Styrene polymerization, kinetic behavior

Our final goal in the present paper is to devise an optimal type of the first stage reactor and its operation method which will maximize the number of polymer particles produced in continuous emulsion polymerization. For this purpose, we need a mathematical reaction model which explains particle formation and other kinetic behavior of continuous emulsion polymerization of styrene. 

Let us determine the value of e by comparing the transient kinetic behavior of monomer conversion in continuous emulsion polymerization of styrene with the model prediction by the Nomura and Harada model. It is reported in the literature that sustained... 

Table I. Kinetic Behavior of Styrene Polymerization Initiated by Radiation Under Wet (Radical) and Dry (Ionic) Conditions...

The polymerization kinetics is governed by the droplet size. Tang et al. found that the polymerization of styrene miniemulsions created by the microfluidizer was faster than that of miniemulsions created by the omnimixer [64]. This behavior can mainly be attributed to the different droplet size prior to polymerization. In the first case, the droplets are smaller than in the second case [65]. Fontenot and Schork observed similar behavior for MMA miniemulsions. With increasing shear and increasing concentration of surfactant, the polymerization rate increases [22]. This again can be explained by different sizes of the initial droplets. 

It was discovered that the addition of 1,3-cyclohexadiene to the Rh -catalyzed reactions increased the rate of butadiene polymerization by a factor of over 20 [20]. Considering the reducing properties of 1,3-cyclohexadiene, this effect could be due to the reduction of Rh to Rh and stabilization of this low oxidation state by the diene ligands. With neat 1,3-cyclohexadiene, Rh is reduced to the metallic state. These emulsion polymerizations are sensitive to the presence of Lewis basic functional groups. A stoichiometric amount of amine (based on Rh) is sufficient to inhibit polymerization completely. It was also discovered that styrene could be polymerized using the Rh catalyst. However, the atactic nature of the polymer, along with the kinetic behavior of the reaction, indicated that a free-radical process, rather than a coordination-insertion mechanism, was operative. 

A good example of this kinetic behavior was found in the study of the n-butyllithium-styrene system in benzene, in which a kinetic order dependency on n-butyllithium concentration was observed, consistent with the predominantly hexameric degree of association of n-butyllithium (Worsfold and By water, 1960). However, this expected correspondence between the degree of association of the alkyllithium compound and the fractional kinetic order dependence of the initiation reaction on alkyllithium concentration was not always observed (Young et al., 1984). One source of this discrepancy is the assumption that only the unassociated alkyllithium molecule can initiate polymerization. With certain reactive initiators, such as 5 c-butyllithium in hexane solution, the initial rate of initiation exhibits approximately a first-order dependence on alkyllithium concentration, suggesting that the aggregate can react directly with monomer to initiate polymerization (Bywater and Worsfold, 1967a). A further... 

A comparison of the observed propagation rate constants for styrene polymerization with different alkali metal counterions is shown in Table 2. Poly(styryl)sodium was presumably associated into dimers since kinetic orders of one-half were observed for the rate dependence on the active chain-end concentration. Poly(styryl)potassium exhibits intermediate behavior dependence on chain-end concentration was one-half order at higher concentrations, but first order at low concentrations. Poly(styryl)rubidium and poly(styryl)cesium exhibit first-order dependencies on chain-end concentrations which is consistent with unassociated chain ends in cyclohexane. The counterion dependence is K+ > Rb+ > Cs+ Li+ in cyclohexane and K+ > Na+ > Li+ in benzene. The interpretation of these results is complicated by the fact that the complex observed rate constants ( obs) reflect both the fact that the dissociation constant for the dimers increases with increasing cation size (no association for rubidium and cesium) and also the fact that the requisite energy associated with charge separation in the transition state would be less for the larger counterions. 

The kinetic behavior of PMS in thermal and peroxide-initiated polymerizations was compared to that of styrene. The isothermal polymerization rates for PMS and styrene were measured over the conventional thermal polymerization temperature range of 110 C to 150 C. The conversions for inhibitor-free monomers were monitored as a function of time and were very similar (Figure 3). The effects of solvent on polymerization rates were also measured and found to be similar. In 10% solvent, either ethylbenzene or para-ethyltoluene, the conversion rates are about 12% lower than without solvent for both styrene and PMS. The molecular weights and... 

