Chemical Recycling of Commercial Poly(l-lactic acid) to l-Lactide Using a High-Performance Sn(II)/Alcohol Catalyst System

Abstract

Poly(L-lactic acid) (PLLA) is a leading commercial polymer produced from biomass, showing useful properties for plastics and fiber applications; after use, it is compostable. One area for improvement is postconsumer waste PLLA chemical recycling to monomer (CRM), i.e., the formation of L-lactide (L-LA) from waste plastic. This process is currently feasible at high reaction temperatures and shows low catalytic activity accompanied, in some cases, by side reactions, including epimerization. Here, a commercial Sn(II) catalyst, applied with nonvolatile commercial alcohol, enables highly efficient CRM of PLLA to yield L-LA in excellent yield and purity (92% yield, >99% L-LA from theoretical max.). The depolymerization is performed using neat polymer films at low temperatures (160 °C) under a nitrogen flow or vacuum. The chemical recycling operates with outstanding activity, achieving turnover frequencies which are up to 3000× higher than previously excellent catalysts and applied at loadings up to 6000× lower than previously leading catalysts. The catalyst system achieves a TOF = 3000 h −1 at 0.01 mol % or 1:10,000 catalyst:PLLA loading. The depolymerization of waste PLLA plastic packaging (coffee cup lids) produces pure L-LA in excellent yield and selectivity. The new catalyst system (Sn + alcohol) can itself be recycled four times in different PLLA "batch degradations" and maintains its high catalytic productivity, activity, and selectivity.

Divergent methods for polyester and polycarbonate depolymerization with a cobalt catalyst

“…While this is not optimized, and does not compete with the Sn( ii )/alcohol catalyst system reported by Williams and coworkers, this represents one of only a few examples of CDP of PLA. 36,39,40,44 Most notably, all polymers which were able to undergo any amount of CDP fared worse under the same conditions with KO t Bu added (see ESI†). These results indicate that the DEP catalyst is more important than the presence of KO t Bu for CDP.…”

“…9 There are few examples of CDP of commercial polycarbonates and polyesters to their respective cyclic monomers. 41–44 In this case, CDP of the polycarbonate often leads to a cis -cyclic carbonate, a product which lacks examples of being polymerized. Many of these examples have shown the ability to perform CDP on specially designed polymers, which have been modified or synthesized with the intention that these polymers also cyclodepolymerize to polymerizable monomers.…”

“…Examples of CDP for common polyesters, such as poly(ε-caprolactone) (PCL), poly(δ-valerolactone) (PVL), polydioxanone (PDO), poly(caprolactam) (nylon-6), and poly(lactic acid) (PLA) are known with metal catalysts. 35,40,44,45,54…”

Synthesis of Poly(δ-caprolactone) via Bis(phenolate) Rare-Earth Metal Complexes Mediated Ring-Opening Polymerization and Its Chemical Recycling

“…Many types of catalysts have been successfully explored for the chemical recycling of polyesters. − Li and co-workers investigated the depolymerization behaviors of PδCL with organobases, where the cyclic trimeric phosphazene base (CTPB) mediated the depolymerization of PδCL back to pristine monomer, while weaker commercial organobases TBD, 1,8-diazabicyclo[5.4.0]­undec-7-ene (DBU), and triethylamine (TEA) did not work . Inspired by this work, we speculated that basic metallic compounds with a rational choice could also promote the depolymerization of PδCL.…”

Alternatives to fluorinated binders: recyclable copolyester/carbonate electrolytes for high-capacity solid composite cathodes

“…[41][42][43] Additionally, ester/carbonate linkages are known to enable recyclability to monomers which can be initiated by the addition of catalysts, heat and/or manipulation of reaction conditions. [44][45][46][47] On its own, P(CL-r-TMC) is a so (low T g ) amorphous polymer. To modify the mechanical properties for optimal binder performance including accommodating volume changes caused by internal stresses at higher voltages, hard (high T g ) polymer blocks need to be introduced.…”