Termination rate chains

The chain termination rate varies inversely with the viscosity of the polymerization medium because of the Trommsdorff Effect (i.e., the reduction of the macroradical mobility with increasing reaction viscosity). This effect significantly influences reaction rate[ ,2, 10]. 

In many real polymerisation reactions, the kinetic scheme given above will be inadequate. Other reaction steps may have to be included amd the results of chain transfer to polymer are not always easy to describe. There is clear evidence which suggests that the chain termination rate coefficient is reduced in value when the concentration of polymer is high [43, 44]. The quantitative assessment for such changes is still a subject of much research [45, 46]. At very high concentrations, the value of kp may also be reduced [47]. Other physical events may also be important, particularly when the reaction becomes heterogeneous. 

Ri) and the (corrected) chain termination rate constant (2kt) reported by Melville and Richards (37) i.e.,... 

One further comment. Molecular weights are determined by the ratio of the propagation rate to chain termination rate. If dismutation, which terminates a given chain, is the main source of chain transfer, we would find ... 

The next basic aspect in the studies of dehydrogenation mechanism is the determination of the H202 interaction type with hydrocarbons. Under high-temperature oxidation conditions this interaction produces unsaturated compounds. Is this reaction the chain of a simple bimolecular type If the process is chain, variations of experimental conditions (presence of inert diluters, small additives, treatment of the reactor surface by various salts, the effect of the surfaceireaction zone volume ratio—the so-called SIV factor, etc.) must significantly change the initiation and chain termination rates. 

Our readsorption model shows that carbon number distributions can be accurately described using Flory kinetics as long as olefin readsorption does not occur (/3r = 0), because primary chain termination rate constants are independent of chain size (Fig. 24). The resulting constant value of the chain termination probability equals the sum of the intrinsic rates of chain termination to olefins and paraffins (j8o + Ph)- As a result, FT synthesis products become much lighter than those formed on Co catalysts at our reaction conditions (Fig. 24, jSr = 1.2), where chain termination probabilities are much lower than jS -I- Ph for most hydrocarbon chains. The product distribution for /3r = 12 corresponds to the intermediate olefin readsorption rates experimentally observed on Co/Ti02 catalysts, where intrapellet transport restrictions limit the rate of removal of larger olefins, enhance their secondary chain initiation reactions, and increase the average chain size of FT synthesis products. 

Carbon number distribution plots also become linear when olefins readsorb very rapidly (large /3r) or when severe intrapellet transport restrictions (large ) prevent their removal from catalysts pellets before they convert to paraffins during chain termination (Fig. 24, jSr = 100). In this case, chain termination to olefins is totally reversed by fast readsorption, even for light olefins. Chain termination occurs only by hydrogen addition to form paraffins, a step that is not affected by secondary reactions and for which intrinsic kinetics depend only on the nature of the catalytic surface. The product distribution again obeys Flory kinetics, but the constant chain termination probability is given by )8h, instead of po + pH- Clearly, bed and pellet residence times above those required to convert all olefins cannot affect the extent of readsorption or the net chain termination rates and lead to Flory distributions that become independent of bed residence time. 

Radical polymerization, including the question of the dependence of chain termination rate constant on the length of the macroradical chain, the possibility for continuous radical polymerization to be achieved through the complexing and stabilization of free radicals and of catalytic chain transfer in radical polymerization. 

I. Dependence of Chain Termination Rate Constant on the Macroradical... 

It was recently shown12) that in radical polymerization the chain termination rate constant is observed to decrease with the introduction of a polyfunctional complex-ing agent into the system. An especially sharp decrease of the termination rate, up to the formation, under certain conditions, of living radical polymerization centers, was noted in the methyl methacrylate-orthophosphoric acid system. 

Of special interest is the possibility of a direct effect on the magnitude of the rate constant of chain transfer to the monomer and the chain termination rate constant. 

American and English investigators in studies121,122) proposed a model of heterogeneous ethylene polymerization on Ziegler-Natta catalysts, which takes into account the dependence of chain termination rate constants on the chain length. 

We found that at the catalysis Fe(II,III)(acac)n ([Cat]=(0.5-5)10 3 mol/l)) products MPC and AP were formed parallel to PEH, at stages of chain propagation Cat + R02 — and quadratic termination of chain 2R02 —and Cat was inactive in the reaction of PEH homolysis [22], In the framework of radical-chain mechanism the chain termination rate in this case will be (1) ... 

The main feature of the linear polymerisation is that this chemical process proceeds in a medium, whose properties sharply varied until the transition into a new phase state. We cannot obtain the qnantitative kinetic model without using the formalism of diffusion control with a qnadratic chain termination rate. 

Where P s the inverse value of M and the measnre of chain termination probability. P = VM Tfjtp at the tp polymer growth rate and the total chain termination rate rg, (M is the MW of a polymer). 

We suggest the original method for evaluation of catalytic activity of complexes formed in situ at the beginning of reaction and in developed process, at elementary stages of oxidation process [33, 90-93] by simplified scheme assuming quadratic termination of chain and equality to zero of rate of homolytic decomposition of ROOH. In the If amework of radical-chain mechanism the chain termination rate in this case will be Eq. (1) ... 

A range of models in the diffusion-controlled reactions conception takes into account the dependence of a constant chain termination rate on the current concentration of monomer (that is. on the conversion) and on the quantity of polymer formed via the polymerization process [54,56]. 

According to Pravednikov s conception [69] in a case, when the average degree of a macromolecule s chain polymerization is small in comparison with some characteristic value P, the process can be described by the usual equation of classic kinetics. Radicals, the length / of which is more than this value of the polymerization degree P, sharply lose their mobility in comparison with the radicals of shorter chain length and in this case the kinetics are described by a dependence of the constant chain termination rate on the chain length. 

Two types of functions in the models for the description of the polymerization processes at moderate and high conversions have been described in the literature [47-53, 57-61, 63, 70-75]. For example, in Ref. [63] until the depth of conversion 7-0,6, the kinetics of the polymerization process are described by a change of the bimolecular chain termination rate constant parameters of the propagation and initiation efficiency are assumed to be constant values. Decreasing the chain termination constant rate via especial empirical dependence has been introduced for values of F > 0,6. Decreasing the chain termination constant rate for F < 0,6 is determined via the coefficients of the translational and segmental diffusion of the macroradicals ... 

In accordance with the model [70] the chain termination rate constant depends on the molecular weight only in the early conversion states, when the translational diffusion is the limiting factor. 

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