A-Level Chemistry: Types and Degradation Mechanisms of Degradable Polymers | A-Level 化学：可降解聚合物的类型与降解机理

📚 A-Level Chemistry: Types and Degradation Mechanisms of Degradable Polymers | A-Level 化学：可降解聚合物的类型与降解机理

Polymers such as polyethene, polypropene and poly(chloroethene) are extremely durable, which makes them useful packaging and structural materials. However, their resistance to breakdown is also a major environmental problem because plastic waste can persist in landfill and oceans for centuries. Degradable polymers are designed to undergo chemical breakdown under specific environmental conditions, offering one strategy to reduce long-lived plastic pollution.

聚乙烯、聚丙烯和聚氯乙烯等聚合物极其耐用，这使得它们成为有用的包装和结构材料。然而，它们的抗分解性也是重大的环境问题，因为塑料废物可以在垃圾填埋场和海洋中存留数个世纪。可降解聚合物被设计为在特定环境条件下发生化学分解，为减少长期塑料污染提供了一种策略。

1. Why Degradation Matters | 为什么降解很重要

Traditional polymers contain strong covalent carbon-carbon and carbon-hydrogen bonds. These bonds are not readily attacked by water, oxygen or microbes, so the polymer remains intact after disposal. Over time, plastic waste accumulates, harming wildlife and entering food chains. Degradable polymers offer a route to materials that can be broken down into smaller molecules after their useful lifetime has ended.

传统聚合物含有强力的碳-碳键和碳-氢键。这些键不容易被水、氧气或微生物攻击，因此聚合物在废弃后仍能保持完整。随着时间推移，塑料废物不断累积，损害野生动物并进入食物链。可降解聚合物为实现材料在有效寿命结束后分解成较小分子提供了一条途径。

Degradable polymers are not a single class of material. They include natural polymers, synthetic polymers with hydrolysable links, and conventional polymers modified with additives that promote breakdown. It is important to distinguish between different types and to know how each one degrades.

可降解聚合物并不是单一类别的材料。它们包括天然聚合物、含有可水解键的合成聚合物，以及通过添加剂促进分解的传统聚合物。区分不同类型并了解每种聚合物如何降解非常重要。

2. Definitions: Degradable, Biodegradable and Compostable | 定义：可降解、生物可降解与可堆肥

The terms ‘degradable’, ‘biodegradable’ and ‘compostable’ are often confused. In exam answers, precise definitions are essential.

“可降解”“生物可降解”和“可堆肥”这三个术语经常被混淆。在考试作答中，精确的定义至关重要。

Term	Meaning
Degradable	Breaks down by chemical, physical or biological processes into smaller fragments or molecules.
Biodegradable	Can be broken down by microorganisms such as bacteria and fungi into water, carbon dioxide, methane and biomass.
Compostable	Biodegrades at a specified rate in a compost facility, leaving no toxic residues.

A polymer may be degradable without being biodegradable. For example, photodegradable plastics may simply fragment into small particles, which are still plastic pollutants. Examiners often test this distinction.

一种聚合物可能是可降解的，但不一定是生物可降解的。例如，光降解塑料可能只是碎裂成小颗粒，而这些小颗粒仍然是塑料污染物。考官经常考查这一区别。

3. Main Types of Degradable Polymers | 可降解聚合物的主要类型

Degradable polymers can be grouped into several broad categories based on their origin and mechanism of breakdown.

可降解聚合物可以根据来源和分解机理分为几个大类。

Natural biodegradable polymers: starch, cellulose, chitin, proteins.

天然生物可降解聚合物：淀粉、纤维素、几丁质、蛋白质。
Synthetic biodegradable polymers: poly(lactic acid) (PLA), polyhydroxyalkanoates (PHA), poly(glycolic acid).

合成生物可降解聚合物：聚乳酸（PLA）、聚羟基烷酸酯（PHA）、聚乙醇酸。
Photodegradable polymers: conventional polymers containing light-sensitive groups or additives that absorb ultraviolet (UV) light.

光降解聚合物：含有光敏基团或能吸收紫外（UV）光的添加剂的传统聚合物。
Oxo-degradable polymers: polyolefins mixed with pro-oxidant additives that accelerate oxidative breakdown.

氧降解聚合物：混有促氧化剂以加速氧化分解的聚烯烃。

4. Natural Biodegradable Polymers | 天然生物可降解聚合物

Natural polymers such as starch and cellulose are produced by living organisms. They contain glycosidic bonds, which can be hydrolysed by enzymes called glycosidases. Microorganisms in soil or water break these polymers down into glucose or other sugars, which are then respired to release energy.

淀粉、纤维素等天然聚合物由生物体产生。它们含有糖苷键，可以被称为糖苷酶的酶水解。土壤或水中的微生物将这些聚合物分解为葡萄糖或其他糖类，然后通过呼吸作用释放能量。

Starch is often blended with synthetic polymers to make them more biodegradable. However, the synthetic component may still remain intact, so such blends are only partially degradable. A careful exam answer should explain that the starch portion is degraded while the synthetic portion may not be.

