Abstract: The present invention provides a process for producing 4,4?-dihydroxydiphenylsulfone of very high purity. In particular, the present invention provides a process for producing 4,4?-dihydroxydiphenylsulfone of high purity comprising the steps of subjecting phenol in combination with a sulfonating agent or phenolsulfonic acid to a dehydration reaction in the presence of an aromatic nonpolar solvent while suspending the resulting dihydroxydiphenylsulfone therein, mixing the reaction suspension with a polar solvent to at least partially dissolve the dihydroxydiphenylsulfone, and precipitating 4,4?-dihydroxydiphenylsulfone.

Abstract: The present invention provides a process for producing dihydroxydiphenylsulfone wherein trihydroxytriphenylsulfone and coloring impurities are effectively removed without altering the isomeric composition of dihydroxydiphenylsulfone. In particular, the present invention provides a process for producing dihydroxydiphenylsulfone comprising the steps of dissolveing or suspending in an aqueous solvent crude dihydroxydiphenylsulfone containing trihydroxytriphenylsulfone, adjusting the pH to 5–7, optionally cooling, and separating the crystalline dihydroxydiphenylsulfone thus precipitated.

Abstract: A thermal recording material includes a thermosensitive coloring layer, which is made by applying a dispersion liquid, in which the developer and a coloring dye are dispersed, onto a supporting base, and then drying the base. The developer contains 4,4?-dihydroxydiphenyl sulfone as a major component; and 2,4?dihydroxydiphenyl sulfone, the 4,4?-dihydroxydiphenyl sulfone and the 2,4?-dihydroxydiphenyl sulfone being contained at a weight ratio greater than 90:10 and no more than 100:0, and the developer containing phenolsulfonic acid phenyl ester no more than 1.5% by weight with respect to a sum quantity of the 4,4?-dihydroxydiphenyl sulfone and the 2,4?-dihydroxydiphenyl sulfone. The present invention thus provides a thermal recording material causing less surface texture fogging and is superior in preservation ability, and also provides a developer suitably used as a component of the thermal recording material.

Abstract: The present invention provides a process for producing 4,4?-dihydroxydiphenylsulfone of very high purity. In particular, the present invention provides a process for producing 4,4?-dihydroxydiphenylsulfone of high purity comprising the steps of subjecting phenol in combination with a sulfonating agent or phenolsulfonic acid to a dehydration reaction in the presence of an aromatic nonpolar solvent while suspending the resulting dihydroxydiphenylsulfone therein, mixing the reaction suspension with a polar solvent to at least partially dissolve the dihydroxydiphenylsulfone, and precipitating 4,4?-dihydroxydiphenylsulfone.

Abstract: The present invention provides a process for producing dihydroxydiphenylsulfone wherein trihydroxytriphenylsulfone and coloring impurities are effectively removed without altering the isomeric composition of dihydroxydiphenylsulfone. In particular, the present invention provides a process for producing dihydroxydiphenylsulfone comprising the steps of dissolveing or suspending in an aqueous solvent crude dihydroxydiphenylsulfone containing trihydroxytriphenylsulfone, adjusting the pH to 5-7, optionally cooling, and separating the crystalline dihydroxydiphenylsulfone thus precipitated.

Abstract: A process for preparing 4,4'-dihydroxydiphenylsulfone, the process including heating 4,4'-dihydroxydiphenylsulfone and 2,4'-dihydroxydiphenylsulfone in the presence of an acid catalyst to isomerize the 2,4'-dihydroxydiphenylsulfone into 4,4'-dihydroxydiphenylsulfone, the process being characterized in that a suspension of 4,4'-dihydroxydiphenylsulfone and 2,4'-dihydroxydiphenylsulfone in a liquid dispersing medium is heated for isomerization while the liquid portion is distilled off or a crystal mixture of 4,4'-dihydroxydiphenylsulfone and 2,4'-dihydroxydiphenylsulfone is heated for isomerization to produce 4,4'-dihydroxydiphenylsulfone in the form of crystal powder.

Abstract: The present invention provides a process for preparing 4,4'-dihydroxydiphenylsulfone which comprises subjecting a phenol and a sulfonating agent to dehydration reaction in the presence of mesitylene as a reaction medium while the produced 4,4'-dihydroxydiphenylsulfone is suspended in the mesitylene, and a process for preparing 4,4'-dihydroxydiphenylsulfone, characterized in that after the foregoing dehydration reaction, the 4,4'-dihydroxydiphenysulfone obtained as suspended in the mesitylene and/or retained in a liquid-free solid form is subjected to reaction for isomerization of 2,4'-dihydroxydiphenylsulfone produced as a by-product into 4,4'-dihydroxydiphenylsulfone with heating at a temperature of not lower than that for isomerization of 2,4'-dihydroxydiphenylsulfone into 4,4'-dihydroxydiphenylsulfone.

Abstract: The process of the present invention is a process for preparing 4,4'-dihydroxydiphenyl sulfone by subjecting phenol, and a sulfonating agent or phenolsulfonic acid to a dehydration reaction in the presence of a solvent, subsequently maintaining the reaction system at a temperature of between 100.degree. to 200.degree. C. and removing the solvent from the reaction system to cause the dehydration reaction to further proceed while allowing 4,4'-dihydroxydiphenyl sulfone to separate out and to subject dissolved 2,4'-dihydroxydiphenyl sulfone formed as a by-product to an isomerization reaction for converting the by-product to 4,4'-dihydroxydiphenyl sulfone, and eventually removing the solvent and the unreacted phenol from the system substantially entirely to complete the isomerization reaction, the process being characterized in that an aromatic sulfonic acid represented by the formula ##STR1## wherein X is a halogen atom, n is 0, 1 or 2, and m is 1 of 2 is caused to be present in the reaction system.