Polymers, also known as polymer compounds, generally refer to compounds with relative molecular weights up to thousands to millions. The vast majority of polymer compounds are mixtures of many homologs with different relative molecular weights, so the relative molecular weight of polymer compounds is the average relative molecular weight. Polymer compounds are made up of thousands of atoms connected to each other by covalent bonds. Although their relative molecular mass is very large, they are all connected in a simple structural unit and repetitive way. The science of studying polymers is called polymer science.
According to the classification of properties, polymers can be divided into three categories: plastic, rubber, and fiber.
- Plastics: According to their hot melt properties, they can be divided into two categories: thermoplastics (such as polyethylene, polyvinyl chloride, etc.) and thermosetting plastics (such as phenolic resin, epoxy resin, unsaturated polyester resin, etc.). The former is a polymer with a linear structure, which can soften and flow when heated. They can be plasticized many times, and their defective and waste products can be recycled and reprocessed into products. The latter is a polymer with body structure, which is cured as soon as it is molded. They can no longer be heated and softened, nor can they be processed repeatedly. Therefore, the defective products and waste products of thermosetting plastics have no value for recycling. The common characteristic of plastics is that they have good mechanical strength (especially polymers with body shape and structure) and can be used as structural materials.
- Fiber: It can be divided into natural fiber and chemical fiber. Chemical fiber can be divided into man-made fiber (such as viscose fiber, acetate fiber, etc.) and synthetic fiber (such as nylon, polyester, etc.). Man-made fiber is made of natural polymers (such as short cotton wool, bamboo, wood, hair, etc.) through chemical processing and silk drawing. Synthetic fibers are synthesized from low molecular weight materials. The characteristic of the fiber is that it can be drawn and formed, and it has good strength and flexural properties, so it can be used as textile materials.
- Rubber: It includes natural rubber and synthetic rubber. Natural rubber is made by extracting gum from rubber trees, rubber grass, and other plants. Synthetic rubber includes cis-polybutadiene rubber, neoprene rubber, and styrene-butadiene rubber and so on, which are obtained by polymerization of various monomers. Rubber is characterized by good high elastic properties and is used as an elastic material.
- Polycondensation reaction: Polycondensation refers to the polymerization of monomers with two or more functional groups that contract and merges with each other to produce small molecular by-products (water, alcohol, ammonia, hydrogen halide, etc.) to form polymer compounds. For example, although phenol and formaldehyde are monofunctional compounds, the initial products of their reactions are multifunctional. These multifunctional molecules are condensed into linear or bulk polymers, namely phenolic resin.
Figure 1. Phenol and formaldehyde were condensed to form phenolic resin.
- Addition polymerization reaction: Addition polymerization refers to the synthesis of polymers from one or more monomers, in which no low molecular substances are formed. The resulting polymer has the same chemical composition as the raw material, and its relative molecular weight is several times the relative molecular weight of the raw material. For example, vinyl chloride is synthesized into polyvinyl chloride.
Figure 2. Synthesis of Polyvinyl Chloride by addition Polymerization of Vinyl Chloride
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