淀粉常与合成聚合物共混以使其更易生物降解。然而，合成成分可能仍然保持完整，因此这种共混物只是部分可降解。考试中严谨的回答应说明淀粉部分被降解，而合成部分可能不被降解。

5. Synthetic Biodegradable Polymers | 合成生物可降解聚合物

Synthetic biodegradable polymers are designed with hydrolysable ester or amide links in their backbones. Two important examples in the A-Level syllabus are poly(lactic acid) and polyhydroxybutyrate.

合成生物可降解聚合物被设计为主链含有可水解的酯键或酰胺键。A-Level 课程中两个重要的例子是聚乳酸和聚羟基丁酸酯。

Poly(lactic acid) (PLA) is produced by condensation polymerisation of lactic acid monomers. The repeat unit is -OCH(CH₃)CO- and the polymer contains ester links. It is compostable industrially and degrades by hydrolysis to lactic acid.

聚乳酸（PLA）由乳酸单体经缩合聚合而成。其重复单元为 -OCH(CH₃)CO-，聚合物含有酯键。它可在工业堆肥条件下降解，通过水解生成乳酸。

Polyhydroxybutyrate (PHB) is a type of polyhydroxyalkanoate produced by bacteria as an energy store. It has the repeat unit -OCH(CH₃)CH₂CO- and is fully biodegradable in environmental conditions. PHB is thermoplastic and its properties resemble polypropene, but it is much more readily degraded.

聚羟基丁酸酯（PHB）是聚羟基烷酸酯的一种，由细菌作为能量储备产生。其重复单元为 -OCH(CH₃)CH₂CO-，在环境条件下可以完全生物降解。PHB 是热塑性塑料，性能类似于聚丙烯，但更容易降解。

6. Degradation Mechanism: Hydrolysis | 降解机理：水解

Hydrolysis is the cleavage of a chemical bond by water. Polyesters such as PLA and PHB contain ester functional groups, R-COO-R’. Water attacks the ester link and breaks the polymer chain into smaller fragments, eventually yielding the monomer acid and alcohol.

水解是水对化学键的断裂作用。PLA、PHB 等聚酯含有酯官能团 R-COO-R’。水攻击酯键并将聚合物链打断成较小片段，最终生成单体酸和醇。

R-COO-R’ + H₂O → R-COOH + R’-OH

For PLA, the overall equation is:

对于 PLA，总方程式为：

(C₃H₄O₂)ₙ + nH₂O → n C₃H₆O₃

The rate of hydrolysis is increased by acidic or alkaline conditions and by heat. Under neutral, dry conditions, hydrolysis is very slow. This explains why PLA cups can hold cold drinks but degrade far faster in a warm, moist compost heap.

酸性或碱性条件以及热量都会加快水解速率。在中性、干燥条件下，水解非常缓慢。这解释了为什么 PLA 杯可以盛放冷饮，但在温暖潮湿的堆肥堆中降解快得多。

7. Degradation Mechanism: Enzymatic and Microbial Action | 降解机理：酶促与微生物作用

Biodegradation is not simply hydrolysis in water. It involves enzymes secreted by microorganisms binding to the polymer surface and catalysing the breakdown of the polymer chains. The small molecules produced are then absorbed and metabolised by the microbes.

生物降解不仅仅是在水中的水解。它涉及微生物分泌的酶与聚合物表面结合并催化聚合物链的分解。产生的小分子随后被微生物吸收和代谢。

In aerobic conditions, complete biodegradation produces carbon dioxide and water:

在需氧条件下，完全生物降解产生二氧化碳和水：

Polymer + O₂ → CO₂ + H₂O + biomass

In anaerobic conditions, such as deep landfill, methane and carbon dioxide are produced. The presence of oxygen, moisture, temperature and nutrient availability all affect how fast this biological process occurs.

在厌氧条件下，例如深层垃圾填埋场，会产生甲烷和二氧化碳。氧气、水分、温度和养分供应都会影响这一生物过程的快慢。

8. Photodegradation | 光降解

Photodegradation is the breakdown of polymers caused by ultraviolet light. UV radiation provides enough energy to break C-C and C-H bonds, especially in polymers that contain carbonyl groups, C=O. The carbonyl group absorbs UV light and undergoes Norrish-type reactions, leading to chain scission and oxidation.

光降解是由紫外线引起的聚合物分解。紫外辐射提供足够的能量来断裂 C-C 键和 C-H 键，尤其是含有羰基 C=O 的聚合物。羰基吸收紫外线并发生 Norrish 型反应，导致链断裂和氧化。

Most commercial polymers do not absorb UV light strongly, so photodegradable plastics contain additives such as ketone groups introduced by copolymerisation, or metal salts that catalyse photo-oxidation. Exposure to sunlight causes embrittlement and fragmentation.

大多数商业聚合物对紫外线吸收不强，因此光降解塑料含有通过共聚引入的酮基或催化光氧化的金属盐等添加剂。暴露在阳光下会导致变脆和碎裂。

A limitation is that photodegradation stops when the plastic is buried or covered, because sunlight cannot reach it. Thus photodegradable plastics may reduce visible litter but still contribute to microplastic pollution.

