The Fire Chief was talking about the phase change occurring inside the MMA tank. These are the critical temps for that to happen.
86°F to 113°F: This is the Induction Phase.
The inhibitor is being consumed, and the monomer is slowly converting into short-chain polymers (oligomers). The liquid will gradually become more viscous—taking on a syrupy consistency. It isn’t “solid” yet, but the heat-transfer and agitation systems will begin to fail as the liquid thickens.
113°F to 149°F: This is the Runaway Phase.
Once the heat of the reaction exceeds the rate at which the vessel can dump heat to the environment, the temperature spikes exponentially. The monomer rapidly converts to high-molecular-weight PMMA (poly-methyl methacrylate). At this stage, the material will transition from a viscous syrup to a hard, rigid plastic “plug” within minutes—often within the confines of the tank or piping
Source: Methacrylate Esters, Safe Handling Guide (2019)