Recent progress in the use of iron-based complex pre-catalysts for ethylene reactivity is reviewed, illustrating the current state-of-the-art and the potential usefulness of such systems for delivering solely ethylene oligomerization or polymerization products. The problems associated with the industrial use of late transition metal complex pre-catalysts are generally regarded as catalyst deactivation and the formation of more products of lower molecular weight at elevated temperature. These problems have been addressed for iron-based complex pre-catalysts via the fine tuning of substituents of existing ligands and/or the design of new ligand sets. Results revealed that modified bis(imino)pyridyliron dichlorides were capable of operating at elevated temperatures, and were capable of delivering highly linear polyethylene. Other new models of iron complexes have achieved high activity for ethylene oligomerization and/or polymerization. Particularly successful has been the use of the 2-iminophenanthrolyliron pre-catalyst, which have now been utilized in a 500 tonne pilot plant.

Ethylene polymerization was performed using a series of 2-[1-(2,6-dibenzhydrylphenylimino)ethyl]-6-[1-(arylimino)ethyl]pyridyliron(II) chlorides with the activity in the range of 10⁷ g PE mol⁻¹ (Fe) h⁻¹, which is the highest observed in iron procatalysts at elevated reaction temperatures such as 80 °C in the presence of MMAO and 60 °C in the presence of MAO, without any trace of ethylene oligomerization.

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.