Termination steps

The termination of the polymer radicals occurs by bimolecular recombination  

Termination reactions also occur between low molecular radicals such as hydroxyl (HO ) and hydroperoxy (HOj) and other available radicals (R )  

When the oxygen pressure is high (atmospheric pressure), the termination reaction almost exclusively occurs by reaction 2.38. At low oxygen pressure other termination reactions take place to some extent. The termination 

Several factors influence recombination reactions, such as  

Effect of steric control and radical mobility in a polymer matrix, 

The polymerization reaction may terminate by the interaction of two free radicals as shown below, 

Polymer A AAAAA-C.. C- AAAAAA/polymer - polymer/ AAAA/-CH2-CH2-AAAAAAA/-polymer 

Similar to other ethylene polymerization processes, removal of the heat of polymerization from the reactor is a critical feature of the process. In this regard, the tubular high-pressure process has a more efficient heat removal process than the stirred autoclave reactor. 

6 Organic Peroxides as Free-Radical Source for Initiation Process 

The replacement of oxygen as the free-radical source with organic peroxides has led to a more controlled polymerization process which has essentially 

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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 ... 

This termination step stops the subsequent growth of the polymer chain. The period during which the chain length grows, i.e., before termination, is known as the active life of the polymer. Other termination steps are possible. 

An example of a polymerization without a termination step is polycondensation ... 

Here the polymer grows by successive esterification with elimination of water and no termination step. 

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 are m general less likely to occur than the propagation steps Each of the termination steps requires two free radicals to encounter each other m a medium that contains far greater quantities of other materials (methane and chlorine mol ecules) with which they can react Although some chloromethane undoubtedly arises via direct combination of methyl radicals with chlorine atoms most of it is formed by the propagation sequence shown m Figure 4 21... 

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... 

Tlie formation of initiator radicals is not the only process that determines the concentration of free radicals in a polymerization system. Polymer propagation itself does not change the radical concentration it merely changes one radical to another. Termination steps also occur, however, and these remove radicals from the system. We shall discuss combination and disproportionation reactions as modes of termination. 

In Chap. 5, p was defined as the fraction (or probability) of functional groups that had reacted at a certain point in the polymerization. According to the current definition provided by Eq. (6.66), p is the fraction (or probability) of propagation steps among the combined total of propagation and termination steps. The quantity 1 - p is therefore the fraction (or... 

We shall consider these points below. The mechanism for cationic polymerization continues to include initiation, propagation, transfer, and termination steps, and the rate of polymerization and the kinetic chain length are the principal quantities of interest. 

Termination steps can involve either similar or dissimilar radicals ... 

At present it is not possible to determine which of these mechanisms or their variations most accurately represents the behavior of Ziegler-Natta catalysts. In view of the number of variables in these catalyzed polymerizations, both mechanisms may be valid, each for different specific systems. In the following example the termination step of coordination polymerizations is considered. 

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 ... 

Tertiary peroxyl radicals also produce chemiluminescence although with lower efficiencies. For example, the intensity from cumene autooxidation, where the peroxyl radical is tertiary, is a factor of 10 less than that from ethylbenzene (132). The chemiluminescent mechanism for cumene may be the same as for secondary hydrocarbons because methylperoxy radical combination is involved in the termination step. The primary methylperoxyl radical terminates according to the chemiluminescent reaction just shown for (36), ie, R = H. 

The two main termination steps for neutral solutions are HO + HO — H2O2 + 2 O3 and HO + HO3 — H2O2 + O3 + O2. An alternative mechanism has been proposed that does not involve HO and HO but has a different initiation step (26). Three ozone molecules are destroyed for each primary event. In the presence of excess HO radical scavengers, ie, bicarbonate, the pseudo-first-order rate constant at 20°C for the initiation step is 175 X. This yields an ozone half-hfe of 66 min at pH 8. In distilled water = 50 mmol/L), the half-hfe is significantly lower, ie, 7 min. 

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. 

The requirements for a polymerisation to be truly living are that the propagating chain ends must not terminate during polymerisation. If the initiation, propagation, and termination steps are sequential, ie, all of the chains are initiated and then propagate at the same time without any termination, then monodisperse (ie, = 1.0) polymer is produced. In general, anionic polymerisation is the only mechanism that yields truly living styrene... 

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. 

The free-radical mechanism iavolves initiation, propagation, and termination steps. During initiation two radicals are produced for every paraffin molecule. 

A factor in addition to the RTD and temperature distribution that affects the molecular weight distribution (MWD) is the nature of the chemical reaciion. If the period during which the molecule is growing is short compared with the residence time in the reactor, the MWD in a batch reactor is broader than in a CSTR. This situation holds for many free radical and ionic polymerization processes where the reaction intermediates are very short hved. In cases where the growth period is the same as the residence time in the reactor, the MWD is narrower in batch than in CSTR. Polymerizations that have no termination step—for instance, polycondensations—are of this type. This topic is treated by Denbigh (J. Applied Chem., 1, 227 [1951]). 

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. 

In the absence of impurities there is frequently no termination step in anionic polymerisations. Hence the monomer will continue to grow until all the monomer is consumed. Under certain conditions addition of further monomer, even after an interval of several weeks, will eause the dormant polymerisation process to proceed. The process is known as living polymerisation and the products as living polymers. Of particular interest is the fact that the follow-up monomer may be of a different species and this enables block copolymers to be produced. This technique is important with certain types of thermoplastic elastomer and some rather specialised styrene-based plastics. 

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. 

The observed rate law is then three-halves order in the reagent A2. In most real systems, the situation is complicated because more than one termination reaction makes a contribution to the total termination rate. A more complete discussion of the effect of termination steps on the form of the rate law has been given by Huyser. ... 

Find a rate equation eonsistent with the proposed meehanism and verify it against the data. Assume that the initiation and termination steps are relatively slow. 

If other possible termination steps are considered, then it can be inferred that recombination or disproportionation of ethyl radicals, c,H5 is more likely than recombination of C and. Step 5... 

Anionic polymerization is better for vinyl monomers with electron withdrawing groups that stabilize the intermediates. Typical monomers best polymerized by anionic initiators include acrylonitrile, styrene, and butadiene. As with cationic polymerization, a counter ion is present with the propagating chain. The propagation and the termination steps are similar to cationic polymerization. 

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. 

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