Degradable Polymers | 可降解聚合物

📚 Degradable Polymers | 可降解聚合物

Polymers have transformed nearly every aspect of modern life, from packaging and textiles to medicine and engineering. However, the same property that makes many synthetic polymers so useful — their remarkable durability — also creates a serious environmental burden. Most commodity plastics, such as polyethylene and polypropylene, possess strong carbon–carbon backbones that resist chemical, photochemical and biological attack, allowing them to persist in the environment for centuries. As a result, vast quantities of plastic waste accumulate in landfills and oceans, fragmenting into microplastics that threaten marine and terrestrial ecosystems. In response to these challenges, scientists have developed degradable polymers that are designed to break down under specific conditions. This topic is central to the Cambridge International A-Level Chemistry syllabus, linking organic chemistry, polymerisation and environmental chemistry.

聚合物几乎改变了现代生活的方方面面，从包装和纺织到医药和工程。然而，许多合成聚合物极其有用的特性——卓越的耐久性——也带来了严重的环境负担。大多数通用塑料，如聚乙烯和聚丙烯，具有牢固的碳–碳主链，能够抵抗化学、光化学和生物侵蚀，因此在环境中可以存留数百年。大量塑料废弃物堆积在填埋场和海洋中，碎裂成微塑料，威胁着海洋和陆地生态系统。为应对这些挑战，科学家们开发了可降解聚合物，使其在特定条件下分解。这一主题是剑桥国际A-Level化学课程的核心内容，连接了有机化学、聚合反应和环境化学。

1. The Environmental Impact of Traditional Polymers | 传统聚合物对环境的影响

Traditional addition polymers such as polyethylene (PE), polypropylene (PP) and polystyrene (PS) are composed of long hydrocarbon chains with strong C–C and C–H bonds. These bonds are not susceptible to hydrolysis because they lack polar functional groups, and they require very high energies to be broken by UV light alone.

传统的加成聚合物，如聚乙烯(PE)、聚丙烯(PP)和聚苯乙烯(PS)，由长烃链构成，具有牢固的C–C和C–H键。这些键不易水解，因为它们缺少极性官能团，并且单独依靠紫外光断裂需要非常高的能量。

Consequently, discarded plastic items accumulate in terrestrial and aquatic environments. Over time, physical abrasion photo-oxidation fragment these materials into microplastics, which can adsorb toxic pollutants and enter the food chain.

因此，废弃的塑料制品在陆地和水生环境中累积。随着时间推移，物理磨损和光氧化将这些材料碎裂成微塑料，它们能够吸附有毒污染物并进入食物链。

Incineration of these plastics can release CO₂ and, if chlorine-containing polymers such as PVC are present, toxic dioxins and hydrogen chloride. Landfill disposal simply defers the problem, as the plastics remain largely unchanged for generations.

焚烧这些塑料会释放二氧化碳，如果存在含氯聚合物如PVC，还会释放有毒的二噁英和氯化氢。填埋处理只是延缓了问题，因为这些塑料在几代人的时间内基本保持不变。

These environmental concerns have driven the search for polymers that can undergo controlled degradation, returning to harmless small molecules that can be assimilated by natural cycles.

这些环境问题推动了对可控制降解的聚合物的探索，使其回归到可被自然循环同化的无害小分子。

2. What Are Degradable Polymers? | 什么是可降解聚合物？

A degradable polymer is a macromolecule designed to break down into smaller fragments through chemical, photochemical or biological processes. The term ‘degradable’ encompasses several distinct mechanisms: hydrolytic degradation, photodegradation and biodegradation.

可降解聚合物是指通过化学、光化学或生物过程分解为较小碎片的大分子。‘可降解’一词涵盖几种不同的机制：水解降解、光降解和生物降解。

In many cases, degradation occurs in a stepwise fashion — first the polymer chains are cleaved into oligomers and monomers, and then these small molecules are mineralised by microorganisms into carbon dioxide, water and biomass under aerobic conditions, or into methane under anaerobic conditions.

在许多情况下，降解是分步进行的——聚合物链首先被切割为低聚物和单体，然后这些小分子被微生物矿化，在有氧条件下生成二氧化碳、水和生物质，在厌氧条件下生成甲烷。

A truly biodegradable polymer must satisfy criteria such as complete conversion to CO₂ and H₂O within a specified time frame and absence of toxic residues. This distinguishes it from merely ‘degradable’ materials, which may fragment without full mineralisation.

