Is it safe to use polypropylene pockets to store CD/DVD recordable media for long durations of time?
Probably not the best choice as they may scratch the discs. Things that happen accidentally can be quite undesirable if the damage accumulates. I'd say storing them in jewel cases, or even in spindles (but out of direct sunlight) is better.
Put them in to jewelcases, 1 disc jewelcases and only 1 in each, store them up in a solid steellocker.