To unlock the Polyatomic mastery challenge, you need to complete 51 Platinum mastery challenges. (At launch, there are 51 weapons in the game, but this can be completed later with any added DLC weapons, so you’ll only need 51 weapons total to unlock the Polyatomic challenges)
The Polyatomic mastery challenge works just like the Platinum challenge. Once you’ve finished the Platinum challenges for a specific weapon class, head back to the Gunsmith and check your new Polyatomic challenges. These challenges will vary from class to class, and this is the last set of challenges to complete to reach the coveted Orion camo.