Styrene chain-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. 

Estimate the chain transfer constants for styrene to isopropylbenzene, ethylbenzene, toluene, and benzene from the data presented in Fig. 6.8. Comment... 

Inhibitors are characterized by inhibition constants which are defined as the ratio of the rate constant for transfer to inhibitor to the propagation constant for the monomer in analogy with Eq. (6.87) for chain transfer constants. For styrene at 50°C the inhibition constant of p-benzoquinone is 518, and that for O2 is 1.5 X 10. The Polymer Handbook (Ref. 3) is an excellent source for these and most other rate constants discussed in this chapter. 

Table 4. Chain-Transfer Constants in Free-Radical Styrene Polymerization...

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. 

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... 

Chain Transfer Constants of Solvent to Styrene in Free Radical Chain Polymerization at 60°C... 

Using the methods described, the values of Cm and Ci in the benzoyl peroxide polymerization of styrene have been found to be 0.00006 and 0.055 respectively [Mayo et al., 1951]. The amount of chain transfer to monomer that occurs is negligible in this polymerization. The chain-transfer constant for benzoyl peroxide is appreciable, and chain transfer with initiator becomes increasingly important as the initiator concentration increases. These effects are shown in Fig. 3-7, where the contributions of the various sources of chain ends are indicated. The topmost plot shows the total number of polymer molecules per 105 styrene monomer units. The difference between successive plots gives the number of polymer molecules terminated by normal coupling termination, transfer to benzoyl peroxide, and transfer to styrene. 

Fig. 3-6 Determination of initiator chain-transfer constants in the t-butyl hydroperoxide initiated polymerization of styrene in benzene solution at 70°C. After Walling and Heaton [1965] (by permission of American Chemical Society, Washington, DC.

Consider the polymerization of styrene initiated by di-t-butyl peroxide at 60°C. For a solution of 0.01 M peroxide and 1.0 M styrene in benzene, the initial rates of initiation and polymerization are 4.0 x 10 11 and 1.5 x 10 7 mol L 1 s 1, respectively. Calculate the values of (jkj), the initial kinetic chain length, and the initial degree of polymerization. Indicate how often on the average chain transfer occurs per each initiating radical from the peroxide. What is the breadth of the molecular weight distribution that is expected, that is, what is the value of Xw/Xnl Use the following chain-transfer constants ... 

It is evident that the values of the transfer constants are dependent on the nature both of the attacking radicals and of the transfer agent itself, and that similar effects should be expected during the synthesis of graft copolymers by chain transfer methods. For example, with respect to toluene the chain transfer constant is a little greater for methyl methacrylate radicals than for styrene radicals on the contrary, with respect to halogenated solvents (CC14) the polystyrene radical is much more effective in the removal of a chlorine atom. Vinyl acetate chains are far more effective than either of the other two polymer radicals. 

Free radical polymerization of styrene, of acrylate and of methacrylate monomers in solutions at 60° C in the presence of this preformed polymer produced graft copolymers in high efficiency, the chain transfer constants for these mercapto groups with styrene and methyl methacrylate being similar to those found with simple mercaptans (80, 85). 

As shown in Figure 1, the addition of 1 part benzene to 4 parts of styrene in the emulsion formulation caused a large reduction in the rate of polymerization. This effect was increased as the concentration of the benzene was increased. The chain transfer constant of benzene is not significant enough to account for this effect but rather the simple fact that... 

Mayo [16] proposed an alternative mechanism that is currently widely supported. Figure 7.7 shows a schematic of the Mayo mechanism. A Diels-Alder reaction between two styrene molecules produces an intermediate dimer (DH), also referred to as Mayo dimer . DH is highly reactive and has never been isolated. To complete the auto-initiation, DH reacts with a third styrene molecule via molecular assisted homolysis [17] to form a phenyltetraline radical (D ) and a phenethyl radical (SH ). A second reaction involving DH is to undergo chain transfer with a growing radical chain to produce a dead polymer chain (PS-H) and a new growing radical. The chain transfer constant (A ct) of DH has been estimated at 10, which is the highest Kcl ever reported for a molecule that contains no heteroatoms [18,19]. 

Problem 32,5 For polymerization of styrene at 60 , the following chain-transfer constants have been measured. Account for the relative effectiveness of the members of each sequence. 

The chain-transfer constants for styrene polymerization with porphyrinic CCT catalysts are given in Table 4. In addition, the following values can be found in the literature kc= 1.4 x 105 M-1 s-1 (40— 70 °C, 9a),314 and k = 6.4 x 104 M-1 s-1, Cs = 400 (40 °C, 9c).40 The low Cs in styrene systems can be partly attributed to the formation of Co—C bonds, reducing the concentration of active Co11 catalyst. The experimental observation that the molecular weight distribution of the polymer formed in CCT remains constant with conversion is not explained.369... 

Table 3 Chain transfer constants (Ctr) of solvents to styrene in free-radical chain polymerization at 60 C...

Ignatz-Hoover, F., Petrukhin, R., Karelson, M. and Katritzky, A.R. (2001) QSRR correlation of free-radical polymerization chain-transfer constants for styrene. J. Chem. Inf. Comput. Sci., 41, 295-299. 

The transfer constants for a number of solvents/additives for polymerization of styrene, methyl methacrylate, and vinyl acetate are listed in Table 6.11. The data indicate dependence of chain transfer constants on the chemical structure of both chain transfer agents and the monomer. The... 

In the field of ionic polymerization, we should mention the investigators87,114) who calculated the relative chain transfer constants for the polymerization of styrene in benzene solutions and its mixture with 1,2-dichloroethane on Friedel-Crafts catalysts. 

Problem 6.22 Weighed amounts of styrene (M) and -butyl mercaptan (S) in sealed glass ampoules were heated at 60° C for different periods of time. The polymers were then precipitated in methanol, dried in oven, and degrees of polymerization evaluated by intrinsic viscosity measurements. From.the data given below calculate the chain transfer constant (Cs) for the st5Tene/dodecyl mercaptan system at 60°C. 

Problem 6.23 In a free-radical polymerization of styrene in solution at 60°C, carbon tetrabromide was used as a chain transfer agent. The initial concentrations of styrene and CBr4 were 1 mol/L and 0.01 mol/L, respectively. In 1 h, these concentrations dropped to 0.85 mol/L and 0.007 mol/L, respectively. What is the chain transfer constant Cs for styrene/CBr4 (Neglect chain transfer to monomer, initiator, and solvent.)... 

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