Polymeric materials that depolymerize into volatiles on command may function as vanishing substrates or packaging for transient electronics. Poly(vinyl -butyl carbonate sulfone) is known to afford low-boiling byproducts upon heat-activated degradation; however, the polymer is rather unstable, even to the degree of being difficult to process and handle. Understanding the origin of this instability is important for the development of robust materials capable of programmed self-destruction. In this work, we show that poly(vinyl -butyl carbonate sulfone)s thermally decompose via carbonate elimination as the rate-determining step, indicating that its thermal instability stems from the lability of the -butyl carbonate group. We further examined the effect of isomeric butyl carbonate side chains on the thermal degradation of poly(vinyl butyl carbonate sulfone)s and found that the degradation onsets range from 91 to 213 °C, yielding as little as 2.77 ± 0.53 wt % residue. Results from our findings will aid in the development of vanishing polymers with tunable thermal degradation.