Vinyl acetate polymerization solvent effects

Solution Polymerization. Solution polymerization of vinyl acetate is carried out mainly as an intermediate step to the manufacture of poly(vinyl alcohol). A small amount of solution-polymerized vinyl acetate is prepared for the merchant market. When solution polymerization is carried out, the solvent acts as a chain-transfer agent, and depending on its transfer constant, has an effect on the molecular weight of the product. The rate of polymerization is also affected by the solvent but not in the same way as the degree of polymerization. The reactivity of the solvent-derived radical plays an important part. Chain-transfer constants for solvents in vinyl acetate polymerizations have been tabulated (13). Continuous solution polymers of poly(vinyl acetate) in tubular reactors have been prepared at high yield and throughput (73,74). 

One of the most dramatic examples of a solvent effect on propagation taken from the early literature is for vinyl acetate polymerization.78,79 Kamachi el al.n reported a ca. 80-fold reduction in kp (30aC) on shifting from ethyl acetate to benzonilrile solvent (Table 8.1). Effects on polymer structure were also reported. Hatada ef a m conducted a H NMR study on the structure of the PVAc formed in various solvents. They found that PVAc (M n 20000) produced in ethyl acetate solvent has 0.7 branches/chain while that formed in aromatic solvents is essentially unbranched. 

Prior to Harwood s work, the existence of a Bootstrap effect in copolymerization was considered but rejected after the failure of efforts to correlate polymer-solvent interaction parameters with observed solvent effects. Kamachi, for instance, estimated the interaction between polymer and solvent by calculating the difference between their solubility parameters. He found that while there was some correlation between polymer-solvent interaction parameters and observed solvent effects for methyl methacrylate, for vinyl acetate there was none. However, it should be noted that evidence for radical-solvent complexes in vinyl acetate systems is fairly strong (see Section 3), so a rejection of a generalized Bootstrap model on the basis of evidence from vinyl acetate polymerization is perhaps unwise. Kratochvil et al." investigated the possible influence of preferential solvation in copolymerizations and concluded that, for systems with weak non-specific interactions, such as STY-MMA, the effect of preferential solvation on kinetics was probably comparable to the experimental error in determining the rate of polymerization ( 5%). Later, Maxwell et al." also concluded that the origin of the Bootstrap effect was not likely to be bulk monomer-polymer thermodynamics since, for a variety of monomers, Flory-Huggins theory predicts that the monomer ratios in the monomer-polymer phase would be equal to that in the bulk phase. 

Imai, K. Shiomi, T. Oda, N. Otsuka, H. Effect of solvent for vinyl-acetate polymerization on microstructure of poly(vinyl-alcohol). J. Polym. Set, Part A Polym. Chem. 1986, 24, 3225-3231. 

Poly(vinyl acetate) polymerization is accomplished by conventional processes, e.g., solution, bulk, or emulsion polymerization. Solution polymerization is favored because the subsequent alcoholysis reaction requires solvent addition. The polymerization step determines the ultimate molecular weight of the PVOH. Catalyst concentration, temperature, and solvent control the degree of polymerization acetaldehyde is an effective chain-transfer agent. It is the agent commonly used. 

A superficially related dependence of on the medium has been observed by Norrish and Smith working with methyl methacrylate, and by Burnett and Melville with vinyl acetate. Rates in poor solvents are high, and determination of by the rotating sector method reveals what appears to be a decrease in kt in the poor solvents. This apparent decrease in kt accounts for the increased rate of polymerization. Actually, precipitation of the polymer seems to be responsible for the effect. The growing radicals become imbedded in precipitated droplets, presumably of very small size. The termination reaction is suppressed owing to isolation of the chain radical in one droplet from that in another. This gel effect is fairly common in systems yield-... 

