Bio-advantaged polyamides from muconic acid

Abstract

The polymer industry has been a critical driver in the research for alternatives to petroleum-derived chemicals, specifically using biomass as renewable carbon feedstock. The current approach for synthesizing bio-derived chemicals focuses largely on reterosynthetic strategies, including “drop-in” chemical replacements that compete directly with their petroleum-derived counterparts. However, it is becoming increasingly evident that this approach is inadequate as the success of biorefineries will suffer from the price volatility of petroleum. A more sustainable alternative includes taking advantage of the unique composition of biomass and targeting bioprivileged platform molecules, which can generate both novel species and commodity chemicals. This versatility is an important attribute as high-price novel chemicals would supplement the economy of biorefineries and diminish challenges associated with the competitive price of petroleum-derived chemicals that have benefited from years of fundamental research. Simultaneously, the bio-derived commodity chemicals would leverage economic risks associated with introducing novel molecules, which do not possess pre-defined markets. Here we demonstrate the diversification of one such bioprivileged molecule, muconic acid (MA), a readily obtained intermediate from sugar or lignin through metabolically engineered yeasts and bacteria. MA was transformed into various novel diacids and copolymerized into a polyamide backbone. Nylon-6,6 was chosen as the base case polymer due to its superior strength, stiffness, and thermal stability, making it applicable in a diverse range of industries. Despite its advantages, nylon-6,6 suffers from drawbacks such as high moisture absorption and ease of flammability. Initially, a cyclic unsaturated diacid was synthesized using MA and further copolymerized with hexamethylene diamine and adipic acid. The introduction of these cyclic sequences in an aliphatic polymer backbone altered the crystallinity of polyamides, thereby suppressing melting point and facilitating processing through injection molding. Other properties, such as degradation temperature and rheological properties (e.g., storage modulus and glass transition temperature) remained largely unaltered. Next, a platform synthesis technique using Diels-Alder chemistry was demonstrated on MA and its derivatives to produce diacids with tailored property enhancements. As a proof of concept, hydrophobic monomers were synthesized using non-polar aliphatic pendant groups of varying chain lengths. The pendant groups were anchored to the diacid using a carbon-carbon linkage that was shown to survive the harsh conditions of polymerization. This novel family of polyamides were fully characterized using gel-permeation chromatography (GPC), dynamic scanning calorimeter (DSC), thermogravimetric analysis (TGA), wide-angle x-ray scattering (WAXS), and dynamic scanning calorimetry (DMA). Dramatic improvements (~70% reduction) in water uptake performance were observed from the novel polyamides at 50% and 100% relative humidity. The robustness of this functionalization technique was extended to aromatic pendant groups to introduce built-in flame retardancy. Charring, which is a key metric in the determination of flame inhibition, doubled in these customized polyamides. These results illustrate a rapid differentiation of MA towards producing diacids exhibiting targeted property enhancements. Further demonstration of MA-differentiation into novel diacids occurred on trans-3-hexenedioic acid (t3HDA), which is obtained through electrochemical hydrogenation of MA. The monounsaturation in this C6 diacid can be utilized as a grafting point for chemical moieties for targeted property enhancements in polymers. We demonstrate a one-pot isomerization and functionalization strategy wherein the double bond in t3HDA derivatives migrates to render this molecule active for Michael-addition (MA). A prominent flame-retardant (FR) molecule, DOPO, was selected as the Michael-donor to form a novel flame-inhibiting diacid. This monomer was copolymerized in a polyamide backbone and was compared to physical mixtures containing comparable amounts of DOPO blended with Nylon-6,6 (PA66). Property characterization of FR-blends and FR-tethered polymers were made through GPC, nuclear magnetic resonance spectroscopy (1H, 13C, 31P NMR), DSC, WAXS, TGA, DMA, and tensile testing. Flame-inhibition of the polymers was analyzed through microscale combustion calorimeter (MCC). FR-tethered samples showed superior crystallinity and mechanical properties, whereas FR-blends sacrificed key properties at high loadings. The synthesis strategy presented herein can be extended for a variety of functional groups for property-modified biopolymers. Finally, concluding remarks are delivered and various strategies for monomer functionalization are outlined. Additionally, future direction to implement bioadvantaged polymers are explored.