I plan to use a polypropylene screw block inside a PVC extrusion. Will there be a reaction when the materials are in constant contact?

I've never heard of plastics reacting with each other.

I had soft plastic artificial fishing worms melt their way through the drawers in my tackle box.

I had soft plastic artificial fishing worms melt their way through the drawers in my tackle box.

Hmmmm... you learn something new every day. I've had chemistry class in the past, and I've known a lot of guys that fish and never heard of that. Maybe it's the right combination of plastics. Like the perfect storm.