High performance biodegradable polymers (e.g., aliphatic polyesters) that can display properties that rival polyolefins are seen as future high demand materials for a variety of applications. Importantly, these types of polymers can be accessed by the ring opening polymerization (ROP) of monomers that can, in some cases, be derived from cheap biorenewable resources highlighting the sustainability of the process. Carefully designed metal complexes that can act as catalysts for such transformations have emerged as useful tools to achieve this goal. This review is concerned with recent progress in the use of well-defined metal complexes based on group 1, 2 and 13 metals to mediate the formation of aliphatic polyesters with a focus on the role played by the auxiliary ligand on influencing catalytic efficiency, controllability, molecular weight as well as stereoselectivity. More specifically, we report on developments in the design, synthesis and structure of such main group metal species supported by various multidentate ligands including bidentate, tridentate and tetradentate families bearing nitrogen, oxygen, sulfur, selenium or phosphorus donor atoms and their catalytic applications in the ROP of cyclic esters. In addition, the fundamental coordination chemistry of the metal complexes is discussed alongside variations in catalytic performance.

Both cage and noncage structures of (Fe2O3)n (n=2–6 and 10) clusters are studied using density functional theory. All the cage structures are stable without imaginary vibrational frequency but the global minima are the noncage clusters for most cases. Our results show that oxidation of Fe4On (n<6) clusters by O2 at room temperature is exothermic, while oxidation of n≥6 clusters is endothermic. This is in qualitative agreement with an experimental observation that only Fe4On+ (n≥6) clusters are produced in a laser vaporization source under saturated O2 growth conditions. Since (Fe2O3)n clusters have high stability and different structural and bonding properties from those of the bulk Fe2O3, they may serve as good models for predicting or interpreting novel properties of Fe2O3 nanomaterials.