The development of peptide amphiphiles containing a beta-sheet-forming sequence not only led to a class of biologically active nanostructures and materials for diverse medical applications but also provided inspiration for the creation of a broader set of functional self-assembling units. Drug amphiphile exploits the assembly potential of therapeutic agents to devise supramolecular medicines that leverage the unique physicochemical and stimuli-responsive features of supramolecular assemblies for improved treatment outcomes. This lecture will detail the rational design of several drug amphiphiles and the emerging properties of their respective assemblies that enable local retention, controlled release, and use as long-acting injectables.
This lecture will describe recent progress in development of backbone-modified peptides intended to mimic information-rich surfaces displayed by specific conformations of natural polypeptides. Backbone modification is achieved through replacement of one α-amino acid residue or more with a β-amino acid residue. The β residue can display the natural side chain, or this residue can be preorganized via a ring. The resulting α/β-peptides can inhibit specific protein-protein interactions, or they can augment signaling through polypeptide-activated receptors. Merits of the α/β-peptides include enhanced resistance to proteolysis and the ability to transmit signals that differ subtly from those of a prototype α-peptide.
Scaffolds for tissue engineering are functionalized with specific peptides to direct desired cell responses. However, scaffolds are typically modified using post-fabrication functionalization techniques, which can affect scaffold properties and have limited spatial resolution. We developed a versatile strategy where peptide-polymer conjugates are 3D printed in user-defined locations without affecting scaffold architecture. The peptide segments of the conjugates become displayed on the surface during fabrication, resulting in scaffolds spatially functionalized with peptides. Peptide surface concentration can also be controlled by simply changing conjugate concentration in the “ink” prior to 3D printing. We recently utilized this platform with peptide-polymer conjugates bearing two different sequences to promote cartilage or bone formation. Separate printer heads were used to localize each peptide in discrete regions to mimic osteochondral (bone-cartilage) tissue. Human mesenchymal stem cells (hMSCs) were seeded in scaffolds to investigate how peptide type and organization influenced cell response and matrix formation (Fig 1). We discovered that the presence of both peptides significantly enhanced hMSC differentiation compared to scaffolds with no peptides or only one peptide. Interestingly, presenting both peptides homogeneously throughout one scaffold induced a significantly different cellular responses compared to scaffolds with spatially organized peptides in separate regions. These results indicated that multi-peptide organization influences local and global cell-material interactions to drive distinct differentiation behavior. This platform enables us to investigate synergistic effects with multiple bioactive peptides to direct regeneration of complex tissues.
Fig 1. Human mesenchymal stem cells were seeded on scaffolds functionalized with no peptides (grey), bone-promoting peptide (red), cartilage-promoting peptide (blue), both peptides spatially organized (blue/red; dual spatial), or both peptides mixed homogeneously (purple; dual mixed). Differentiation was only observed in dual-peptide scaffolds with dual spatial driving osteogenesis while dual mixed induced chondrogenesis.
Biological systems modulate the spatial and temporal character of their peptide and protein populations by regulating synthesis, transport, degradation, secretion, receptor-binding and cellular uptake. Non-canonical amino acid tagging provides a versatile tool for tracking these processes in space and time. This lecture will discuss recent applications of non-canonical amino acid tagging in the study of complex biological systems relevant to human health and disease.
Cationic polypeptides readily complex with nucleic acids. As cationic polypeptide blocks conjugated with neutral water-soluble polymers, such as polyethylene oxide (PEG), they readily assemble into complex core micelles with nucleic acids. We have characterized the shapes and sizes of such micelles very thoroughly, via x-ray scatterind, dynamic light scattering and cryo-TEM, in terms of their dependences on the chain lengths of the various constituents and the nature of the specific nucleic acid involved. This has resulted in the determination of some general scaling laws for size vs chain lengths. We have also deployed this type of micelle as an effective therapeutic for conditions resulting from inflamed vascular endothelium.
