I agree with Maudland: if we were familiar with the names of all the plastics and chemicals surrounding us, we'd have little room in our brains for anything else!
I don't think polypropylene is as obscure as some of the plastics in our lives, but I'm not really surprised that some people don't know the word. In the Corpus of Contemporary American English the word is more frequently used in academic journals than any other type of publication. And although it appears fairly frequently in magazines, many of these are special interest ones - e.g. medical, business, popular science, boating, hiking. In this forum there was one previous occurrence of the word, in a word request (so far unanswered) from ridethetalk: "LAYERABLE – layerable clothing consists of tank tops, skivvies, jumpers, t-shirts as well as polypropylene underwear".
I'll change the word to rare and remove it as a puzzle seed word. In practice this means we won't be seeing it again.