📚 Condensation Polymerisation Mechanisms Explained | 缩合聚合反应机理详解
Condensation polymerisation is a fundamental topic in A-Level Chemistry. It involves the joining of small molecules (monomers) through the formation of covalent bonds, with the simultaneous elimination of a small molecule such as water, hydrogen chloride, or ammonia. Understanding the mechanism is essential for predicting products, drawing repeating units, and explaining polymer properties.
缩合聚合是 A-Level 化学中的核心考点。它通过单体间形成共价键，同时脱去水、氯化氢或氨等小分子而实现。掌握其机理对于预测产物、书写重复单元以及解释聚合物性质至关重要。
1. What is Condensation Polymerisation? | 什么是缩合聚合？
Condensation polymerisation is a type of polymerisation in which monomers with at least two functional groups react to form a polymer, releasing a small molecule (often water) during each bond-forming step. The process is stepwise and the polymer chain grows gradually as monomers and small oligomers react with each other.
缩合聚合是一种聚合反应：含有至少两个官能团的单体相互反应生成聚合物，并在每一步成键过程中释放一个小分子（通常是水）。该过程是逐步进行的，聚合物链随着单体与低聚物之间的反应而逐渐增长。
Each step forms a covalent bond between two monomers.
每一步都会在两个单体之间形成一个共价键。
Small molecules such as H₂O, HCl or NH₃ are eliminated.
同时脱去 H₂O、HCl 或 NH₃ 等小分子。
The polymer chain grows slowly; the monomer concentration decreases continuously.
聚合物链增长缓慢；单体浓度不断下降。
2. Requirements for Monomers | 单体要求
For condensation polymerisation to occur, each monomer must have at least two functional groups capable of reacting with each other. These functional groups are typically carboxylic acids (-COOH), alcohols (-OH), amines (-NH₂), or acyl chlorides (-COCl).
发生缩合聚合时，每个单体必须至少含有两个能够互相反应的官能团。这些官能团通常是羧基（-COOH）、羟基（-OH）、氨基（-NH₂）或酰氯基（-COCl）。
One monomer can have two same groups, e.g. ethane-1,2-diol (HO-CH₂-CH₂-OH).
一种单体可含有两个相同基团，例如乙二醇（HO-CH₂-CH₂-OH）。
Two different monomers with complementary groups may be used, e.g. a dicarboxylic acid and a diamine.
也可使用两种官能团互补的单体，例如二羧酸和二胺。
If a monomer has both -COOH and -OH on the same molecule, it can self-polymerise, e.g. hydroxybutanoic acid.
若一个单体同时含有 -COOH 和 -OH，则可发生自聚，例如羟基丁酸。
Another key point is that at least two reactive groups per monomer are required so that the chain can continue to grow in both directions. A monofunctional molecule would act as a terminating agent.
另一个关键点是每个单体至少需要两个反应基团，以便链在两端都能继续增长。若只含一个官能团，则该分子将起封端剂作用。
3. Mechanism: Polyester Formation | 机理：聚酯的生成
Polyesters are formed by the reaction between a dicarboxylic acid and a diol, or between a hydroxy acid. The key step is a condensation reaction between a -COOH group and an -OH group, producing an ester linkage (-COO-) and releasing water.
聚酯由二羧酸与二醇反应生成，或由羟基酸自聚生成。关键步骤是 -COOH 与 -OH 之间的缩合反应，形成酯键（-COO-）并释放水。
For example, the polymerisation of ethane-1,2-diol and benzene-1,4-dicarboxylic acid proceeds as follows:
例如，乙二醇与对苯二甲酸的聚合反应如下：
n HO-CH₂-CH₂-OH + n HOOC-C₆H₄-COOH → [-O-CH₂-CH₂-O-CO-C₆H₄-CO-]ₙ + 2n H₂O
The mechanism is essentially nucleophilic addition-elimination at the carbonyl carbon. The lone pair on the alcohol oxygen attacks the electron-deficient carbonyl carbon, forming a tetrahedral intermediate. Then a proton is transferred and water leaves, giving the ester linkage.
该机理本质上是在羰基碳上发生的亲核加成-消除反应。醇氧上的孤对电子进攻缺电子的羰基碳，形成四面体中间体，随后发生质子转移并脱去一分子水，最终形成酯键。
Step 1: Nucleophilic attack of -OH on the carbonyl carbon.
第一步：-OH 对羰基碳进行亲核进攻。
Step 2: Proton transfer and elimination of H₂O.
第二步：质子转移并脱去 H₂O。
Step 3: Chain extends as the new terminal -OH or -COOH remains active.
第三步：新的末端 -OH 或 -COOH 继续反应，使链增长。
R-COOH + R'-OH ⇌ R-COO-R' + H₂O
Note that the reaction is reversible. In an exam, you may need to state that water must be removed to drive the equilibrium to the right.