Living systems have evolved to grow and heal through self-assembling processes capable of organizing molecular and cellular building-blocks at multiple size scales. These “biological organization principles” (BOPs) emerge from cooperative interactions and chemical networks between multiple components, which allow biological systems to diversify, respond, and ultimately optimize.1 There is great interest to develop materials and devices that can emulate the structural and functional properties of such systems. Towards this goal, it is critical to develop new fabrication methodologies which enable the assembly of multiple types of building-blocks into functional ensembles integrated hierarchically.
This talk will present recent work form our laboratory focused on integrating supramolecular events found in nature with engineering and biofabrication principles. The first part of the talk will describe approaches harnessing protein order-disorder synergies to regulate organic-inorganic interactions and engineer mineralizing materials for regeneration of hard tissues such as dental enamel and bone.2 The second part will describe methodologies exploiting self-assembly and diffusion-reaction processes to organize multiple types of biomolecules hierarchically for the fabrication of regenerative implants as well as in vitro models of cancer3 or vascular tissue.4
Peptide amphiphile fibers have been synthesized to assemble into long fibers which, when co-assemble or functionalized with various moieties including thermal- and photo-responsive polymers, produce functional hydrogels that are actuated via external stimuli. When co-assemble with magnetic components, photo-responsive hydrogels can be actuated using external magnetic fields to produce locomotion (Figure 1). In this talk, we explain the design and modeling of photo-responsive hybrid hydrogels that integrate supramolecular nanofibers and ferromagnetic nanowires, which when coupled to magnetic fields, produce robotic functions including, walking, swimming, and deliver cargo.
Figure 1. Magnetic fields exert controllable forces that generate microscopic actuation and locomotion in hydrogels with ferromagnetic components The arrows in the figure represent the magnetization direction (same as the direction of Ni nanowires). The color bar on the right side represents the displacement of the robot from the initial configuration. (from Li, Chuang, et al. 49, 12 2020, Science Robotics, Vol. 5, p. eabb9822).
The striking resemblance between biological one-dimensional structures and supramolecular polymers assembled in water inspired Sam Stupp and many other chemists to study these artificial mimics for possible biomedical applications. In our research we use water-soluble 1,3,5-benzene-tricarboxamides (BTAs) that from one-dimensional fibers in aqueous solution. By modifying the periphery of these BTAs, we can create biological relevant copolymers that can use their dynamic nature to adaptive their structure to rearrange the ligands accordingly to the receptors at the cell membrane. In the lecture, we will show that fundamental insights into the dynamic nature of these copolymers are essential to create function.
Dow is dedicated to providing innovative, market relevant polyolefin technology to meet global demand. Polyolefin elastomers are a specialty class of materials within the Plastics & Specialty Polymers division at Dow. In this talk, the evolution of polyolefin technology from random polyethylene copolymers to olefin block copolymers will be discussed. Dow’s INFUSE™ olefin block copolymer (OBC) technology is a novel class of commercially available polyolefins with unique physical properties derived from the block structure. Dow’s INFUSE™ OBCs are used in automotive impact modification, elastic film, and high-performance footwear foams. More recently, polypropylene based olefin block copolymers have been developed and serve as a bridge between PE and PP.
Impact copolymer polypropylene (ICP) and thermoplastic vulcanizates (TPV) are the most important propylene (PP)-based heterogeneous polyolefins. Conventional ICPs and TPVs contain ethylene-propylene (EP)-based copolymers as the rubbery phase, which have poor compatibility with PP, leading to weak interfacial adhesion and large microdomains. Better rubber/PP compatibility would lead to enhanced blend properties. Blends of polypropylene with ethylene (E)/1-butene (B) copolymers show a composition window (between 60 and 90 wt% B) where molecular miscibility is observed. A series of BE copolymers and BEDM (1-butene/ethylene/diene) terpolymers were synthesized and blended with isotactic PP (iPP) to produce fully miscible BE/iPP and BEDM/iPP blends. Upon cooling, iPP crystallization drives nanophase segregation in BE/iPP blends, resulting in soft nano-ICPs with remarkably high stretchability (strain to break ~ 500 %), compared to conventional ICPs (<100%). Dynamic vulcanization of BEDM/iPP blends results in a unique hierarchical microstructure comprising a PP matrix and a rubber-rich dispersed phase with internal nanostructure. The latter consist of iPP and BEDM cocontinuous domains. This novel microstructure produce soft elastomers with improved elastomeric performance, compared to conventional TPVs. This proof of concept study represents a new paradigm for the design of rubbers for ICP and TPV applications with tailored compatibility with iPP matrix, which can be extended to other E/α-olefin copolymers.
