Constant of termination

Ra, Ra symbol of a-type radical or ion and its concentration kap constant of propagation reaction between Ra and M klap constant of termination reaction between Ra and R rap> rfia reactivity ratios for binary free-radical copolymerization of monomers Ma and M ... 

Change in chemical composition of the solvent used can also change the velocity of polymerization. Viscosity of the examined system is another very important parameter which should be taken into account. Templates, as any macromolecular compounds, change viscosity in comparison with the viscosity of polymerizing system in a pure solvent. It is well known that the increase in viscosity can change the rate constant of termination and eventually the rate of polymerization. In many systems, an insoluble complex is formed as a product of template polymerization. It is obvious that the character of polymerization and its kinetics change. 

It is interesting to note that this relation was verified for polymerizations of styrene, p-methoxystyrene, and isobutylvinylether with iodine by Kanoh and Higashimura (19). They have demonstrated that the apparent rate constants of termination, propagation, and monomer transfer increased with increasing dielectric constant of solvent, and the rates of the increase for these three rate constants were of the order of Eq. (21). 

TABLE 14 Relative Rate Constants of Termination Between a Polymer Radical and R /... 

By means of the spatially intermittent reactor, Ito and O Driscoll determined, for example, the absolute rate constant of termination, fct, for methyl methacrylate copolymerization with butyl or dodecyl methacrylate. The value of /ct is a function of the monomer mixture composition [89]. 

He polymerized methacrylonitrile by means of 2,2 -azobisisobutyronitrile. The isobutyronitrile radical generated by initiator decomposition is structurally almost identical with the macroradical end group. Under these conditions, the rate constant of primary radical termination should be very near to the rate constant of termination between oligomeric and polymeric radicals. The studied polymerizations were carried out in dimethylformamide, which is a poor solvent for polymethacrylonitrile. In poor solvents, the change in termination rate with the length of the growing chain should not depend on the excluded volume [10],... 

The rate constant of propagation is then calculated by determining the rate of monomer consumption using estimated values of the rate of initiation and the rate constant of termination. Rates of monomer consumption, defined as G(-M), vary from 102 to 107 molecules/100 eV. Rates of initiation are usually determined from conductivity data. These can be correlated with Gt values [228] of various organic molecules, which are approxi-... 

The branched sulfonium ions are not reactive, thus Reaction (130) is an irreversible termination. The measured ratios of rate constants kp/k, [(rate constant of propagation to rate constant of termination according to Eq. (130)] reflect the general phenomenon that by increasing the number and size of the substituents the contribution of chain transfer to polymer may be considerably reduced [161] (Table 12). 

The second stage of reaction is characterized by considerable growth of polymerization rate as a result of sharp decrease of rate constant of termination of macromolecular radicals, acceleration, with respect to initial rate, growing with the growth of MW of polymer being obtained (fig,4). 

After polymerization has started, the free volume of the reaction system will continuously decrease. The smaller the amount of free volume, the lower is the mobility of the long-chain molecules. At a certain point, the rate constant of termination becomes diffusion-controlled and autoacceleration takes place. The diffusion-controlled rate constant kt will become smaller than the original chemically controlled rate constant kt. To keep track of the diffusion-controlled kt, one can use the following equation ... 

In the polymerization of 2-alkoxy-2-oxo-l, 3,2-dioxaphosphorinanes it Imb been shown that although kp does not depend too much on the structure of the substituent R, the rate constant of termination k, decreases with increasing size of the alkyl group and for the large substituents (e.g. R = ( 113)380 kt becomes eventually so small that it cannot be measured. The corresponding rate constants and activation parameters are given in Table 17. 

The data collected by Goethals on the kp/k ratio in the polymerization of cyclic amines (determined from Eq. (153) permits to show (Fig. 16) that there is a linear relationship between In (kp/kf) and the effective volume of substituents. These volumes were calculated according to Ref. 262. Thus, although the linearity itself has little importance, this dependence is in good agreement with the discussed earlier influence of substituents on the rate constants of termination and fuopagation. 

Equations, which are also applicable to suspension, solution, and bulk polymerization, form an extension of the Smith-Ewart rate theory. They contain an auxiliary parameter which is determined by the rate of initiation, rate constant of termination, and volume of the porticles. The influence of each variable on the kinetics of emulsion polymerization is illustrated. Two other variables are the number of particles formed and monomer concentration in the particles. Modifications of the treatment of emulsion polymerization are required by oil solubility of the initiator, water solubility of the monomer, and insolubility of the polymer in the monomer. 

A complete kinetic scheme, involving slow initiation, propagation, and termination by chain transfer to polymer, has been developed 29,30) and it was conclusively shown that it is applicable for BCMO polymerization. The pertinent kinetic equations were given in Vol. I (Adv. Polymer Sci. 37). The rate constant of termination by chain transfer to polymer was kt = 1.2 10 3 mol-1 - I s-1 (C6H5C1, 70 °C)18). 

When the rate constant of termination is sufficiently smaller compared to kMChi than the rate is first order in total catalyst concentration and first order in monomer. 

Magnitude of rate constant of termination steps, which depend on the complexity of the radicals. 

Figure 10.9 reports the MALDI-TOF mass spectrum of a polymer obtained by pulsed-laser polymerization that displays a series of peaks ranging from 3000 Da to 12000 Da. The MMD resulting from the MS was plotted, and the point of inflection of the MMD (Eq. 10.3) was determined. The data allow one to determine the rate constant of termination and the rate constant of chain transfer.The method based on MS has now become a standard. 

Diffusion theories have been proposed that relate the rate constant of termination to the initial viscosity of the polymerization medium. The rate determining step of termination, the se ental diffusion of the chain ends is inversely proportional to the microviscosity of the solution. °Yokota and Itoh modified the rate equation to include the viscosity of the medium. According to that equation, the overall polymerization rate constant should be proportional to the square root of the initial viscosity of the system. 

In defining it is assumed that the rate constant of cross-termination is equal to the geometric mean of the rate constants of termination in homopolymerization. This relation is applicable in the case of gas reactions. In copolymerizations, however, the value of can increase up to about 400 (methyl methacrylate/vinyl acetate). In addition, depends on the composition of the mixture. The cause of this composition effect is unknown, although it is significant that is particularly large when the monomers tend toward alternating copolymerization. 

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