Studies in ring-opening polymerization. III. 5-methyl-5-propyl and 5-methyl-5-isopropyl-1,3,2-dioxathiolan-4-one 2-oxide

5-Methyl-5-propyl-1,3,2-dioxathiolan-4-one 2-oxide (MPAS) and 5-methyl-5-isopropyl-1,3,2-dioxathiolan-4-one 2-oxide (MiPAS), which are isomers of the previously studied 5,5-diethyl-1,3,2-dioxathiolan-4-one 2-oxide (DEAS), have been synthesized and their polymerizability compared with that of the last compound. The two unsymmetrically substituted monomers polymerize by a mechanism which is substantially identical to that of their symmetrically substituted counterpart. In dry nonhydroxylic solvents the rate-determining process is the primary scission of the ring, which takes place with elimination of sulfur dioxide and concurrent ring contraction to form an α-lactone intermediate. In this reaction, the parent acid, produced by reaction of the monomer with adventitious traces of moisture, acts as the initiating species. The resultant polymers are all hydroxyl/carboxyl-terminated, but, whereas those derived from the two unsymmetrically substituted monomers are amorphous and readily soluble in a variety of organic solvents, those derived from the diethyl-substituted ring have been shown to be highly crystalline materials which dissolve in very few solvents. The relative polymerization rates are illustrated by the first-order rate constants for decomposition in nitrobenzene at 90°C: DEAS, 20.1 × 10^?5 sec^?1; MiPAS, 11.0 × 10^?5 sec^?1; MPAS, 9.7 × 10^?5 sec^?1. The role of the substituents in determining the magnitude of these constants is discussed in terms of both the Thorpe-Ingold effect and electron donation at C-5.