separates into a gaseous fraction at ordinary temperature and pressure to yield a liquid portion as the fourth fraction which contains decreased amount of acidity and hc the thus obtained fourth fraction can be blended with the second fraction in an optional proportion to produce a preparation having an mdi content or viscosity which conforms to respective use the preparation obtained by mixing the second and the fourth fractions has acidity of 0 005 to 0 02 and hc of 0 05 to 0 15 which can be varied by the mixing ratio hence the thus obtained preparation exhibits higher reactivity as compared with the preparation obtained by using the third fraction without heat treatment in the next step the solid obtained in step b can be divided by gradual warming into an initially molten portion the fifth fraction and principally molten portion the sixth fraction for example the solid is gradually warmed to 40 u00b0 c to obtain the fifth fraction as a liquid product the residual solid is further warmed to about 55 u00b0 c to obtain the sixth fraction in the form of a molten product it is preferred for maintaining high purity of the fifth fraction to increase the temperature to 40 u00b0 c as slowly as possible this is because the solid itself is a mixture of crystalline materials the term 34 as slowly as possible 34 is a matter of degree for example the solid obtained in step of b is gradually warmed over about 5 hours from the temperature of the solid itself to 40 u00b0 c the fifth fraction obtained by an ideal treatment is a mixture composed of 2 0 to 7 0 of 2.