Over the weekend, a methyl methacrylate tank in Garden Grove, California experienced a runaway polymerization. The danger was contained, but the incident lays bare why storing bulk reactive monomers demands serious care. MMA is a feedstock for PMMA, the material behind Plexiglas, Perspex, and Lucite.

MMA and related acrylates share a carbon-carbon double bond that's highly reactive toward free radicals. When a radical attacks one molecule, it spawns another that attacks the next. The polymerization releases heat, which accelerates the process. That thermal feedback loop makes bulk storage hazardous.

Commercial MMA contains inhibitors like hydroquinones and BHT that need at least 5% oxygen to work. Tanks are painted white to limit solar heating, and a temperature rise of even one or two degrees per hour signals trouble. Emergency response uses phenothiazine to halt the chain reaction.

Phenothiazine kills the chain without oxygen but may ruin the contents. If the tank is overpressurized, cooling with sprayed water keeps polymerization from rupturing it. The Garden Grove tank is a total loss, but containment prevented a toxic flammable mess across suburban LA.