Octamethyltetrasiloxane,D₄ and hexamethyldisiloxane,MM can be equilibrated by suitable catalyst:
n/4D₄+MM→MD_nM
With the same ratio of D₄ to MM,the products obtained by using concentrated sulfuric acid as catalyst have much lower average molecular weights and greater change of viscosity upon prolong heating and by action of alkali than those obtained by using 70% sulfuric acid.The reason for the difference in the properties of the products is presumably that,in the former,the polydimethylsiloxane molecules are terminated partially by SiOH,which gradually condenses to form SiOSi linkage with increase in molecular weight.The presence of SiOH group had been prow ed by Karl Fischer titration method.Using 70% sulfuric acid,much less SiOH group was found.