Solvents transfer constants

Solvent transfer constants of various solvents for vinyl acetate are shown in Table 4.5 [16]. Polyvinyl acetate, as the raw material of polyvinyl alcohol for fiber, is produced by solution polymerization of vinyl acetate. In most cases, methanol is employed as the solvent. Solvent transfer constant, Cs, is one of the most important features for the selection of the solvent (see below in Section 4.2.2.5). 

Solution polymerization under employment of methanol seems to be the best process from the practical point of view. Methanol is a good solvent of polyvinyl acetate the solvent transfer constant of methanol is small compared to that of a common solvent such as acetone, so that PVA of a sufficiently high DP is obtained even when a comparatively large amount of methanol is used in polymerization. Methanol and vinyl acetate form an azeotrope at 60°C, the latent heat of which is so large that it is easier to remove the heat of polymerization. 

The solvent transfer constants vary by several orders of magnitude (Table 20-7). The more readily transferable atoms that there are per molecule (H in the benzene, toluene, ethyl benzene series), the weaker the bond (carbon tetrachloride, carbon tetrabromide), and the more resonance-stabilized the resulting radical (triphenylmethane, fluorene pentaphenyl-ethane), the higher is the transfer constant. 

This suggests that polymerizations should be conducted at different ratios of [SX]/[M] and the molecular weight measured for each. Equation (6.89) shows that a plot of l/E j. versus [SX]/[M] should be a straight line of slope sx Figure 6.8 shows this type of plot for the polymerization of styrene at 100°C in the presence of four different solvents. The fact that all show a common intercept as required by Eq. (6.89) shows that the rate of initiation is unaffected by the nature of the solvent. The following example examines chain transfer constants evaluated in this situation. 

Chain transfer to initiator or monomer cannot always be ignored. It may be possible, however, to evaluate the transfer constants to these substances by investigating a polymerization without added solvent or in the presence of a solvent for which Cgj is known to be negligibly small. In this case the transfer constants Cjj and Cj determined from experiments in which (via... 

Table 10. Chain-Transfer Constants to Common Solvents for Poly(ethyl acrylate) ...

The molecular weight of a polymer can be controlled through the use of a chain-transfer agent, as well as by initiator concentration and type, monomer concentration, and solvent type and temperature. Chlorinated aUphatic compounds and thiols are particularly effective chain-transfer agents used for regulating the molecular weight of acryUc polymers (94). Chain-transfer constants (C at 60°C) for some typical agents for poly(methyl acrylate) are as follows (87) ... 

Chain transfer is an important consideration in solution polymerizations. Chain transfer to solvent may reduce the rate of polymerization as well as the molecular weight of the polymer. Other chain-transfer reactions may iatroduce dye sites, branching, chromophoric groups, and stmctural defects which reduce thermal stabiUty. Many of the solvents used for acrylonitrile polymerization are very active in chain transfer. DMAC and DME have chain-transfer constants of 4.95-5.1 x lO " and 2.7-2.8 x lO " respectively, very high when compared to a value of only 0.05 x lO " for acrylonitrile itself DMSO (0.1-0.8 X lO " ) and aqueous zinc chloride (0.006 x lO " ), in contrast, have relatively low transfer constants hence, the relative desirabiUty of these two solvents over the former. DME, however, is used by several acryhc fiber producers as a solvent for solution polymerization. 

Chain transfer to solvent is an important factor in controlling the molecular weight of polymers prepared by this method. The chain-transfer constants for poly(methyl methacrylate) in various common solvents (C) and for various chain-transfer agents are Hsted in Table 10. 

Solution Polymerization. Solution polymerization of vinyl acetate is carried out mainly as an intermediate step to the manufacture of poly(vinyl alcohol). A small amount of solution-polymerized vinyl acetate is prepared for the merchant market. When solution polymerization is carried out, the solvent acts as a chain-transfer agent, and depending on its transfer constant, has an effect on the molecular weight of the product. The rate of polymerization is also affected by the solvent but not in the same way as the degree of polymerization. The reactivity of the solvent-derived radical plays an important part. Chain-transfer constants for solvents in vinyl acetate polymerizations have been tabulated (13). Continuous solution polymers of poly(vinyl acetate) in tubular reactors have been prepared at high yield and throughput (73,74). 

Solution Polymerization. This method is not commercially important, although it is convenient and practical, because it provides viscous cements that are difficult to handle. Also, the choice of the solvent is a key parameter due to the high solvent chain-transfer constants for acrylates. 

Donor solvent Formation constant X(20°C)/1 mol- -AH / kJ moC Charge-transfer band W/nm fmax A VI/cm- 2 ... 

The following sections detail the chemistry undergone by specific transfer agents that react by atom or group transfer by a homolytic substitution mechanism. Thiols, disulfides, and sulfides arc covered in Sections 6.2.2.1,6.2.2.2 and 6.2.2.3 respectively, halocarbons in Section 6.2.2.4, and solvents and other agents in Section 6.2.2.5. The transfer constant data provided have not been critically... 