真正的可生物降解聚合物必须满足在特定时间内完全转化为CO₂和H₂O、不产生有毒残留物等标准。这使它与仅仅‘可降解’的材料区分开来，后者可能只碎裂而不完全矿化。

3. Mechanisms of Degradation: Hydrolysis | 降解机理：水解

Hydrolysis is the most common degradation pathway for condensation polymers that contain heteroatom linkages, such as esters, amides and urethanes. Water molecules attack the electrophilic carbonyl carbon, cleaving the polymer backbone.

水解是缩聚聚合物最常见的降解途径，这类聚合物含有酯、酰胺和氨酯等杂原子键。水分子进攻缺电子的羰基碳，切断聚合物主链。

In a polyester, for example, each ester group reacts with water to yield a carboxylic acid and an alcohol. The general reaction can be represented as:

例如，在聚酯中，每个酯基与水反应生成羧酸和醇。一般反应可表示为：

RCOOR’ + H₂O ⇌ RCOOH + R’OH

Hydrolysis is catalysed by both acids and bases. In living systems, enzymes such as lipases and esterases dramatically accelerate hydrolysis under mild conditions. The rate of hydrolytic degradation depends on the accessibility of water to the ester bonds, which is influenced by the polymer’s crystallinity, hydrophilicity and morphology.

水解反应受酸和碱催化。在生命体系中，脂肪酶和酯酶等酶能在温和条件下显著加速水解。水解降解的速率取决于水分子接近酯键的难易程度，这受聚合物结晶度、亲水性和形态的影响。

Aliphatic polyesters such as poly(lactic acid) and poly(glycolic acid) are particularly susceptible to hydrolysis because their open-chain structures allow water penetration, and the resulting monomers are naturally occurring metabolites.

脂肪族聚酯如聚乳酸和聚乙醇酸特别容易水解，因为它们的开链结构允许水渗透，且生成的单体是天然存在的代谢物。

4. Mechanisms of Degradation: Photodegradation | 降解机理：光降解

Photodegradation involves the absorption of ultraviolet (UV) radiation by chromophores within the polymer, leading to bond homolysis and the formation of free radicals. These radicals can initiate chain scission, crosslinking or oxidation.

光降解涉及聚合物中的发色团吸收紫外(UV)辐射，导致键均裂并形成自由基。这些自由基可引发断链、交联或氧化。

Many pure hydrocarbon polymers do not absorb sunlight strongly. However, impurities, catalyst residues or deliberate incorporation of photosensitive groups (such as carbonyl groups from ketones) can render a polymer photodegradable. Ketone-containing polymers undergo Norrish Type I and Type II reactions:

许多纯烃类聚合物对太阳光吸收不强。然而，杂质、催化剂残留或有意引入的光敏基团（如酮的羰基）可以使聚合物变得可光降解。含酮聚合物经历Norrish I型和II型反应：

Norrish I: –CH₂–CO–CH₂– → –CH₂• + •CO–CH₂–

Norrish II: –CH₂–CH₂–CH₂–CO– → –CH=CH₂ + CH₃CO–

These reactions break the polymer backbone, reducing molecular weight and eventually producing brittle, fragmented material. Photodegradable plastics are sometimes used in agricultural mulch films, where exposure to sunlight triggers disintegration after the growing season.

这些反应打断聚合物主链，降低分子量，最终产生脆性碎屑。可光降解塑料有时用于农业地膜，在生长季结束后，暴露于阳光会引发崩解。

However, photodegradation alone is often incomplete, leaving microplastic fragments. Therefore, modern approaches frequently combine photodegradation with subsequent biodegradation.

然而，单纯的光降解往往不完全，会留下微塑料碎片。因此，现代方法常将光降解与后续的生物降解结合起来。

5. Mechanisms of Degradation: Biodegradation | 降解机理：生物降解

Biodegradation is mediated by microorganisms — bacteria, fungi and algae — that secrete extracellular enzymes to depolymerise the macromolecules into water-soluble products small enough to be transported across cell membranes.

生物降解由微生物介导——细菌、真菌和藻类——它们分泌胞外酶，将大分子解聚为水溶性产物，使其小到足以跨过细胞膜。

The process typically occurs in two stages: (i) enzymatic hydrolysis or oxidation of the polymer backbone, and (ii) intracellular metabolism of the resulting monomers to yield energy, CO₂, water and new biomass. For a polymer to be truly biodegradable, both steps must proceed without the accumulation of toxic intermediates.

