A-Level Chemistry: Polymer Structure Determination | A-Level 化学：聚合物结构推断

📚 A-Level Chemistry: Polymer Structure Determination | A-Level 化学：聚合物结构推断

Polymers are large molecules formed from many small monomer units linked together. In CIE A-Level Chemistry, you are expected to deduce the structure of a polymer from its monomer(s) and to work backwards from a polymer to identify the original monomer(s). The key is to recognise the type of polymerisation and to handle the repeating unit systematically.

聚合物是由许多单体单元连接而成的大分子。在 CIE A-Level 化学中，你既需要根据单体推断聚合物的结构，也需要根据聚合物反推原始单体。关键在于识别聚合类型并有条不紊地处理重复单元。

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1. What is a Polymer? | 什么是聚合物？

A polymer is a long-chain molecule built from many small molecules called monomers. In an addition polymer, monomers add together without losing any atoms; in a condensation polymer, monomers join with the loss of small molecules such as water. The repeating unit is the smallest group of atoms that repeats along the chain, and it is written in square brackets with a subscript n.

聚合物是由许多称为单体的小分子连接而成的长链分子。在加成聚合物中，单体直接加合而不失去任何原子；在缩合聚合物中，单体连接时会失去水等小分子。重复单元是沿链重复的最小原子团，写于方括号中并加下标 n。

For example, poly(ethene) has the repeating unit [-CH₂-CH₂-]ₙ. The subscript n represents the degree of polymerisation, which is the number of repeating units in the chain. This notation applies to both addition and condensation polymers.

例如，聚乙烯的重复单元是 [-CH₂-CH₂-]ₙ。下标 n 表示聚合度，即链中重复单元的数目。这种记法同时适用于加成聚合物和缩合聚合物。

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2. Repeating Units vs Monomer Units | 重复单元与单体单元

To avoid exam errors, distinguish clearly between a monomer and a repeating unit. A monomer is the starting molecule, such as CH₂=CH₂. A repeating unit is the part of the polymer chain that repeats, such as -CH₂-CH₂-. In an addition polymer, the repeating unit has exactly the same atoms as the monomer, but the double bond has been replaced by a single bond.

为避免考试错误，需要清晰区分单体和重复单元。单体是起始分子，例如 CH₂=CH₂。重复单元是聚合物链中重复的部分，例如 -CH₂-CH₂-。在加成聚合物中，重复单元与单体的原子完全相同，但双键已被替换为单键。

In a condensation polymer, the repeating unit is not the same as the monomer because water (or another small molecule) is removed. Therefore, when you work backwards, you must add back the elements of water to regenerate the monomers.

在缩合聚合物中，重复单元与单体不同，因为反应中失去了水（或其他小分子）。因此，当反向推导时，必须加回水的元素以还原单体。

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3. Addition Polymers: From Monomer to Polymer | 加成聚合物：从单体到聚合物

Addition polymerisation involves alkenes. For a generic vinyl monomer CH₂=CHR, where R is a hydrogen atom, halogen, alkyl group or other substituent, the polymerisation reaction is:

加成聚合涉及烯烃。对于通用乙烯基单体 CH₂=CHR，其中 R 可以是氢原子、卤素、烷基或其他取代基，聚合反应为：

n CH₂=CHR → [-CH₂-CHR-]ₙ

Common examples include poly(ethene) from ethene, poly(chloroethene) from chloroethene (CH₂=CHCl), and poly(phenylethene) from phenylethene (CH₂=CHC₆H₅). Notice that the double bond opens up and becomes a single bond between two carbon atoms.

常见例子包括：由乙烯得到聚乙烯，由氯乙烯（CH₂=CHCl）得到聚氯乙烯，由苯乙烯（CH₂=CHC₆H₅）得到聚苯乙烯。注意双键打开并在两个碳原子之间形成单键。

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4. Working Backwards: Monomer from Repeating Unit | 逆向推导：从重复单元到单体

When you are given the repeating unit of an addition polymer, the strategy is simple: place a double bond between the two carbon atoms of the main chain. The substituents stay exactly where they are.

当你得到加成聚合物的重复单元时，策略很简单：在主链的两个碳原子之间画一个双键。取代基的位置保持不变。

For example, the repeating unit [-CH₂-CH(CH₃)-]ₙ comes from the monomer CH₂=CHCH₃, which is propene. If the repeating unit is [-CH₂-CHCl-]ₙ, the monomer is CH₂=CHCl, chloroethene.

例如，重复单元 [-CH₂-CH(CH₃)-]ₙ 来自单体 CH₂=CHCH₃，即丙烯。若重复单元为 [-CH₂-CHCl-]ₙ，则单体是 CH₂=CHCl，即氯乙烯。

If both carbon atoms carry substituents, such as in [-CHCl-CHCl-]ₙ, the monomer must be CHCl=CHCl, which is 1,2-dichloroethene. Always check that each carbon atom in the monomer has four bonds after adding the double bond.

如果两个碳原子上都有取代基，例如 [-CHCl-CHCl-]ₙ，则单体必须是 CHCl=CHCl，即 1,2-二氯乙烯。始终检查加入双键后每个碳原子是否满足四价。

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5. Condensation Polymers: Polyesters and Polyamides | 缩合聚合物：聚酯和聚酰胺

Condensation polymers contain functional groups in the main chain. The two most important families are polyesters and polyamides. A polyester forms from a dicarboxylic acid and a diol; a polyamide forms from a dicarboxylic acid and a diamine.

缩合聚合物在主链中含有官能团。两个最重要的类别是聚酯和聚酰胺。聚酯由二元羧酸和二醇形成；聚酰胺由二元羧酸和二胺形成。

n HO-R-OH + n HOOC-R’-COOH → [-O-R-O-CO-R’-CO-]ₙ + 2n H₂O

The equation above shows the formation of a polyester. The repeating unit contains both the diol part (-O-R-O-) and the diacid part (-CO-R’-CO-). The small molecule eliminated is water, and two water molecules are released for each pair of monomers.

上面的方程式表示聚酯的形成。重复单元包含二醇部分 (-O-R-O-) 和二酸部分 (-CO-R’-CO-)。消除的小分子是水，每对单体释放两分子水。

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6. Identifying Linkages in Condensation Polymers | 识别缩合聚合物中的连接键

To find the monomers from a polyester repeating unit, locate the ester link -COO-. The carbonyl carbon (C=O) came from the dicarboxylic acid, while the oxygen that is single-bonded to the next carbon came from the diol.

要从聚酯重复单元中找单体，先定位酯键 -COO-。羰基碳（C=O）来自二元羧酸，而与下一个碳成单键的氧来自二醇。

Now break the C-O single bond in the ester. Add -OH to the carbonyl carbon to restore a carboxylic acid group, and add -H to the oxygen to restore an alcohol group. This gives you the two monomers.

然后断开酯键中的 C-O 单键。在羰基碳上加 -OH 恢复羧基，在氧上加 -H 恢复羟基。这样就得到了两个单体。

For a polyamide, break the C-N single bond in the amide link -CONH-. Add -OH to the carbonyl carbon to reform a carboxylic acid, and add -H to the nitrogen atom to reform an amine. This method works for nylon and Kevlar type polymers.

对于聚酰胺，断开酰胺键 -CONH-

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