Vinyl acetate monomer, polymerization

A batch reactor of length 2.5 m and diameter 5.046 m is filled with vinyl acetate monomer. Polymerization is carried out isothermaUy at 50°C. The reactor is jacketed for heat removal. What is the temperature of the coolant ... 

Buffers are frequently added to emulsion recipes and serve two main purposes. The rate of hydrolysis of vinyl acetate and some comonomers is pH-sensitive. Hydrolysis of monomer produces acetic acid, which can affect the initiator, and acetaldehyde which as a chain-transfer agent may lower the molecular weight of the polymer undesirably. The rates of decomposition of some initiators are affected by pH and the buffer is added to stabilize those rates, since decomposition of the initiator frequently changes the pH in an unbuffered system. Vinyl acetate emulsion polymerization recipes are usually buffered to pH 4—5, eg, with phosphate or acetate, but buffering at neutral pH with bicarbonate also gives excellent results. The pH of most commercially available emulsions is 4—6. 

Mechanisms. Because of its considerable industrial importance as well as its intrinsic interest, emulsion polymerization of vinyl acetate in the presence of surfactants has been extensively studied (75—77). The Smith-Ewart theory, which describes emulsion polymerization of monomers such as styrene, does not apply to vinyl acetate. Reasons for this are the substantial water solubiUty of vinyl acetate monomer, and the different reactivities of the vinyl acetate and styrene radicals the chain transfer to monomer is much higher for vinyl acetate. The kinetics of the polymerization of vinyl acetate has been studied and mechanisms have been proposed (78—82). 

Partially hydrolyzed poly(vinyl alcohol) grades are preferred because they have a hydrophobic /hydrophilic balance that make them uniquely suited for emulsion polymerization. The compatibUity of the residual acetate units with the poly(vinyl acetate) latex particles partly explains the observed stabilization effect. The amount of PVA employed is normally 4—10% on the weight of vinyl acetate monomer. The viscosity of the resulting latex increases with increasing molecular weight and decreasing hydrolysis of the PVA (318). 

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... 

Erbil, H.Y., 2000, Vinyl Acetate Emulsion Polymerization and Copolymerization with Acrylic Monomers, CRC, Boca-Raton, FL. 

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. 

Poly(vinyl alcohol) used to manufacture the poly(vinyl acetal)s is made from poly(vinyl acetate) homopolymer (see Vinyl polymers, vinyl alcohol polymers Vinyl POLYMERS, VINYL ACETATE POLYMERS). Hydrolysis of poly(vinyl acetate) homopolymer produces a polyol with predominantly 1,3-glycol units. The polyol also contains up to 2 wt % 1,2-glycol units that come from head-to-head bonding during the polymerization of vinyl acetate monomer. Poly(vinyl acetate) hydrolysis is seldom complete, and for some applications, not desired. For example, commercial PVF resins may contain up to 13 wt % unhydrolyzed poly(vinyl acetate). Residual vinyl acetate units on the polymer help improve resin solubility and processibility (15). On the other hand, the poly (vinyl alcohol) preferred for commercial PVB resins has less than 3 wt % residual poly (vinyl acetate) units on the polymer chain. 

Chain transfer also occurs to the emulsifying agents, leading to their permanent incorporation into the product. Chain transfer to aldehydes, which may be formed as a result of the hydrolysis of the vinyl acetate monomer, tends to lower the molecular weight and slow the polymerization rate because of the lower activity of the radical that is formed. Thus, the presence of acetaldehyde condensates as a poly (vinyl alcohol) impurity strongly retards polymerization (91). Some of the initiators such as lauryl peroxide are also chain-transfer agents andlower the molecular weight of the product. 

Vinyl acetate is polymerized in aqueous emulsion and used widely in surface coating and in adhesives. Copolymerized with vinyl esters of branched carboxylic acids and small quantities of acrylic acid, it gives paint latices of excellent performance characteristics. G. C. Vegter found that a coagulum-free latex of very low residual monomer content can be produced from a mixture of an anionic and a nonionic emulsifier according to a specific operating procedure. The freeze/thaw stability of polymeric latices has been investigated by H. Naidus and R. Hanzes. 

