Substitution Reactions of Poly (vinyl alcohol)

There are many practical uses for products from reactions of poly (vinyl alcohol). Among them are commercial preparations of poly (vinyl acetal) s formed through condensations with aldehydes. Two materials that are currently being marketed are poly (vinyl formal) and poly (vinyl butyral). The first one is formed from partially hydrolyzed poly (vinyl acetate) that is dissolved in aqueous acetic acid and excess formaldehyde. The mixture is heated, sulfuric acid is added, and the reaction is allowed to proceed at 70–90C for 6 h. Sulfuric acid is then neutralized and the formal precipitates out.

Two different industrial processes are used for preparations of the butyral. In both of them, acetate free poly(vinyl alcohol) is used. In the first one, 10% solutions of the starting material are treated with butyraldehyde and sulfuric acid. The mixtures are heated to 90C for 1½ h and the products precipitate. They are neutralized, washed, and dried. In the second one, poly(vinyl alcohol) is suspended in ethanol/ethyl acetate and butyraldehyde together with a strong mineral acid is added. The solutions are then neutralized. The butyrals separate out. They are neutralized and the resin are washed and dried. If poly(vinyl alcohol) films are reacted with formaldehyde in water containing salt and an acid catalyst (heterogeneous formalization), cross-linking occurs. The number of the cross-links increases with decreasing acid concentration and fixed amounts of formaldehyde and salt [257]. Direct reactions of poly (vinyl alcohol) with aldehydes in the Kornblum reaction result in formations of acetals that also contain residual hydroxyl group and often acetate groups. The acetate groups can be there from incomplete hydrolysis of the parent poly (vinyl acetate) that was used to form the poly (vinyl alcohol). Reactions of poly (vinyl alcohol) with ketones yield similar ketals. At present, no ketals are offered commercially. Alkyl etherification of poly (vinyl alcohol) occurs when the polymer is combined with n-alkyl halides in dimethyl sulfoxide combined with pyridine [258, 259]. It was suggested that the alkyl halides convert to aldehydes and acids and then act as intermediates in the dimethyl sulfoxide pyridine solution [258, 259]:

Many modifications of poly (vinyl alcohol) were carried out to form photosensitive materials. Thus, unsaturation was introduced into the pendant groups for photocross-linking. One example is a condensation with pyridinium and quinolinium salts [260]:

The material cyclodimerizes on exposure to light [261]. Schotten-Baumann esterifications of poly (vinyl alcohol) are used extensively in preparations of various derivatives. The reactions appear to proceed well when acid chlorides are employed in two phase systems [262]. The polymers are dissolved in water and the solutions are blended in 1:1:1 equal volume with NaOH solutions and cyclohexanone. They are then mixed thoroughly with solutions of the acid chlorides in mixtures of cyclohexanone and toluene. The reaction mixture is stirred vigorously for about 90 min at 5to5C to obtain the desired product. Metalation of poly (vinyl alcohol) is used to form ether derivatives [263]:

The Ritter reaction on poly (vinyl alcohol) yields soluble products. Only some of the hydroxyl groups, however, are converted to amide structures [264]:

Also, it is possible that neighboring group interactions may lead to cyclizations and formations of 1,3-oxazines [264].

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مواضيع ذات صلة

Block Copolyesters

Block Copolymers

Miscellaneous Graft Copolymerizations

Preparation of Graft Copolymers with Ionic Chain-Growth and Step-Growth Polymerization Reactions

Graft Copolymer Formation with the Aid of High-Energy Radiation

Photochemical Syntheses of Graft Copolymers

Initiations of Polymerizations from the Backbone of Polymers

Preparation of Graft Copolymers with the Aid of Macromonomers

Free-Radical Grafting Reactions to Polymers with Double Bonds

Free-Radical Grafting by Chain-Transferring Process

Vulcanization of Elastomers

Miscellaneous Exchange Reactions