Chemically Recyclable Polyester Thermosets from Activated Adipic Acid and Renewable Polyols

Abstract

This study outlines a method for producing chemically recyclable crosslinked polyesters using renewable polyols-glycerol and sorbitol-combined with adipic acid (AA), which is transformed/activated into a polyanhydride mixture prior to use. A three-step procedure has been designed: 1) an acid-catalyzed reaction of AA with nontoxic isopropenyl acetate or acetic anhydride to form a crosslinking mixture (CLM) made of adipic-acetic mixed polyanhydrides; 2) a solvent- and additive-free process where glycerol or sorbitol, or a combination thereof, is reacted with the CLM to achieve a prepolymer, and 3) a casting/molding of the liquid viscous prepolymer to yield a thermoset as the end product. Different thermosets (eight examples) are prepared by changing the reagents ratio. These solids are thoroughly characterized by tensile tests, DMA, high-resolution magic angle spinning and solid-state NMR, thermal gravimetric analysis, DSC, and fourier transformed infra red (FT-IR) spectroscopy. The formation of cross-linked polyesters is confirmed in all cases, but mechanical properties varied significantly from one specimen to another. Interestingly, a tensile strength up to 18 MPa-approximately an order of magnitude higher than similar polymers-is achieved when sorbitol and the CLM are used in a 1:1 wt% ratio. The chemical recycle of the resulting polymers is achieved via methanolysis with quantitative recovery of the monomeric units.

Keywords: bioplastics; chemical recycling; isopropenyl acetate; solvent‐free; upcycling.

Figure 1

Shares of biobased polymers in different market segments in 2023. Data.^{[ 1 ]}

Figure 2

A) Schematic view of a hyperbranched polymer: linear defects (L), dendritic (D) and terminal (T) units and B) a biomass‐based hyperbranched polyol from triethyl citrate and glycidol.

Figure 3

Top: the synthesis of the adipic acid (AA)‐based CLM with isopropenyl acetate (iPAc). Bottom: the trend of the reaction over time. Symmetric anhydrides Acetyl anhydrides Isopropenyl esters Adipic anhydride (1). Determined by ¹H NMR with dioxane as the internal standard. The decline in isopropenyl ester signals was due to the formation of mixed anhydrides via an acid‐catalyzed transesterification with carboxylic acid groups, accompanied by the release of acetone.

Figure 4

A) Representation of the preparation of glycerol and sorbitol poly‐adipates. B) Flat and C) bent typical polyester sample (S₁). D) Prepolymerization and polymerization conditions and labelling of the synthesized polyester samples. Glyc:Sorb:CLM = glycerol:sorbitol:CLM weight ratio in the prepolymer synthesis.

Figure 5

Examples of dyed samples of polyesters obtained by the reaction of glycerol with CLM. Conditions of prepolymerization and curing were those of Figure 4.

Figure 6

Evaluation of the mass balance for a typical preparation of the polyester G₁.

Figure 7

FT‐IR spectra of samples S₁ (left) and G₁ (right) over time.

Figure 8

a,b): aliphatic regions of HR‐MAS HSQC NMR spectra of samples G₁ and S₁, respectively. c,d): oxygenated aliphatic regions of HR‐MAS HSQC NMR spectra of samples G₁ and S₁, respectively. e) Overlay of ¹³C HR‐MAS, INEPT, DP‐MAS, and CP‐MAS NMR spectra. (e) and f): Overlay of 13C HR‐MAS, INEPT, DP‐MAS, and CP‐MAS NMR spectra for G1 nd S1 respectively.

Figure 9

Synthesis and recycle of the polyesters.