During the COVID-19 pandemic, there was a dramatic rise in demand for N95 filtering face piece respirators (FFRs) to help protect healthcare and frontline workers from the SARS-CoV-2 virus. As one of the largest manufacturers of N95 respirators, 3M was confronted with the global supply chain challenge of increasing its production to over 2 billion respirators worldwide in 2020 to meet this unprecedented demand. Because electrostatically charged filter media is a critical component in N95 FFRs a key enabler in this attainment was prior technical work to establish a robust polyolefin resin supply chain. Here we present the results of studies to identify which material properties are critical for ensuring long-term stability of the electrostatic charge present within N95 filter media that allows it to provide high filtration efficiency at a comfortable breathing resistance.
The Science Behind Respiratory Protection
Our unique manufacturing process injects a powerful electrostatic charge into an open formation material, improving the efficiency of particle trapping while allowing more air to pass through. This results in highly effective respirators—NIOSH-approved and FDA-cleared for fluid resistance.
Take a Closer Look at Trapping Airborne Particles
Illustration of how 3M’s electrostatically charged microfibers—magnified 10,000 times—attract and capture particles from the air.
Hydrogenated styrenic block copolymers (HSBC) are soft thermoplastic elastomers, whose molecular structure can be tailored to be compatible with multiple types of polymers, including polyolefins. This leads HSBCs to find applications in a wide range of market segments like automotive, packaging, medical, recycling of plastics/bioplastics, and in consumer durables and electronics. HSBCs can be blended with polypropylenes at various ratios to enhance impact strength at low temperatures in rigid applications and modify hardness and flexibility in soft applications. Newer grades being developed at Kraton can be used to improve processability while maintaining the same performance, for instance, in medical film and bag applications, and in flame retardant compounds used in wire and cable and consumer durable applications. In this paper, we will provide an overview of the structure and characteristics of HSBC polymers, along with the performance attributes such as flame retardant performance, impact modification and mechanical property enhancement, and compatibilization.
Functional polyolefins (f-PO) have long been synthetic targets in polymer chemistry. Catalysts competent to form f-PO by insertion polymerization are nevertheless dominated by palladium, which is impractical solution for large scale f-PO applications. This presentation will discuss two distinct approaches to generate f-PO architectures using non-precious metal catalysts. Nickel(II) catalysts capable of producing linear, statistical f-PO copolymers will be discussed, along with studies to understand their ligand-dependent control of activity, molecular weight, and competitive monomer enchainment. Alternatively, a coordinative chain transfer polymerization can be exploited to generate f-PO with chain end functionality that complement the main chain functional group incorporation of the nickel catalysts.
A demand still exists for new molecular polyolefin catalysts that can lead to differentiated ethylene/alpha-olefin copolymer microstructures, including polymer molecular weight and comonomer incorporation. The combination of carbazole and 3,5-di-tert-butylphenyl groups on bis-biphenylphenol based ligand structures was used to study the effect of catalyst symmetry on an ethylene/alpha-olefin copolymerization reaction in a continuously-stirred tank reactor (CSTR). Synthesis of the catalysts along with characterization of the polymers will be discussed.
