Free radical polymerization propagation, Chain termination

When vinyl chloride is polymerized in solution, there is no autoacceleration. Also, a major feature of vinyl chloride free-radical polymerization is chain transferring to monomer [2%]. This is supported by experimental evidence [297,298]. In addition, the growing radical chains can terminate by chain transferring to dead polymer molecules. The propagations then proceed from the polymer backbone [297]. Such new growth radicals, however, are probably short lived as they are destroyed by transfer to monomer [299]. 

If the reactions involved in free-radical polymerization proceeded precisely in the maimer outlined above (initiation, propagation, termination), in all instances, one would obtain long and linear polymer chains. However, in free radical polymerization, linear chains are, in fact, not formed. Instead, extensive branching is observed particularly with monomers such as ethylene. In this context a slight clarification of branching is required. Thus, for a polymer obtained from a substituted vinyl monomer regular side groups are present (Fig. 2.3). 

Figure 15.18 Steps in the free-rac8cal polymerization of ethylene. In this polymerization method, free radicals initiate, propagate, and terminate the formation of an addition polymer. An initiator (Y—O—O— Y) is spit to form two molecules of a free radktal (Y—0-). The free radktal attacks the ir bond of a morxjmer and creates another free radical (Y—O—CHg—CHg-). The process continues, and the chain grows (propagates) until an inhibitor is added (not shown) or two free radk als combine.

Free-radical polymerization can be terminated with a transfer agent such as benzoquinone which consumes free radicals. Mercaptans (thiols) such as butyl mercaptans are commonly used transfer agents. Free radicals at macromolecule chain ends can selectively react with transfer agents to terminate chain growth but the free radical is transferred to another macromolecule that continues to grow. Transfer can occur with the initiator, monomer, macromolecule, and solvent. Monomer propagation and transfer to the monomer are... 

Both modes of ionic polymerization are described by the same vocabulary as the corresponding steps in the free-radical mechanism for chain-growth polymerization. However, initiation, propagation, transfer, and termination are quite different than in the free-radical case and, in fact, different in many ways between anionic and cationic mechanisms. Our comments on the ionic mechanisms will touch many of the same points as the free-radical discussion, although in a far more abbreviated form. 

The free-radical polymerization of methacrylic monomers follows a classical chain mechanism in which the chain-propagation step entails the head-to-taH growth of the polymeric free radical by attack on the double bond of the monomer. Chain termination can occur by either combination or disproportionation, depending on the conditions of the process (36). 

In the period 1910-1950 many contributed to the development of free-radical polymerization.1 The basic mechanism as we know it today (Scheme 1.1), was laid out in the 1940s and 50s.7 9 The essential features of this mechanism are initiation and propagation steps, which involve radicals adding to the less substituted end of the double bond ("tail addition"), and a termination step, which involves disproportionation or combination between two growing chains. 

The polymerization rate equations are based on a classical free radical polymerization mechanism (i.e., initiation, propagation, and termination of the polymer chains). 

Figure 2,3 Chain growth polymerization exemplified by free radical polymerization of polyethylene a) initiation, b) propagation, c) chain transfer, and d) termination...

As with other chain reactions, free radical polymerization is a rapid reaction which consists of the characteristic steps of initiation, propagation, and termination. Free radical initiators are produced by the homolytic cleavage of covalent bonds as well as numerous radiation-associated methods. 

For classical free radical polymerizations the rate of propagation is proportional to the concentration of monomer and the square root of the initiator concentration. Termination usually occurs through a coupling of two live radical chains but can occur through disproportionation. The rate of termination for coupling is directly proportional to initiator concentration. The DP is directly proportional to monomer concentration and inversely proportional to the square root of the initiator concentration. 

On the basis of the nature of the initiation step, polymerization reactions of unsaturated hydrocarbons can be classified as cationic, anionic, and free-radical polymerization. Ziegler-Natta or coordination polymerization, though, which may be considered as an anionic polymerization, usually is treated separately. The further steps of the polymerization process (propagation, chain transfer, termination) similarly are characteristic of each type of polymerization. Since most unsaturated hydrocarbons capable of polymerization are of the structure of CH2=CHR, vinyl polymerization as a general term is often used. 

Polymerization of monomer by vinyl addition polymerization is a typical chain process. Three main reaction stages can be identified initiation, propagation and termination. During the initiation event, free radicals are created. In a photopolymerization, the initiating free radicals are formed in a photoprocess. Propagation is the process of addition of monomer to the growing free radical chain. The destruction of the free radical center occurs during termination. 

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