# Metal-Size Influence in Iso-Selective Lactide Polymerization

## TL;DR

Researchers found that lutetium-based initiators can efficiently produce iso-selective polylactide, highlighting the importance of metal choice in controlling polymer stereochemistry.

## Contribution

The study introduces the first highly iso-selective lutetium initiators for lactide polymerization and demonstrates metal size's influence on stereocontrol.

## Key findings

- Phosphasalen lutetium ethoxide complex achieves high iso-selectivity (Pi=0.81–0.84) and excellent polymerization control.
- Lanthanum derivative shows moderate heteroselectivity (Ps=0.74), indicating metal center choice affects stereocontrol.
- Polymerization rates correlate inversely with metallic covalent radius (La>Y>Lu).

## Abstract

Iso-selective initiators for the ring-opening polymerization (ROP) of rac-lactide are rare outside of Group 13. We describe the first examples of highly iso-selective lutetium initiators. The phosphasalen lutetium ethoxide complex shows excellent iso-selectivity, with a Pi value of 0.81–0.84 at 298 K, excellent rates, and high degrees of polymerization control. Conversely, the corresponding La derivative exhibits moderate heteroselectivity (Ps=0.74, 298 K). Thus, the choice of metal center is shown to be crucial in determining the level and mode of stereocontrol. The relative order of rates for the series of complexes is inversely related to metallic covalent radius: that is, La>Y>Lu.