Selective trimerization of ethylene to produce 1-hexene is a commercially practiced process that yields valuable comonomer for linear low density polyethylene production. Several years ago, ExxonMobil chemists developed a family of chromium catalysts useful for ethylene trimerization, but a mechanistic understanding of the catalysis remained elusive. This talk presents a mechanistic proposal to explain the catalytic selectivity, supported by a computational exploration of proposed cycle. Results will be discussed in terms of geometric requirements for reaction and the fundamental steps involved in catalysis.
The synthesis and self-assembly of block copolymers into 1D, 2D and 3D nano- and microstructures is of great interest for a wide range of applications such as drug delivery, catalysis and filtration. A key challenge in this field is obtaining independent control over molecular structure and hierarchical structure in all dimensions using scalable one-pot chemistry. To address this challenge, we report on the ring opening polymerization-induced crystallization-driven self-assembly (ROPI-CDSA) of polyester-based block copolymers into 1D, 2D and 3D nanostructures with 10-20% solids weight. A key feature of ROPI-CDSA is that the polymerization time is generally much shorter than the crystallization-driven self-assembly relaxation time, resulting in a non-equilibrium self-assembly process. The self-assembly mechanism is analyzed by cryo-transmission electron microscopy, wide-angle x-ray scattering, Fourier transform infrared spectroscopy, and turbidity studies. Resultant organogels were further studied with scanning electron microscopy and rheology. The analysis revealed that the self-assembly mechanism is dependent on both the polymer molecular structure and concentration. Knowledge of the self-assembly mechanism enabled the kinetic trapping of multiple hierarchical structures from a single block copolymer. The non-equilibrium nature of the self-assembly process was studied by modifying the polymerization kinetics using variety of ring-opening catalysts. Modification of the polymerization process leads to different outcomes in structural and morphological properties, with slower polymerizations resulting in stronger gels. This research greatly expands the dimensionality of existing polymerization-induced self-assembly processes by allowing for independent control over the morphological structure, irrespective of the molecular structure. Additionally, mechanistic knowledge and synthetic variation of catalyst and polymer is being applied to make colloidal suspensions and bulk materials for a variety of drug delivery applications from conjugation to sutures.
Degradable polymers are an important field of research in polymer science and have been used in a wide range of applications including (nano)medicine, microelectronics, and environmental protection. As one of the most efficient methods to construct olefinic polymers, ring-opening metathesis polymerization (ROMP) can endow the obtained polymers with an impressive range of functionalities, but the materials are usually persistent in the environment due to the high stability of all carbon backbone. Ruthenium Fischer-type carbenes have usually been demonstrated as inactive species in metathesis reactions and are frequently regarded as inert species in ring-opening metathesis polymerization. In this presentation, these complexes play a crucial role in cascade alternating ring-opening metathesis polymerization targeting to generate degradable poly(enol ether)s. When enyne or acyclic diyne monomers are combined with low-strain cyclic enol ethers, a controlled chain-growth copolymerization occurs resulting in degradable polymers decorated by hydrolytically labile enol ether backbones with high degrees of alternation (>90% alternating dyads), low dispersities, and targetable molecular weights. Due to the controlled behavior, this methodology is amenable to the synthesis of alternating diblock polymers with combinations of enyne-enyne or diyne-enyne blocks. All the obtained polymers can be degraded into small-molecule fragments under aqueous acidic conditions. This work furthers the potential of Fischer-type ruthenium alkylidenes in polymerization strategies and presents new avenues for the generation of functional metathesis materials.
Ring-opening metathesis polymerization (ROMP) is one of the most widely used strategies for controlled polymerizations. Although metal alkylidene initiators enable the synthesis of functional polymers with excellent stereocontrol, residual metal contamination and high costs of metal reagents are major limitations of ROMP. In a departure from traditional metal initiators, our group recently reported a metal free variant of ROMP (MF-ROMP) which utilizes enol ether initiators, and organic photocatalysts to achieve ROMP via radical cationic intermediates. Within the unique mechanism of MF-ROMP, we have discovered that ion-pairing interactions provide a means to impart stereocontrol during polymerization. During this talk, we will describe our initial discovery of stereocontrol in MF-ROMP and discuss our mechanistic hypotheses. Key parameters include examination of enol ether influences, pyrylium counteranion structure reactivity relationships, and solvent effects. We will conclude with a discussion about the materials properties of stereocontrolled MF-ROMP products.