光降解的一个局限性是，当塑料被掩埋或覆盖时，阳光无法照射到它，降解就会停止。因此光降解塑料可能减少可见垃圾，但仍会加剧微塑料污染。

9. Oxo-Degradable and Hydrolytically Degradable Polymers | 氧降解与水降解聚合物

Oxo-degradable plastics are conventional polyolefins, such as low-density polyethene, containing pro-oxidant additives. These additives promote oxidative breaking of the polymer chains in the presence of oxygen and UV light. The process produces low-molar-mass fragments that are theoretically biodegradable, but complete mineralisation is often slow and incomplete in real environments.

氧降解塑料是含有促氧化剂添加剂的传统聚烯烃，例如低密度聚乙烯。这些添加剂在氧气和紫外线存在下促进聚合物链的氧化断裂。该过程产生低摩尔质量的碎片，理论上可生物降解，但在真实环境中完全矿化通常缓慢且不彻底。

Hydrolytically degradable polymers, in contrast, contain functional groups such as esters, amides or anhydrides that are susceptible to hydrolysis. PLA, PHB and poly(glycolic acid) all belong to this category. They degrade by bulk or surface erosion depending on their structure and dimensions.

相比之下，水降解聚合物含有易被水解的酯、酰胺或酸酐等官能团。PLA、PHB 和聚乙醇酸都属于这一类。它们的降解方式根据结构和尺寸可以是本体侵蚀或表面侵蚀。

10. Factors Affecting Degradation | 影响降解的因素

Degradation is not a fixed property of a polymer. It depends on both the polymer structure and the environment. A biodegradable polymer may remain intact for years if heat, moisture and microbes are absent.

降解不是聚合物的固定属性。它取决于聚合物结构和环境。如果缺少热量、水分和微生物，可生物降解聚合物也可能保持完整多年。

Moisture: water is required for hydrolysis; dry conditions slow degradation.

水分：水解需要水；干燥条件会减慢降解。
Temperature: higher temperatures increase reaction rates and microbial metabolism.

温度：较高温度会提高反应速率和微生物代谢。
Oxygen: aerobic oxidation and microbial respiration require oxygen.

氧气：有氧氧化和微生物呼吸需要氧气。
UV light: sunlight drives photodegradation in chain-breaking reactions.

紫外光：日光在断链反应中驱动光降解。
Surface area: smaller particles have larger surface area-to-volume ratios and degrade faster.

表面积：较小颗粒具有更大的表面积-体积比，降解更快。
Crystallinity: crystalline regions are less accessible to water and enzymes than amorphous regions.

结晶度：结晶区比无定形区更难被水和酶接触。
pH: acidic or alkaline conditions catalyse ester hydrolysis.

pH：酸性或碱性条件催化酯水解。

11. Exam Comparisons: PLA vs PHB | 考试对比：PLA 与 PHB

Examiners often ask candidates to compare two biodegradable polyesters. A useful approach is to structure the answer around source, structure, degradation and products.

考官经常要求考生比较两种可生物降解聚酯。一种有用的答题思路是围绕来源、结构、降解和产物来组织答案。

Feature	PLA	PHB
Source	Lactic acid from corn starch fermentation	Produced by bacteria from glucose
Monomer	2-hydroxypropanoic acid (lactic acid)	3-hydroxybutanoic acid
Repeat unit	-OCH(CH₃)CO-	-OCH(CH₃)CH₂CO-
Degradation product	Lactic acid	3-hydroxybutanoic acid
Degradation type	Hydrolysis, often in industrial compost	Hydrolysis by microbial enzymes in soil or water

12. Conclusion and Exam Tips | 结论与备考提示

Degradable polymers are an important application of polymer chemistry to environmental problems. The key ideas are the presence of hydrolysable bonds, the role of water, enzymes and UV light, and the final products of degradation. You should be able to draw the repeat units of PLA and PHB, identify ester links, and explain why the rate of degradation depends on environmental conditions.

可降解聚合物是高分子化学应对环境问题的重要应用。关键要点包括可水解键的存在、水、酶和紫外光的作用，以及降解的最终产物。你应该能够画出 PLA 和 PHB 的重复单元、识别酯键，并解释为什么降解速率取决于环境条件。

In a multiple-choice or short-answer question, check whether the question asks for ‘biodegradable’ or ‘photodegradable’. The definitions are different. In a longer question, always name the functional group being broken and the small molecule product. Avoid simply saying ‘the polymer breaks down in the environment’; be specific about the mechanism.

在做选择题或简答题时，注意题目问的是“生物可降解”还是“光降解”。两者的定义不同。在长答题中，一定要指明被断裂的官能团和小分子产物。避免只说“聚合物在环境中分解”，要对机理给出具体说明。

Published by TutorHao | Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

A-Level Chemistry: Types and Degradation Mechanisms of Degradable Polymers | A-Level 化学：可降解聚合物的类型与降解机理