When did they get the patent? When was it set to expire?
It wasn’t so much that the patent would run out or that duPont was the sole vendor, it was the fact that the die had been cast and R-12 was about to be banned and that dupont was one of the first to develop an acceptable replacement, R-134A; the history of the invention, patenting and widespread use is covered well by the historic background part of the ozone story released by NOAA:
http://www.esrl.noaa.gov/gmd/hats/publictn/elkins/cfcs.html