Kinetics of Chain Transfer

The monomer must also be considered a potential transfer agent. It may perform the function of the solvent in reaction (30), an atom being transferred from the monomer to saturate the radical. Possibly a proton may be transferred from the carbon atom of the radical chain to the unsaturated monomer as follows  

Chain transfer may also occur with a previously formed polymer molecule. The latter type of transfer produces no net increase in the number of molecules hence it need not concern us here. Its effect on the polymer structure will be considered in Chapter VI. 

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Bamford43,59 63 has proposed a general treatment for solving polymerization kinetics with chain length dependent kt and considered in some detail the ramifications with respect to molecular weight distributions and the kinetics of chain transfer, retardation, etc. 

S.3.7 Kinetics of Chain Transfer to Polymer. Determination of Rate Constants. Summary... 

The kinetics of chain transfer to ammonia has been investigated for potassium amide-initiated polymerization of styrene in liquid ammonia at —33.5 °C. The calculated chain transfer constant ( / ) was 2.34 x 10 " [109]. The chain transfer reaction of poly(styryl)lithium with toluene at 60 °C was investigated during the polymerization of styrene using " C-labeled toluene. The calculated chain transfer constant was 5 x 10 [41]. A much larger chain... 

The kinetics of chain transfer to ammonia have been investigated for potassium amide-initiated poljunerization of styrene in liquid ammonia at —33.5°C. 

The kinetics of chain transfer to ammonia have been investigated for potassium amide-initiated polymerization of styrene in liquid ammonia at -33.5 °C. The calculated chain transfer constant (K/kp) was 2.34 x 10". ... 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

The kinetic chain length has a slightly different definition in the presence of chain transfer. Instead of being simply the ratio Rp/R, it is redefined to be the rate of propagation relative to the rates of all other steps that compete with propagation specifically, termination and transfer (subscript tr) ... 

The kinetic treatment corresponds to that given above for the case of chain transfer with polymer. We have merely to write for the rate of generation of cross-linked units... 

Three kinetics parameters in particular give estimates of the relative rates of chain transfer and shuttling to propagation ... 

Jin, Q. and C. M. Bethke, 2002, Kinetics of electron transfer through the respiratory chain. Biophysical Journal 83, 1797-1808. 

The mechanistic issues to be discussed are the initiation modes of the reaction, the propagation mechanism, the perfect alternation of the polymerisation reaction, chain termination reactions, and the combined result of initiation and termination as a process of chain transfer. Where appropriate, the regio- and stereoselectivity should be discussed as well. A complete mechanistic picture cannot be given without a detailed study of the kinetics. The material published so far on the kinetics comprises only work carried out at temperatures of -82 to 25 °C, which is well below the temperature of the catalytic process. 

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. 

The two types of chain transfer to monomer are kinetically indistinguishable, but one (Eq. 5-17) results in unsaturated end groups, while the other (Eq. 5-18) results in saturated end groups. 

Chain transfer to counterion differs kinetically from chain transfer to monomer in that the rate of chain transfer to monomer has a first-order dependence on monomer while chain... 

Unfortunately, there is no direct evidence for this kind of chain transfer step, since all kinetic studies have been carried out under conditions where j5-elimination would dominate. However, the statement by Wilke [30] that the Aufbau process produces chains of at most =100 units agrees well with the predicted average degree of polymerization of =70 units [7]. 

Thus, the quality of the SAM does play a role in the overall growth kinetics and resulting film thickness. The role of chain transfer has not been quantified but it clearly does not prevent photo-SlP. 

The emulsion polymerization of vinyl hexanoate has been studied to determine the effect of chain transfer on the polymerization kinetics of a water-insoluble monomer. Both unseeded and seeded runs were made. For unseeded polymerizations, the dependence of particle concentration on soap is much higher than Smith-Ewart predictions, indicating multiple particle formation per radical because of chain transfer. Once the particles have formed, the kinetics are much like those of styrene. The lower water solubility of vinyl hexanoate when compared with styrene apparently negates its increased chain transfer, since the monomer radicals cannot diffuse out of the particles. 

A few very important points have been neglected by some authors in their evaluation of chain transfer constants by means of kinetic measurements. Frequently, a retardation of the overall rate is to be observed in the presence of chain transfer agents. A correct value of the chain transfer constant can result only if the reactions which lead to this retardation are properly considered in the kinetic scheme. In addition, the equation which one must use to calculate the chain transfer constant depends on the type of molecular weight average which is measured. Failure to... 

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