Initiators, free radical

Free-radical initiators, Anionic initiators, Cationic initiators, 

Free-radical initiators are chemical substances that, under certain conditions, initiate chemical reactions by producing free radicals  

The principal commercial initiators used to generate radicals are peroxides and azo compounds. Lesser amounts of carbon—carbon initiators and photoinitiators, and high energy ionizing radiation are also employed commercially to generate radicals. 

Radicals are employed widely in the polymer industry, where their chain-propagating behavior transforms vinyl monomers into polymers and copolymers. The mechanism of addition polymerization involves all three types of reactions discussed above, ie, initiation, propagation by addition to carbon—carbon double bonds, and termination  

In these equations I is the initiator and I- is the radical intermediate, M is a vinyl monomer, I—M- is an initial monomer radical, I—MnM- is a propagating polymer radical, and and M7 are polymer end groups that result from termination by disproportionation. Common vinyl monomers that can be homo-or copolymerized by radical initiation include ethylene, butadiene, styrene, vinyl chloride, vinyl acetate, acrylic and methacrylic acid esters, acrylonitrile, IV-vinylimidazole, IV-vinyl-2-pyrrolidinone, and others (2). 

Fiee-iadical initiatois, Anionic initiatois, Cationic initiatois, 

Fiee-iadical initiatois aie chemical substances that, undei ceitain conditions, initiate chemical leactions by pioducing fiee ladicals  

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Polymerization reactions. There are two broad types of polymerization reactions, those which involve a termination step and those which do not. An example that involves a termination step is free-radical polymerization of an alkene molecule. The polymerization requires a free radical from an initiator compound such as a peroxide. The initiator breaks down to form a free radical (e.g., CH3 or OH), which attaches to a molecule of alkene and in so doing generates another free radical. Consider the polymerization of vinyl chloride from a free-radical initiator R. An initiation step first occurs ... 

Step 1 Homolytic dissociation of a peroxide produces alkoxy radicals that serve as free radical initiators... 

Any one of these expressions gives the rate of initiation Rj for the particular catalytic system employed. We shall focus attention on the homolytic decomposition of a single initiator as the mode of initiation throughout most of this chapter, since this reaction typifies the most widely used free-radical initiators. Appropriate expressions for initiation which follows Eq. (6.6) are readily derived. 

DIITIATORS - FREE-RADICAL INITIATORS] (Voll4) l,ly-Azobis(cyanocyclohexane) [2094-98-6]... 

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