**1. Introduction**

First discovered by J. Paul Hogan and Robert L. Banks in 1951, polypropylene (PP) is polymerized from propylene out of crude oil and is the most widely used commodity thermoplastic by volume [1]. Over the past 70 years, significant progress has been achieved to manufacture and commercialize PP (**Figure 1**) [2, 3]. The most recent milestone in the field of polypropylene is from PureCycle Technology, where the waste carpet has been successfully purified into clear, odorless ultrapure recycled polypropylene (UPRP) resin [4]. Through not fully commercialized yet, the innovation opens up a new venue of recycling processed PP into raw materials as a resin.

As one of the cheapest plastics with great processability, chemical resistance, and moisture barriers, PP with different tacticity found various downstream applications in textile, automotive, cosmetics, and consumer packaging. In 2016, 26% of polymer demand in the world by volume was from PP (**Figure 2**) [5].
