Termination steps, chain reactions

Free radical polymerization Relatively insensitive to trace impurities Reactions can occur in aqueous media Can use chain transfer to solvent to modify polymerization process Structural irregularities are introduced during initiation and termination steps Chain transfer reactions lead to reduced molecular weight and branching Limited control of tacticity High pressures often required... 

We can also terminate a chain reaction by intentionally adding a scavenger species S that readily scavenges the chain propagator R. We write this reaction step as... 

Chain transfer/termination steps involve reactions with methanol [schemes (84) to (86)], spontaneous transfer [scheme (87)] or reaction with acids [scheme (88)] [107] ... 

The combination of any two free radicals is a termination step because it decreases the number of free radicals. Other termination steps involve reactions of free radicals with the walls of the vessel or other contaminants. Although the first of these termination steps gives chloromethane, one of the products, it consumes the free radicals that are necessary for the reaction to continue, thus breaking the chain. Its contribution to the amount of product obtained from the reaction is small compared with the contribution of the propagation steps. 

The regenerated bromine radical reacts with another molecule of the alkene, continuing the chain reaction. Both of the propagation steps are moderately exothermic, allowing them to proceed faster than the termination steps. Note that each propagation step starts with one free radical and ends with another free radical. The number of free radicals is constant, until the reactants are consumed, and free radicals come together and terminate the chain reaction. 

Reactions like this are known as termination steps and are actually an important part of any chain reaction without termination steps the reaction would be uncontrollable. 

Chain termination step The combination of reactive species (such as radicals) which terminates the chain reaction. 

In this mechanism of polymerization, a small amount of free radicals is generated. These attack the carbon-carbon double bonds of monomer molecules, bond to one carbon, and produce the more stable free radical this is the initiation step. Since few chains are initiated, the free radical attacks yet another monomer, adds to the double bond, and forms another free radical that, in turn, continues the process this is propagation. Eventually two developing free radical chains may bond together and terminate the chain reaction. 

As for the termination step, different reactions have been detected, according to the structure of the polymers, such as terminal vinylic groups, which are evidence of -hydride elimination from the polymeric chains. 

Oxidative degradation of polymers typically follows a free-radical mechanism involving crosslinking and/or chain scission initiated by free radicals from peroxides formed during the initial oxidation step [1-11]. Enhanced stability has been achieved by the use of additives which are frequently called antioxidants or heat stabilizers. One approach employed to reduce the oxidation of polyolefins like PE and PP is to terminate the chain reaction by introducing an antioxidant with a greater affinity than a polyolefin for the peroxy radical RO. Such antioxidants (AH) function by reacting with RO2 to form a relatively inactive radical A, i.e.,... 

Regarding the termination step, chain transfer to a monomer, polymer, solvent or counterion can terminate the growth of chains. If X+ is not loss and rejoins the anion to reform the acid catalyst, the termination is called transfer to counterion if X" " initiates another monomer molecule to start a new chain, termination is defined as transfer to monomeT [6]. When a distinct termination reaction does not occur, living cationic polymers are obtained. 

In practice side reactions intervene to reduce the efficiency of the propagation steps The chain sequence is interrupted whenever two odd electron species combine to give an even electron product Reactions of this type are called chain terminating steps Some commonly observed chain terminating steps m the chlorination of methane are shown m the following equations... 

Termination steps (Section 4 17) Reactions that halt a chain reaction In a free radical chain reaction termination steps consume free radicals without generating new radicals to continue the chain... 

One characteristic of chain reactions is that frequentiy some initiating process is required. In hydrocarbon oxidations radicals must be introduced and to be self-sustained, some source of radicals must be produced in a chain-branching step. Moreover, new radicals must be suppHed at a rate sufficient to replace those lost by chain termination. In hydrocarbon oxidation, this usually involves the hydroperoxide cycle (eqs. 1—5). 

Autooxidation. Liquid-phase oxidation of hydrocarbons, alcohols, and aldehydes by oxygen produces chemiluminescence in quantum yields of 10 to 10 ° ein/mol (128—130). Although the efficiency is low, the chemiluminescent reaction is important because it provides an easy tool for study of the kinetics and properties of autooxidation reactions including industrially important processes (128,131). The light is derived from combination of peroxyl radicals (132), which are primarily responsible for the propagation and termination of the autooxidation chain reaction. The chemiluminescent termination step for secondary peroxy radicals is as follows ... 

If the initiation reaction is much faster than the propagation reaction, then all chains start to grow at the same time. Because there is no inherent termination step, the statistical distribution of chain lengths is very narrow. The average molecular weight is calculated from the mole ratio of monomer-to-initiator sites. Chain termination is usually accompHshed by adding proton donors, eg, water or alcohols, or electrophiles such as carbon dioxide. 

Autoca.ta.Iysis. The oxidation rate at the start of aging is usually low and increases with time. Radicals, produced by the homolytic decomposition of hydroperoxides and peroxides (eqs. 2—4) accumulated during the propagation and termination steps, initiate new oxidative chain reactions, thereby increasing the oxidation rate. 

Addition polymerisation is effected by the activation of the double bond of a vinyl monomer, thus enabling it to link up to other molecules. It has been shown that this reaction occurs in the form of a chain addition process with initiation, propagation and termination steps. 

The step in which the reactive intermediate, in this case A-, is generated is called the initiation step. In the next four equations in the example above, a sequence of two reactions is repeated this is the propagation phase. Chain reactions are characterized by a chain length, which is the number of propagation steps that take place per initiation step. Finally, there are termination steps, which include any reactions that destroy one of the reactive intermediates necessary for the propagation of the chain. Clearly, the greater the frequency of termination steps, the lower the chain length will be. 

The result of the steady-state condition is that the overall rate of initiation must equal the total rate of termination. The application of the steady-state approximation and the resulting equality of the initiation and termination rates permits formulation of a rate law for the reaction mechanism above. The overall stoichiometry of a free-radical chain reaction is independent of the initiating and termination steps because the reactants are consumed and products formed almost entirely in the propagation steps. 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

Termination Occasionally, two radicals might collide and combine to form a stable product. When that happens, the reaction cycle is broken and the chain is ended. Such termination steps occur infrequently, however, because the concentration of radicals in the reaction at any given moment is very small. Thus, the likelihood that two radicals will collide is also small. 

---