Showing papers by "Marc A. Hillmyer published in 2018"

Approaches to Sustainable and Continually Recyclable Cross-Linked Polymers

Abstract: Cross-linked polymers are ubiquitous in daily life, finding applications as tires, insulation, adhesives, automotive parts, and countless other products. The covalent cross-links in these materials render them mechanically robust, chemically resistant, and thermally stable, but they also prevent recycling of these materials into similar-value goods. Furthermore, cross-linked polymers are typically produced from petroleum-based feedstocks, and their hydrocarbon backbones render them nondegradable, making them unsustainable in the long term. In recent years, much effort has focused on the development of recycling strategies for cross-linked polymeric materials. In the following Perspective, we discuss many of these approaches, and highlight efforts to sustainably produce recyclable cross-linked polymers. We present our thoughts on future challenges that must be overcome to enable widespread, viable, and more sustainable and practical implementation of these materials, including the sustainable sourcing of f...

Reprocessable Acid-Degradable Polycarbonate Vitrimers

Abstract: Vitrimers are cross-linked polymer networks containing linkages that undergo thermally activated, associative exchange reactions, such that the cross-link density and overall network connectivity are preserved. Polycarbonates are industrially relevant polymers that, to our knowledge, have not yet been explored as vitrimers. We developed hydroxyl-functionalized polycarbonate networks that undergo transcarbonation exchange reactions at elevated temperatures in the presence of catalytic Ti(IV) alkoxides. The rate of transcarbonation within the networks, estimated through stress relaxation experiments, was tuned by adjusting the catalyst loading or hydroxyl group concentration in the networks. The polymer networks exhibit recovery of their tensile strength and plateau storage modulus (71–133%) after reprocessing. In addition to being reprocessable, the networks were hydrolyzed and decarboxylated in aqueous acid to recover 80 wt % of the precursor to the bifunctional cyclic carbonate monomer. These observation...

Sustainable Polyester Elastomers from Lactones: Synthesis, Properties, and Enzymatic Hydrolyzability

Abstract: Chemically cross-linked elastomers are an important class of polymeric materials with excellent temperature and solvent resistance. However, nearly all elastomers are petroleum-derived and persist in the environment or in landfills long after they are discarded; this work strives to address these issues by demonstrating the synthesis of renewable, enzymatically hydrolyzable, and mechanically competitive polyester elastomers. The elastomers described were synthesized using a novel bis(β-lactone) cross-linker and star-shaped, hydroxyl-terminated poly(γ-methyl-e-caprolactone). Using model compounds, we determined that the bis(β-lactone) cross-linker undergoes acyl bond cleavage to afford β-hydroxyesters at the junctions. The mechanical properties of the cross-linked materials were tunable and competitive with a commodity rubber band. Furthermore, the elastomers demonstrated high thermal stability and a low glass transition (−50 °C), indicating a wide range of use temperatures. The polyester networks were als...

Consequences of Grafting Density on the Linear Viscoelastic Behavior of Graft Polymers

Abstract: The linear viscoelastic behavior of poly(norbornene)-graft-poly(±-lactide) was investigated as a function of grafting density and overall molar mass. Eight sets of polymers with grafting densities ranging from 0 to 100% were synthesized by living ring-opening metathesis copolymerization. Within each set, the graft chain molar mass and spacing between grafts were fixed, while the total backbone length was varied. Dynamic master curves reveal that these polymers display Rouse and reptation dynamics with a sharp transition in the zero-shear viscosity data, demonstrating that grafting density strongly impacts the entanglement molar mass. The entanglement modulus (Ge) scales with inverse grafting density (ng) as Ge ∼ ng1.2 and Ge ∼ ng0 in accordance with scaling theory in the high and low grafting density limits, respectively. However, a sharp transition between these limiting behaviors occurs, which does not conform to existing theoretical models for graft polymers. A molecular interpretation based on thin fl...

Multiblock Polyesters Demonstrating High Elasticity and Shape Memory Effects

Abstract: Polyester block polymers containing polylactide have garnered significant attention as renewable, degradable alternatives to traditional elastomers. However, the low glass transition of the PLA blocks limits the upper-use temperatures of the resulting elastomers. To improve the thermal performance, we explore a series of multiblock polyesters composed of poly(e-decalactone) (PDL) and poly(cyclohexene phthalate) (PCHPE). These materials are prepared using switchable polymerization catalysis followed by chain extension. The strategy involves (i) alternating ring-opening copolymerization (ROCOP) of cyclohexene oxide and phthalic anhydride, (ii) e-decalactone ring-opening polymerization (ROP), and (iii) diisocyanate coupling of the telechelic triblocks to increase molar mass. The resulting multiblock polyesters are amorphous, and the blocks are phase separated; glass transition temperatures are ∼−45 and 100 °C. They show thermal resistance to mass loss with Td5% ∼ 285 °C and higher upper use temperatures comp...

