PURPOSE: To obtain the titled composition most suitable for antithrombotic materials having the selective permeability, by polymerizing octamethylcyclotetrasiloxane in the presence of bis(3-aminopropyl)tetramethyldisiloxane having amino groups at both ends.

CONSTITUTION: Bis(3-aminopropyl)tetramethyldisiloxane having amino groups at both ends is mixed with octamethylcyclotetrasiloxane, and a strongly basic catalyst, e.g. a solution prepared by dissolving 1.0g KOH dried at 70°C under reduced pressure for 5hr in isopropanol purified by the distillation, and concentrating the solution to 100ml total volume, is added to the resultant mixture to carry out the anionic polymerization at about 135°C for 24hr under stirring and give the aimed polydimethylsiloxane having amino groups at both ends.