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

Abstract

Abstract5‐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.