Both the poly (vinyl acetate)-poly (vinyl alcohol) and styrene-shellac examples of colloid participation in the polymerization require much investigation especially with respect to whether grafting to the colloid occurs in aqueous solutions or at the particle surface. The former would appear more likely for the relatively water soluble vinyl acetate monomer, while the latter should be favored for the less soluble styrene. In any case, both would result in identical particle stabilization. 

The copolymer composition in miniemulsion copolymerization of vinyl acetate and butyl acrylate during the initial 70% conversion was found to be less rich in vinyl acetate monomer units [34]. Miniemulsion polymerization also allowed the synthesis of particles in which butyl acrylate and a PMMA macromonomer [83, 84] or styrene and a PMMA macromonomer [85] were copolymerized. The macromonomer acts as compatibilizing agent for the preparation of core/shell PBA/PMMA particles. The degree of phase separation between the two polymers in the composite particles is affected by the amount of macromonomer used in the seed latex preparation. 

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. 

This type of result is expected for low-conversion VA polymerizations because of the high transfer reaction rate with vinyl acetate monomer. 

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. 

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 ... 

Suspension polymerization is designed to combine the advantages of both the bulk and solution polymerization techniques. It is one of the extensively employed techniques in the mass production of vinyl and related polymers. Suspension polymerization (also referred to as bead or pearl polymerization) is carried out by suspending the monomer as droplets by efficient agitation in a large mass (continuous phase) of nonsolvent, commonly referred to as the dispersion or. suspension medium. Water is invariably used as the suspension medium for all water insoluble monomers because of the many advantages that go with it. Styrene, methyl methacrylate, vinyl chloride, and vinyl acetate are polymerized by the suspension... 

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.)... 

Figure 1 shows the typical steps in the manufacture of suspension (solution type) resins. The vinyl chloride and vinyl acetate monomers are "charged" (added) to a pressure vessel. Water and suspending agent, etc., are added, the actual polymerization being carried out under conditions of controlled pressure and temperature. 

VINYL ACETATE or VINYL ACETATE HQ or VINYL ACETATE MONOMER (108-05-4) C4HSO2 Forms explosive mixture with air [explosion limits in air (vol %) 2.6 to 13.4 fiashpoint 18°F/-7°C autoignition temp 756°F/402°C Fire Rating 3]. Polymerizes readily if not inhibited elevated temperatures and the influence of light, air, oxygen, water, or peroxides initiates violent reaction. Must be stabilized (hydroquinone or diphenylamine has been recommended) to prevent polymerization. Violent reaction with strong oxidizers, non-oxidizing mineral acids, 2-aminoethanol, butyl acrylate ... 

Emulsion Copolymerizations. Due to the good copolymerizability of VEC with vinyl ester monomers, it seemed likely that VEC could be incorporated into a vinyl acetate/butyl acrylate latex. First, it was important to determine if VEC is prone to hydrolysis in the acidic medium used for vinyl acetate emulsion polymerization. As a check, a single experiment was carried out using an acetic acid-sodium acetate buffer at pH=4 and heating for 4 hours at 80°C. In this experiment, 6.1% of the VEC was hydrolyzed to the 3-butene-1,2-diol. Since VEC is only soluble in water up to 3.3 %, it is expected that most of the VEC will be in the oil phase during the emulsion polymerization and that only a small amount will be hydrolyzed. 

Among the observable facts it was found that there is no significant effect of the concentration of emulsifier on this system. Therefore, the implication is that the polymerization initially takes place exclusively in the aqueous phase [136]. The resulting polymer particle precipitates as it forms [134]. In this case we may assume, that only a microscopic phase-separation takes place. The polymer particles which form adsorb emulsifier fiom the aqueous environment and remain dispersed. Then the particles may absorb more monomer somewhat in the manner called for by the Smith-Ewart theory. Of course, other dissolved vinyl acetate monomer molecules may continue to be polymerized in aqueous solution, thus accounting for the increase in the number of particles as the polymerization proceeds to high conversion. The classical Smith-Ewart treatment states that the number of particles is determined by the surfactant to monomer ratio and, in effect remains constant throughout the process. 

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