Polymerizations conducted in nonaqueous media in which the polymer is insoluble also display the characteristics of emulsion polymerization. When either vinyl acetate or methyl methacrylate is polymerized in a poor solvent for the polymer, for example, the rate accelerates as the polymerization progresses. This acceleration, which has been called the gel effect,probably is associated with the precipitation of minute droplets of polymer highly swollen with monomer. These droplets may provide polymerization loci in which a single chain radical may be isolated from all others. A similar heterophase polymerization is observed even in the polymerization of the pure monomer in those cases in which the polymer is insoluble in its own monomer. Vinyl chloride, vinylidene chloride, acrylonitrile, and methacryloni-trile polymerize with precipitation of the polymer in a finely divided dispersion as rapidly as it is formed. The reaction rate increases as these polymer particles are generated. In the case of vinyl chloride ... 

There have been efforts to enhance stereoselectivity in radical polymerization by using fluoroalcohols or Lewis acids that complex with monomers such as MMA and vinyl acetate [Isobe et al., 2000, 2001a Okamoto et al., 2002], In almost all instances the effects are nil or very small. For example, the use of perfluoro-t-butyl alcohol as solvent instead of toluene changes (rr) from 0.89 to 0.91 in the polymerization of MMA at —78°C. An exception is in the polymerization of acrylamide in the presence of some rare-earth Lewis acids such as ytterbium triflate. The polymer is atactic at 0°C, (m) = 0.46, in the absence of the Lewis acid, but significantly isotactic, (m) — 0.80, in the presence of the Lewis acid. The reason for this effect is unclear. More highly isoselective polymerization occurs in some radical polymerizations of MMA (Sec. 8-14b). 

Commercial Hydrolysis Process. The process of converting poly (vinyl acetate) to poly(vinyl alcohol) on a commercial scale is complicated on account of the significant physical changes that accompany the conversion. The viscosity of the poly (vinyl acetate) solution increases rapidly as the conversion proceeds, because the resulting poly(vinyl alcohol) is insoluble in the most common solvents used for the polymerization of vinyl acetate. The outcome is the formation of a gel swollen with the resulting acetic acid ester and the alcohol used to effect the transesterification. 

Norrish and Smith [29] and later Tromsdorff et al. [30] described a polymerization of methyl methacrylate, the rate of which increased from a certain conversion. The number of monomers of similar behaviour was extended by methyl acrylate [31 ], butyl acrylate [32] and other acrylates [33] and methacrylates [34], and vinyl acetate. The effect was explained by the reduction of the termination rate caused by hindered macroradical mobil-ity in viscous medium it was called the gel effect, or the Norrish-Tromsdorff effect. The gel effect is clearly manifested in radical polymerizations of weakly transferring monomers in bulk. It is significant also in the presence of a good solvent. The gel effect is suppressed by the presence of poor solvents++ and by... 

A lot of papers have been published on the effect of solvent on free radical polymerization rate. Studies on this effect have greatly been stimulated by (1) Norrish-Trommsdorff effect, (2) Q, e-scheme in copolymerization, (3) Retardation of the polymerization rate of vinyl acetate, (4) radical complex. 

It was found by Burnett and Melville36 in 1947 that the radical polymerization of vinyl acetate was retarded in aromatic solvents. This retardation effect was confirmed by several researchers37-42. It is characterized by three features all of which cannot be simultaneously explained by the conventional kinetic scheme involving degradative chain transfer to solvent. (1) The rate of polymerization is markedly reduced in comparison with that in many aliphatic solvents. (2) The order with respect to initiator remains close to one-half over a wide range of initiator concentration. 

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. 

The polymerization of vinyl acetate in various alcohols has been suggested by many workers. Certainly when methanol is used as a solvent, the solvent concentration has a profound influence on the molecular weight of the polymer [67,99,103]. This effect has been attributed to the formation of acetaldehyde, a well-known chain-transfer agent, by a transesterification reaction involving the monomer [103] ... 

TABLE XV The Effect of Solvents on the Polymerization of Vinyl Acetate [96] ... 

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... 

Recently, highly syndiotactic polyvinyl acetate was obtained polymerization of vinyl acetate was carried out in fluoroalcohols [57]. l,l,l-Tri(trifluoromethyl) methanol is the most effective solvent, where syndiotacticity of the polymers in dyad polymerized at 20 and —78°C are 62.3 and 72.2%, respectively. 

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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