Polyesters of hydroxybenzoic acids

Abstract

INTRODUCTIONOiie of the first man-made condensation polymers was prepared from p-hydroxybenzoic acid by Fisher' in 1910. The method of preparation, however, involved a step-wise condensation to a polymer of only four units. Several investigators have subsequently reported the preparation of polyesters from the isomeric hydroxybenzoic Only one of these investigators3 was successful in preparing high molecular weight polymers containing only the hydroxybenzoic repeat unit. These polymers were sufficiently high in molecular weight to produce satisfactory fibers and films.In our study, techniques used in the original work were improved, and the polymers were more fully evaluated. The purpose of this paper is to describe the method of preparation and certain physical and chemical properties of the polymers from m-and phydroxybenzoic acids. I. PREPARATION OF POLYMERS A. Starting MaterialsThe materials used in the polymerization were macetoxybenzoic and p-acetoxybenzoic acids. The melting point of the meta isomer was 130.5-131.5"C. and that of the para isomer was 178-179°C. The isomers were prepared by acetylating the corresponding hydroxybenzoic acids and were purified by crystallization. B. Method of PolymerizationIt is difficult to obtain high molecular weight polymers by direct self-esterification of hydroxybenzoic acids. An acid-interchange reaction involving the acetoxy derivative is faster and leads to higher polymers. This reaction proceeds according to eq. (1).The acetic acid formed is displaced by the aromatic carboxy group, and elimination of the acetic acid results in tthe formation of a high polymer. This reaction proceeds without a catalyst, but higher molecular weight polymers are formed and a faster reaction results when a catalyst such as magnesium is used. The reaction is preferably carried out a t reduced pressure to aid in the removal of acetic acid.In a typical experiment, 5.5 g. of m-acetoxybenzoic acid and 4.5 g. of p-acetoxybenzoic acid were placed in a 100-ml., round-bottomed flask provided with an inlet and an outlet for nitrogen, a motordriven glass stirrer, and an attachment for applying vacuum. The flask was heated by a temperaturecontrolled metal bath. A small chip of magnesium (about 0.01% on the basis of the amount of monomers) was added to the flask. The contents were melted and heated at 220°C. in a nitrogen atmosphere.The pressure was lowered to about 60 mm. while nitrogen was admitted slowly, and acetic acid was distilled out. As the melt viscosity of the polymer increased, heat was applied to the bath until a final temperature of 300°C. was reached. The pressure was then reduced to 0.2 mm. In one hour or less the melt viscosity was so high that stirring was no longer effective. The inherent viscosity at this point was approximately 0.5. Inherent viscosities of the polymers were determined with the use of a 60/40 phenol/tetrachloroethane mixture a t a concentration of 0.23 g./100 ml.An additional increase in molecular weight could be obtained by continuing the polymerization in the so...

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