Reactions of Poly vinyl acetate

In addition to its use as a plastic, poly(vinyl acetate) is used to produce two polymers that cannot be synthesized directly since their monomers do not exist. Poly(vinyl alcohol) is obtained by alcoholysis of poly(vinyl acetate) with methanol  

Both acids and bases catalyze the reaction, but base is usually employed because of the more rapid rates and freedom from side reactions. 

Reaction of poly(vinyl alcohol) with an aldehyde yields the corresponding poly(vinyl acetal)  

The reaction is usually carried out with an acid catalyst. Acetal formation does not proceed to completion because of isolation of single hydroxyl groups between pairs of acetal structures. The two most important acetals are the formal and butyral (R = H and C3H7, respectively). The applications of poly(vinyl alcohol) and its acetals are described in Sec. 3-14c-2. 

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An industrial example of acidolysis is the reaction of poly(vinyl acetate) with butyric acid to form poly(vinyl butyrate). Often a butyric acid—methanol... 

Transesterification has a number of important commercial uses. Methyl esters of fatty acids are produced from fats and oils. Transesterification is also the basis of recycling technology to break up poly(ethylene terephthalate) [25038-59-9] to monomer for reuse (29) (see Recycling, plastics). Because vinyl alcohol does not exist, poly(vinyl alcohol) [9002-89-5] is produced commercially by base-cataly2ed alcoholysis of poly(vinyl acetate) [9003-20-7] (see Vinyl polymers). An industrial example of acidolysis is the reaction of poly(vinyl acetate) with butyric acid to form poly(vinyl butyrate) [24991-31-9]. 

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. 

Analogously, poly(vinyl ketals) can be prepared from ketones, but since poly(vinyl ketals) are not commercially important, they are not discussed here. The acetalization reaction strongly favors formation of the 1,3-dioxane ring, which is a characteristic feature of this class of resins. The first of this family, poly(vinyl ben2al), was prepared in 1924 by the reaction of poly(vinyl alcohol) with ben2aldehyde in concentrated hydrochloric acid (2). Although many members of this class of resins have been made since then, only poly(vinyl formal) [9003-33-2] (PVF) and poly(vinyl butyral) [63148-65-2] (PVB) continue to be made in significant commercial quantities. 

Manufacture. PVBs are manufactured by a variety of two-stage heterogeneous processes. In one of these an alcohol solution of poly(vinyl acetate) and an acid catalyst are heated to 60—80°C with strong agitation. As the poly(vinyl alcohol) forms, it precipitates from solution (77). Ethyl acetate, the principle by-product, is stripped off and sold. The precipitated poly(vinyl alcohol) is washed to remove by-products and excess acid. The poly(vinyl alcohol) is then suspended in a mixture of ethyl alcohol, butyraldehyde, and mineral acid at temperatures above 70°C. As the reaction approaches completion the reactants go into solution. When the reaction is complete, the catalyst is neutralized and the PVB is precipitated from solution with water, washed, centrifuged, and dried. Resin from this process has very low residual vinyl acetate and very low levels of gel from intermolecular acetalization. 

N. J. Earhart, The Grafting Reactions ofPoly(vinyl alcohol) During the Emulsion Copolymerisyation of Poly(vinyl acetate—co-butyl acrylate), Ph.D. dissertation. 

In the slurry process, the hydrolysis is accompHshed using two stirred-tank reactors in series (266). Solutions of poly(vinyl acetate) and catalyst are continuously added to the first reactor, where 90% of the conversion occur, and then transferred to the second reactor to reach hiU conversion. Alkyl acetate and alcohols are continuously distilled off in order to drive the equiUbrium of the reaction. The resulting poly(vinyl alcohol) particles tend to be very fine, resulting in a dusty product. The process has been modified to yield a less dusty product through process changes (267,268) and the use of additives (269). Partially hydroly2ed products having a narrow hydrolysis distribution cannot be prepared by this method. 

The products are amorphous resins whose rigidity and softening point depend on the aldehyde used. Poly(vinyl butyral), with the larger side chain, is softer than poly(vinyl formal). Since the reaction between the aldehyde and the hydroxyl groups occurs at random, some hydroxyl groups become isolated and are incapable of reaction. A poly(vinyl acetal) molecule will thus contain ... 

In a typical process, 100 parts of poly(vinyl acetate) are added to a mixture of 200 parts acetic acid and 70 parts water, which has been warmed to about 70°C, and stirred to complete solution. Sixty parts of 40% formalin and 4 parts sulphuric acid (catalyst) are added and reaction is carried out for 24 hours at 1Q°C. Water is added to the mixture with rapid agitation to precipitate the granules, which are then washed free from acid and dried. 

