The Kaydiar strategy was used to construct a polyolefin-like recyclable polyester platform

Recently, Professor Xu Tieqi of Dalian University of Technology and Eugene Y.-X. of Colorado State University in the United States. Chen collaborated to build a polyolefin-like recycled polyester platform through the Kaydia strategy, and developed a new generation of recycled polyester plastics with excellent physical properties.

On November 8, 2022, Beijing time, the research results were published in the journal Nature Chemistry with the title “A circular polyester platform based on simple gem-disubstituted valerolactones”.

Considered one of the greatest inventions of the 20th century, plastic is inexpensive, lightweight, malleable and durable, and has a wide range of applications. However, the chemical inertness of plastics themselves causes them to accumulate in the environment for a long time and cause “white pollution”, and most plastics need to be terminated by incineration and landfill, thus further polluting the environment. The best solution to prevent plastic pollution, as with other types of pollution, is to stop pollution at the source.

Designing closed-loop polymers or biodegradable polymers with intrinsic chemical recyclability is an effective means of solving current plastic problems. The ability to return polymers to polymer precursors with high selectivity and purity drives the circular economy. The design of monomers is the core of building circular polymers, but the design of monomers faces great challenges because it is necessary to balance the cyclability and properties of the polymer with the polymerizability and degradability of the polymer. The difficulty of this challenge stems from the multiple requirements of kinetics, thermodynamics, and practical properties required to develop a closed-loop polymer that is both chemically recyclable and useful.

In this work, the authors used commercial bio-based δ-valelactone as raw materials to construct a high-performance closed-loop polyester platform based on alkyl substituted six-membered cyclic esters. The valeolide diakyl group not only achieves the chemical cyclability of the polymer but also gives the polymer excellent physical properties.

Figure: Kayl substitution strategy to build high-performance recyclable polyester

The author obtained a series of alkyl substituted valelactones from the “one-pot method” with high yield. Thermodynamic experiments show that the upper limit temperature of the polymerization reaction of vale-substituted valelactone (VLR2) is significantly lower than that of δ-valelactone, which indicates that the polymer formed has better depolymerization. Dialkyl-substituted lactones generate a variable molecular weight polyester PVLR2 under mild conditions, and the resulting PVLR2 is selectively depolymerized under mild conditions (pyrolysis at 300 °C or catalytic pyrolysis at 150 °C) and completely converted into monomers that achieve an ideal balance in polymerization/depolymerization. The recovered monomers can be repolymerized to form the same polymer without purification, thus achieving a closed monomer-polymer-monomer cycle.

Figure 1: (a). α, synthesis and polymerization and depolymerization cycle of α-dialkyl-δ-pentanolactone (VLR2); (b) Upper polymerization temperature (Tc) of the monomer

The length of the substituent chain effectively regulates the physical properties of the polyester material. As the length of the substitution alkyl chain increases, the melting point of the polymer increases first, and the melting transition temperature Tm value of PVLEt2 is the highest, reaching 140 °C, which is comparable to HDPE and exceeds PBAT (138 °C). Further increasing the length of the substituted alkyl chain, the melting point and glass transition temperature of the polymer gradually decreased, and the Tm of PVLPr2 was 123 °C comparable to the Tm value of LDPE.

Figure 2 :(a). DSC curve of PVLEt2; (b). DSC curve for PVLPr2; (c). Tm and Tg values and trends of PVLR2

In terms of mechanical properties, PVLPr2 has high tensile strength (σB = 44.0±2.6 MPa), good ductility (εB = 209±13%) and toughness (UT = 57.2 MJ m-3), and has significant strain hardening phenomenon (σy = 20.7±0.3 MPa) after the yield point, and its mechanical properties are better than LDPE. The tensile strength of PVLPr2 is even higher than HDPE (σB = 21.4±0.5 MPa). The mechanical properties of VLEt2 (11%) and VLPr2 (89%) copolymers PVLEt2/Pr2 were further improved, and the ultimate tensile strength reached σB = 47.1±0.3 MPa, the elongation at break reached εB = 322±1.4%, and the toughness reached UT = 79.7 MJ m-3. In addition, PVLPr2 and PVLEt2/Pr2 also have excellent barrier performance, the oxygen permeability value of PVLPr2 and PVLEt2/Pr2 is significantly lower than LDPE, and the water vapor transmission rate WVTR value is comparable to LDPE. These results indicate that PVLPr2 and PVLEt2/Pr2 are excellent candidates to replace LDPE in packaging applications.

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Fig. 3: (a). Stress/strain curve; (b). Oxygen permeability; (c). Water vapor transmission rate

This work provides a new way to build high-performance recyclable polymers, which is expected to accelerate the discovery of high-performance recyclable polymers and promote the application of recyclable polymers in the real world. (Source: Web of Science)

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