Bottom-up self-assembly of molecular building blocks to form hierarchical nanostructures across a broad range of length scales holds great promise for the synthesis of next-generation multi-functional materials. In this presentation, a synthetic platform for the preparation of multi-component graft block copolymers (GBCPs) as compositionally anisotropic molecular building blocks will be introduced for hierarchical self-assembly. For instance, a series of (A-branch-B)n-block-Cm GBCPs were synthesized by sequential ring-opening metathesis polymerization of “A-branch-B” and “C” macromonomers (MMs), where “A-branch-B” represents a branched MM containing a polymerizable norbornene group tethered with both A and B polymer chains. The chemical incompatibility of A and B side chains results in intramolecular phase separation, creating a pre-organized interface of A- and B-rich substructure with an interface normal to the backbone. The C-grafted block forms a superstructure with the A/B-grafted block with a spatial periodicity determined by the characteristic length of the backbone. Four distinct hierarchical morphologies including lamellae-in-lamellae, lamellae-in-cylinders, cylinders-in-lamellae, and cylinders-in-cylinders were readily prepared by varying the lengths of the backbone or side chains. The impact of composition, molecular dimension, and environment on the resulting super- and substructures was studied by X-ray scattering and electron microscopy. (A-branch-B)n-block-(A-branch-C)m and (A-branch-B)n-block-(C-branch-D)m type GBCPs were also prepared with hierarchical morphologies containing hetero-substructures. A more scalable synthetic route that employs random copolymerization of monofunctionalized A and B MMs for the formation of the A/B-grafted block was also developed and the self-assembly behaviors were compared with the previous GBCPs containing A-branch-B MM.
Block copolymer (BCP) materials and their self-assembly has been widely studied in the last two decades. BCPs are advantageous in generating periodic patterns at nanoscale over large areas as well as providing access to a wide range of morphologies and size of features. Hence, BCPs are considered to be a promising candidate for microelectronics as sub-10 nm feature sizes can be achieved in a scalable and cost-effective manner. Herein, we demonstrate the synthesis and self-assembly of poly(3-hydroxystyrene)-b-poly-(dimethylsiloxane)-b-poly(3-hydroxystyrene) (P3HS-b-PDMS-b-P3HS) triblock copolymers. The incorporation of P3HS and PDMS dramatically increases the effective interaction parameter and offers inherent etch contrast between the blocks. The triblock architecture also has a higher tendency towards forming perpendicular nanostructures compared with the diblocks. Hydroxy-terminated PDMS polymers were functionalized to initiate the atom transfer radical polymerization of an acetal protected 3-hydroxystyrene monomer. The target triblocks were achieved after a subsequent deprotection of the acetal group under mild acidic condition. The resulting triblocks have dispersities ranging from 1.10 to 1.26 and the synthesis provides robust control over molecular weights and volume fractions. The large chemical incompatibility of the blocks enabled the formation of ordered structures at low molecular weights that yielded lamellar periodicities as small as 9.3 nm. Smaller dimensions were obtained by triblocks than that of diblocks with similar molecular weights. The phase diagram of this triblock was mapped out based on the volume fractions and morphologies. The lamellar and cylinder morphology windows were observed at lower volume fractions of the P3HS block, relative to the diblocks. This phase diagram shift is ascribed to the dispersity of the middle block relative to the end blocks, which was confirmed by cleaving and characterizing the end blocks. Thin-film study on diblock and triblock also confirmed the advantages of triblock architecture for thin-film assembly.
