Plastic (polymer) cases were tried for pistol ammunition about 40 years ago. I can't remember the brand but the cases were triangular, quite thick, the bullet conventional round. They were used in a specially chambered revolver by the same company. For whatever reason the idea never gained any traction, probably because it was an unfamiliar concept for traditionalists and promoted and sold by the inventor, not one of the majors like Smith and Wesson, Colt, or Federal who had their own turf to protect. It worked, it was expensive, I don't remember reading much negative comment but nobody got really excited about the concept either.
But the concept is alive and well with our modern plastic shotgun ammunition, this was controversial when it appeared in the 1950's, now it's normal.