The PVF is made by acidic reaction between poly(vinyl alcohol) (PVA) and formaldehyde. The poly(vinyl alcohol) is, in turn, made by hydrolysis of poly(vinyl acetate) or transesterification of poly(vinyl acetate). Thus, residual alcohol and ester functionality is usually present. Cure reportedly occurs through reaction of phenolic polymer hydroxyls with the residual hydroxyls of the PVA [199]. The ester residues are observed to reduce bond strength in PVF-based systems [199]. This does not necessarily extend to PVF-P adhesives. PVF is stable in strong alkali, so participation of the acetals in curing is probably unimportant in most situations involving resoles. PVF is physically compatible with many phenolic resins. 

Production of hydrogels by chemical conversion uses chemical reactions to convert one type of gel to another. This can involve the conversion of a nonhydrogel to a hydrogel or the conversion of one type of hydrogel to another. Examples of the former include the hydrolysis of polyacrylonitrile to polyacrylamide or of poly(vinyl acetate) to poly(vinyl alcohol) an example of the latter... 

PVA Formation Reaction. Poly(vinyl alcohol) is itself a modified polymer being made by the alcoholysis of poly(vinyl acetate) under acid or base catalysis as shown in Equation 1 (6.7). This polymer cannot be made by a direct polymerization because the vinyl alcohol monomer only exists in the tautomeric form of acetaldehyde. This saponification reaction can also be run on vinyl acetate copolymers and this affords a means of making vinyl alcohol copolymers. The homopolymer is water soluble and softens with decomposition at about 200°C while the properties of the copolymers would vary widely. Poly(vinyl alcohol) has been widely utilized in polymer modification because ... 

Complexes with Iodine. One of the simplest "reactions" of poly(vinyl alcohol) is the formation of a blue complex with iodine. This complex formation, which requires the presence of KI, has been studied extensively by many workers (26-31). This complex also forms with partially hydrolyzed poly(vinyl acetates) (26) and is known to be affected by the 1,2-glycol content and the isotacticity of the polymer both of which tend to reduce complex formation (31). The complex also depends on the molecular weight of the poly(vinyl alcohol) and the iodine concentration. 

In the above-mentioned example of the polymer-analogous saponification of poly(vinyl acetate) the reactant and the product differ in their properties, for example, in their solubility however, both compounds have the same average degree of polymerization. The poly(vinyl alcohol) obtained by saponification can, in principle, be esterified back to poly(vinyl acetate) with the original molecular weight the reacetylated polymer then has the same properties as the original material. The viscosity number may be used to check whether in fact any chain scission has occurred during the reaction sequence of saponification and reacetylation (see Example 5-1). 

The base-catalyzed alcoholysis of poly(vinyl acetate) is quite rapid and is thought to be autocatalytic. Under usual reaction conditions, the reaction goes to approximately 90% completion. To reach 100% conversion requires specialized conditions. Achieving partial hydrolysis is difficult because of the rapidity of the process. 

It has been known for some time [see Ref. (176) for earlier work] that if poly(vinyl alcohol), produced by hydrolysis of poly(vinyl acetate) is reacetylated, the PVAc so obtained has a lower MW than the original PVAc prior to hydrolysis, though the MW of the material is not lowered any further by subsequent cycles of hydrolysis and reacetylation. Various explanations had been advanced for this phenomenon Wheeler explained it as a consequence of the presence of branches joined to the main chain through ester linkages which would be broken on hydrolysis and not re-formed on reacetylation. These branches were ascribed to chain transfer reactions with acetate groups, either in the polymer, or in monomer molecule subsequently polymerized at their double bonds. Transfer reactions by attack on hydrogen atoms other than those in... 

The synthesis of poly(vinyl acetals) (252) represents another example of generating a heterocycle, in this case the 1,3-dioxane nucleus, by application of a polymer modification reaction. Generally, the polymer modified is poly(vinyl alcohol) (180) or one of its copolymers. The 1,3-dioxane ring is generated (Scheme 122) by an acid-catalyzed acetalization reaction with an aldehyde, although ketones have also been reacted. A review (71MI11102) is available covering synthesis, properties and applications of the two most common and industrially important poly(vinyl acetals), poly(vinyl butyral) and poly(vinyl formal), as well as many other functional aldehydes that have been attached. 

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