amide link in nylon. Proteins can be hydrolysed back into their constituent amino acids by heating with aqueous acid or alkali.

肽键与尼龙中的酰胺键完全相同。蛋白质可以通过与稀酸或稀碱共热而水解回其组成氨基酸。

8. Hydrolysis of Polymers | 聚合物的水解

Addition polymers such as poly(ethene) and poly(propene) are generally resistant to hydrolysis because their backbones consist of strong, non-polar C–C and C–H bonds. They do not contain polar functional groups that water can attack easily, so they persist in the environment for a very long time.

聚乙烯和聚丙烯等加成聚合物通常难以水解，因为其主链由强非极性的 C–C 和 C–H 键构成。它们不含水容易进攻的极性官能团，因此在环境中持久存在。

Condensation polymers, in contrast, contain polar ester or amide linkages that can be hydrolysed. Polyesters undergo hydrolysis to give back the diol and dicarboxylic acid, while polyamides hydrolyse to give the diamine and dicarboxylic acid.

相反，缩合聚合物含有极性的酯键或酰胺键，可被水解。聚酯水解后重新生成二元醇和二元羧酸，聚酰胺水解后生成二元胺和二元羧酸。

Acid hydrolysis uses dilute hydrochloric acid or sulfuric acid and heat. For example, a polyester repeat unit can be hydrolysed as follows:

酸水解使用稀盐酸或稀硫酸并加热。例如，聚酯重复单元可按如下方式水解：

–[–OCH₂CH₂OOC–C₆H₄–CO–]ₙ– + 2n H₂O → n HOCH₂CH₂OH + n HOOC–C₆H₄–COOH

Alkaline hydrolysis of a polyester produces the dicarboxylate salt rather than the free acid. In the case of a polyamide, acid or alkaline hydrolysis breaks the amide link, giving an amine and a carboxylic acid or its salt. This is why condensation polymers can be designed to be biodegradable under certain conditions.

聚酯的碱性水解生成二羧酸盐而非游离酸。对于聚酰胺，酸或碱水解会断裂酰胺键，生成胺和羧酸或其盐。这正是缩合聚合物在一定条件下可被设计为可生物降解的原因。

9. Identifying Monomers from a Polymer | 由聚合物识别单体

A common A Level skill is to deduce the monomer structure from a given polymer repeat unit. For addition polymers, the repeat unit has a saturated carbon backbone with no ester or amide groups. To find the monomer, take two adjacent carbon atoms in the backbone, remove the side groups from one carbon if necessary, and convert the C–C single bond into a C=C double bond.

A Level 常见技能是根据给定的聚合物重复单元推断单体结构。对于加成聚合物，重复单元具有饱和碳骨架，没有酯基或酰胺基。要找到单体，取主链上两个相邻碳原子，必要时移去侧基，并将 C–C 单键改为 C=C 双键。

For example, the repeat unit –[–CH₂–CHCl–]ₙ– comes from the monomer chloroethene, CH₂=CHCl. The repeat unit –[–CH₂–CH(CH₃)–]ₙ– comes from propene, CH₂=CHCH₃.

例如，重复单元 –[–CH₂–CHCl–]ₙ– 来自单体氯乙烯 CH₂=CHCl；重复单元 –[–CH₂–CH(CH₃)–]ₙ– 来自丙烯 CH₂=CHCH₃。

For condensation polymers, identify the ester or amide link in the repeat unit. Break the bond at the link, and then add an –OH group to the carbonyl carbon and an –H atom to the oxygen or nitrogen atom. This restores the original functional groups: a carboxyl group and a hydroxyl group for polyesters, or a carboxyl group and an amine group for polyamides.

对于缩合聚合物，先识别重复单元中的酯键或酰胺键。在连接处断开，将 –OH 加到羰基碳上，将 –H 加到氧或氮原子上。这样可恢复原始官能团：聚酯得到羧基和羟基，聚酰胺得到羧基和氨基。

Always count the carbon atoms carefully when reconstructing the monomer. For example, if the repeat unit contains –OCH₂CH₂OOC–C₆H₄–CO–, the two monomers are HOCH₂CH₂OH and HOOC–C₆H₄–COOH.

重建单体时务必仔细数碳原子。例如，若重复单元含有 –OCH₂CH₂OOC–C₆H₄–CO–，则两种单体为 HOCH₂CH₂OH 和 HOOC–C₆H₄–COOH。

10. Environmental Impact of Polymers

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Polymerisation | 聚合反应