Vinyl acetate polymerization benzene, effect

Many substituents stabilize the monomer but have no appreciable effect on polymer stability, since resonance is only possible with the former. The net effect is to decrease the exothermicity of the polymerization. Thus hyperconjugation of alkyl groups with the C=C lowers AH for propylene and 1-butene polymerizations. Conjugation of the C=C with substituents such as the benzene ring (styrene and a-methylstyrene), and alkene double bond (butadiene and isoprene), the carbonyl linkage (acrylic acid, methyl acrylate, methyl methacrylate), and the nitrile group (acrylonitrile) similarly leads to stabilization of the monomer and decreases enthalpies of polymerization. When the substituent is poorly conjugating as in vinyl acetate, the AH is close to the value for ethylene. 

Radical chain polymerizations are characterized by the presence of an autoacceleration in the polymerization rate as the reaction proceeds [North, 1974], One would normally expect a reaction rate to fall with time (i.e., the extent of conversion), since the monomer and initiator concentrations decrease with time. However, the exact opposite behavior is observed in many polymerizations—the reaction rate increases with conversion. A typical example is shown in Fig. 3-15 for the polymerization of methyl methacrylate in benzene solution [Schulz and Haborth, 1948]. The plot for the 10% methyl methacrylate solution shows the behavior that would generally be expected. The plot for neat (pure) monomer shows a dramatic autoacceleration in the polymerization rate. Such behavior is referred to as the gel effect. (The term gel as used here is different from its usage in Sec. 2-10 it does not refer to the formation of a crosslinked polymer.) The terms Trommsdorff effect and Norrish-Smith effect are also used in recognition of the early workers in the field. Similar behavior has been observed for a variety of monomers, including styrene, vinyl acetate, and methyl methacrylate [Balke and Hamielec, 1973 Cardenas and O Driscoll, 1976, 1977 Small, 1975 Turner, 1977 Yamamoto and Sugimoto, 1979]. It turns out that the gel effect is the normal ... 

Azad and Fitch (5) investigated the effect of low molecular weight hydrocarbon additives on the formation of colloidafr particles in suspension polymerization of methyl methacrylate and vinyl acetate. It was found that the additives n-octane, n-dodecane, n-octadecane, n-tetracosane and mineral oil exerted a thermodynamic affect depending upon water-solubility and molecular weight. Since these effects on emulsion polymerization have not been considered by the earlier investigators, we have chosen n-pentane and ethyl benzene as additives with limited water-solubility and n-octadecane, and n-tetracosane as water-insoluble ones. Seeded emulsion polymerization was chosen so that the number of particles could be kept constant throughout the experiments and only the effect of the other parameters on the rate could be determined. 

The reduction in molecular weight of the polymer is slight as compared to that in many other solvents. Stockmayer et al.41,42 once interpreted this retardation effect in terms of copolymerization involving the aromatic ring, but the failure of the copolymerization of benzene with vinyl monomers was confirmed by the application of the isotope technique43-47. Therefore, the influence of aromatic compounds on the polymerization rate of vinyl acetate has remained unsolved. 

Benzene, in small quantities, reduces the rate of polymerization of vinyl acetate. In effect it acts as an inhibitor for this polymerization [86]. Actually, the function of benzene in the polymerization of vinyl acetate is quite complex and has been discussed above. Benzene may form a complex with the monomer, it may act as a chain-transfer agent, or it may actually copolymerize [cf. 37-43]. 

The solution polymerization of allyl acetate was studied in an effort to determine the effect of monomer concentrations on the reaction kinetics [14]. These studies were limited to the use of benzene. The growing allyl acetate radicals formed stable adducts with the solvent much as vinyl acetate does. The stabilized adduct is terminated by combination with a growing radical. The twinning reaction is said to account for the relatively high molecular weight of the polymer despite the fact that the reaction with benzene is a chain-transfer reaction. Ethyl acetate, on the other hand, would have been a preferable solvent... 

U Propagation In a classical study of solvent effect on propagation, Hatada et,al found that the polymerization of vinyl acetate is subjected to significant influence by the solvent. With benzene as solvent essentially no branching was observed. However, in ethyl acetate 0.7 branch per chain (for Mn = 20000) was observed. The reason postulated is that the benzene complexes with the propagating radical to make it more selective. 

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