polymer

Polymers are also known as addition polymers. A product obtained by polymerization (addition polymerization) of a monomer. Molecules have repeating structural units. A low molecular weight oligomer, such as paraformaldehyde. High molecular weight, thousands or even millions of high polymer or polymer compounds. Polymers are both natural and synthetic. Natural polymers, such as proteins are polymers of amino acids, starch and cellulose are cyclic polyhydroxy polymers (saccharides), etc., also known as biopolymers. Most of them can be biodegraded, which may cause an increase in the biological oxygen demand of water in a short period of time. Under anaerobic conditions, they will decompose to produce pollutants such as ammonia, hydrogen sulfide and methane, but will not cause long-term environmental impact. There are many types of synthetic polymers, and the products are widely used, such as polyvinyl chloride and polystyrene plastics, resins, polyesters and rubbers. They are refractory substances. In the environment, a large amount of waste products will be produced for a long time, and the plasticizer will evaporate to pollute the environment and affect human health. Most of the monomers of the polymer are derived from petroleum, and many of them are toxic and harmful substances such as vinyl chloride as a carcinogen. Nowadays, oxides, hydrogen peroxide-containing compounds and carbonyl-containing compounds of various polymers have been developed. The polymer is photolyzed into low-molecular organic matter under sunlight and can be further degraded by the organism to become harmless, so that it does not pollute the environment. A high molecular polymer is a high molecular weight compound mainly composed of one or several structural units which are mainly covalently bonded, and is also called a high polymer, a high molecular compound, a macromolecular compound or the like. For example, polyethylene [—CH 2 —CH 2 —] n, nylon 6 [—NH(CH 2 ) 5 CO—] n , their structural units are —CH 2 —CH 2 —, —NH(CH 2 ) 5 CO—, and the degree of polymerization is n. . Due to its high molecular weight (usually 104-106), it shows many special properties, such as being more difficult to dissolve or even insoluble; it dissolves first; the solution viscosity is much higher than that of an equal concentration of small molecule solution; the intermolecular force is large, Usually only viscous liquid or solid, can not be gasified; solid has a certain mechanical strength, can be drawn, can pull the film. Polymers are composed of many large molecular chains of varying lengths. This property is called polydispersity of molecular weight and its molecular weight distribution can be used to describe its polydispersity. The molecular weight of the so-called polymer is an average value. According to the statistical average method, there is a number average molecular weight [equal to the total mass of the high polymer (in g) divided by the total amount of molecules of various molecular weight molecules to represent Mm], mass average molecular weight (equal to high polymer) The sum of the mass fraction of molecules of various molecular weights and their corresponding molecular weights, expressed in Mm). Mm/Mm is called the polydispersion coefficient and is used to measure the width and width of the molecular weight distribution. The larger the polydispersity coefficient, the wider the molecular weight distribution and the greater the degree of dispersion. The properties of the polymer are also largely determined by the shape of the molecular chain. According to the shape of the molecular chain, it can be divided into several structures: linear, spherical, mesh and body. The main chain atoms of the linear polymer are often arranged in a long chain shape, and more or less branches of different lengths are attached to the long chain. Polymers of this type tend to melt when heated, and are also soluble in specific organic solvents, have the potential to form crystals, and can be manually oriented. Since a small amount of branches causes the molecular spacing to increase, the structure becomes loose, so that the mechanical strength is lowered, and the solvency and plasticity are increased. Commonly used such polymers are polyethylene, polyvinyl chloride, rubber, and the like. The main structure of the spherical polymer is also a long chain shape, but has a large number of branches and surrounds the main chain to make the molecules spherical. Its strength and elasticity are not as high as linear structure, no significant melting point, but good solubility. The intermediate products of the reaction processes such as phenolic resin and urea-formaldehyde resin are all such structures. The main structure of the network structure polymer is also a long chain shape, but is crosslinked to form a network. It does not melt at high temperatures, but it can be soft and plastic; it does not dissolve in organic flux, but it can swell, and vulcanized rubber belongs to such polymers. The bulk type polymer has a long chain as a main chain and is crosslinked with many other molecules in a three-dimensional space, but can be gradually crosslinked in the polymerization process of the monomer. The high-polymerization material of the bulk structure is hard and brittle, neither melts nor plasticity at high temperatures, and cannot be dissolved in an organic solvent. The final products of resins such as phenolic, aminaldehyde, epoxy and polyester belong to this structure. We can classify polymers from different angles. Divided into chemical fiber, plastic, rubber, paint and adhesive according to nature and use; divided into addition polymer (such as polyethylene, polypropylene, etc.), condensation polymer (such as polyester, polyamide) according to the name of the synthesis reaction Etc.), ring-opening polymers (such as polyethers, etc.); classified by source into natural polymers (such as starch, cellulose, etc.), synthetic polymers (such as olefinic polymers, etc.), semi-synthetic polymers (such as acetic acid) Cellulose, etc.); according to the main chain element composition is divided into carbon chain polymer (main chain mainly composed of carbon atoms), heterochain polymer (in addition to carbon atoms in the main chain, there are also hetero atoms such as oxygen, nitrogen, sulfur, etc.), Elemental organic polymer (the main chain is mainly composed of boron, silicon, aluminum, oxygen, nitrogen, sulfur, phosphorus, etc., and the side chain is an organic group such as methyl, ethyl, etc.); Functional polymers, etc. The indexes of use of the polymer mainly include strength, hardness, heat resistance, corrosion resistance, abrasion resistance, solvent resistance, light transmittance, airtightness, electrical insulation, and the like. The density of the polymer is small, much smaller than that of the same volume of steel; some have electrical conductivity, magnetic properties, some have high temperature resistance, low temperature resistance, and radiation resistance; some have good air tightness and light transmittance. Widely used in medical, electrical, heat, construction, packaging materials, plastics and other aspects. The production method of the polymer includes bulk polymerization (including melt polymerization, which is a polymerization of a monomer under an initiator or light, heat, radiation, generally without adding other medium), solution polymerization (monomer, initiator dissolved in a suitable solution) Polymerization), emulsion polymerization (polymerization of the monomer in water as an emulsion under stirring and emulsifier), suspension polymerization (under stirring and dispersing agent, the monomer is dispersed into monomer droplets, suspended in Water is polymerized, etc., depending on the requirements of use, different polymerization methods can be employed.