Additives transfer constants

Various methods for estimating transfer constants in radical polymerization have been devised. The methods are applicable irrespective of whether the mechanism involves homolytic substitution or addition-fragmentation. 

Compounds with a thiocarbonyl a to the S-S bond such as the dithiuram (e.g. 8f2Al and xanthogen disulfides (e.g. 9)M have transfer constants that are much higher than other disulfides. In part, this may be due to the availability of another mechanism for induced decomposition (Scheme 6.9) involving addition to the C S double bond and subsequent fragmentation. Thiocarbonyl double bonds are very reactive towards addition and an addition-fragmentation mechanism has been demonstrated for related compounds (Section 6.2.3.5). 

Most monosulfides generally have very low transfer constants. Exceptions to this rule are allyl sulfides (Section 6,2.3.2) and thiocarbonylthio compounds such as the trithiocarbonatcs and dithiocstcrs (Section 9.5.3) that react by an addition-fragmentation mechanism. 

Many solvents and additives have measurable transfer constants (Table 6.5). The accuracy of much of the transfer constant data in the literature is questionable with values for a given system often spanning an order of magnitude. In some cases the discrepancies may be real and reflect differences in experimental conditions. In other cases they are less dear and may be due to difficulties in molecular weight measurements or other problems. 

Table 6.5 Transfer Constants (60 °C, bulk) for Selected Solvents and Additives...

Rates of addition to transfer agents 11,12 are determined by the same factors that determine rates of addition to monomers Section 23). Substituents on the remote terminus of a double bond typically have only a minor influence. Thus, in most cases, the double bonds of the transfer agents have a reactivity towards propagating radicals that is comparable with that of the common monomers they resemble. With efficient fragmentation, transfer constants can be close to unity. The radicals formed by addition typically have low reactivity towards further propagation and other intermolecular reactions because of steric crowding about the radical center. 

In the case of allyl peroxides (12 X= CH2, A=CH2, BO),1 1 1 intramolecular homolytic substitution on the 0-0 bond gives an epoxy end group as shown in Scheme 6.18 (1,3-Sn/ mechanism). The peroxides 52-59 are thermally stable under the conditions used to determine their chain transfer activity (Table 6.10). The transfer constants are more than two orders of magnitude higher than those for dialkyi peroxides such as di-f-butyl peroxide (Q=0.00023-0.0013) or di-isopropyl peroxide (C =0.0003) which are believed to give chain transfer by direct attack on the 0-0 bond.49 This is circumstantial evidence in favor of the addition-fragmentation mechanism. 

The rate constants ( add) for addition of the MMA propagating radical30 (and other radicals 9) to 66-68 arc believed to be similar. The transfer constant of 66 is thought to be lower than 67 and 68 by more than an order of magnitude because of... 

The first five telomers (n = 1 - 5) were isolated and identificated. The authors showed that telomers T2 and T3 are preferentially formed in one stereoisomeric form (with minor amounts of other possible isomers), i.e. the radical addition reaction makes it possible to perform asymmetric control at the steps of chain transfer and chain growth. The partial chain transfer constants Cn are given in this work, which are within the range from 0.3 to 0.5 for radicals C2-C5. We consider... 

Transfer constants for polystyrene chain radicals at 60° and 100°C, obtained from the slopes of these plots and others like them, are given in the second and third columns of Table XIII. Almost any solvent is susceptible to attack by the propagating free radical. Even cyclohexane and benzene enter into chain transfer, although to a comparatively small extent only. The specific reaction rate at 100°C for transfer with either of these solvents is less than two ten-thousandths of the rate for the addition of the chain radical to styrene monomer. A fifteenfold dilution with benzene was required to halve the molecular weight, i.e., to double l/xn from its value (l/ rjo for pure styrene (see Fig. 16). Other hydrocarbons are more effective in lowering the degree of polymerization through chain transfer. 

