Step propagation

The key reaction in the propagation sequences is the formation of polymer peroxy radicals (POO ) by the reaction of polymer alkyl radicals (P ) with oxygen  

This reaction is very fast but diffusion-controlled (cf. section 2.17.1) [571, 2299, 2300]. 

The next propagation step is the abstraction of a hydrogen atom by the polymer peroxy radical (POO ) to generate a new polymer alkyl radical (P ) and polymer hydroperoxide (POOH) [368, 697, 1316, 1921, 2300]. 

The propagation step is very much dependent on the eflSciency of the decomposition (photolysis and/or thermolysis) of polymer hydroperoxides (POOH) during which new free radicals such as polymer oxy radical (PO ) and hydoxyl radical (HO ) are formed  

This reaction is mainly initiated by an energy transfer process from a carbonyl group (CO) to a hydroperoxide group (OOH) (cf. section 2.5) and is dependent on the cage recombination reaction (cf. section 2.1.3). 

The three steps required for the free-radical polymerization mechanism process are (1) Initiation Step, (2) Propagation Step, and (3) Termination Step. 

Molecular oxygen was utilized by the ICI scientists in 1933 as the free-radical source to initiate the high-pressure ethylene polymerization reaction that provided high molecular weight polyethylene. (A free radical is a highly reactive, unstable, chemical intermediate comprised of an unpaired electron designated by a [ ] symbol.) Presently, organic peroxides (R-O-O-R) are utilized as the free-radical source as shown below. 

The free radicals are formed by the thermal decomposition of the initiating organic peroxide. For example, if R = CgH5-C=0, then this free radical eliminates a CO molecule to provide the CgH radical that reacts with ethylene as shown below to provide the polymerization initiation free radical. 

The initiation rate is controlled by the peroxide type, peroxide concentration and reactor temperature. 

Branching along the polymer backbone is the result of both intramolecular and intermolecular reactions involving the growing free radical illustrated above. 

The HBr so formed then reacts with NBS to produce a bromine molecule and sucdnimide  

The Br2 molecule then reacts with the anthracenyl radical formed above to yield the product and a bromine radical  

This bromine radical starts the sequence over again. That is, a chain reaction is initiated. In the present reaction, a trace amount of an iodine-carbon tetrachloride solution is added to the reaction mixture. The iodine acts as a moderator or a 

Reagents and Equipment. Weigh and add 50.0 mg (0.28 mmol) of anthracene and 50 mg (0.28 mmol) of N-bromosucdnimide to a 3.0-mL conical vial containing a magnetic spin vane. To this mixture add 0.4 mL of carbon tetrachloride followed by one drop of I2-CQ4 solution delivered from a Fhsteur pipet. 

CAUTION Carbon tetrachloride is a cancer suspect agent. Dispense it in the hood using an automatic delivery pipet. 

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A propagation step involving growth around an active center follows RCH2—CHCl -h CH2=CHC1 —> RCH2—CHCl—CH2—CHCl and so on, leading to molecules of the structure... 

FIGURE 4 21 The initiation and propagation steps in the free radical mechanism for the chlorination of methane Together the two propaga tion steps give the overall equation for the reaction... 

Write equations for the initiation and propagation steps for the formation of dichloromethane by free radical chlorination of chloromethane... 

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... 

These two products arise because m one of the propagation steps a chlorine atom may abstract a hydrogen atom from either a methyl or a methylene group of butane... 

Can you write an equation for the initiation step that precedes these propagation steps ... 

The propagation steps in the formation of benzyl chloride involve benzyl radical as an intermediate... 

Propagation steps (Section 4 17) Elementary steps that repeat over and over again in a chain reaction Almost all of the products in a chain reaction arise from the propagation steps... 

In the next three sections we consider initiation, termination, and propagation steps in the free-radical mechanism for addition polymerization. One should bear in mind that two additional steps, inhibition and chain transfer, are being ignored at this point. We shall take up these latter topics in Sec. 6.8. 

Polymer propagation steps do not change the total radical concentration, so we recognize that the two opposing processes, initiation and termination, will eventually reach a point of balance. This condition is called the stationary state and is characterized by a constant concentration of free radicals. Under stationary-state conditions (subscript s) the rate of initiation equals the rate of termination. Using Eq. (6.2) for the rate of initiation (that is, two radicals produced per initiator molecule) and Eq. (6.14) for termination, we write... 