该过程通常分两个阶段：(i) 酶催化水解或氧化聚合物主链，(ii) 生成的单体进入胞内代谢，产生能量、CO₂、水和新的生物质。要使聚合物真正可生物降解，两个步骤都必须进行，且不积累有毒中间产物。

Aliphatic polyesters are especially amenable to biodegradation because their ester bonds resemble those in natural lipids, making them substrates for a wide range of microbial enzymes. Polymers with aromatic rings or quaternary carbon atoms in the backbone generally resist biodegradation.

脂肪族聚酯特别适合生物降解，因为它们的酯键与天然脂质中的酯键相似，成为多种微生物酶的底物。主链中含有芳香环或季碳原子的聚合物通常能抵抗生物降解。

Environmental conditions — temperature, moisture, pH, oxygen availability and the presence of a suitable microbial community — critically influence the rate of biodegradation.

环境条件——温度、湿度、pH值、氧气供应以及是否存在合适的微生物群落——对生物降解速率有决定性影响。

6. Poly(lactic acid) (PLA): A Leading Biodegradable Polymer | 聚乳酸 (PLA)：领先的可生物降解聚合物

Poly(lactic acid), or PLA, is an aliphatic polyester derived from renewable resources such as corn starch or sugarcane. Its monomer, lactic acid (2-hydroxypropanoic acid), is produced by bacterial fermentation of carbohydrates.

聚乳酸(PLA)是一种来源于可再生资源（如玉米淀粉或甘蔗）的脂肪族聚酯。其单体乳酸（2-羟基丙酸）由碳水化合物经细菌发酵制得。

PLA can be synthesised either by direct condensation polymerisation of lactic acid or, more commonly, by ring-opening polymerisation of the cyclic dimer lactide. The latter yields high-molecular-weight polymer suitable for fibre and film applications.

PLA既可通过乳酸直接缩聚合成，也可更常见地通过环状二聚体丙交酯的开环聚合制备。后者可得到适用于纤维和薄膜的高分子量聚合物。

Hydrolysis of PLA cleaves the ester linkages, regenerating lactic acid, which is non-toxic and can be metabolised by the body or by microorganisms:

PLA水解断裂酯键，再生出乳酸。乳酸无毒，可被人体或微生物代谢：

(C₃H₄O₂)ₙ + n H₂O → n C₃H₆O₃ (lactic acid)

PLA finds extensive use in biodegradable packaging, disposable cups, compost bags and biomedical devices such as sutures and drug-delivery implants. In the body, PLA sutures hydrolyse slowly, eliminating the need for surgical removal.

PLA广泛用于可降解包装、一次性杯子、堆肥袋以及生物医学器件，如缝合线和药物输送植入物。在体内，PLA缝线缓慢水解，无需手术拆除。

However, PLA requires industrial composting conditions (temperatures around 58 °C) to degrade within weeks; in home compost or marine environments, degradation is extremely slow.

然而，PLA需要工业堆肥条件（温度约58 °C）才能在数周内降解；在家庭堆肥或海洋环境中，降解极为缓慢。

7. Polyhydroxyalkanoates (PHA and PHB) | 聚羟基烷酸酯 (PHA 和 PHB)

Polyhydroxyalkanoates (PHAs) are a family of polyesters naturally synthesised by bacteria as intracellular carbon and energy storage granules. The most studied member is poly(3-hydroxybutyrate), or PHB.

聚羟基烷酸酯(PHA)是一类由细菌在胞内合成的聚酯，作为碳源和能量储存颗粒。研究最多的成员是聚(3-羟基丁酸酯)，即PHB。

PHB is produced by bacteria such as Cupriavidus necator (formerly Alcaligenes eutrophus) when they are grown under nutrient limitation but with excess carbon source. The polymer accumulates as granules that can be extracted and processed into a thermoplastic.