Transfer constants for mercaptans with several monomers are given in Table XV. Results for the two methods described above are in satisfactory agreement. The rate of reaction with mercaptan relative to the rate of monomer addition (i.e., the transfer constant) varies considerably for different chain radicals (see Table XIV). Temperature coefBcients of the transfer constants for mercaptans are very small, which fact indicates that the activation energy for removal of a hydrogen atom from the sulfhydryl group of a mercaptan is nearly equal to that for monomer addition. 

This model is representative for the conditions described in the previous section, except for the mode of administration which can be oral, rectal or parenteral by means of injection into muscle, fat, under the skin, etc. (Fig. 39.7). In addition to the central plasma compartment, the model involves an absorption compartment to which the drug is rapidly delivered. This may be to the gut in the case of tablets, syrups and suppositories or into adipose, muscle or skin tissues in the case of injections. The transport from the absorption site to the central compartment is assumed to be one-way and governed by the transfer constant (Fig. 39.7a). The linear differential model for this problem can be defined in the following way ... 

The chain transfer constant for an additive or solvent in the polymerization can be determined. This value can then be compared with the transfer constants for the same substance in the polymerization of the same monomer by known radical, cationic, and anionic initiators. 

We can tentatively conclude, therefore, that the effect of chain transfer is still making itself felt in the polymerization of vinyl caproate in spite of its low water solubility. Except at the lowest particle concentrations, chain transfer is important. The polymerization in these regions is midway betwen Case I and Case II. When variables are considered separately, there is some dependence of polymerization rate on particle concentration, and also some dependence on initiator concentration. In addition, at constant organic volume, while the rate of polymerization increases as the particle concentration increases (Rp oc 2V- ), the rate per particle decreases as the particles get smaller. This shows that transferred radicals are mainly trapped in the particles, but some diffuse out and can undergo termination with other growing radicals. 

In general, deuterium substitution can be used to decide which hydrogen atom in a molecule is the one most concerned in transfer processes. Introduction of deuterium at this position will reduce the transfer constant, and if transfer is accompanied by retardation, it will also reduce the extent of retardation. For work of this type, tritiumlabelling of the additive is not suitable substances so labelled usually contain only a very small proportion of molecules actually containing tritium and although there is a large isotope effect with these molecules, their number is so small relative to that of the unlabelled molecules that their influence cannot be detected. 

If the rates at which monomer and an additive become incorporated in a polymer are compared by analyzing the polymer, it may be possible to calculate the transfer constant for the additive. In this connection, it is important to recognize that transfer is a two-stage process consisting of radical-displacement followed by re-initiation. In the simplest case, in which a hydrogen atom is abstracted by the polymer radical, the relevant reactions can be written as... 

For example isopropylbenzene, which is considered as a model substance for polystyrene, has a chain transfer constant with polymethyl methacrylate radicals equal at 80° C to 1.9 10 4 (30). This means that at equimolecular concentration of monomer (methyl methacrylate) and transfer agent (isopropylbenzene, or in our case polystyrene) only one transfer reaction will occur against five thousand normal monomer addition steps. 

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... 

The impedance for the study of materials and electrochemical processes is of major importance. In principle, each property or external parameter that has an influence on the electrical conductivity of an electrochemical system can be studied by measurement of the impedance. The measured data can provide information for a pure phase, such as electrical conductivity, dielectrical constant or mobility of equilibrium concentration of charge carriers. In addition, parameters related to properties of the interface of a system can be studied in this way heterogeneous electron-transfer constants between ion and electron conductors, or capacity of the electrical double layer. In particular, measurement of the impedance is useful in those systems that cannot be studied with DC methods, e.g. because of the presence of a poor conductive surface coating. 

They are prepared from the addition of CS2 to the corresponding alcohols in basic medium. They are known to exhibit high transfer constants (Cr = 1-20) [21]. Constanza et al. [210], Otsu and Yoshida [49], Uraneck [121] and Fokina et al. [229] intensively studied the polymerization of styrene, MMA and butadiene with these compounds in order to obtain telechelics with functional xanthogens. Furthermore, chemical change (e.g., hydrolysis) was also performed to achieve the synthesis of corresponding a,co-dithiols [229] such as from 49 ... 

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