Robust, thermally stable and impurity-tolerant aluminum-based catalyst system for polylactide production under industrial conditions

This study is led by Xiaosa Zhang (College of Science, Shenyang University of Chemical Technology, Shenyang 110142, China) and Bin Wang (Tianjin Key Laboratory of Composite & Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300350, China).

Synthetic plastics have become indispensable in human’s daily life. The petroleum-based plastics (e.g., polyolefins) for disposable products generally show extremely slow natural

degradation processes and generate microplastics that will be spread and enriched in the biological chain. The landfill and incineration treatments will cause even more serious secondary pollution. Consequently, sustainable and environmentally friendly bio-based plastics have attracted ever-increasing attentions in both academia and industry. Polylactide (PLA) has emerged as one of the leading sustainable and bio-degradable plastics, with a production of about 300,000 tons/year. PLA can be decomposed into water and carbon dioxide in industrial composting, while it will be degraded to lactic acid in just a few weeks in vivo. PLA also exhibits good mechanical properties as well as easy processability. Thus, PLA has been used for both disposable and durable goods including packaging, textile, additive manufacturing and medical applications. Ring-opening polymerization of _L-lactide (_L-LA) is a powerful approach that leverages a renewable monomer to generate high-molecular-weight PLA, and the mechanical, thermal and degradation properties can be largely modulated by the catalyst system. Industrially scalable production of PLA is performed at elevated temperature (140−200 °C) and in solvent-free (melt) conditions. Tin(II) bis(2-ethyl-hexanoate) [Sn(Oct)₂] combined with an alcohol is the dominant industrial catalyst for PLA production, thanks to its low cost, fewer epimerization side reactions, relative tolerance to impurities and high activity even at low catalyst concentrations. In addition, Sn(Oct)₂ leads to colorless polymer, and no decoloring treatment is needed after polymerization. Nonetheless, Sn(Oct)₂ is recognized as toxic, and additional polymer purification is required to remove toxic residual Sn. Thus, it is highly desired to develop benign alternative catalysts that are compatible with existing infrastructures and suitable for PLA production under industrial conditions.

Low-toxicity/nontoxic metals (e.g., Mg, Al, Zn, Fe, Co, Ge, Zr) have been used for _L-LA polymerization over the past decades. Special attention has been paid to aluminum-based complex because of the high Lewis acidity and natural abundance. In addition, Al shows much lower cytotoxicity than Sn, and short-term and low-level Al exposure will not affect the neuronal cells. Although the aluminum-based catalysts were highly active in ε-caprolactone polymerization, they only showed low and moderate activity in ROP of _L-LA. Moreover, most of these catalysts addressed solution polymerizations that are typically performed at 25−100 °C, and only a few could be used in the solventless polymerization at elevated temperatures. The Salen-aluminum complex could promote _L-LA polymerization under solvent-free conditions. However, the high catalyst loading ([Al] > 0.3 mol %) severely limits the potential application in commercial production of high-performance PLA. Recent elegant work by Romain et al. somewhat overcomes this limitation, who reported that chiral catam−aluminum complex exhibited very high activity and 9,200 equiv of _L-LA was polymerized in 4.5 min at 150 °C. Nonetheless, robust, thermally stable and protonic agent-tolerant aluminum catalysts for industrial PLA production have been lacking.

The ligand backbone and coordination environment strongly affected the catalyst’s stability, tolerance to impurities, and the subsequent polymerization behavior. We found that the thermostability and impurity tolerance of the aluminum complex could be significantly improved by substituting typical salen with structurally similar bipyridine bisphenolate ligand, thus showing excellent tolerance to protonic impurities and high activity at low catalyst concentration in cyclic anhydride / epoxide ring-opening copolymerization. However, the pentacoordinated (BpyBph)Al alkoxide exhibited extremely low activity in _L-LA ROP. While the binary catalytic system consisting of (BpyBph)Al alkoxide and alkoxide salt was highly active, it induced severe epimerization side reaction and afforded amorphous PLA at high temperature.

Here, the author designed and synthesized tetracoordinated aluminum methyl complexes bearing (amidoalkyl)pyridine−phenolate (AmPyPh) pincers. They envisioned that AmPyPh pincers are more flexible than BpyBph ligand and easily distorted to form an optimal transition state in LA polymerization, which will lead to an increased polymerization rate. Besides, the thermostability and impurity tolerance will be further improved in that the aluminum center is embedded entirely in the AmPyPh pincer. These (AmPyPh)AlMe complexes exhibited much higher catalytic performance than most of currently reported aluminum-based complexes. (AmPyPh)AlMe could promote LA polymerizations under industrially relevant conditions (150−180 °C, melt) with catalyst concentrations as low as 0.0005 mol % (monomer/ catalyst feed ratio = 200000:1), affording colorless semicrystalline PLLA. PLLA-diols (precursors for polyurethane) with narrow distribution and tailored molecular weight could be readily prepared under industrial conditions by using bifunctional alcohol as an initiator.

See the article:

Robust, Thermally Stable and Impurity-Tolerant Aluminum-Based Catalyst System for Polylactide Production under Industrial Conditions

https://doi.org/10.1021/polymscitech.4c00020