Lab 12 — Polymer Chemistry
Polymerizing Polypropylene
Addition polymerization: the C=C double bond of propylene (CH₂=CH–CH₃) opens and links monomer after monomer into a chain. The backbone becomes a repeating –CH₂–CH(CH₃)–, and the arrangement of the methyl side groups — the tacticity — decides the material. All on one side, and it crystallizes into the strong everyday plastic (isotactic). Strictly alternating also crystallizes (syndiotactic). Random, and it can't crystallize at all — you get a soft gum (atactic). The Ziegler–Natta catalysts that control this arrangement earned a Nobel Prize.
Pick a tacticity and the methyl groups line up differently — same formula, different material
From the exact same composition (just propylene), the methyl arrangement alone turns it into a hard plastic or a soft gum — the essence of polymer science. Isotactic PP crystallizes thanks to its regular order: it melts around 160 °C and, at 0.90 g/cm³, floats on water. It fills containers, fibers, and car parts. Being addition polymerization, no byproduct forms — every atom ends up in the chain (100% atom economy).