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... 

The polymerization mechanism continues to include initiation, termination, and propagation steps. This time, however, there are four distinctly different propagation reactions ... 

In any application of a copolymer the rate of formation of the product, its molecular weight, and the uniformity of its composition during manufacture are also important considerations. While the composition of a copolymer depends only on the relative rates of the various propagation steps, the rate of formation and the molecular weight depend on the initiation and termination rates as well. We shall not discuss these points in any detail, but merely indicate that the situation parallels the presentation of these items for homopolymers as given in Chap. 6. The following can be shown ... 

The free-radical polymerization of acrylic monomers follows a classical chain mechanism in which the chain-propagation step entails the head-to-tail growth of the polymeric free radical by attack on the double bond of the monomer. 

Carbon-centered radicals generally react very rapidly with oxygen to generate peroxy radicals (eq. 2). The peroxy radicals can abstract hydrogen from a hydrocarbon molecule to yield a hydroperoxide and a new radical (eq. 3). This new radical can participate in reaction 2 and continue the chain. Reactions 2 and 3 are the propagation steps. Except under oxygen starved conditions, reaction 3 is rate limiting. 

Under these conditions, a component with a low rate constant for propagation for peroxy radicals may be cooxidized at a higher relative rate because a larger fraction of the propagation steps is carried out by the more reactive (less selective) alkoxy and hydroxy radicals produced in reaction 4. 

An important descriptor of a chain reaction is the kinetic chain length, ie, the number of cycles of the propagation steps (eqs. 2 and 3) for each new radical introduced into the system. The chain length for a hydroperoxide reaction is given by equation (10) where HPE = efficiency to hydroperoxide, %, and 2/ = number of effective radicals generated per mol of hydroperoxide decomposed. For 100% radical generation efficiency, / = 1. For 90% efficiency to hydroperoxide, the minimum chain length (/ = 1) is 14. 

A typical example of a nonpolymeric chain-propagating radical reaction is the anti-Markovnikov addition of hydrogen sulfide to a terminal olefin. The mechanism involves alternating abstraction and addition reactions in the propagating steps ... 

Chemistry. Free-radical nitrations consist of rather compHcated nitration and oxidation reactions (31). When nitric acid is used in vapor-phase nitrations, the reaction of equation 5 is the main initiating step where NO2 is a free radical, either -N02 or -ON02. Temperatures of >ca 350° are required to obtain a significant amount of initiation, and equation 5 is the rate-controlling step for the overall reaction. Reactions 6 and 7 are chain-propagating steps. 

The degree of polymerization is controlled by the rate of addition of the initiator. Reaction in the presence of an initiator proceeds in two steps. First, the rate-determining decomposition of initiator to free radicals. Secondly, the addition of a monomer unit to form a chain radical, the propagation step (Fig. 2) (9). Such regeneration of the radical is characteristic of chain reactions. Some of the mote common initiators and their half-life values are Hsted in Table 3 (10). 

Halophenols without 2,6-disubstitution do not polymerize under oxidative displacement conditions. Oxidative side reactions at the ortho position may consume the initiator or intermpt the propagation step of the chain process. To prepare poly(phenylene oxide)s from unsubstituted 4-halophenols, it is necessary to employ the more drastic conditions of the Ullmaim ether synthesis. A cuprous chloride—pyridine complex in 1,4-dimethoxybenzene at 200°C converts the sodium salt of 4-bromophenol to poly(phenylene oxide) (1) ... 

Rea.CtlVltyRa.tlO Scheme. The composition of a copolymer at any point in time depends on the relative rates that each monomer can add to a chain end. If it is assumed that the chemical reactivity of a propagating chain depends only on the terminal unit and is not affected by any penultimate units, then four possible propagation steps in the copolymerisation of two monomers, and M2, with two growing chain ends, M and M2, can be written as follows ... 

The ratio describes the relative reactivity of polymer chain M toward monomer M and monomer M2. Likewise, describes the relative reactivity of polymer chain M2 toward M2 and M. With a steady-state assumption, the copolymerisation equation can be derived from the propagation steps in equations 3—6. 

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. 

Following the steady-state approximation, both propagation steps must proceed at the same rate otherwise, the concentration of A- or C- would build up. By substituting for the concentration of the intermediate C-, we obtain ... 

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