The present invention relates to pure polyurethanes containing aliphatic and/or cycloaliphatic diisocyanates, macrodiols having a molecular weight of from 500 to 10,000 and chain extenders having a molecular weight of from 62 to 400, obtainable by reacting aliphatic and/or cycloaliphatic diisocyanates with a macrodiol to give an NCO-containing preadduct, where from 1.5 to 22 mol of diisocyanate are employed per mole of macrodiol, and the chain extension of the preadduct is carried out by means of low-molecular-weight aliphatic and/or cycloaliphatic diols, optionally further diisocyanate - with retention of the abovementioned NCO/OH ratio - is added as chain extender, where the preadduct formation is carried out at 90-150@C and the chain extension is carried out at temperatures of 120-230@C, with the proviso that said reaction conditions are maintained at least until a melt viscosity of at least 2,500 mPas, measured at 120@C, has been reached, and the polyaddition is subsequently continued to completion at any desired time, if necessary at elevated temperatures of up to 240@C. The polyurethanes are preferably used in medical technology.
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