Kinetics propagation constants

In-situ Raman spectroscopy experiments were used to determine effective kinetic propagation constants for a series of unsteady-state divinyl ether polymerisations at different isothermal temperatures and light intensities. A series of Raman experiments were performed on cationic photopolymerisations of a divinyl ether initiated with a diaryliodonium salt of hexafluoroantimonate photosensitised by anthracene. Isothermal Raman experiments were performed for a series of reaction temperatures and were used to determine the overall activation energy of the polymerisation reaction. 20 refs. USA... 

Kinetic studies of the polymerization of mono-functional polymethyl methacrylate led to the determination of the propagation constants, k , of the sodium, potassium, and cesium salts 29- 35 36) of polymethyl methacrylates anions. Surprisingly, they... 

The observations discussed above suggest that the kinetic order of lithium poly-isoprene propagation should vary with the living polymer concentration. The effect is imperceptible in aliphatic hydrocarbons, but is observed in benzene solutions. The apparent propagation constants of lithium polyisoprene (MW 2 2 10 ) were determined in benzene and the results are displayed in Fig. 16 in the form of a plot of log kapp vs log c, c denoting the total living polymer concentration. 

The reasons behind this accelerated rate behavior have been attributed to a decrease in chain transfer processes (28,29) and a decreased termination rate (24,25) indicated by molecular weight measurements (26). Recently, direct evidence of decreases in the termination rate have been shown (27) and in these studies both the termination and propagation kinetic constants were determined for polymerizations exhibiting enhanced rates in a smectic phase. The propagation constant, kp, decreases slightly in the ordered phase from the isotropic polymerization. Such a decrease would be expected because of the lower temperature in the smectic phase. The termination kinetic constant, kt, however, decreases almost two orders of magnitude for the ordered polymerization, indicating a dramatically suppressed termination rate. 

C olvents have different effects on polymerization processes. In radical polymerizations, their viscosity influences the diffusion-controlled bimolecular reactions of two radicals, such as the recombination of the initiator radicals (efficiency) or the deactivation of the radical chain ends (termination reaction). These phenomena are treated in the first section. In anionic polymerization processes, the different polarities of the solvents cause a more or less strong solvation of the counter ion. Depending on this effect, the carbanion exists in three different forms with very different propagation constants. These effects are treated in the second section. The final section shows that the kinetics of the... 

Sufficient experimental data from several laboratories now exist to describe the conditions under which the radiation-induced ionic propagation of many pure liquid vinyl monomers can be observed. The kinetic data and electrical conductivity measurements establish the ionic nature of the reaction scavenger studies appear to establish the preponderant role played by the carbonium ion in propagating the polymerization. On the basis of a single propagating species, it is possible to write a simple mechanism to describe the process. Limiting values of several of the kinetic rate constants can be estimated, notably the rate constant for reaction between a bare carbonium ion and a vinyl double bond. These rate constants are compared with similar constants arrived at in chemically initiated free radical, carbonium ion and carbanion polymerization. Several shortcomings of the present scheme are discussed. 

It follows that the kinetic results permit us to find kf and Kdiss because in the absence of the boride the slope of the line kp vs. 1/C1/2 gives (kf — k )Kdiss172, whereas the slope of the line kp vs. 1/[counterion], obtained in the presence of the salt gives (kf — k )Kdiss. The propagation constant k may be found from either plot it is given by the intercept of the respective lines. 

Propagation constants, k , of the polystyryl ion-pairs in THP were derived, like in the other studies, by extrapolating the plots of kp vs. l/c[a to infinite concentration of living polymers, or by suppression of the dissociation of ion-pairs of living polymers caused by the addition of appropriate tetraphenyl boride salts. The kinetic and conductometric results of all of these studies, summarized in Table 8, show a fair agreement between the data reported by all the investigators. 

