Are these seedlings?
Planted specimen trees?
Either way... Strongly urge you to keep these valuable trees!
Fast growing attractive shade trees, butterfly host plants... What's not to love?
If you only intend to grow turf...
Taking out poplars is no big deal, certainly no need for chemicals.
Just shovel-dig... Insert shovel next to seedling, grab sapling and tug as you prise it out of dirt with shovel.