Vinyl acetate common production reaction

Probably the most widely used industrial emulsion or dispersion adhesives are those based on poly(vinyl acetate), commonly referred to as PVA. These product are normally manufactured by emulsion polymerization whereby, basically, vinyl acetate monomer is emulsified in water with a suitable colloid-emulsifier system, such as poly(vinyl alcohol) and sodium lauryl sulfate, and, with the use of water soluble initiator such as potassium persulfate, is polymerized. The polymerization takes place over a period of four hours at 70°C. Because the reaction is exothermic, provisions must be made for cooling when the batch size exceeds a few liters. The presence of surfactants (emulsifiers) and water-soluble protective colloids facilitates the process resulting in a stable dispersion of discrete polymer particles in the aqueous phase. 

Among the most commonly applied chiral moiety for nitrones (2) is the N-a-methylbenzyl substituent (Scheme 12.6) (18-25). The nitrones 8 with this substituent are available from 1 -phenethylamine, and the substituent has the advantage that it can be removed from the resulting isoxazolidine products 9 by hydrogeno-lysis. This type of 1,3-dipole has been applied in numerous 1,3-dipolar cycloadditions with alkenes such as styrenes (21,23), allyl alcohol (24), vinyl acetate (20), crotonates (22,25), and in a recent report with ketene acetals (26) for the synthesis of natural products. Reviewing these reactions shows that the a-methylbenzyl group... 

Some monomers are also polymerized by a cationic mechanism in a series of steps not too unlike those of anionic chain-growth. Initiators are often Lewis acids such as AICI3. The polymerization is not quite as straightforward as anionic, because for one thing cationic intermediates are subject to more side reactions. Common monomers that undergo cationic polymerization include styrene, isobutylene, and vinyl acetate. Some commercial products... 

Epoxides derived from vinyl halides and vinyl acetates tend to rearrange readily on heating. McDonald and his coworkers uncovered several mechanistically intriguing examples of thermal rearrangements involving a-chloro epoxides. In some instances the epoxides could not be isolated, but presumably are reactive intermediates, e.g. in the peroxy acid reaction of chlorostilbenes. A common intermediate was proposed to account for the formation of the same (chlorine-migrated) product from both geometrical isomers of (51 equation 22). 

Discovered more than 70 years ago, hydroformylation is nowadays one of the most important reactions in the chemical industry because aldehydes can be transformed to many other products. In the enantioselective version, rhodium/ diphosphorus ligand complexes are the most important catalytic precursors, although cobalt and platinum complexes have also been widely used. For these systems, the active species are pentacoordinated trigonal-bipyramidal rhodium hydride complexes, [HRh(P-P)(CO)2]. In those complexes, the coordination mode of the bidentate ligand (equatorial-equatorial or equatorial-apical) is an important parameter to explain the outcome of the process. The most common substrates of enantioselective hydroformylation are styrenes followed by vinyl acetate and allyl cyanide. With these substrates, mixtures of the branched (b, chiral) and linear (1, not chiral) aldehydes are usually obtained. In addition, some hydrogenation of the double bond is often observed. Therefore, chemo- and regioselectivity are prerequisites to enan-tioselectivity and all of them must be controlled. An additional eomplieation is that chiral aldehydes are prone to racemise in the presenee of rhodium spe-... 

Reaction temperature is about 75-80°C and reaction time about 2 hours. The reaction is highly exothermic and in order to achieve better control and a product with smaller particle size it is common practice to polymerize firstly only a portion of the monomer and then add the remainder slowly over 2—4 hours. The buffer is added to the system to minimize hydrolysis of the vinyl acetate. As mentioned previously, the resulting latex is normally used as such and the solid polymer is not isolated. 

Poly(vinyl acetate) is readily hydrolyzed by treating an alcoholic solution with aqueous acid or alkali. Acid hydrolysis results in traces of acid in the poly(vinyl alcohol) which are difficult to remove and which lead to instability of the polymer alkaline hydrolysis results in contamination of the product by a large amount of sodium acetate which is also difficult to remove and which has little intrinsic value. These difficulties are avoided if poly(vinyl alcohol) is prepared from poly(vinyl acetate) by alcoholysis using a small amount of base as catalyst. The reaction is commonly carried out by treating poly(vinyl acetate) with methanol in the presence of sodium methoxide ... 

