I don't understand. I AM trying to go from copper to plastic. Are you
talking about the ones with the hose type, rotating fitting?

You are going from a female copper/brass to male plastic (if I read things
correctly.)

I was suggest that (if the situation permits) you should go from 1/2" copper
tubing to a female plastic adapter. The plastic "adaptor" could be a
transition to all plastic with glued joints (plastic glued joints are VERY
reliable when done right) or it could be an elbow or even a valve.

I used a 1/2" plastic valve when I needed to temporarily cap on 1/2" copper
tubing. When I reconnect, I will not have to shut off the water.

These adaptors (tubing to plastic) "work" as well even when there is water
present. You can't say the same for pipe dope, solder, or glue. Maybe
you can have some water flowing when working with teflon tape.