Initiator chain transfer constants

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.

Table 6.6 Some Values of Initiator Chain Transfer Constants...

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. 

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) ... 

Cumene hydroperoxide [95], benzoyl peroxide, or tert-h iiy peroxide [96]. can be used as accelerators with alkylboron initiators. The chain transfer constant for MMA to tributylborane has been estimated to be 0.647, which is comparable to tripropylamine [97]. 

As shown by the data in Fig. 31, the chain transfer constant of this initiator, Q = 1.0. In this context it is of interest to remember that the effect of initiator concentration on the molecular weight of HSi-PaMeSt was negligible, probably because of unfavorable thermodynamics (Sect. III.B.3.b.iv.). In contrast, with isobutylene chain transfer from the propagating carbenium ion to initiator is thermodynamically favorable (see Sect. IH.B.4.b.i.). Thus it is not surprising to find a large Q. The chain transfer mechanism has been illustrated in Scheme 5. 

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... 

The situation becomes more complex for semi-reversible chain transfer, where kcl and /cRT are both positive, but kCT > kRT As demonstrated in Fig. 7, MJMn can be greater than, less than, or equal to 2.0, depending on the conversion and the magnitudes of the chain transfer constants. The Mn of the polymer is simply a function of C ° the value of C has no effect on M up to C = C °. However, M is dramatically affected by lower values of Ct. If C° Ca > 0, then the initial increase in M IM is dramatic, and M IM does not dip below 2 until high conver-sion. However, as C approaches C 0, the initial increase in MJMn is negligible, and MJMn drops below 2.0 at low conversion. In any case, if Ca > 0, then MJMn asymptotically approaches two from the low side. 

A chain-transfer constant C for a substance is defined as the ratio of the rate constant klr for the chain transfer of a propagating radical with that substance to the rate constant kp for propagation of the radical. The chain-transfer constants for monomer, chain-transfer agent, and initiator are then given by... 

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. 

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. 

Here [M] is monomer concentration, [S] stabilizer concentration, [I] initiator concentration A and b are constants in the function of radius of gyration rg = A(Mw)b (Mw is the molecular weight of the stabilizer) and Mm is the molecular weight of the monomer, p is density, and Cs is the chain transfer constant to the stabilizer. 

Graft ABS polymers were prepared by the reaction of an approximate azeotropic mole ratio of styrene-acrylonitrile monomer mixture in the presence of different weights of diene substrate latex. Initiator, chain transfer agent, and soap levels were constant. Grafting reactions were... 

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