注意该反应是可逆的。考试中可能需要你指出，必须移除水才能使平衡向右移动。
4. Mechanism: Polyamide Formation | 机理：聚酰胺的生成
Polyamides are formed by the reaction between a dicarboxylic acid and a diamine, or by the self-polymerisation of an amino acid. The linkage formed is an amide bond (-CONH-), and water is eliminated.
聚酰胺由二羧酸与二胺反应生成，或由氨基酸自聚生成。形成的键为酰胺键（-CONH-），同时脱去水。
n H₂N-(CH₂)₆-NH₂ + n HOOC-(CH₂)₄-COOH → [-NH-(CH₂)₆-NH-CO-(CH₂)₄-CO-]ₙ + 2n H₂O
The mechanism is similar to esterification: the nitrogen lone pair attacks the carbonyl carbon, then a proton is lost and water is eliminated. However, the reaction conditions are often more vigorous, and acid or base catalysis may be needed.
其机理与酯化相似：氮上的孤对电子进攻羰基碳，随后失去质子并脱去水。但反应条件通常更剧烈，可能需要酸或碱催化。
The amide bond is more polar than the ester bond, leading to stronger intermolecular hydrogen bonding.
酰胺键比酯键极性更强，因此分子间氢键更强。
Nylon-6,6 is prepared using hexanedioic acid and hexamethylenediamine.
尼龙-6,6 由己二酸和己二胺制备。
Kevlar is a polyamide made from 1,4-diaminobenzene and benzene-1,4-dicarboxylic acid.
凯夫拉（Kevlar）是由 1,4-二氨基苯和对苯二甲酸制成的聚酰胺。
In an acidic medium, the amine is protonated and becomes unable to act as a nucleophile. Therefore, the polymerisation is often performed under controlled pH or at high temperature using the nylon-66 salt (a 1:1 salt of the diamine and diacid).
在酸性介质中，胺会被质子化而无法作为亲核试剂。因此，聚合常在受控 pH 条件下进行，或通过尼龙-6,6 盐（二胺与二酸的 1:1 盐）在高温下进行。
5. Reaction Conditions and Catalysts | 反应条件和催化剂
Typical condensation polymerisation is carried out at high temperature (150–350 °C) with a catalyst. The small molecule by-product is continuously removed by vacuum or distillation.
典型的缩聚反应在高温（150–350 °C）和催化剂存在下进行。小分子副产物通过真空或蒸馏不断移除。
| Polymer | Monomers | Catalyst / Condition |
| Polyester | Diol + diacid | H₂SO₄ or Ti-based catalyst, heat |
| Polyamide | Diamine + diacid | High temperature, pressure; sometimes enzyme |
| Aromatic polyamide | Diamine + acyl chloride | Low temperature, no water, base to neutralize HCl |
When acyl chlorides are used instead of carboxylic acids, the reaction is much faster and produces HCl as the by-product. For example, benzenedicarbonyl dichloride reacts with a diamine to form a polyamide and HCl.
若使用酰氯代替羧酸，反应速率更快，副产物为 HCl。例如，苯二甲酰氯与二胺反应生成聚酰胺并放出 HCl。
6. Degree of Polymerisation and End Groups | 聚合度和端基
The degree of polymerisation (n) equals the average number of monomer units in a polymer chain. In a condensation polymer, the end groups are often still reactive unless deliberately terminated.
聚合度（n）等于聚合物链中单体单元的平均数目。在缩聚物中，除非刻意封端，否则端基通常仍具有反应活性。
If a dicarboxylic acid and diol are used in a 1:1 molar ratio, one end is -COOH and the other is -OH (statistically).
当二羧酸与二醇以 1:1 摩尔比反应时，统计上链的一端是 -COOH，另一端是 -OH。
If one monomer is in excess, the polymer chain will have a majority of that monomer's end group.
若某种单体过量，聚合物链将主要以其对应的端基结尾。
The presence of end groups allows further reactions, such as cross-linking or block copolymerisation.
端基的存在允许进一步反应，例如交联或嵌段共聚。
To calculate the composition of a repeating unit, remember that each bond formed loses one water molecule, so the combined formula of the repeating unit is the sum of both monomers minus 2H₂O (for a 1:1 reaction of two bifunctional monomers with each other).
要计算重复单元组成，请记住每个成键步骤失去一分子水，因此重复单元式是两种单体组合式减去 2H₂O（对两种双官能团单体等摩尔反应而言）。
7. Linkages and Repeating Units | 键合类型和重复单元
In condensation polymers, the repeating unit contains the functional group formed during polymerisation. For polyesters, the repeating unit includes the ester group -COO-. For polyamides, it includes the amide group -CONH-.
