Looks to me to be a transition fitting. It's first soldered to the copper, then you attach the pex. The plastic pipe should have some markings on it to identify the maker and the type. They're usually repeated all along the entire length of the roll of tubing. It might be Wirsbo/Uphonor, as that's not a crimp connection (Wirsbo/Uphonor use an expansion system with a reinforcing ring). I don't deal with the stuff enough to be sure, though.