Poly(tetramethyleneoxide) contg. soft segments (60% by mass, mol. mass 1000) was converted in reaction with 4,4'-methylenebis(phenyl isocyanate) (6% excess) and 1,4-butanediol at 120 degrees C for 30 min under pressure of 10 Pa to a pre-polymer annealed then to a multiblock elastomer at 60-120 degrees C. Phase sepn. of resulting polyurethanes was studied by Fourier-transform IR spectroscopy, while the mol. mass was detd. by intrinsic viscosity measurements. The increase in annealing temp. resulted in raise in phase sepn. and mol. mass, whilst no effect on the polymer d. was obsd.