First, a polymer of formula (IV'), in which both R1' and R2' are an ethylene group, X' is general formula (III'), both R4' and R5' are a methyl group, Z' is a hydrogen atom and m' is 4, was prepared by adding a 1,7-dichloro-octamethyltetrasiloxane into a solution of 2,2'-thiodiethanol in pyridine and thereafter heating the mixture as described in Example 20.