Addition polymerization

摘要

CHAIN growth polymerization is called addition polymerization. All chain growth polymerization are addition reaction products but it is not necessary that all addition reactions are chain growth polymers. Polymers are synthesized in two ways. First ones are the addition polymers, which are formed by addition reactions that link together monomers containing multiple bonds. These polymers usually form via a free-radical mechanism in which the multiple bond is homolytically cleaved by light, heat or with the assistance of a radical initiator, such as benzoyl peroxide, cumene peroxide or other peroxide derivatives. Addition polymerization can be any of the two types. In simple terms, addition polymers are formed when organic compounds containing one or more C=C double bonds add to each other. Addition reactions typically require a catalyst to initiate the reaction, but once a reaction starts, it will continue as a chain reaction until thousands of monomer units have been added together Polymerization with chain growth by free radicals and ionic polymerization: Anionic polymerization and Cationic polymerization. Generally, most of the polymers are polymerized by free radical formation because it is not steric hindrance. Addition polymers are extremely large molecules. A classic example is the number of alkene monomer units that make up a typical polymer molecule, such as polyethylene. High-density polyethylene or HDPE is a linear polymer with an average molecular weight (molar mass) of 500,000 g/mole. Assuming the basic or repeating structural unit in polyethylene is - (CH_2-CH_2) n-, the value of n is 500,000/28, or about 18,000 ethylene molecules. Another example is Polystyrene, which is used to make rigid, clear plastics, is a common addition polymer. It is made by reacting styrene, CH_2=CH-C_6H_5, with dibenzoyl peroxide. Dibenzoyl peroxide is a free-radical catalyst which breaks apart in the presence of heat or light to produce benzoyl radicals. Free radicals contain unpaired electrons and are thus very reactive. There are three main steps in the addition reaction for the preparation of polystyrene.