缩聚物的重复单元中包含聚合过程中形成的官能团。聚酯的重复单元包含酯基 -COO-；聚酰胺的重复单元包含酰胺基 -CONH-。
For a polymer formed from two different monomers, the repeating unit must be written as the combined segment from both monomers, arranged in the correct order:
对于由两种不同单体形成的聚合物，重复单元必须写成来自两种单体的组合片段，并按正确顺序排列：
[-O-CH₂-CH₂-O-CO-C₆H₄-CO-]ₙ
Each repeating unit is derived from one diol and one diacid. The end groups are not usually included in the repeating unit unless a specific oligomer is asked.
每个重复单元来自一个二醇分子和一个二酸分子。除非题目要求写特定低聚物，否则端基一般不包含在重复单元中。
8. Comparison with Addition Polymerisation | 与加成聚合的对比
s ethene, opening the double bond to form long chains. No small molecule is eliminated, and the repeating unit has the same atomic composition as the monomer.
加成聚合涉及含 C=C 双键的单体（如乙烯），双键打开形成长链。没有小分子脱去，重复单元的原子组成与单体完全相同。
| Feature | Addition Polymerisation | Condensation Polymerisation |
| Monomer type | Alkenes | Bifunctional molecules |
| By-product | None | Small molecules (H₂O, HCl) |
| Mechanism | Free radical / ionic | Stepwise nucleophilic substitution/addition-elimination |
| Repeating unit formula | Same as monomer | Loses atoms from small molecule |
Another difference is that condensation polymers often contain heteroatoms (O, N) in the main chain, giving them higher polarity and stronger intermolecular forces than typical addition polymers like polyethene.
另一个区别是缩聚物的主链通常含有杂原子（O、N），使其极性更强，分子间作用力比聚乙烯等典型加成聚合物更强。
9. Common Exam Questions and Pitfalls | 常见例题和陷阱
In CIE A-Level exams, students are often asked to draw the repeating unit from given monomers, deduce the monomers from a polymer structure, or suggest reaction conditions.
在剑桥 CIE A-Level 考试中，学生常被要求根据给定单体画出重复单元、根据聚合物结构推断单体，或提出反应条件。
Pitfall 1: Forgetting to remove water when calculating the formula of a polyester repeating unit.
陷阱一：计算聚酯重复单元式时忘记减去水分子。
Pitfall 2: Writing the repeating unit from only one monomer when a copolymer is formed from two different monomers.
陷阱二：当共聚物由两种不同单体形成时，只写其中一种单体的重复单元。
Pitfall 3: Confusing polyester with polyamide. Check for -COO- vs -CONH-.
陷阱三：混淆聚酯与聚酰胺。注意区分 -COO- 和 -CONH-。
Pitfall 4: Using "addition" terminology for condensation polymerisation.
陷阱四：在缩聚反应中误用"加成聚合"的术语。
If given the polymer structure, look for the ester or amide link. Cut the bond between the carbonyl carbon and the oxygen/nitrogen, then add -OH to the carbonyl side and -H to the oxygen/nitrogen side to get the monomer(s).
如果题目给出聚合物结构，请找到酯键或酰胺键。断开羰基碳与氧/氮之间的键，然后在羰基一侧加上 -OH，在氧/氮一侧加上 -H，即可得到单体。
10. Summary | 小结
Condensation polymerisation is a step-growth process involving bifunctional monomers, elimination of small molecules, and the formation of ester or amide links. It is distinct from addition polymerisation and is essential for understanding natural polymers like proteins and polysaccharides, as well as synthetic fibres like nylon and polyester.
缩合聚合是一种逐步增长过程，涉及双官能团单体、小分子脱去以及酯键或酰胺键的形成。它与加成聚合有本质区别，对于理解蛋白质和多糖等天然聚合物，以及尼龙、聚酯等合成纤维至关重要。
Key functional groups: -COOH, -OH, -NH₂, -COCl.
关键官能团：-COOH、-OH、-NH₂、-COCl。
Key linkages: -COO- (ester), -CONH- (amide).
关键连接：-COO-（酯键）、-CONH-（酰胺键）。
By-products: H₂O for acid+alcohol/amine; HCl for acyl chloride+amine.
副产物：酸+醇/胺时产生 H₂O；酰氯+胺时产生 HCl。
Always balance the number of atoms when writing the polymer equation.
书写聚合反应方程式时始终注意原子守恒。
Mastering these basics will help you to solve mechanistic, structural and practical questions on condensation polymerisation with confidence.
掌握这些基础知识将帮助您自信地解决关于缩合聚合的机理、结构和实验类问题。
Published by TutorHao | Chemistry Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
或截图后用「扫一扫 → 相册」选择本图