Reduction of the Ester Groups

Perhaps the most reported reactions of these polymers are reductions of the functional groups. Among them is the reaction with lithium aluminum hydride to reduce the ester groups. The success, however, depends upon the reaction medium. Poly (methyl methacrylate) can be reduced to poly (methallyl alcohol) in ether solvents [236]:

The results, however, are inconclusive, because combustion analyses fail to match the theoretical composition for poly (methallyl alcohol). It is impossible to tell to what extent the reduction takes place [236]. Inconclusive results are also obtained in similar reductions of poly (methyl acrylate) in mixtures of tetrahydrofuran and benzene. The product of such reduction is acetylated with acetic anhydride in pyridine [237] as follows:

Hydrolysis in water of the product of acetylation, followed by treatment with hot m-cresol, and subsequent extraction with hydrochloric acid to remove the suspended inorganic matter [237], yields a material that is still only soluble in pyridine and m-cresol. Somewhat similar results are obtained in reductions of high molecular weight poly(methyl acrylate) with lithium aluminum hydride [238] in tetrahydrofuran. The reaction yields a product that is only soluble in mixtures of hydrochloric acid with either methyl alcohol, dioxane, or tetrahydrofuran. The problem is apparently due to some residual aluminum that is hard to remove [239]. If, however, the reduction is carried out in a N-methylmorpholine solution, followed by addition of potassium tartrate, a pure product can be isolated [240]. N-methylmorpholine is a good solvent for reductions of various macromolecules with metal hydrides [236]. In addition, the solvent permits use of strong NaOH solutions to hydrolyze the addition complexes that form. Other polymers that can be reduced in it are those bearing nitrile, amide, imide, lactam, and oxime pendant groups. Reduction of polymethacrylonitrile, however, yields a product with only 70% of primary amine groups [241]. Complete reductions of pendant carbonyl groups with LiAlH₄ in solvents other than N methylmorpholine, however, were reported. Thus, a copolymer of methyl vinyl ketone with styrene was fully reduced in tetrahydrofuran [242]. Reductions with metal hydrides are often preliminary steps for additional reactions. For instance, a product of LiAlH₄ reduction of syndiotactic poly (methyl methacrylate) can be reacted with succinic anhydride and then converted to an amide [243]:

Substituted succinic anhydride can be used as well. When poly (dimethyl itaconate) is reduced with LiAlH₄ in THF, the product contains some ash, but 93% of the functional groups are reduced [239]:

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المزيد

مواضيع ذات صلة

Nucleophilic and Electrophilic Substitutions

Metalation Reactions

Nitration, Reduction, and Diazotization

Sulfonation Reactions

Reactions of Halomethylated Polymers

Chloromethylation Reactions

Reactions of Polystyrene

Substitution Reactions of Halogen-Bearing Polymers

Substitution Reactions of Saturated Polymeric Hydrocarbons

Cyclizations and Intramolecular Rearrangements

Isomerization Reactions

Epoxidation Reactions