A comparison of the observed propagation rate constants for styrene polymerization with diffaent alkali metal counterions in hydrocarbon solution is shown in Table l. Poly(stytyl)sodium was presumably assodated into dimeis since kinetic orders of one-half were observed for the rate dependence on the active chain-end concenttation. Poly(styryl)potassium edtibits intermediate behavior dependence on chain-end concentration was... 

CuPFg, complexed with two molecules of pyridine, is an efficient system for ATRP of styrene and methyl acrylate [8]. Notably, ligand exchange occurring with mixed systems such as CuBr/R-Q was not observed due to the noncoordinating nature of CuPF. For styrene polymerization employing phenylethylchloride as the alkyl halide, better control of the molecular weight and linear kinetic behavior was observed. The rates of polymerization were enhanced in methyl acrylate polymerization. 

I have discussed two cases of chain microstructure determination in SO2 copolymers. First, the styrene-S02 system, which exhibits the general kinetic and compositional behavior of such systems, particularly as a function of polymerization temperature. Second, and considerably more complicated, is the chloroprene-S02 system. This one represents the limit of what can be handled and more or less completely solved at the present time. To do so requires about all o ir resources at superconducting frequen-... 

In the examples described above, the transition is shown from ideal (n = /2) to nonideal (n > /2) behavior. There are, however, systems for which ideal emulsion polymerization practically cannot be achieved. It is nevertheless possible to describe the kinetics of such systems quantitatively. Recently, Gerrens has obtained values of the propagation and termination rate constants at diflFerent temperatures for vinyltoluene and vinylxylene (28). The termination rate of polymer radicals of these monomers is so low that even at small rates of initiation in small particles, n is larger than /2. From measurements of the reaction rate before and after injection of additional initiator in the polymerizing system it was possible to calculate n both at the original and at the boosted initiation rate with the aid of Equation 5. Consistent results were obtained when the additional amount of initiator was varied. From these rate data, the termination rate constant was found to be 10 and 17 liters mole- sec. at 45° C. for vinyltoluene and vinylxylene, respectively. These values are to be compared with 10 for styrene (Table IV). 

Even when conditions are scrupulously controlled, the kinetics of cationic polymerization are rarely simple. Water is highly reactive towards organic cations and if present as initiator, any excess will terminate polymer chains. Excess water may also destroy the coinitiator in some cases, or compete successfully with monomer for the initiator-coinitiator complex (see later). The kinetic influence of water is thus complicated. In some systems, the initial rate of polymerization increases with concentration of water at low concentrations and becomes independent as this concentration increases. Such behavior has been reported for the polymerization of isobutene in dichloromethane initiated by titanium tetrachloride and water [21]. In other systems, the initial rate of polymerization may rise to a maximum and then decline with increasing concentrations of water. Such behavior has been observed in the SnCl4/H20 initiated polymerization of styrene in carbon tetrachloride [22]. 

At low impurity content, the polymerization takes place with kinetics that is characteristic of free-radical polymerization (O Eq. (23.100)). Such behavior was observed, for instance, in the cationic polymerization of styrene (Swallow 1973). 

FRP leads to the formation of statistical copolymers, where the arrangement of monomers within the chains is dictated purely by kinetic factors. However, reactivity of a monomer in copolymerization cannot be predicted from its behavior in homopolymerization. Vinyl acetate polymerizes about 30 times more quickly than styrene (see Table 4.2), yet the product is almost pure polystyrene if the two monomers are copolymerized together in a 50 50 mixture. a-Methylstyrene cannot be ho-mopolymerized to form high-MW polymer due to its low ceiling temperature (see Table 4.6), yet is readily incorporated into copolymer at elevated temperatures. These and other similar observations can be understood by considering copolymerization mechanisms and kinetics. 

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