Vinyls acetate, radical polymerization

When the conversion is larger than 25% a shoulder appears on the GPC molecular weight distribution at approximately twice that of the most probable Mn which is interpreted as resulting from hydrogen abstraction from the VAc polymer chain and radical coupling.The vinyl acetate radical polymerization at 333 K thus has inherent non-ideal behavior that limit the precision of the controlled radical polymerization. 

The Q-e Scheme. The magnitude of and T2 can frequentiy be correlated with stmctural effects, such as polar and resonance factors. For example, in the free-radical polymerization of vinyl acetate with styrene, both styrene and vinyl acetate radicals preferentially add styrene because of the formation of the resonance stabilized polystyrene radical. 

For the remaining three systems, styrene-vinyl acetate, vinyl acetate-vinyl chloride, and methyl acrylate-vinyl chloride, one reactivity ratio is greater than unity and the other is less than unity. They are therefore nonazeotropic. Furthermore, since both ri and 1/7 2 are either greater than or less than unity, both radicals prefer the same monomer. In other words, the same monomer—styrene, vinyl chloride, and methyl acrylate in the three systems, respectively—is more reactive than the other with respect to either radical. This preference is extreme in the styrene-vinyl acetate system where styrene is about fifty times as reactive as vinyl acetate toward the styrene radical the vinyl acetate radical prefers to add the styrene monomer by a factor of about one hundred as compared with addition of vinyl acetate. Hence polymerization of a mixture of similar amounts of styrene and vinyl acetate yields an initial product which is almost pure polystyrene. Only after most of the styrene has polymerized is a copolymer formed... 

The initiators used m vinyl acetate polymerizations are the familiar free-radical types, Buffers are frequently added to emulsion recipes. Vinyl acetate emulsion polymerization recipes are usually buffered to pH 4-5. The pH of most commercially available emulsions is 4-6. 

Vinyl acetate is polymerized commercially using free-radical polymerization in either methanol or. in some circumstances, ethanol. 

The results definitely prove our hypotheses in the kinetic model for vinyl acetate emulsion polymerization (10), that vinyl radical, CH2=C-0Ac, is the major monomer radical formed and is a stable radical which reinitiates relatively slowly compared to the propagation step. 

Chemistry. Vinyl acetate is polymerized commercially using free-radical polymerization in either methanol or, in some circumstances, ethanol. Suitable thermal initiators include organic peroxides such as butyl peroxypivalate, di(2-ethylhexyl) peroxydicarbonate, butyl peroxyneodecanoate, benzoyl peroxide, and lauroyl peroxide, and diazo compounds such as 2,2 -azobisisobutyronitrile (205—215). The temperatures of commercial interest range from... 

In vinyl acetate emulsion polymerization radical desorption is important, i.e. with this monomer 0 and therefore m 0. Typical values of m and a lie in the intervals 10"1 - 10 3 and 10 3 - 10 6 respectively in the early stages of polymerization. 

A linear polymer is one in which each repealing unit is linked only to two others. Polystyrene (1-1), poly(methyl methacrylate) (1-34), and poly(4-methyl pentene-1) (1-35) are called linear polymers although they contain short branches which arc part of the monomer structure. By conirast, when vinyl acetate is polymerized by free-radical initiation, the polymer produced contains branches which were not present in the monomers. Some repeating units in these species are linked to three or four other monomer residues, and such polymers would therefore be classified as branched. 

Atom transfer to form a stable radical which does not reinitiate polymerization, as in the reaction of poly(vinyl acetate) radical and diphenylamine to yield a diphenyl nitrogen radical which will not add vinyl acetate but may terminate a macroradical. 

Vinyl acetate was polymerized in a free-radical reaction. The initial monomer concentration was 1 mol/liter and its concentration after I h was 0.85 mol/liter. Chloroform was present as a chain transfer agent, with concentrations 0.01 mol/liter at time zero and 0.007 mol/liter after I h. What is the chain transfer constant C in this case ... 