Thiols react more rapidly with nucleophilic radicals than with electrophilic radicals. They have very large Ctr with S and VAc, but near ideal transfer constants (C - 1.0) with acrylic monomers (Table 6.2). Aromatic thiols have higher C,r than aliphatic thiols but also give more retardation. This is a consequence of the poor reinitiation efficiency shown by the phenylthiyl radical. The substitution pattern of the alkanethiol appears to have only a small (<2-fokl) effect on the transfer constant. Studies on the reactions of small alkyl radicals with thiols indicate that the rate of the transfer reaction is accelerated in polar solvents and, in particular, water.5 Similar trends arc observed for transfer to 1 in S polymerization with Clr = 1.4 in benzene 3.6 in CUT and 6.1 in 5% aqueous CifiCN.1 In copolymerizations, the thiyl radicals react preferentially with electron-rich monomers (Section 3.4.3.2). 

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...

Cs transfer constant to solvent or added transfer agent... 

ESI mass spectrometry ive mass spectrometry ESR spectroscopy set EPR spectroscopy ethyl acetate, chain transfer to 295 ethyl acrylate (EA) polymerizalion, transfer constants, to macromonomers 307 ethyl methacrylate (EMA) polymerization combination v.v disproportionation 255, 262 kinetic parameters 219 tacticity, solvent effects 428 thermodynamics 215 ethyl radicals... 

The effect of temperature on Mv has been studied by a number of workers (Table 8) and in all cases, a decrease in temperature increased PIB molecular weight. Since solvent dielectric constant increases with decreasing temperatures, molecular weights also areexpected to decrease. Apparently such effect is small as shown by the increase in Mv s with decreasing temperature. At very low temperatures, Mv suddenly drops as shown above. This was explained4 by assuming a reduced rate of initiation leading to an increase in transfer to initiator. 

CEP = ratio of the rate constant for polymer transfer (long-chain branching) to the constant for propagation CFS = ratio of the rate constant for solvent transfer to the constant for propagation... 

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. 

The results of chain transfer studies with different polymer radicals are compared in Table XIV. Chain transfer constants with hydrocarbon solvents are consistently a little greater for methyl methacrylate radicals than for styrene radicals. The methyl methacrylate chain radical is far less effective in the removal of chlorine from chlorinated solvents, however. Vinyl acetate chains are much more susceptible to chain transfer than are either of the other two polymer radicals. As will appear later, the propagation constants kp for styrene, methyl methacrylate, and vinyl acetate are in the approximate ratio 1 2 20. It follows from the transfer constants with toluene, that the rate constants ktr,s for the removal of benzylic hydrogen by the respective chain radicals are in the ratio 1 3.5 6000. Chain transfer studies offer a convenient means for comparing radical reactivities, provided the absolute propagation constants also are known. 

Chain transfer constants (ktr/kp) for monomer, solvent, and polymer (Chaps. IV, V, and IX). 

Free radical copolymerizations of the alkyl methacrylates were carried out in toluene at 60°C with 0.1 weight percent (based on monomer) AIBN initiator, while the styrenic systems were polymerized in cyclohexane. The solvent choices were primarily based on systems which would be homogeneous but also show low chain transfer constants. Methacrylate polymerizations were carried out at 20 weight percent solids... 

This potential was developed to ensure that the molecules inside the sphere never escape and maintain a fully solvated system during molecular dynamics. Here, es, Rs, ew and Rw are the van der Waals constants for the solvent and the wall and rj is the distance between the molecule i and the center of the water sphere, Ro is the radius of the sphere. The quantities A, B and Rb are determined by imposing the condition that W and dW/dr, vanish at r, = Ro. The restraining potential W is set to zero for r, < R0. The van der Waals parameters Es, ew, Rs and Rw can also be specifically defined for different solvents. The constants Awaii and Cwan are computed using a well depth of es = ew = 0.1 kcal and the radius of Rs = Rw = 1.25 A. For the other set of simulations, especially for the hydride ion transfer, we applied periodic boundary conditions by using a spherical boundary shell of 10.0 A of TIP3P40 water to cover the edges of the protein. 

Chain-transfer constants, 25 571t Chain-transfer rate constants, 19 832 Chain-transfer rates, 19 839 Chain transfer to solvent (CTS), 23 385 Chalcanthite, 7 772 Chalcogenide glasses, 12 575, 584 semiconductivity in, 12 587 Chalcogenides acidic, 12 190-191 gallium, 12 359 in photocatalysis, 19 75 plutonium, 19 691 zirconium, 26 641... 

The most important of these in chemically initiated polymerizations are the transfer reactions with solvent, rate Rs, and rate-constant ks, and with monomer, rate Rm, and rate-constant km. Solvent transfer was shown to be important by Ueno etal. (1966c) for the polymerization of styrene in toluene, and it will be discussed below. The chemistry of the transfer with an aromatic compound ArH, discovered by Plesch et al. (Plesch 1953 Brackman Plesch 1958 Penfold Plesch 1961), can be represented as... 

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