Addition polymers, a cornerstone of modern materials science, are formed from unsaturated monomers that undergo polymerisation. Their widespread use in various industries underscores their importance, but it also brings to light environmental concerns. For more information on the formation and types of specific addition polymers, see Polyethylene (PE) - Formation and Types.
Mechanism of Formation
Addition polymers are formed when unsaturated monomers, typically containing carbon-carbon double bonds, join together in a chain reaction. The process is facilitated by initiators that can form covalent bonds with the monomer.
- Free Radical Mechanism:
- Initiation: A molecule, often a peroxide, dissociates upon heating, producing free radicals. These radicals are highly reactive and can initiate the polymerisation process.
- Propagation: The free radical attacks the double bond of a monomer, opening it up and forming a larger radical. This new radical can then attack other monomers, allowing the chain to grow.
- Termination: Eventually, two radicals might collide, combining to form a stable molecule and ending the chain growth. Alternatively, radicals can be terminated by reaction with inhibitors or by disproportionation.
Properties of Addition Polymers
Physical Properties:
- Density: The arrangement of polymer chains affects the density. For instance, LDPE has a branched structure leading to a lower density, while HDPE has a linear structure, resulting in a higher density.
- Tensile Strength: Polymers with cross-linking, like vulcanised rubber, exhibit higher tensile strength due to the additional bonds between polymer chains.
- Insolubility: Their high molecular weight renders them insoluble in most solvents.
- Thermal Stability: They can endure elevated temperatures without decomposing, though this varies among polymers.
Chemical Properties:
- Inertness: Generally, addition polymers are chemically inert, making them resistant to most chemicals, which is why they are used in containers for storing chemicals.
- Reactivity: Some polymers can undergo further reactions. For instance, polypropylene can be oxidised, or PVC can be chlorinated.
Examples of Addition Polymers
Polyethylene (PE):
- Formation: Derived from the polymerisation of ethylene.
- Uses: Ubiquitous in daily life, it's found in plastic bags, bottles, toys, and more.
- Types:
- Low-Density Polyethylene (LDPE): Has a branched structure, making it flexible. Used in cling films and flexible containers.
- High-Density Polyethylene (HDPE): Linear structure gives it rigidity. Used in milk jugs and pipe fittings.
Polypropylene (PP):
- Formation: Results from the polymerisation of propylene.
- Uses: Due to its resilience against fatigue, it's used in automotive parts, containers, carpets, and ropes.
- Properties: It possesses a higher melting point than polyethylene, making it suitable for applications requiring heat resistance. For more detailed properties of alkanes, see Alkanes - Properties.
Environmental Concerns and Recycling
Non-biodegradability:
- Addition polymers, especially polyethylene, can persist in the environment for centuries, leading to significant pollution. This longevity is due to their strong carbon-carbon bonds and the absence of functional groups that can be readily attacked by microbial enzymes. The Environmental Impact and Alternatives page offers insights into efforts to address these issues.
Microplastics:
- As these polymers degrade physically, they break into smaller particles, termed microplastics. These particles can infiltrate the food chain, posing threats to marine life and, potentially, humans.
Recycling:
- Mechanical Recycling: The most common method, it involves melting the polymer and reshaping it. However, each recycling cycle degrades the polymer's properties.
- Chemical Recycling: Though less common due to its cost, this method breaks the polymer into its monomers, which can then be repolymerised. This method can produce virgin-quality polymers.
Environmental Impact:
- The production and disposal of addition polymers contribute to greenhouse gas emissions. Recycling helps mitigate this by reducing the demand for new polymers.
Alternatives:
- Bioplastics, derived from renewable resources like cornstarch or sugarcane, are emerging as alternatives. They are often biodegradable and have a reduced carbon footprint compared to traditional polymers.
Understanding the structure of polymers is crucial, and concepts like Structural Isomerism and Stereoisomerism provide foundational knowledge for studying various types of polymers.
FAQ
Polypropylene is synthesised from propylene monomers, which inherently have a methyl group attached to one of the carbon atoms in the double bond. Upon polymerisation, this methyl group persists as a side chain on the polymer backbone. In contrast, polyethylene originates from ethylene, a simpler molecule without such side groups. This structural difference has profound implications for the properties of the resulting polymers. For instance, the presence of the methyl group in polypropylene introduces steric hindrance, which can affect its crystallinity. As a result, polypropylene generally exhibits a higher melting point and different mechanical properties compared to polyethylene.
Tacticity is a term that describes the spatial arrangement of side groups, such as the methyl groups in polypropylene, along the polymer chain. There are three primary configurations: isotactic (uniform arrangement with all side groups on the same side), atactic (random arrangement), and syndiotactic (alternating arrangement). The tacticity profoundly influences the polymer's physical properties. For instance, isotactic polypropylene, with its regular structure, tends to be more crystalline, translating to a higher melting point and greater tensile strength than the amorphous atactic polypropylene. By controlling the tacticity during the polymerisation process, manufacturers can tailor the properties of polypropylene for a wide range of applications, from textiles to automotive components.
Indeed, while many addition polymers are notoriously persistent in the environment, there are biodegradable variants. These polymers often incorporate functional groups or weaker bonds into their structure, rendering them more susceptible to microbial degradation or environmental breakdown. For instance, polylactic acid (PLA) is an addition polymer derived from renewable resources like corn starch. When exposed to specific conditions, microorganisms can metabolise PLA, breaking it down into harmless by-products. Another example is polyhydroxyalkanoates (PHA), which are produced by bacteria as energy storage molecules and can be degraded in various environments.
Addition polymers, such as polyethylene, are characterised by strong carbon-carbon single bonds and a lack of functional groups that can be vulnerable to environmental agents. Their saturated nature, devoid of double bonds or other reactive sites, makes them highly resistant to microbial degradation and various chemical reactions. This inherent stability, while advantageous in terms of product longevity and durability, also means they remain in the environment for extended periods, leading to concerns about plastic pollution and environmental sustainability.
Addition polymers are formed through the direct polymerisation of unsaturated monomers, typically alkenes, without the elimination or release of any small molecules. This means that every atom present in the monomer is incorporated into the polymer chain. On the other hand, condensation polymers are formed when two different monomers react, leading to the elimination of a small molecule, typically water. This process often involves the reaction of functional groups present on the monomers. The distinction is fundamental as it dictates the formation mechanisms, resulting molecular structures, and the inherent properties of the polymers.
Practice Questions
Polyethylene is formed through the addition polymerisation of ethylene monomers. The process begins with the initiation step, where a molecule, often a peroxide, dissociates upon heating to produce free radicals. These radicals are highly reactive and can initiate the polymerisation process. In the propagation step, the free radical attacks the double bond of an ethylene monomer, opening it up and forming a larger radical. This new radical can then attack other ethylene monomers, allowing the chain to grow. The process concludes with the termination step, where two radicals might collide, forming a stable molecule and ending the chain growth.
Addition polymers, especially polyethylene, are non-biodegradable and can persist in the environment for centuries, leading to significant pollution. Their strong carbon-carbon bonds and lack of functional groups make them resistant to microbial degradation. As they degrade physically, they break into microplastics, which can infiltrate the food chain, posing threats to marine life and potentially humans. One method to address these concerns is recycling. Mechanical recycling, the most common method, involves melting the polymer and reshaping it. Although each recycling cycle can degrade the polymer's properties, recycling reduces the demand for new polymers and helps mitigate greenhouse gas emissions associated with their production and disposal.
