The methyl methacrylate (MMA) runaway situation in Garden Grove, CA, seems to have plateaued. The aerospace company whose storage tanks are in the news produces what it calls “transparencies”. This is their trade name for plexiglass aircraft canopies, wind shields and windows. The word “plexiglass” is in common usage but derives from the trademark “Plexiglass“. Its not to be confused with “Lexan” which is a polycarbonate.

It dawned on me that the MMA runaway situation will be self-limiting. As the runaway polymerization proceeds, the rate of heat evolution should taper off as the reaction rate slows due to consumption of MMA. Just simple kinetics. The runaway reaction converts liquid MMA to solid poly[methyl methacrylate], or just PMMA. The MMA is turning onto a solid brick of plastic. This slows down the rate of reaction which necessarily slows down the rate of heat release.

The upshot is that if the tanks are kept cool enough to prevent rupture, yet warm enough to allow the reaction to creep forward, the runaway is under control.

The very fact that this runaway happened at all in “normal” storage suggests that the MMA was insufficiently passivated with stabilizer. The usual stabilizers are typically some variety of phenolic additives. This would include BHT, BHA and other catechols and phenols chemically modified for organic solubility.

The polymerization of MMA occurs via a radical chain reaction mechanism. Stabilizers like the phenolics are able to release H-dot, or hydrogen radical, which interferes with the polymerization by terminating the free radicals propagating the reaction. Each H-dot combines with any other radical it comes in contact with and halts MMA participation in the reaction.

Below is a graphic from an article published on PubMed. Watch the dots, the dots are the radical electron. The upper reaction is chain propagation which is the desired reaction. See how the double line in the MMA structures changes to a single line? One of the two electrons in the second line is taken by an incoming free radical to form a new link in the growing polymer. The other electron is the leftover free radical which awaits a collision with another MMA molecule. This is how the polymer grows.

Let’s assume that the first reaction is the desired reaction and I’ll refer to it as “proper”. The critical feature of MMA is the double line. That is the location of the transformations in the polymerization. Each double line in the MMA has two ends- one end is the terminus and the other end is internal. The addition of the next MMA is desired to add its terminus of the growing end of the chain. This transformation leaves a radical at the internal end for further desired polymerization.

The second reaction shows how propagation can run afoul when the double line connects at the wrong position. The shape of this connection error- or kink- is different than that of the intended connection. The result is a polymer backbone linkage of 2 carbon atoms rather than the desired single carbon. This doesn’t happen just once but many times. The cumulative effect is that a chemically distinct polymer is produced that degrades or dilutes the desired physicochemical properties of the intended PMMA.

The third and lower reaction shows the undesired linkage reaction with both “properly” linked MMA and “improperly” linked chains to give a succeeding undesired linkage.

Note: “Desired” and “undesired” refer to intended connectivity and the unintended, kinked, connectivity. In reality, the desirability of any polymerization process is determined by the resulting set of physicochemical properties of the bulk polymer. Knobs are twisted, levers pulled and temperatures set in the plant to give the desired product with its combination of linkages.

I wonder if during the event that if a trickle of stabilizer were added, would it have made an observable difference? I assume that no agitation in the tank is available beyond convection currents, of course. it would be a large scale science experiment on an evolving and novel public emergency situation. Responders and, uh, everyone else, would disapprove.