Vinyl acetate polymerizes by a free-radical mechanism. Free radicals generated by the decomposition of organic peroxides such as benzoyl or hydrogen peroxide or of inorganic per salts such as potassium or ammonium persulfate are commonly used to initiate polymerization. Reactions ordinarily are accomplished at temperatures above room temperature. Other techniques of polymerization have been used to make novel products low temperature redox polymerization, irradiation, and ionic catalysis. 

Figure 12.11 shows the pyrograms of vinyl paints from two monochromes by the Italian artist Piero Manzoni. The two paints are clearly different in composition acetic acid (peak 1) and benzene (peak 2) are present as common markers of the PVAc binder in both cases, but sample (a) contains dibutyl phthalate (peak 6) as external plasticizer. Peak 5 was recognized as bis(2-methylpropyl)-phthalate which is formed from dibutylphthalate isomerization, while butyl acetate (peak 3) and butyl benzoate (peak 4) are secondary products of recombination reactions occurring during the pyrolysis. Sample (b), however,... 

Many such activated acyl derivatives have been developed, and the field has been reviewed [7-9]. The most commonly used irreversible acyl donors are various types of vinyl esters. During the acylation of the enzyme, vinyl alcohols are liberated, which rapidly tautomerize to non-nucleophilic carbonyl compounds (Scheme 4.5). The acyl-enzyme then reacts with the racemic nucleophile (e.g., an alcohol or amine). Many vinyl esters and isopropenyl acetate are commercially available, and others can be made from vinyl and isopropenyl acetate by Lewis acid- or palladium-catalyzed reactions with acids [10-12] or from transition metal-catalyzed additions to acetylenes [13-15]. If ethoxyacetylene is used in such reactions, R1 in the resulting acyl donor will be OEt (Scheme 4.5), and hence the end product from the acyl donor leaving group will be the innocuous ethyl acetate [16]. Other frequently used acylation agents that act as more or less irreversible acyl donors are the easily prepared 2,2,2-trifluoro- and 2,2,2-trichloro-ethyl esters [17-23]. Less frequently used are oxime esters and cyanomethyl ester [7]. S-ethyl thioesters such as the thiooctanoate has also been used, and here the ethanethiol formed is allowed to evaporate to displace the equilibrium [24, 25]. Some anhydrides can also serve as irreversible acyl donors. 

While alcohol oxidations have been the most common metal promoted reactions involving molecular oxygen, a number of other metal catalyzed oxidations of potential synthetic interest have been reported. Supported palladium catalysts are comparable to many soluble palladium catalysts in promoting the selective oxidations of alkenes and aromatics. 2-Butene was oxidized primarily to crotonic acid over Pd/C in water but methyl vinyl ketone and crotonaldehyde were also formed in significant amounts. When this oxidation was run in acetic acid the allyl acetates were the major products, particularly when a Pd/Al203 catalyst... 

In an extension of their earlier work, Ojima and coworkers have described analogous reactions using vinyl silyl ketene acetals (160 equation 19). 9 xhe carbon-carbon bond forming step in these reactions occurs exclusively at the terminal carbon of the vinyl silyl ketene acetal (160) to give cyclized 5,6-dihy-dro-2-pyridones (162), methyl 5-amino-2-pentenoates (163), or mixtures. Steric and electronic factors play an important role in the formation of cyclized products, which are favored by the lack of an a-sub-stituent (R ) and the presence of an N-alkyl group on the imine (R ). Formation of (162) and (163) proceeds by a common metalated intermediate, since in reactions that form cyclized products (162), quenching at -50 °C leads to acyclic products. Yields are good to excellent when corrected for recovered imine (161), but a 100% excess of vinyl silyl ketene acetal (160) must be employed. It is curious to note that these reactions require much lower initial temperatures (-100 C) than those of silyl ketene acetals. 

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