Preparation, Characterization, and Formulation Development of Drug-Drug Protic Ionic Liquids of Diphenhydramine with Ibuprofen and Naproxen.

Abstract: Diphenhydramine (DPH) has been used with ibuprofen (IBU) or naproxen (NAP) in combined therapies to provide better clinical efficacy as an analgesic and sleep aid. We discovered that DPH can form protic ionic liquids (PILs) with IBU and NAP, which opens the opportunity for a new delivery mode of these combination drugs. [DPH][IBU] and [DPH][NAP] PILs exhibit low ionicity, as confirmed by Fourier transform infrared and 1H NMR spectroscopy, and accompanied by low diffusivity, high viscosity, and poor ionic conductivity. Evaluation of pharmaceutical properties of the two PILs showed that these PILs, despite high solubility and good wettability, exhibited low dissolution rates, owing to the poor dispersion of the PIL drops and the resultant small surface area during dissolution. However, when loaded into a mesoporous carrier, the PIL-carrier composites exhibited improved dissolution rates along with excellent flow properties and easy handling. Oral capsules of both PILs were developed using such composites. S...

Techno-economic Analysis of a Chemical Process To Manufacture Methyl-ε-caprolactone from Cresols

Abstract: Methyl-e-caprolactone is a monomer used in the manufacture of elastomeric biodegradable polymers with enhanced properties. We present here the conceptual process design for the production of methyl-e-caprolactone from para-cresol, a biorenewable feedstock obtained from lignin. The two-reaction process consists of cresol hydrogenation to methyl-cyclohexanone followed by Baeyer–Villiger oxidation to methyl-e-caprolactone. Details for designing an optimized process include unit operation design of two reactors, one decanter, one flash tank, and five distillation columns. Distillation and integrated heat transfer were determined via process simulation, with the objective of optimizing the process for net present value. For cresol obtained at $1.00 kg–1, the minimum selling price of methyl-e-caprolactone (defined as the product selling price that results in a zero net present value of the entire 30 year project) was $3.521, $2.798, and $2.557 kg–1 ($1.600, $1.272, and $1.162 lbm–1) at three process scales of 1...

Poly(allyl alcohol) Homo- and Block Polymers by Postpolymerization Reduction of an Activated Polyacrylamide

Abstract: Direct polymerization of allyl alcohol generally results in low molar mass oligomers or highly branched or cross-linked structures, and the properties and applications of linear, high molar mass poly(allyl alcohol) (PAA) are relatively unexplored Such macromolecular materials that cannot otherwise be obtained directly can be accessed by postpolymerization modification strategies Herein we describe the synthesis and characterization of linear, high molar mass PAA by borohydride reduction of a new activated polyacrylamide The facile polymerization and mild reduction conditions enable the preparation of PAA of targeted molar masses and low dispersity, as well as PAA block polymers via chemoselective reduction of the activated amide moiety

Activated Polyacrylamides as Versatile Substrates for Postpolymerization Modification

Abstract: The displacement of an activated leaving group in polymeric repeat units is a powerful method of postpolymerization modification. This strategy enables the synthesis of polymers otherwise unobtainable by direct polymerization as well as the preparation of a diverse array of macromolecular structures. We demonstrate that the activation of acrylamide through the introduction of two tert-butyloxycarbamate (Boc) groups followed by radical polymerization leads to a new class of activated polyacrylamides analogous to well-known activated polyacrylates. Transamidation of poly(di(Boc)-acrylamide) utilizing primary amines proceeds to high conversion under mild conditions, and the products can be readily purified. Less nucleophilic secondary amines and alcohols require more forcing conditions. We demonstrate the utility of this approach by preparing copolymers capable of on-demand gel formation and the synthesis of block polymers using controlled radical polymerization.

Computational Design of High-χ Block Oligomers for Accessing 1 nm Domains.

Abstract: Molecular dynamics simulations are used to design a series of high-χ block oligomers (HCBOs) that can self-assemble into a variety of mesophases with domain sizes as small as 1 nm. The exploration of these oligomers with various chain lengths, volume fractions, and chain architectures at multiple temperatures reveals the presence of ordered lamellae, perforated lamellae, and hexagonally packed cylinders. The achieved periods are as small as 3.0 and 2.1 nm for lamellae and cylinders, respectively, which correspond to polar domains of approximately 1 nm. Interestingly, the detailed phase behavior of these oligomers is distinct from that of either solvent-free surfactants or block polymers. The simulations reveal that the behavior of these HCBOs is a product of an interplay between both “surfactant factors” (headgroup interactions, chain flexibility, and interfacial curvature) and “block polymer factors” (χ, chain length N, and volume fraction f). This insight promotes the understanding of molecular features...

