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Dead polymer refers to a polymer that has no longer reactive due to chain termination in a chain polymerization reaction. All polymers formed by chain free radical polymerization of olefin monomers are dead polymers. Only in anionic polymerization, due to the absence of chain termination and chain transfer reaction, the growing chain can remain active, and active polymers can be obtained at this time.
Such as low-density polyethylene, polystyrene, polyvinyl chloride, etc.
Free radical polymerization is a polymerization reaction initiated by free radicals to make the chain growth (chain growth) free radicals continue to grow. It is also called free radical polymerization. Most addition polymerization reactions use olefin monomers containing unsaturated double bonds as raw materials. By opening the double bonds in the monomer molecules, repeated addition reactions are carried out between molecules to connect many monomers to form macromolecules. It is mainly used in addition polymerization of olefins. The most commonly used method to generate free radicals is the thermal decomposition of the initiator or the redox decomposition reaction of the two-component initiator. Free radicals can also be generated by heating, ultraviolet irradiation, high-energy irradiation, electrolysis and plasma initiation.
Free radical polymerization occupies an extremely important position in polymer chemistry. It is the earliest and most thoroughly studied polymerization process developed by humans. More than 60% of polymers are obtained through free radical polymerization, such as low-density polyethylene, polystyrene, polyvinyl chloride, polymethyl methacrylate, polyacrylonitrile, polyvinyl acetate, styrene-butadiene rubber, nitrile rubber, chloroprene rubber, etc.
This polymerization reaction is a chain polymerization reaction, which is divided into four elementary reactions: chain initiation, chain growth, chain termination and chain transfer.
Also known as the beginning of the chain, the main reaction has two steps: the formation of active centers - free radicals, and then free radicals initiate monomers. The main side reaction is that oxygen and impurities interact with primary free radicals or active monomers to hinder the polymerization reaction. Generally, an initiator is required for initiation. Commonly used initiators include azo initiators, peroxide initiators and redox initiators. Azo initiators include azobisisobutyronitrile, dimethyl azobisisobutyrate initiator, V-50 initiator, etc. Peroxides include BPO, etc. Light, heat and radiation can also be used for initiation.
is the process in which active monomers repeatedly and rapidly add to monomer molecules to form macromolecular free radicals. Whether the chain growth reaction can proceed smoothly is mainly determined by factors such as the structural characteristics of the free radicals converted from the monomers, the concentration of the monomers in the system and the ratio to the concentration of the active chain, the impurity content and the reaction temperature.
is mainly formed by the interaction of two free radicals, which refers to the disappearance of the activity of the active chain, that is, the disappearance of the free radicals to form a stable molecule of the polymer. The main ways of termination are single-radical termination and double-radical termination. Double-radical termination is the combination of two active chain free radicals and double-radical termination of disproportionation reaction, or the existence of both. When the viscosity of the system is too high, double radical termination is not possible and only single radical termination is possible.
The chain free radical takes an atom from a low molecule such as a monomer, solvent, initiator or a formed macromolecule and terminates, and causes these molecules that have lost an atom to form a new free radical. Chain termination transfers the active species to another molecule, while the original active species itself terminates.
Polyethylene (PE) is a thermoplastic resin obtained by polymerization of ethylene. In industry, it also includes copolymers of ethylene and a small amount of ¦Á-olefins. Polyethylene is odorless, non-toxic, feels like wax, has excellent low temperature resistance (the lowest operating temperature can reach -100~-70¡ãC), good chemical stability, and can withstand the erosion of most acids and alkalis (not resistant to acids with oxidizing properties). It is insoluble in general solvents at room temperature, has low water absorption, and has excellent electrical insulation.
Polyethylene (PE) is a polymer formed by the polymerization of ethylene. The product has a history of about 60 years. The global polyethylene production ranks first among the five major general-purpose resins.
Depending on the polymerization method, molecular weight and chain structure, polyethylene is divided into high-density polyethylene, low-density polyethylene and linear low-density polyethylene.
Low-density polyethylene (LDPE) is commonly known as high-pressure polyethylene. Because of its low density and the softest material, it is mainly used in plastic bags, agricultural films, etc. High-density polyethylene (HDPE) is commonly known as low-pressure polyethylene. Compared with LDPE and LLDPE, it has higher heat resistance, oil resistance, steam permeation resistance and environmental stress cracking resistance. In addition, it has good electrical insulation, impact resistance and cold resistance. It is mainly used in blow molding, injection molding and other fields.
Linear low-density polyethylene (LLDPE) is a copolymer formed by the polymerization of ethylene and a small amount of high-olefins in the presence of a catalyst. LLDPE is similar in appearance to LDPE, with poorer transparency but good surface gloss, low-temperature toughness, high modulus, bending resistance and stress cracking resistance, and good impact strength at low temperatures.
LLDPE's application areas have penetrated almost all LDPE markets. At present, LLDPE and HDPE are in the growth stage of their life cycle; LDPE gradually entered the mature stage of development in the late 1980s, and there are few LDPE equipment in the world. Polyethylene can be formed by extrusion, injection, molding, blow molding and melt spinning, and is widely used in industry, agriculture, packaging and daily industry. It is widely used in China. Film is its largest user, consuming about 77% of low-density polyethylene and 18% of high-density polyethylene. In addition, injection molded products, wires and cables, hollow products, etc. all account for a large proportion of its consumption structure, and occupy a pivotal position in the plastics industry.
Polyethylene materials are difficult to print (unless they are modified in the bulk or surface), so most of them are colorless or light-colored products. Of course, due to its good resistance to environmental aging, most artificial turf on sports fields is made of polyethylene. The simplest identification method is to ignite a small piece of sample with a gas flame (such as a lighter). The sample will continue to burn, with smoke and the smell of burning candles. Scratch it with a fingernail. The one with a scratch is low-density polyethylene (LDPE), otherwise it is high-density polyethylene (HDPE).
1. Polyethylene has excellent chemical stability. It is resistant to corrosion by various chemicals such as hydrochloric acid, hydrofluoric acid, phosphoric acid, formic acid, amines, sodium hydroxide, potassium hydroxide, etc. at room temperature, but nitric acid and sulfuric acid have a strong destructive effect on polyethylene;
2. Polyethylene is easily photo-oxidized, thermally oxidized, and ozone-decomposed. It is easy to degrade under the action of ultraviolet rays. Carbon black has an excellent light-shielding effect on polyethylene. After being irradiated, cross-linking, chain breaking, and the formation of unsaturated groups may occur.
A milky white, translucent thermoplastic made by homopolymerization of ethylene and copolymerization with a small amount of ¦Á-olefins. Density 0.86-0.96g/cm. According to density, there are low-density polyethylene (including linear low-density polyethylene), ultra-low-density polyethylene, etc. Odorless and non-toxic. Resistant to chemicals, insoluble in solvents at room temperature. Resistant to low temperatures, the lowest operating temperature is -70--100¡æ. Good electrical insulation and low water absorption. Physical and mechanical properties vary with density. In industry, low-density polyethylene mainly adopts high-pressure (110-200MPa) and high-temperature (150-300¡æ) free radical polymerization. Others use low-pressure coordination polymerization. Sometimes the same set of equipment can produce polyethylene products with a density of 0.87-0.96g/cm, which is called full-density polyethylene process technology. Polyethylene can be processed into films, wire and cable sheaths, pipes, various hollow products, injection molded products, fibers, etc. It is widely used in agriculture, packaging, electronics and electrical, machinery, automobiles, daily necessities, etc.
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