Polymerization initiator chain transfer constant

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.

Effect of Initiator Chain Transfer Constant on the Polymerization Mechanism If the amount of Mn2(CO)iQ is less than that of the initiator, depending on the value of its CT constant, (CT = / pagation) ... 

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. 

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... 

S]/[M], a plot often called a CLD plot. Cs is the straight line slope of the CLD plot. Cm can he obtained from polymerizations at very low initiator concentrations and no chain-transfer agent present. The last term on the right side of Eq. 3-121 is zero, and the first term is very small. Under these conditions the slope of the CLD plot is Cm- Chain transfer constants obtained by the CLD method are essentially indistinguishable from those of the Mayo plot [Heuts et al., 1999]. 

Five different types of rate constants are of concern in radical chain polymerization—those for initiation, propagation, termination, chain transfer, and inhibition. The use of polymerization data under steady-state conditions allows the evaluation of only the initiation rate constant kd (or kt for thermal initiation). The ratio kp/k J2 or kp/kl can be obtained from Eq. 3-25, since Rp, Rj, and [M] are measurable. Similarly, the chain-transfer constant k /kp and the inhibition constant kz/kp can be obtained by any one of several methods discussed. However, the evaluation of the individual kp, k ktr, and kz values under steady-state conditions requires the accurate determination of the propagating radical concentration. This would allow the determination of kp from Eq. 3-22 followed by the calculation of kt, kIr, and kz from the ratios kp/ltj2, ktr/kp, and kz/kp. 

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 ... 

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. 

Vinyl acetate was polymerized in a free-radical reaction. The initial monomer concentration was 1 mol/liter and its concentration after I h was 0.85 mol/liter. Chloroform was present as a chain transfer agent, with concentrations 0.01 mol/liter at time zero and 0.007 mol/liter after I h. What is the chain transfer constant C in this case ... 

Generally, efficiency of CCT catalysis drops in emulsion polymerization. The following values of CCT chain-transfer constants may be compared with solution and bulk polymerization CcMMA =1100 M 1 s-1, CcEMA = 640 M-1 s 1, Ccn BMA = 520 M s-1, Cc2 EHMA = 400 M 1 s 1 (75 °C, water, 9a).342 In miniemulsion polymerization, the choice of catalyst depends on the choice of initiator (see Table 10).345... 

Problem 6.26 From the polymerization data given in Problem 6.25, determine the chain transfer constant of the initiator I for the polymerization of monomer M at 60°C. 

Fig. 14A-C. Isobutylene polymerization by TMPCl/TiCl4 and HX /TiCl4 at — 60 °C A,B determination of chain transfer constants in systems with slow initiation plus chain transfer to monomer C N vs conversion plot at various ED concentrations in the presence of initiator ([TEA] = (mol/l) =5-10 A = 1.5-HT3, = 3-l(T3, = 6-103) and in the absence of initiator (O, A, , V)...

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.)... 

A class of peroxides which has received little attention in styrene polymerization is hydroperoxides. In inert solvents, hydroperoxides are relatively stable. However, in a FR environment, they undergo an induced decomposition. The induced decomposition results from the relatively high chain transfer constant (Cj) of the hydroperoxide hydrogen (C, 0.05). Abstraction of the hydrogen by a growing polystyryl radical produces a peroxy radical which adds to styrene. As mentioned peviously, the styrene adduct likely decomposes to form styrene oxide and a tert-alkoxy radical which subsequently initiates polymerization. 

The kinetics of chain transfer to ammonia has been investigated for potassium amide-initiated polymerization of styrene in liquid ammonia at —33.5 °C. The calculated chain transfer constant ( / ) was 2.34 x 10 " [109]. The chain transfer reaction of poly(styryl)lithium with toluene at 60 °C was investigated during the polymerization of styrene using " C-labeled toluene. The calculated chain transfer constant was 5 x 10 [41]. A much larger chain... 

---