Studies in ring‐opening polymerization. IV. Thermal polymerization of phenyl‐substituted 1,3‐dioxolan‐2,4‐diones

The effect of C‐5 phenyl substituents on the thermal decomposition of the 1,3‐dioxolan‐2,4‐dione ring has been examined. Unlike the dimethyl‐substituted ring, 5‐methyl‐5‐phenyl‐1,3‐dioxolan‐2,4‐dione decomposes smoothly in dry nonhydroxylic solvents to yield polymer and carbon dioxide. The introduction of a second C‐5 phenyl substituent produces a similar but more rapid decomposition, with the added complication of a competing ring fragmentation leading to ketone formation. An analogy is drawn between the observed behavior of the phenyl‐substituted 1,3‐dioxolan‐2,4‐diones and that of the previously studied 1,3,2‐dioxanthiolan‐4‐one 2‐oxides. These monomers therefore provide another example of the thermal polymerisation mechanism first observed with 5,5‐dimethyl‐1,3,2‐dioxathiolan‐4‐one 2‐oxides. In these reactions the rate‐determining step is the first‐order ring‐scission process leading to the formation of an α‐lactone intermediate. This intermediate then takes part in a very rapid chain‐growth process which governs the characteristics of the polymer formed.