As the demand for new materials in organic electronic devices (photovoltaics, field effect transistors, etc.) continues to grow, the routes by which these materials are synthesized becomes increasingly important in order to maximize their utility. This also brings a need for these procedures to become increasingly green in terms of the reagents used, the amount of waste produced, and the total number of synthetic steps. For conjugated polymeric materials, a current synthetic method that has shown promise in all these aspects is direct arylation polymerization (DArP). DArP has shown significant improvements over Stille, Suzuki, Negishi, Kumada, and Grignard metathesis by removing the need for stoichiometric quantities of reactive main group elements prior to cross-coupling, limiting the amount of toxic byproducts and waste, and reducing the overall number of steps in the reaction sequence. 2,3-Alkyl functionalized thieno[3,4-b]pyrazine (TP) has been shown to be an excellent candidate in the synthesis of alternating copolymers via DArP due to the existence of only two C-H bonds available for activation, which are also suitably acidic. When paired with other acceptors, TP produces remarkably low band gap (Eg) copolymers as low as 0.97 eV. This level of bandgap reduction is on the scale of some of the lowest Eg via alternating donor-acceptor copolymers to date and provides evidence of the significant electron-donating capabilities of the TP unit. This presentation will outline the modifications of DArP required to produce low Eg TP-acceptor copolymers, while highlighting that these conditions can be applied to a variety of dibromo-acceptor precursors without additional adjustments. Additional specifics such as solubility enhancement due to side chain tuning and the resulting physical and electronic properties of the polymers produced will also be discussed.
Despite recent progress in synthesis and utility of semiconducting nanoparticles (NPs) due to their intriguing optoelectronic properties, the compositional instability of some of them remains a great challenge for their practical applications. Herein, we report a unique strategy via capitalizing on a set of star-like block copolymers as nanoreactors for in situ crafting of uniform semiconducting NPs with readily tailored sizes, surface chemistry, optoelectronic properties, and more importantly, an array of markedly enhanced stabilities. The diameter of the resulting NPs can be conveniently tuned by regulating the molecular weight of the inner hydrophilic blocks of star-like block copolymers. The intimate and permanent tethering of the outer blocks of star-like block copolymers on the surface of semiconducting NPs imparts their effective dispersion in both the solution and dry state. Intriguingly, judiciously alternating the compositions and chain lengths of the outer blocks of the star-like block copolymers renders controllable, remarkably improved stability and additional functionality of semiconducting NPs. On the other hand, the incorporation of conjugated polymer connected outside semiconducting NPs manifested efficient separation of photogenerated charge carriers at their interfaces due to the appropriate electronic band alignment between conjugated polymer and semiconducting NPs. As a result, the polymer-ligated semiconducting NPs with both enhanced stability and efficient charge carriers separation can be utilized in novel application that previously hindered by their intrinsic instability, for instance, photo-induced polymerization. In principle, our star-like block copolymer nanoreactor strategy can be easily extended to synthesize functional NPs other than semiconductors for investigation into their dimension-dependent physical properties and self-assembly as well as various applications.
Although ring opening metathesis polymerization was known since the 60's, the first living ROMP systems required to discovery of well defined metathesis initiators In the mid 80's. Since that time many ROMP catalysts have been discovered that will Initiate living polymerization. These systems have opened the way to the synthesis of a wide array of well devined polymer systems.
The talk will include discussion and analysis of strategies and tactics aimed at the synthesis of natural products targets with an emphasis on a class found in humans. The presentation will also include the design and synthesis of probes inpsired by natural products with which to study and decode biology.
The use of just two types of building blocks, linear and angular, in conjunction with symmetry considerations allows the rational design of a wide range of metallocyciic polygons and polyhedra via the coordination motif. We have used this approach to self-assemble a variety of 2D supramolecular polygons such as triangles, rectangles, squares, hexagons, etc. as well as a number of 3D supramolecular polyhedra: truncated tetrahedral, triginal prisms, cubooctahedra, dodecahedra as well as metallapolymers. More recently we have functionalized these rigid supramolecular scaffolds with different electroactive, host-guest, dendritic, and hydrophobic/hydrophilic moieties and have investigated the properties of these multi-functionalized supramolecular species. Additionally, we have explored the self-assembly of 2D polygons and 3D polyhedra on a variety of surfaces with the aim of developing their potential in device settings. These supramolecular ensembles are characterized by physical and spectral means. The design strategy, formation, characterization and biomedical activities of these assemblies will be discussed, along with our very recent results.
