Performance polyamides built on a sustainable carbohydrate core

nature sustainability

Performance polyamides built on a sustainable carbohydrate core

Sustainably producing plastics with performance properties across a variety of materials chemistries is a major challenge-especially considering that most performance materials use aromatic precursors that are still difficult to source sustainably. Here we demonstrate catalyst-free, melt polymerization of dimethyl glyoxylate xylose, a stabilized carbohydrate that can be synthesized from agricultural waste with ninety-seven percent atom efficiency, into amorphous polyamides with performances comparable to fossil-based semi-aromatic alternatives. Despite the presence of a carbohydrate core, these materials retain their thermomechanical properties through multiple rounds of high-shear mechanical recycling and could be chemically recycled. Techno-economic and life-cycle analyses suggest selling prices close to those of nylon sixty-six with a reduction of global warming potential of up to seventy-five percent. This work illustrates the versatility of a carbohydrate moiety to impart performance that can compete with that of semi-aromatic polymers across two important materials chemistries.

Finding plastic precursors that can be synthesized with high efficiency from abundant, renewable feedstocks that are compatible with multiple material chemistries and that can offer performance properties akin to aromatic, phthalate-based monomers would greatly facilitate competition with petro-based products. Bio-based monomers on the Department of Energy's list of top value-added chemicals, notably two, five-furandicarboxylic acid, have been successfully used to create high-performance polyesters such as poly(ethylene furanoate). However, this molecule illustrates the challenge of finding versatile polymer precursors as its polymerization into useful polyamides has been hindered by incompatibility with the harsh reaction conditions of polyamide synthesis. For example, when polymerized with diamines in the melt as is done industrially, FDCA tends to undergo substantial decarboxylation, resulting in low-molecular-weight polymers (Mn less than ten kiloDaltons). Researchers have therefore resorted to solvent polymerization using Yamazaki-Higashi conditions, interfacial polymerization, or enzymatic polymerization to study FDCA-based polyamides. However, these techniques are unlikely to be industrially viable, or sustainable, due to high solvent usage, the use of difficult-to-recycle catalysts or enzymes and increased downstream purification. Moreover, succinic acid, a less-rigid diacid on the Department of Energy's list of value-added chemicals from biomass, which also has a proven track record in synthesizing useful polyesters (for example, poly(butylene succinate)), similarly cannot currently be polymerized into high-molecular-weight polyamides due to intracyclization reactions at high temperatures. Even when direct solid-state polymerization is used to minimize these reactions, MnS of only eight kiloDaltons are achieved. Finding sustainable diacids that are compatible with both polyester and polyamide chemistries, akin to phthalic acids, is a notable challenge.

Producing sustainable monomers that can be incorporated into engineering polyamides could reduce the environmental footprint of this sector but also offers a higher-value market (as compared with commodity polyesters and polyolefins) to aid in the profitability of lignocellulosic biomass valorization, which is an ongoing challenge. Polyamide production currently faces substantial sustainability challenges, particularly in terms of global warming potential. For example, the most common polyamide, nylon sixty-six, carries a global warming potential of approximately eight to nine kilograms CO2-equivalent per kilogram, with adipic acid carrying a large majority of the burden (global warming potential equals eight point five kilograms CO2-equivalent per kilogram adipic acid). Furthermore, polyamides are sold at high market prices, ranging from three to seven dollars per kilogram for nylon sixty-six to as high as seven to twenty dollars per kilogram for the high-performance, semi-aromatic polyphthalamides based on phthalic acids, and their copolymers. These factors could enable cost competitiveness of bio-based monomers, especially for high-performance specialty polyamides, while also offering large sustainability enhancements. In part for these reasons, there are already several commercial bio-based polyamides on the market, the large majority of which stem from the C ten and C n acid/amine precursors that can be produced from castor oil (for example, PA-eleven; PA-ten, ten; PA-ten, T; and so on). Despite their comparatively low mechanical strength and glass transitions relative to nylon sixty-six, these materials offer performance advantages due to their exceptionally low water absorption and high flexibility, making them suitable for a range of applications, such as in specialty tubing (for example, fuel lines) or luxury sporting goods (for example, ski boots). However, there is still a lack of economically viable rigid, bio-based monomers for the synthesis of more high-performance polyamides with mechanical strengths and glass transitions akin to PPAs and their copolymers.

Building on a previously reported polymer precursor, dimethyl glyoxylate xylose, that could be directly produced from abundant biomass at high yields and leads to degradable polyesters with well-rounded performance when polymerized with dialcohols, in this Article, we demonstrate a simple synthetic pathway to also produce high-molecular-weight polyamides from dimethyl glyoxylate xylose in quantitative yields, in the melt at two hundred fifty degrees Celsius, with three-hour reaction times and without a catalyst or specialized reactor systems. The incorporation of this largely unmodified natural carbohydrate cycle into the backbone of polyamides leads to performance properties competitive with those of complex amorphous engineering polyamides while enabling an efficient, atom-economical and sustainable synthesis from abundant renewable feedstocks. Mechanical testing demonstrates that these materials are sufficiently robust for industrial processing and several rounds of mechanical recycling. Alternatively, we introduce a process for chemical recycling these materials to monomers at yields greater than ninety percent with straightforward downstream separations. Finally, we estimate selling prices to be in the range of commercial nylon sixty-six (three to six dollars per kilogram, depending on the diamine used) with a fifty-six to seventy-five percent reduction in global warming potential, even when using fossil-derived glyoxylic acid.