PHB由细菌如Cupriavidus necator（原Alcaligenes eutrophus）在营养受限而碳源过量的条件下产生。聚合物以颗粒形式积累，可被提取并加工成热塑性塑料。

The repeating unit of PHB is –O–CH(CH₃)–CH₂–CO–, and its hydrolysis yields 3-hydroxybutanoic acid (3-hydroxybutyric acid):

PHB的重复单元为–O–CH(CH₃)–CH₂–CO–，其水解生成3-羟基丁酸：

(C₄H₆O₂)ₙ + n H₂O → n C₄H₈O₃ (3-hydroxybutyric acid)

Unlike PLA, PHB is fully biodegradable in a wide range of environments, including soil, freshwater and marine ecosystems, because many microorganisms possess PHA depolymerase enzymes that rapidly cleave the polymer.

与PLA不同，PHB在包括土壤、淡水和海洋生态系统在内的多种环境中均可完全生物降解，因为许多微生物拥有PHA解聚酶，能迅速切割该聚合物。

Despite its excellent biodegradability, PHB has limitations: it is relatively brittle, has a narrow processing window and is more expensive to produce than PLA or petrochemical plastics. Blending with plasticisers or other biopolymers can improve its mechanical properties.

尽管PHB具有出色的生物降解性，但它也有局限性：相对脆性、加工窗口窄，且生产成本高于PLA或石化塑料。与增塑剂或其他生物聚合物共混可改善其力学性能。

8. Other Degradable Polymers: PGA, PCL and PVA | 其他可降解聚合物：PGA、PCL 和 PVA

Poly(glycolic acid) (PGA) is the simplest linear aliphatic polyester, synthesised from glycolic acid. Its dense crystal packing makes it highly impermeable to water, yet it undergoes relatively rapid hydrolysis due to the absence of side-chain steric hindrance. PGA is widely used in absorbable surgical sutures.

聚乙醇酸(PGA)是最简单的线性脂肪族聚酯，由乙醇酸合成。其致密的晶体堆积使其对水高度不渗透，但由于缺乏侧链的空间位阻，它仍能相对快速地水解。PGA广泛用于可吸收手术缝线。

Poly(ε-caprolactone) (PCL) is a semi-crystalline polyester with a low melting point (around 60 °C). Its hydrolysis is slower than that of PLA and PGA, making it suitable for long-term drug delivery devices and tissue-engineering scaffolds.

聚(ε-己内酯)(PCL)是一种半结晶聚酯，熔点低（约60 °C）。其水解速度比PLA和PGA慢，适合用于长期药物释放器件和组织工程支架。

Poly(vinyl alcohol) (PVA) is a water-soluble polymer containing –OH side groups. It is produced by hydrolysis of poly(vinyl acetate). PVA can be biodegraded by specific microorganisms under both aerobic and anaerobic conditions. It is used in water-soluble packaging, laundry pods and hospital laundry bags.

聚乙烯醇(PVA)是一种含有–OH侧基的水溶性聚合物，由聚醋酸乙烯酯水解得到。在特定微生物作用下，PVA可在需氧和厌氧条件下生物降解。它被用于水溶性包装、洗衣凝珠和医院洗衣袋。

The degradation of these polymers generally occurs via hydrolysis of their ester or acetate linkages, followed by microbial mineralisation. The choice of polymer for a given application balances degradation rate, mechanical strength and cost.

这些聚合物的降解通常通过其酯键或醋酸酯键的水解进行，随后由微生物矿化。为特定应用选择聚合物时需在降解速率、力学强度和成本之间取得平衡。

9. Degradation Conditions and Composting | 降解条件与堆肥

The term ‘biodegradable’ can be misleading if not qualified by the environment and timescale. A polymer that degrades rapidly in an industrial composting facility may persist for years in a marine or landfill setting.

如果不限定环境和时间尺度，‘可生物降解’一词可能产生误导。在工业堆肥设施中能快速降解的聚合物，在海洋或填埋场中可能存留多年。

Industrial composting standards (e.g. EN 13432) require a material to achieve at least 90 % mineralisation to CO₂ within 180 days at 58 ± 2 °C, alongside disintegration and the absence of ecotoxicity. PLA meets these criteria in industrial facilities but not in typical home compost heaps where temperatures rarely exceed 35 °C.

工业堆肥标准（如EN 13432）要求材料在58 ± 2 °C下、180天内至少实现90 %矿化为CO₂，同时发生崩解且不具有生态毒性。PLA在工业设施中满足这些标准，但在典型的家庭堆肥堆中（温度很少超过35 °C

Published by TutorHao | A-Level Chemistry Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Degradable Polymers | 可降解聚合物