Vinyl alcohol results

If hydration of acetylene followed the same pattern as hydration of alkenes, we would expect addition of H— and —OH to the triple bond to yield the structure that we would call vinyl alcohol. But all attempts to prepare vinyl alcohol result— like hydration of acetylene—in the formation of acetaldehyde. 

A comprehensive study of the photodegradation of poly(vinyl esters) has been published by Buchanan and McGill. " Photo-oxidative decomposition of poly(vinyl alcohol) results in the production of carbonyl functions. " " It has been observed that the formation of Ni complexes increased the photostability of polybenzoxazoles, ... 

Many of these reactions are reversible, and for the stronger nucleophiles they usually proceed the fastest. Typical examples are the addition of ammonia, amines, phosphines, and bisulfite. Alkaline conditions permit the addition of mercaptans, sulfides, ketones, nitroalkanes, and alcohols to acrylamide. Good examples of alcohol reactions are those involving polymeric alcohols such as poly(vinyl alcohol), cellulose, and starch. The alkaline conditions employed with these reactions result in partial hydrolysis of the amide, yielding mixed carbamojdethyl and carboxyethyl products. 

Vinyl alcohol does not exist as a monomer, but Herrmann and Haehnel (1) were able to obtain the desired product poly(vinyl alcohol) [9002-89-5] (PVA), by polymerizing vinyl acetate and then hydrolyzing the resultant poly(vinyl acetate). This process is employed for the commercial production of PVA even now. The principal concern of the discoverers was development of a suture for surgical operations the fiber then obtained was not suited for clothing use (2). 

The selectivity of pervaporation membranes varies considerably and has a critical effect on the overall separation obtained. The range of results that can be obtained for the same solutions and different membranes is illustrated in Figure 41 for the separation of acetone from water using two types of membrane (89). The figure shows the concentration of acetone in the permeate as a function of the concentration in the feed. The two membranes shown have dramatically different properties. The siUcone mbber membrane removes acetone selectively, whereas the cross-linked poly(vinyl alcohol) (PVA) membrane removes water selectively. This difference occurs because siUcone mbber is hydrophobic and mbbery, thus permeates the acetone preferentially. PVA, on the other hand, is hydrophilic and glassy, thus permeates the small hydrophilic water molecules preferentially. 

Figure 5 illustrates the type of encapsulation process shown in Figure 4a when the core material is a water-immiscible Hquid. Reactant X, a multihmctional acid chloride, isocyanate, or combination of these reactants, is dissolved in the core material. The resulting mixture is emulsified in an aqueous phase that contains an emulsifier such as partially hydroly2ed poly(vinyl alcohol) or a lignosulfonate. Reactant Y, a multihmctional amine or combination of amines such as ethylenediamine, hexamethylenediamine, or triethylenetetramine, is added to the aqueous phase thereby initiating interfacial polymerisation and formation of a capsule shell. If reactant X is an acid chloride, base is added to the aqueous phase in order to act as an acid scavenger. 

Chain transfer also occurs to the emulsifying agents, leading to their permanent iacorporation iato 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 polymerisation 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 polymerisation (91). Some of the initiators such as lauryl peroxide are also chain-transfer agents and lower the molecular weight of the product. 

Poly(vinyl alcohol) [9002-89-5] was discovered through the addition of alkaU to a clear alcohoHc solution of poly(vinyl acetate), which resulted in the ivory-colored poly(vinyl alcohol) (4). The same discovery has been made by studying the reversible transformation between poly(vinyl alcohol) and poly(vinyl acetate) via esterification and saponification (5). The first scientific reports on poly(vinyl alcohol) were pubUshed in 1927 (6,7). 

The excellent chemical resistance and physical properties of PVA resins have resulted in broad industrial use. The polymer is an excellent adhesive and possesses solvent-, oil-, and grease-resistant properties matched by few other polymers. Poly(vinyl alcohol) films exhibit high tensile strength, abrasion resistance, and oxygen barrier properties which, under dry conditions, are superior to those of any other known polymer. The polymer s low surface tension provides for excellent emulsification and protective coUoid properties. 

Ethylene oxide reacts with poly(vinyl alcohol) under normal ethoxylation conditions (135—142). The resulting products have properties that make them usehil as cold-water-soluble films. 

The presence of catalyst residues, such as alkali hydroxide or alkali acetate, a by-product of the hydrolysis reaction, is known to decrease the thermal stability of poly(vinyl alcohol). Transforming these compounds into mote inert compounds and removal through washing are both methods that have been pursued. The use of mineral acids such as sulfuric acid (258), phosphoric acid (259), and OfXv o-phosphotic acid (260) has been reported as means for achieving increased thermal stability of the resulting poly(vinyl alcohol). 

Commercial Hydrolysis Process. The process of converting poly(vinyl acetate) to poly(vinyl alcohol) on a commercial scale is compHcated 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 polymeri2ation 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. 

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. 

Drying of the poly(vinyl alcohol) is critical to both the color and solubiHty of the final product. Excessive drying temperatures result in high product color and an increase in the crystallinity, which in turn reduces the solubiHty of the product. Drying is initially subjected to a flash regime, where the solvent not contained within the particles is flashed off. This first phase is foUowed by a period where the rate is controUed by the diffusion rate of solvent from the poly(vinyl alcohol) particles. Because the diffusion rate falls as the material dries, complete drying is not practical. The polymer is therefore generally sold at a specification of 95% soHds. 

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). 

Poly(ethylene-i (9-vinyl alcohol) is made by saponification of ethylene—vinyl acetate copolymers. The properties of these materials depend on the amount of vinyl alcohol present in the copolymer. High vinyl alcohol content results in more hydrophilic materials possessing higher densities, stiffness, and moduh. They are used commercially as barrier resins for packaging. Important producers include Du Pont and EVALCA (74) (see Barrier polymers). 

In contrast to the hydrolysis of prochiral esters performed in aqueous solutions, the enzymatic acylation of prochiral diols is usually carried out in an inert organic solvent such as hexane, ether, toluene, or ethyl acetate. In order to increase the reaction rate and the degree of conversion, activated esters such as vinyl carboxylates are often used as acylating agents. The vinyl alcohol formed as a result of transesterification tautomerizes to acetaldehyde, making the reaction practically irreversible. The presence of a bulky substituent in the 2-position helps the enzyme to discriminate between enantiotopic faces as a result the enzymatic acylation of prochiral 2-benzoxy-l,3-propanediol (34) proceeds with excellent selectivity (ee > 96%) (49). In the case of the 2-methyl substituted diol (33) the selectivity is only moderate (50). 

Commerical grades of EVOH typically have vinyl alcohol contents in the range 56-71%, but in contrast to the corresponding EVA materials these copolymers are crystalline. Furthermore, an increase in the vinyl alcohol content results in an increase in such properties as crystalline melting point, tensile strength and tensile modulus together with a decrease in oxygen permeability. This is a reflection of the fact that the ethylene and vinyl alcohol units in the chain are essentially isomorphous (see Sections 4.4 and 14.3.1). 

Almog et al. [80] studied the dispersion polymerization of styrene in different alcohols as the continuous medium by using AIBN and vinyl alcohol-vinyl acetate copolymer as the initiator and the stabilizer, respectively. Their results showed that the final particle size decreased with the alcohol type according to the following order ... 

---