Results

Synthesis and characterization

In a previous study, we reported a scalable process to produce dimethyl glyoxylate xylose by acid-catalysed acetalization of xylose with glyoxylic acid, followed by esterification with methanol. The dimethyl glyoxylate xylose product comprised a mixture of four stereoisomers and was purified by extraction, distillation and crystallization. The recrystallization parameters could also be adjusted to selectively crystallize the most prevalent stereoisomer

S S isomer. This synthesis was demonstrated with a yield of ninety-five percent from purified xylose and a yield of seventy to eighty-three percent (molar basis from hemicellulose) when directly utilizing non-edible biomass such as wood or corn cobs. In addition to DMGX, this process generated a stabilized lignin and a digestible cellulose as valuable co-products. This resulted in overall biomass utilization efficiencies of up to eighty-six percent, which represents the weight percentage of the cellulose, lignin, and hemicellulose fractions that were successfully incorporated into valuable products. High biomass utilization efficiencies are key to achieving both sustainability and profitability and are often low for bio-based aromatic plastic precursors. Bio-based terephthalic acid and FDCA have maximum theoretical biomass utilization efficiencies of forty-two percent and fifty-nine percent, respectively, with actual efficiencies estimated to be on the order of fifteen to thirty percent when produced industrially via five-chloromethyl furfural. Retaining the carbohydrate core within the polymer backbone via DMGX enables a theoretical biomass utilization efficiency of ninety-seven percent and an actual biomass utilization efficiency of eighty percent, which has large implications in terms of cost and sustainability.

In this study, a series of polyamides were synthesized at ninety-eight to one hundred percent yield by direct melt polycondensation of crystalline DMGX with stoichiometric amounts of various aliphatic diamines (C six, C nine, C ten and C twelve) in the presence of a secondary antioxidant (zero point five weight percent triphenyl phosphite) and without addition of a polymerization catalyst. The antioxidant was used to prevent heavy discolouration of the polymer, probably by reducing hydroperoxide formation during polymerization. DMGX/diamine/triphenyl phosphite mixtures were heated to one hundred forty degrees Celsius under nitrogen and mechanical stirring, resulting in rapid reaction to form a white solid with distillation of methanol. Stirring was stopped, and the reaction temperature was then brought to two hundred fifty to two hundred sixty degrees Celsius, where stirring was restarted to form a yellow or orange, viscous polymer melt. Discolouration during the melt polycondensation of polyamides by formation of conjugated species is a notorious problem and, in other systems, has been addressed by increased purification of the monomers combined with additive set optimization. Vacuum was then applied to drive the polycondensation, and the polymerizations were deemed complete when mechanical stirring became inefficient (total reaction time of three to four hours). The polymers were directly discharged from the reaction flasks without dissolution or precipitation. The most-abundant single stereoisomer of DMGX (S S stereoisomer) was used for synthesis of the polyamides to simplify chemical characterization. The more industrially relevant four-stereisomer DMGX mixture, which requires fewer separation steps and utilizes all four DMGX stereoisomers, was also used to synthesize one of the fully bio-based polyamides with bio-based one, ten-diaminodecane. We also scaled up the highest performing polyamide with one, eight-diaminooctane, both at a four hundred gram scale.

The molecular weights of the resulting polymers were determined via size-exclusion chromatography with multi-angle static light scattering. All polymers from the single stereoisomer of DMGX (prefixed with one S hereafter) yielded high-molecular-weight polyamides (M n equals twenty to thirty kiloDaltons) with dispersities typical of a well-behaved polycondensation process (D j equals one point five to one point eight). If higher molecular weights were desired, longer reaction times in high-torque industrial reactor systems could be used. However, this is unlikely to substantially enhance useful properties and would potentially decrease processability due to increased melt viscosities. The polymer produced from the four-stereoisomer mixture of DMGX and one, eight-diaminooctane at a four hundred gram scale, four S-poly(octamethylene xylosediglyoxylamide) (four S P A-eight, DGX), had an essentially identical molecular weight distribution as the single-isomer counterpart (one S P A-eight, DGX) produced at a thirty-five gram scale. The four S-poly(decamethylene xylosediglyoxylamide) (four S P A-ten, DGX) polymer also had a similar molecular weight to the corresponding single-isomer polymer (one S P A-ten, DGX) but a higher dispersity (D j equals two point nine). Full chemical characterization of the polyamides by nuclear magnetic resonance spectroscopy and matrix-assisted laser desorption-ionization time-of-flight mass spectrometry.

revealed that the intended polymers were synthesized and with a final one to one stoichiometry of DMGX to diamine.