The 20th century has been described as the age of plastic. From the serendipidous discovery of polythene in 1898 by the German Chemist Hans von Pechmann to the equally fortunate accidental discovery of an industrial process for synthesising poly(ethene) in 1933 by the English ICI (Imperial Chemical Industry) scientists Fawcett and Gibson, synthetic polymers have become both ubiquitous and an environmental concern in everyday life.
Syllabus referenceStructure 2.4.5 - Addition polymers form by the breaking of a double bond in each monomer.
- Represent the repeating unit of an addition polymer from given monomer structures.
Guidance
- Examples should include polymerization reactions of alkenes.
- Structures of monomers do not have to be learned, but will be provided or will need to be deduced from the polymer.
Tools and links
- Structure 3.2 - What functional groups in molecules can enable them to act as monomers for addition reactions?
- Reactivity 2.1 - Why is the atom economy 100% for an addition polymerization reaction?
Addition reactions
Addition reactions involving unsaturated hydrocarbons, such as alkenes and alkynes, are fundamental processes in organic chemistry. Unsaturated hydrocarbons are characterized by the presence of double or triple bonds between carbon atoms, which are regions of high electron density and reactivity.
In an addition reaction, the double or triple bond opens up, allowing atoms or groups of atoms to add across the bond. This process converts unsaturated hydrocarbons into saturated molecules. The most common types of addition reactions include hydrogenation, halogenation, hydrohalogenation, and hydration.
Hydrogenation involves adding hydrogen (H2) across the double or triple bond, typically in the presence of a catalyst like palladium or nickel, leading to the formation of alkanes. Halogenation is the addition of halogen atoms (such as chlorine or bromine) to the unsaturated hydrocarbon, resulting in dihalogenated products. Hydrohalogenation involves the addition of hydrogen halides (like HCl or HBr) to the hydrocarbon, while hydration adds water (H2O) across the double bond, forming alcohols.
These reactions are not only important for understanding chemical properties and reactivity of organic compounds but also have significant practical applications. For instance, the hydrogenation of vegetable oils forms margarine, a solid form of fat, and the production of important industrial chemicals and polymers often involves addition reactions of unsaturated hydrocarbons.
Addition polymers
Addition polymerization is a key chemical process where unsaturated monomers, typically alkenes or alkynes, join together without the loss of any atoms or molecules. Each monomer has a double or triple bond, which opens up to form a bond with another monomer, creating long chains or polymers. This process is integral to the production of many synthetic materials that play a vital role in modern society.
In the context of chemistry, addition polymerization may proceed via a free-radical process giving rise to products with limited structural symmetry, or using a Zeigler-Natta catalytic process that build the polymer via ionic intermediates producing highly symmetrical polymers.
This latter process allows the polymer to be designed with structures that are more crystalline, and propperties that are more consistent and predictable.
Society greatly benefits from addition polymerization, as it is the foundation for creating a variety of plastics and synthetic rubbers. For instance, polyethylene, formed from the polymerization of ethylene, is used in packaging, containers, and insulation. Polystyrene, another product of addition polymerization, is utilized in packaging, disposable food containers, and insulation materials. Synthetic rubbers produced through this process are essential in the automotive and manufacturing industries for tires and other rubber products.
The widespread use of these polymers highlights their importance but also raises concerns about environmental sustainability. As many of these polymers are not biodegradable, their disposal poses ecological challenges, prompting research into recyclable and biodegradable alternatives. Nevertheless, the contribution of addition polymerization to various facets of modern life is undeniable, making it a significant area of study in both chemistry and material science.
