The tax-free treatment of the spin-offs Dow Chemical Co DOW.N and DuPont DD.N plan to carry out after they merge their businesses is a prime driver of the deal, potentially saving tens of billions of dollars, industry experts said. The $120 billion merger, announced last week, comes less than a month after drug maker Pfizer Inc PFE.N said it would use its $160 billion acquisition of Allergan Plc AGN.N as a way to cut its taxes. It underscores the growing use of mergers and acquisitions as a way to slash corporate America’s tax bill.