Synthesis of 4,4-bis(nonafluorobiphenyl-4-oxyphenyl)-bis(trifluoromethyl)methane and a ladder polyether with spirobisindane fragments on its base - I. M. Tkachenko; Ja. L. Kobzar; O. V. Shekera; V. V. Shevchenko (Reports of the National Academy of Sciences of Ukraine, 2015)