Isobutylene (2-Methylpropene, CAS: 115-11-7; UN 1055; Hazard Class 2.1 Flammable Gas) is a valuable olefin monomer from the C4 hydrocarbon chain. Two mainstream grades are widely available: 99.5% industrial grade and 99.9% polymer grade. Both can be supplied in 118L LPG-style cylinders with 50kg filling. However, they differ greatly in impurity control, reaction stability, and end-use suitability. Many buyers treat them as identical material, which may lead to polymerization failure, color deviation, and inconsistent product performance.