We will consider the MWD in two simple cases. The first is when chain transfer is sufficiently rapid to ensure that all other chain-stopping events can be ignored. In such a situation, whereas the compartmentalized nature of the reaction may affect the rate of initiation of new chains, it will not affect the lifetime distributions of the chains once they are formed. The MWD may then be found from the bulk formulas, provided only that the average number of free radicals per particle, is known. Such an approach has been used by Friis et al. (1974) to calculate the MWD evolved in a vinyl acetate emukion polymerization. These authors included in addition the mechanisms of terminal bond polymerization and of transfer to polymer (both of which cause broadening). The formulas required for the in corporation of these mechanisms could be taken from bulk theory. 

Recently, Ugelstad et al. l969i proposed a semiempirtcal rate coefficient for radical desorption in vinyl chloride emulsion polymerization. On the other hand, Nomura et al. (1971, 1976) have derived a rate coefficient for radical desorption theoretically with both stochastic and deterministic approaches and have successfully applied it to vinyl acetate emulsion polymerization. They also pointed out that radical desorption from the particles and micelles played an important role in micellar particle formation, Fiiis et al. 1973 also derived the rate coefficient for radical desorption in a different way. Lift et al. (1981) discussed in more detail the chemical reactions incorporated in the physical process of radical desorption in the emulsion polymerization of vinyl acetate. 

On the other hand, in vinyl acetate emulsion polymerization the value of p. was 1.2 X 10 (Nomura et al., 1976), This value is also about 10 times greater than that predicted hy the diffusion theory. The reason for this may be that radicals have greater difficulty m entering micelles than polymer panicles, or it may be that radicals, having entered a micelle, may escape from the micelle too rapidly to cause initiation, because the micelle has too small a volume. Both factors will decrease the apparjsnt value of k, and hence increase the value of c. Therefore, e can be regarded as a factor that represents the radical capture efficiency of a micelle relative to a particle. ... 

The function of the chelator is to complex the ferrous ion and thus limit the concentration of free iron. Redox systems appear very versatile, permitting polymerization at ambient temperatures and the possibility of control of the rate of radical initiation versus polymerization time. This would thus permit control of heal generation and the minimization of reaction time. The use of the redox system ammonium persulfate (2 mmol) together with sodium pyrosulfite (Na S Oj 2.5 mmol) together with copper sulfate (0.002 mmol) buffered with sodium bicarbonate in I liter of water form an effective redox system for vinyl acetate emulsion polymerization. The reaction was started at 25 C and run nonisothermally to 70 C. The time to almost complete conversion was 30 min (Warson, 1976 and Edelhauser, 1975). 

Araki et at. (1967, 1969) carried out a more systematic study of the kinetics and other features of the y-iniliated emulsion polymerization of vinyl acetate using sodium lauryl sulfate as the emulsifier. This system had been thoroughly investigated with potassium persulfate as the initiator (Litt et cL. 1960,1970). Some post ei cts have been observed with vinyl acetate, particularly above 50% conversion (Friis, 1973 Sunardi, 1979). These effects had been used by Allen cr at. (1960,1962) for the possible synthesis of block and graft polymers and will be described later in this chapter. The half-life of the radicals in a vinyl acetate latex polymerization was determinad by Hummel et at. (1969) as 0.8 min at 53.8% conversion. Araki et fll. (1967, 1969) determined all the normal rate dependencies and included some seeded latex studies. Their results and those of other investigators are summarized in Table II together with those found with potassium persulfate initiation and those predicted by the Smith-Ewart Case 2 theory. The... 

In addition, with these monomers the substituent not only preferentially complexes the electrophile but may even reduce the nucleophilicity of the double bond by electron attraction. Acrylates (and similarly vinyl acetate) thus do not polymerize cationically. (It may be noted that vinyl acetate is also not polymerized by anionic initiators as they attack the acetate linkage. Vinyl acetate is polymerized only by free radicals.)... 

