Master of Chaos and Order: Opposite Microstructures of PCL-co-PGA-co-PLA Accessible by a Single Catalyst**

One catalyst, two reaction set-ups, three monomers and unlimited macromolecular microstructural designs: The iron guanidine complex [FeCl2(TMG5NMe(2)asme)] (1) polymerizes lactide faster than the industrially used Sn(Oct)(2) and shows high activity towards glycolide and epsilon-caprolactone. Its distinguished features enable the synthesis of both block and random-like copolymers in the melt by a simple change of the polymerization set-up. Sequential addition of monomers yields highly ordered block copolymers including the symmetrical PLA-b-PGA-b-PCL-b-PGA-b-PLA pentablock copolymers, while polymerizations of monomer mixtures feature enhanced transesterifications and pave the way to di- and terpolymers with highly dispersed repeating unit distributions. A robust catalyst active under industrially applicable conditions and producing copolymers with desired microstructures is a major step towards biocompatible polymers with tailor-made properties as alternatives for traditional plastics on the way towards a sustainable, circular material flow.

Automated summary

This article introduces a iron guanidine complex catalyst for the synthesis of copolymers with different microstructures. The catalyst exhibits high activity and fast polymerization of lactide, glycolide, and epsilon-caprolactone. By changing the polymerization set-up or sequentially adding monomers, both block and random-like copolymers, as well as copolymers with highly dispersed repeating unit distributions, can be obtained. This research represents an important step towards biocompatible polymers with tailor-made properties, contributing to a sustainable and circular material flow.