Zn(HMDS)₂ as a Versatile Transesterification Catalyst for Polyesters Synthesis and Degradation toward a Circular Materials Economy Approach

Abstract

Zn­(HMDS)2 as a simple, biocompatible, efficient, and supporting ligand-free metal catalyst has been achieved for polyesters synthesis and degradation. The catalyst not only has a powerful capacity for ring-opening polymerization of various lactones to obtain diverse sustainable polyesters, such as poly­(lactide), poly­(3-hydroxybutyrate), and poly­(caprolactone), etc., with controlled molecular weights and narrow molecular weight distributions, but also can be used to degrade polyesters into valuable depolymerization products with excellent yields and chemoselectivity, demonstrating a truly circular materials economy approach to sustainability.

The Chemical Recycling of Polyesters for a Circular Plastics Economy: Challenges and Emerging Opportunities

“…A recent kinetic study revealed these complexes to adopt unusual behaviour, noting curved Arrhenius plots and variable activation energies, whilst observing the formation of Me-LA as low as À 20°C. [153] Yang et al [154] recently reported Zn(HMDS) 2 as a highly efficient catalyst for the transesterification of a variety of polyesters including PLA, poly(β-butyrolactone) (PBL), poly(δ-valerolactone) (PVL) and PCL. Promisingly, 99 % Me-LA yield was achieved within 2 h at room temperature, although a high catalyst loading (1.0 mol %) and large excess of MeOH (24.7 equiv.)…”

“… [153] Yang et al. [154] recently reported Zn(HMDS) 2 as a highly efficient catalyst for the transesterification of a variety of polyesters including PLA, poly( β‐ butyrolactone) (PBL), poly( δ ‐valerolactone) (PVL) and PCL. Promisingly, 99 % Me‐LA yield was achieved within 2 h at room temperature, although a high catalyst loading (1.0 mol %) and large excess of MeOH (24.7 equiv.)…”

Chemically recyclable polymer materials: polymerization and depolymerization cycles

“…[164][165][166] Different types of catalysts can achieve this strategy, such as imino monophenolate Zn complexes, Zn(OAc) 2 , TBD, Mg n Bu 2 , DMAP, Sn(Oct) 2 , Zn(HMDS) 2 , etc. [167][168][169][170][171][172][173][174][175][176][177][178][179] Furthermore, the tetramethyl glycolide (TMG), a cyclic dimer of hydroxyisobutyric acid (HIBA), was also explored as a degradable polymer by Nishida. 180 A high molecular weight PTMG (M n up to 90 kg mol −1 ) was obtained by t BuLi catalyzed anionic ring-opening polymerization.…”

“…Methanolysis of PCL is an interesting route to obtain high value-added methyl 6-hydroxyhexanoate, which has been achieved by Wang under mild conditions. 167 A new type of closed-loop recyclable polyester poly[2-(2hydroxyethoxybenzoate)] (P2HEB) with an aromatic ring in the backbone was reported by Shaver in 2015 (Scheme 25). 211,212 Using MeAl[Salen] (Cat.14) as a catalyst, a temperature dependent and concentration dependent monomer/polymer equilibrium process was observed.…”

Simple Zn(ii) complexes for the production and degradation of polyesters

“…More recent work has demonstrated an ethylene diamine-based ligand with a catam (N-H) moiety that far exceeds the activity of the direct imine-based counterpart, achieving 85% Me-LA yields aer 30 minutes at 50 C. 70,72 The aforementioned ligand is a building block of the tetradentate catalen system, zinc complexes of which were also active for PLA degradation to methyl lactate at moderate yields aer 8 hours at 80 C. 71 Recent examples have also achieved PLA alcoholysis at ambient conditions and with no solvent, in some cases demonstrating full PLA conversion aer 1 hour. 73,74 Organocatalysts, such as DMAP, 75 TBD, 76 TMC 77 and ionic liquids, [78][79][80] have also been reported but typically require high catalyst loading and can be limited by issues relating to corrosivity, toxicity and high cost.…”