And it will be the machines we create and build to remove the plastics that it will start with. Probably some nano machine. We'll give them the simple job of "find and remove all micro plastics from air/soil/water" while we continue to use plastic. And eventually the little AI will come to the inevitable conclusion that the only way to achieve this goal is remove the cause of micro plastics aka humans.

And if you haven't played Universal Paperclips you should. Also read "I Have No Mouth and I Must Scream" by Harlan Ellison. Both work off the same concept as above and is the most common argument against the use of AI. Or at least making AI that can learn.