It will never happen. (of course, now that I said it...
)
DHL would block such a merger or purchase.
"...In November 2001, Polar was acquired by Atlas Air Worldwide Holdings (AAWW), whose Atlas Air subsidiary is a provider of ACMI (aircraft, crew, maintenance and insurance) freighter leasing. Polar became the scheduled service provider for AAWW, while Atlas continued to supply Boeing 747 freighters on a wet-lease basis to major airlines.
In October 2006, it was announced that DHL Express would be acquiring a 49% stake in Polar. Polar is still majority-owned by Atlas Air Worldwide Holdings (51%), and has 736 employees (as of March 2007)..."
https://en.wikipedia.org/wiki/Polar_Air_Cargo