Addition or Chain Polymerization

In chain polymerization, a simple, low-molecular-weight molecule possessing a double bond, referred to in this context as a monomer, is treated so that the double bond opens up and the resulting free valences join with those of other molecules 

It is evident that no side products are formed consequently the composition of the mer or repeating unit of the polymer (-CH2-CHCI-) is identical to that of the monomer (CH2=CHC1). The identical composition of the repeating unit of a polymer and its monomer(s) is, in most cases, an indication that the polymer is an addition polymer formed by chain polymerization process. The comihon addition polymers and the monomers from which they are produced are shown in Table 

Chain polymerization involves three processes chain initiation, chain propagation, and chain termination. (A fourth process, chain transfer, may also be involved, but it may be regarded as a combination of chain termination and chain initiation.) Chain initiation occurs by an attack on the monomer molecule by a free radical, a cation, or an anion accordingly, the chain polymerization processes are called free-radical polymerization, cationic polymerization, or anionic polymerization. A free radical is a reactive substance having an unpaired electron and is usually formed by the decomposition of a relatively unstable material called an initiator. Benzoyl peroxide is a common free-radical initiator and can produce free radicals by thermal decomposition as 

Free radicals are, in general, very active because of the presence of unpaired electrons (denoted by dot). A free-radical species can thus react to open the double bond of a vinyl monomer and add to one side of the broken bond, with the reactive center (unpaired electron) being transferred to the other side of the broken bond  

The new species, which is also a free radical, is able to attack a second monomer molecule in a similar way, transferring its reactive center to the attacked molecule. The process is repeated, and the chain continues to grow as a large 

Chain polymerization involves three processes chain initiation, chain propagation, and chain termination. (A fourth process, chain transfer, may also be involved, but it may be reg irded as a combination of chain termination and chain initiation.) Chain initiation occurs by an attack on 

Methyl meiheerylate Poly(methyl methacrylate) (PMMA) 

Lo r density. stifFer. and higher temperature resistance than PE used for water pipes, integral hinges, sterilizable hospital equipment. e.g. Propathenet Novoien, Moplen, Hostalen, Mariex. 

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The kinetics of polycondensation and polyaddition reactions follow the same general scheme, but both differ sharply from the kinetics of addition or chain polymerization. 

Illustration of addition or chain polymerization. A free radical R opens a double bond of an ethylene molecule, transferring the unpaired electron to its opposite end where it can open the double bond of another polyethylene molecule and so on. 

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