Since the solubility of vinyl acetate is 2.1% at 50°C and 3.5% at 70°C [15], deviations from the Smith-Ewart treatment are not entirely surprising. The water solubility of vinyl acetate was one of the significant factors in the deviation from the conventional theory of emulsion polymerization. Another factor is the reactivity of the vinyl acetate radicals toward other materials present in the system such as the surfactants. 

Vinyl acetate is polymerized in dispersion form using various initiators. Exanples of ionic initiators commonly used for free-radical emulsion polymerizations are ammonium, sodium or potassium persulfate. Topical nonionic hydrophobic initiators include 2,2 -azobis(isobutyronitrile) (AIBN) and benzoyl peroxide. Water-soluble nonionic initiators such as tertiary-butyl hydroperoxide are also employed. The initiator 4,4 -azobis(4-cyanovaleric acid) in its acid state is oil soluble, while neutralization causes it to become water soluble providing for further diversity in initiators. 

Figure 5. Polymerization mechanism and VAc/linseed oil copolymer structure (in the example, the triglyceride contains one linoleic and two linolenic chains). The poly(vinyl acetate) radical intermediate (I) reacts with a double bond of the triglyceride (II) and produces the radical intermediate (III), which after other reaction steps is transformed into the product (IV).

Vinyl acetate is polymerized free radically in bulk, emulsion, or suspension. Bulk polymerization occurs at the boiling temperature of the monomer (72.5 C at 1 bar), and yields highly branched polymer because of chain transfer via the ester groups (see also Section 20.4.3). Commercially, the polymerization is taken to a specific yield and the residual monomer is removed by thin-layer evaporation. Alternatively, continuous polymerization can be carried out in a tower. But this method only produces moderate degrees of polymerization since the tower process requires that the polymer should flow and the flow temperature should lie below the decomposition tempera-... 

An example of the macromonomer method is the preparation of graft copolymers of PEO by the free radical polymerization of vinyl acetate in the presence of PEO. The growing vinyl acetate radical would abstract a hydrogen atom from the PEO chain, creating a radical at this site. The newly created radical would then polymerize vinyl acetate to form a branch on the chain. The rather randomly occurring chain transfer reaction would form a graft copolymer of PEO and poly(vinyl acetate). 

The practically most important copolymer is made from ethene and propene. Titanium- and vanadium-based catalysts have been used to synthesize copolymers that have a prevailingly random, block, or alternating structure. Only with Ziegler or single site catalyst, longer-chain a-olefins can be used as comonomer (e.g., propene, 1-butene, 1-hexene, 1-octene). In contrast to this, by radical high-pressure polymerization it is also possible to incorporate functional monomers (e.g., carbon monoxide, vinyl acetate). The polymerization could be carried out in solution, slurry, or gas phase. It is generally accepted [173] that the best way to compare monomer reactivities in a particular polymerization reaction is by comparison of their reactivity ratios in copolymerization reactions. 

When vinyl acetate is polymerized to degrees of conversion above about 30%, appreciable branching occurs because of chain transfer. Since a growing radical may abstract a hydrogen atom from either the chain or the methyl group, two kinds of branches are possible ... 

For the styrene-vinyl acetate-MA polymerization very significant deviations from the classical concept of free-radical copolymerization were observed.Due to almost identical rates of reaction of MA with styrene and vinyl acetate, there is strict alternation of the monomers along the polymer chain and the copolymer composition regularly corresponds to a 1 1 1 ratio. 

Kahrs and Zimmermann " have shown that if vinyl acetate is polymerized in the presence of poly(ethylene oxide) a graft copolymer results. Other examples have been given since then Whether the radicals formed by transfer to the backbone initiate the polymerization of the monomer or recombine with growing macroradicals is not clearly established. 

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