Amides as Non-polymerizable Catalytic Adjuncts Enable the Ring-Opening Polymerization of Lactide With Ferrous Acetate Under Mild Conditions

Toufik Naolou,Andreas Lendlein,Axel T Neffe

doi:10.3389/fchem.2019.00346

Abstract

Sn-based catalysts are effective in the ring-opening polymerization (ROP) but are toxic. Fe(OAc)2 used as an alternative catalyst is suitable for the ROP of lactide only at higher temperatures (>170°C), associated with racemization. In the ROP of ester and amide group containing morpholinediones with Fe(OAc)2 to polydepsipeptides at 135°C, ester bonds were selectively opened. Here, it was hypothesized that ROP of lactones is possible with Fe(OAc)2 when amides are present in the reactions mixture as Fe-ligands could increase the solubility and activity of the metal catalytic center. The ROP of lactide in the melt with Fe(OAc)2 is possible at temperatures as low as 105°C, in the presence of N-ethylacetamide or N-methylbenzamide as non-polymerizable catalytic adjuncts (NPCA), with high conversion (up to 99 mol%) and yield (up to 88 mol%). Polydispersities of polylactide decreased with decreasing reaction temperature to ≤ 1.1. NMR as well as polarimetric studies showed that no racemization occurred at reaction temperatures ≤145°C. A kinetic study demonstrated a living chain-growth mechanism. MALDI analysis revealed that no side reactions (e.g., cyclization) occurred, though transesterification took place.

Highlights

Fe(II) and Fe(III) compounds have been proposed as a less toxic alternative to Sn-based catalysts in the ring-opening polymerization (ROP) of lactones, such as dilactide (Gibson et al, 2002; Wang et al, 2005; Chen et al, 2006; Hege and Schiller, 2014; Geng et al, 2016)
We recently showed that in the case of morpholinediones, Fe(OAc)2 was effective in the ROP by selectively opening the ester bond, while the amide bond was preserved (Naolou et al, 2016)
While the amount of non-polymerizable amides as catalyst adjuncts (NPCA) did have an influence on the catalytic ROP at 105◦C as the lowest investigated reaction temperature, N-ethylacetamide as NPCA resulted in more effective polymerization compared to N-methylbenzamide, even if the latter was used in higher quantities

Summary

Introduction

Fe(II) and Fe(III) compounds have been proposed as a less toxic alternative to Sn-based catalysts in the ROP of lactones, such as dilactide (Gibson et al, 2002; Wang et al, 2005; Chen et al, 2006; Hege and Schiller, 2014; Geng et al, 2016). The catalysts may promote transesterification of oligomers and polymers (Figure 1C), which is typically observed at high temperatures and/or high conversions.

Results

Conclusion