By combining the kinetic and fractionation data, the authors calculated the propagation constant of associated ends to be 27 M-1s-1 (per active end). Using a different approach this writer concluded that the respective constant is smaller than 15 this M-1s-1. Both evaluations reveal a substantial decrease of reactivity arising from the association. 

The most interesting results were obtained in polymerization studies performed with kryptated cations. Kinetics of propylene sulphide polymerization in THF at — 30 °C with Na+, (2,2,2) led to the results shown in Fig. 69. The propagation of these ion-pairs turned out to be higher than of the free anions. On the whole propagation constant of ion-pairs increases with the radius of the cation, viz. at - 30°C in THF ... 

As Reiser describes it, once a radical chain is initiated, it propagates spontaneously until it is terminated by an encounter with another radical, by disproportionation, or in some other way. The average chain length, which ultimately determines the photosensitivity of the system, according to Reiser, is linked to the kinetic rate constants of the individual processes. Furthermore, the rate of initiation (i), propagation (p), and termination (t) can be fairly well described by the following equations ... 

Lachinov and co-workers [52] have performed a more detailed study of the kinetics of block radical polymerisation of perfluoroalkylmethacrylates (FMA) in the solid state. The main method of investigation of the kinetics of FMA polymerisation was isothermic calorimetry [58]. Due to the absence of data on the heat effects of their polymerisation in the reference literature, these values were measured [52]. The values of AQ and glass transition point (T ) of polymers formed are shown in Table 8.3. Obviously heat of polymerisation of monomers of the fluoroacrylate sequence is quite close to heat of polymerisation of non-substituted monomers of the AMA sequence [59], and a significant influence of the length of the fluoroalkyl radical on this parameter is absent [52]. In accordance with the Polyani-Semenov rule, the present result makes it possible to consider that chain propagation constant of FMA with the accuracy of the pre-exponential multiplicand being equal to each other [57]. 

We see that the influence of Kp is more significant than of Kq. Therefore, the parameter controlling the polymerization kinetics is the propagation constant. The concentration is strongly affected for higher Kp values. 

The kinetic propagation reaction equations for radical copolymerization of two monomers, Mj and M2, are written in Fig. 3.45. The complications due to additional, different monomers, as well as transfer, reverse, and termination reactions increase the numbers of equations and rate constants, so that the resulting reaction equations are unhandy, and it needs much experimental work to establish the rate constants. Also, the computational effort to solve the many equations becomes excessive. 

Quantitative analysis of the kinetic measurements shows that the propagation constant kp remains constant, while the termination constant Kipp) decreases. In allyl acetate, on the other hand, the termination constant Kipm) remains constant even at high yields. Therefore the termination by mutual deactivation of two polymer free radicals must be prevented by the high viscosity, causing the free radical concentration, and hence the rate of polymerization, to increase. Since the termination constants Kipp) are diffusion controlled (Section 20.2.4.2) even at low viscosities, however, the effect cannot be due to the beginning of diffusion control. It must rather develop from the fact that diffusion-controlled effects are altered. The interpretation of the gel effect is assumed to be diffusion control caused by the whole polymer molecule. On the other hand, mobility of segments governs the rate constants for termination. 

Here, [M+] corresponds to the concentration of propagating (imtrapped) active centers, while kyt represents the first-order kinetic rate constant for termi-nation/trapping of active centers. 

In high-pressure ethylene polymerization, the kinetic rate constants are also dependent on pressure. For example, the propagation rate constant is given by... 

In writing Eqs. (7.1)-(7.4) we make the customary assumption that the kinetic constants are independent of the size of the radical and we indicate the concentration of all radicals, whatever their chain length, ending with the Mj repeat unit by the notation [Mj ], This formalism therefore assumes that only the nature of the radical chain end influences the rate constant for propagation. We refer to this as the terminal control mechanism. If we wished to consider the effect of the next-to-last repeat unit in the radical, each of these reactions and the associated rate laws would be replaced by two alternatives. Thus reaction (7. A) becomes... 

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