Solvent chain transfer, styrene polymerization

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. 

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. 

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... 

This may explain the fact that the degree of swelling with ethyl benzene, given in Figure 4, is identical to that for styrene alone. In the ease of pentane, since the interaction parameters and J2 are not identical to that of the "solvent," the swelling would be lower than that due to simple dilution. A large difference between X13 and X23 avors selective absorption of the better solvent in the polymer particle. The effect of pentane on the rate of polymerization is most probably similar to that of ethyl benzene, but with a lower [m) in the particles, and with less chain transfer, as discussed below. ... 

A plot of the reciprocal of the number average degree of polymerization versus [T f[M should then be linear, if the analysis is correct. And indeed it is, as illustrated for the polymerization of styrene in various solvents, which act as chain transfer agents (hence the use of the symbol [5] instead of [7] in Figure 4-22). 

Clearly the benzylic H-atoms attached to the polystyrene backbone are not as labile as in cumene. This is likely due to the steric effect of the coil configuration of the polymer chain which blocks access of the /er/-butoxy radicals. Nonetheless, some backbone H-atom abstraction from the polystyrene backbone does occur during radical polymerization of styrene. The extent of abstraction is proportional to the concentration of peroxide initiator added to the process. Typically, in commercial continuous bulk polymerization processes the concentration of peroxide initiator is kept below 500 ppm. Also a few percent of a solvent having some chain transfer activity (ethylbenzene) is added to the styrene feed. This is done so that the extent of branching is small. If the concentration of initiator is increased to >500 ppm and/or the chain transfer solvent falls below a certain level, the extent of branching can increase to a level where gels began to appear in the product. The mechanism of... 

Poly(styrene)s containing acylperoxide groups are thus obtained by selective photolysis of the azo moieties at 350 or 371 nm. These prepolymers are successively used as macronitiators for the free radical polymerization of vinyl chloride at 70 °C. Styrene/vinyl chloride block copolymers are thus produced [55] by the above two-step route, although relevant amounts (50-60%) of poly(styrene) and poly(vinyl chloride), due to both low peroxide content ( 0.6 groups per macromolecule of polystyrene) and chain transfer with solvent and monomer, are also pre t. 

Determine the molecular weight distribution for the formation of polystyrene for an initiiUor concentralwn of 10 molar and a monomer concentration of 3 molar. What are M , M , and the polydispersity, D, after 40 hrs How would the change if chain transfer were neglected The polymerization of styrene is carried out in a batch reactor. Plot the mole fraction of polystyrene of chain length 10 as a function of time for an initial concentration of 3M and 0.0 IM of monomer an initiator respectively, The solvent concentration is 10 molar,... 

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

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 cationic polymerization of styrene in aromatic solvents such as toluene, the chain transfer reaction to a solvent molecule usually occurs as follows 45-49... 

---