Poly(alkyl methacrylate)-Grafted Polyolefins as Viscosity Modifiers for Engine Oil: A New Mechanism for Improved Performance

Abstract: Lubricant additives based on graft copolymers with polyolefin backbones and poly(alkyl methacrylate) side chains were examined for their viscosity modifying behavior in base oil. A combination of ring-opening metathesis polymerization, atom transfer radical polymerization of alkyl methacrylates, and hydrogenation was used to prepare the target materials. Viscometric measurements reveal that larger side chain molar mass provides better thickening efficiency. More importantly, increasing the side chain polarity favorably impacts the performance of these graft copolymers as viscosity modifiers. While competitive modifiers must simultaneously meet several technical requirements, the most promising graft copolymer exhibits similar or larger viscosity index at a lower concentration (treat rate) compared to state-of-the-art polymeric additives. This feature bodes well for future additive design. Dynamic light scattering and pulsed field gradient NMR on dilute solutions of the graft copolymer in dodecane (a model...

Bioderived Acrylates from Alkyl Lactates via Pd-Catalyzed Hydroesterification

Abstract: The conversion of methyl and ethyl lactate to their corresponding alkyl 2-(propionyloxy)propanoate esters was achieved by Pd catalyzed hydroesterificative coupling with carbon monoxide (CO) and ethylene (C2H4) at moderate temperatures and CO/C2H4 pressures. A screening of reaction conditions showed that the reaction could be carried out at low loadings of catalyst, which was generated in situ from inexpensive and commercially available reagents. High conversions and product yields were obtained in a variety of solvents and even under neat conditions. Product analysis identified transesterification to be the primary competing reaction, which could be mitigated by changing solvents, as well as minimizing the amount of excess acid present in solution. Pyrolysis of methyl and ethyl 2-(propionyloxy)propanoate transformed these esters into their respective acrylates suitable for subsequent polymerization, along with propionic acid as a valuable coproduct.

Revisiting the Anionic Polymerization of Methyl Ethacrylate

Abstract: Synthesis of poly(methyl ethacrylate), (PMEA), in tetrahydrofuran at −78 °C using anionic polymerization techniques results in high molar mass (>30 kg mol−1), low dispersity (1.3), and high conversion (>81%). The molar masses of a series of samples are consistent with values anticipated by the monomer-to-initiator ratio and conversion. These results represent a significant improvement to earlier reported attempts to prepare PMEA using anionic methods. Successful diblock polymerization of polystyrene-block-PMEA, (PS-PMEA), and poly(4-tert-butylstyrene)-block-PMEA, (PtBS-PMEA), is achieved through sequential anionic polymerization techniques with dispersities as low as 1.06 and segment molar fractions close to those targeted. Broad principal scattering peaks observed by small-angle X-ray scattering (SAXS) for symmetric PS-PMEA at relatively high molar mass (39 kg mol−1) suggests an effective interaction parameter (χeff) that is smaller than for PS-block-poly(methyl methacrylate). On the other hand, PtBS-PMEA block polymers form a well-ordered morphology based on SAXS measurements and is attributable to the more hydrophobic PtBS segment. These results confirm the viability of PMEA as a new constituent in the expanding suite of polymers suitable for preparing nanostructured block polymers.

Functionalized Mesoporous Polymers with Enhanced Performance as Reference Electrode Frits

Abstract: Mesoporous glasses (∼10 nm pores) and macroporous polymers (∼1 μm pores) are often used as frits in the fabrication of aqueous reference electrodes. These frits function as salt bridges that allow for electrical contact between the sample and reference solutions while slowing cross contamination of the two solutions. Unfortunately, mesoporous glass and macroporous polymer frits used for these purposes inherently result in sample-dependent potentials or in rapid cross contamination of the sample and reference solutions, respectively. To address these issues, we synthesized mesoporous polymer frits, with much smaller pore sizes (∼10 nm) and electrically neutral hydrophilic pore walls. These monoliths were prepared from a bicontinuous, microphase-separated, and cross-linked block polymer precursor, that is, poly(lactide)-b-poly(isoprene)-b-poly(styrene-co-divinylbenzene), PLA-b-PI-b-P(S-co-DVB). The PLA serves as a selectively etchable sacrificial block, the PI provides latent reactive sites on the pore wall...