Termination mechanism for

Transfer to initiator can be a major complication in polymerizations initiated by diacyl peroxides. The importance of the process typically increases with monomer conversion and the consequent increase in the [initiator] [monomer] ratio.9 105160 162 In BPO initiated S polymerization, transfer to initiator may be lire major chain termination mechanism. For bulk S polymerization with 0.1 M BPO at 60 °C up to 75% of chains are terminated by transfer to initiator or primary radical termination (<75% conversion).7 A further consequence of the high incidence of chain transfer is that high conversion PS formed with BPO initiator tends to have a much narrower molecular weight distribution than that prepared with other initiators (e.g. AIBN) under similar conditions. 

Unstable structures are known to arise by chain termination. Mechanisms for radical-radical termination in MMA polymerization have been discussed in Sections 5.2.2.1.2 and 5.2.2.2.2 and these are summarized in Scheme 8.5. It is established that both disproportionation and combination occur to substantial extents. The head-to-head linkages 1 and the unsaturated chain ends 2 both constitute weak links in PMMA.26 2 "33 The presence of these groups account for... 

At low temperatures (below —85 °C), the majority of the product molecules are keto-esters, with only small but balancing quantities of diesters and diketones. At higher temperatures, the same product molecules are produced in a 2 3 4 ratio close to 2 1 1. These observations have been explained [13] by assuming two initiation and two termination mechanisms for polyketone formation. 

Neurons in the central nervous system communicate by chemical transmission. Of relevance to the present discussion are monoamine neurons that release dopamine, norepinephrine, or serotonin as one of their transmitters in response to an action potential. Reuptake transporter proteins embedded in the neuronal plasma membrane then clear the synapse of monoamines, typically taking up 70-80%) of the released transmitter. This reuptake is thought to be the major termination mechanism for the monoamine chemical signaling process. 

From our discussion in the preceding section, termination mechanisms for generating polymer chains and assuming all transfer species are as reactive as the original (i.e., no inhibition), the expression for V becomes ... 

When a polymer chain stops its growth after chain transfer, an active center is vacated to allow the formation of a new polymer chain. The chain transfer by the elimina tion of the p-H group is not important for most Ziegler-Natta catalysts, but it is the major chain termination reaction for most metallocene catalysts. The elimination of the p-methyl group does not occur in multiphase catalysis, but is the most important chain termination mechanism for the metallocene catalysts containing CpzMClz-MAO, where M is zirconium (Zr) or hafnium... 

Scheme III. A proposed termination mechanism for Ti-catalyzed poty-merization of acetylene.

To determine the prevailing termination mechanism in SI-PMP over a broad range of relevant reaction conditions, experimental data on film thickness evolution such as that shown in Figure 12.4 were fit in the brush regime (transitions marked by arrows in Figure 12.4) by the kinetic models that incorporate one or more termination mechanisms. For example, Rahane et al. combined Equation 12.1 with expressions for STR based on either bimolecular termination or chain transfer to monomer to develop models for how layer thickness should evolve as a function of exposure time. These models, shown as Equations 12.3 and 12.4, respectively, can be compared to experimental data of polymer layer thickness as a function of time to deduce which irreversible termination mechanisms are prevalent. 

The polydispersity will also remain constant, with the natural instantaneous value characteristic of the type of termination mechanism for example, for termination by disproportionation, the instantaneous values of the n-, w-, and z-averages of the polymer mass (M, M, and M ) stand in the ratio 1 2 3, respectively [49]. 

Another possible explanation for the trend observed in Fig. 5 is the recently proposed mechanism of catalytic radical terminatiOTi in solution ATRP [104]. The proposed termination mechanism for solution ATRP might also be applicable for SI-ATRP. According to that mechanism, the presence of catalyst can increase the amount of termination, which contributes to the observed decrease in growth rate. Another factor to be considered is the possibdity that the resulting grafting density is affected by different concentrations of catalyst. However, as mentioned previously, the grafting density is not known a priori, nor can it be measured accurately for a flat substrate. 

The termination mechanisms for yeast FAS (transfer to Co A) and mammalian FAS (liberation of free acid) are different. 

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. 

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. 

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. 

Use the values determined in Example 7.6 for the vinylidene chloride (M )-isobutylene (M2) systemf to calculate for various values of fi according to the terminal mechanism. Prepare a plot of the results. On the same graph, plot the following experimentally measured values of fi and ... 

Under acidic conditions, furfuryl alcohol polymerizes to black polymers, which eventually become crosslinked and insoluble in the reaction medium. The reaction can be very violent and extreme care must be taken when furfuryl alcohol is mixed with any strong Lewis acid or Brn nstad acid. Copolymer resins are formed with phenoHc compounds, formaldehyde and/or other aldehydes. In dilute aqueous acid, the predominant reaction is a ring opening hydrolysis to form levulinic acid [123-76-2] (52). In acidic alcohoHc media, levulinic esters are formed. The mechanism for this unusual reaction in which the hydroxymethyl group of furfuryl alcohol is converted to the terminal methyl group of levulinic acid has recendy been elucidated (53). 

The early kinetic models for copolymerization, Mayo s terminal mechanism (41) and Alfrey s penultimate model (42), did not adequately predict the behavior of SAN systems. Copolymerizations in DMF and toluene indicated that both penultimate and antepenultimate effects had to be considered (43,44). The resulting reactivity model is somewhat compHcated, since there are eight reactivity ratios to consider. 

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. 

The reaction of water with isocyanate is shown in the third item of Fig. 1 [5]. The water/isocyanate reaction is the major curing mechanism for the one-component urethane adhesives. Most one-component urethanes are based on an isocyanate-terminated prepolymer (I). Usually, the moisture in the air is used to cure the adhesive, but in some instances, a fine mist of water may be introduced on top of the adhesive before the bond is closed, in order to facilitate cure ... 

In addition to providing some control of price risk, futures and options markets are also very useful mechanisms for price discovery and for gauging market sentiment. There is a world-wide need for accurate, real-time information about the prices established through futures and options trading, that is, a need for price transparency. Exchange prices are simultaneously transmitted around the world via a network of information vendors terminal seiwices directly to clients, thereby allowing users to follow the market in real time wherever they may be. Energy futures prices are also widely reported in the financial press. These markets thus enable an open, equitable and competitive environment. 

Problem 7.19 Oik- of the chain-termination steps that sometimes occurs to interrupt polymerization is the following reaction between two radicals. Propose a mechanism for the reaction, using fishhook arrows to indicate electron flow. 

Figure 1.26 Abbreviated mechanism for the catalytic hydrogenation of a terminal alkene using...

The principal mechanism for terminating dopamine signaling is reuptake by the presynaptic neuron via the dopamine transporter (DAT). Dopamine that is not taken up is metabolized by the enzymes monoamine oxidase (MAO) and catechol-O-methyl transferase... 

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