Two of the biggest corporate borrowers in America are preparing to buy back billions of dollars in debt as they reorganize in an environment that includes reduced federal tax deductions on interest payments, and uncertain demand for industrial products during the U.S. trade fight with China. The DuPont Co., which is scheduled to be spun off by DowDuPont in the spring, said Tuesday that it is offering to buy back $6.2 billion of its debt, at premiums of $30 per $1,000 outstanding, on a range of notes that had been scheduled to mature from March 2019 through 2038. DowDuPont expects to pay for the DuPont Co.