Synthetic efficiency demands that asymmetric synthesis, must be the long term objective for the synthesis of chiral molecules. While most attention has focused on transition metal catalysis, less effort has involved main group asymmetric catalysis. Furthermore, most metal catalysts are mononuclear. Given the nature of most chemical reactions wherein two groups are added, dinuclear catalysts have much more potential due to the ability to tailor the choice of metal to each reactant. A ligand derived from a,a-diarylprolinols and phenols provides spontaneous self-assembly of dinuclear metal complexes with zinc and/or magnesium as the metal. The effect of ligand structure on performance is evaluated. This catalyst has shown promise in asymmetric polymerization. Extension to hetero dinuclear chiral complexes is also illustrated.
The 1955 demonstration by Lord Todd that dinucleotides could be constructed using P(V)-based chemistry was rapidly discarded with the advent of Carruthers pioneering phosphoramidate platform. This discovery revolutionized the field and enabled much of what we take for granted today in the oligonucleotide world. Our lab has been interested in reinvestigating whether the native P(V)-based chemistry that Nature uses in her approach has any place in mainstream oligonucleotide synthesis. This talk will focus on our latest strides in this area of inquiry.
Due to the increasing demands for environmentally friendly materials, bio-derived and degradable materials are of major interest. Lipoic acid and its derivatives, which contain a reactive 5-membered cyclic disulfide ring, undergo copolymerization with vinyl monomers leading to S-S units along the backbone. This has the potential of unique dynamic properties such as self-healing, reconfigurability, and degradability. In this presentation, fundamental studies of copolymerization of lipoate derivatives (lipoic acid ester) with conventional vinyl monomer (acrylate, styrene and etc.) will be described. The properties of the resulting copolymers are found to be tunable and illustrate a path towards sustainability for vinyl-based materials.
Epoxy resins contain an oxirane ring prior to cure and require a curing agent or catalyst to create a three-dimensional, cross-linked network. Curing agents include acid anhydrides and primary or secondary amines. Common catalysts are tertiary amines. The final polymer properties depend on the type and quantity of curing agent or catalyst used. Epoxy resins are known for having high tensile strength, good chemical resistance, and dimensional stability. Despite having a range of favorable properties, epoxy resins are highly flammable making them challenging to use in high temperature applications. One pathway to improve thermal stability is to incorporate molecules with inherently high char yields. A high char yield is favorable in high temperature applications because the char acts as a thermal insulator and protects the bulk material from excess heat and mass transfer.
Epoxy resins have an additional concern. Many commercially available epoxy resins are synthesized from bisphenol A (BPA). BPA is a known human endocrine disruptor and sourced from petroleum-based feedstocks. Therefore, there is a need to find a suitable renewable alternative to BPA that can be used in synthesizing epoxy resins for high temperature applications.
Furan is a five-membered, heterocyclic, aromatic compound from renewable polysaccharides. The aromatic nature and renewable feedstock make furan a good alternative to benzene. Additionally, furan is known to increase the char yield when used in polymer networks. Poly(furfuryl alcohol), a furan-based thermoset, is primarily used as a high temperature foundry binder further confirming furan’s ability to increase thermal stability.
In this study, furan-based furfuryl amine was epoxidized to create a furan-based, renewable diepoxy monomer. The diepoxy monomer contained two tertiary amines eliminating the need for a curing agent or catalyst to form a cross-linked polymer. The curing behavior was investigated using Differential Scanning Calorimetry. Thermogravimetric and viscoelastic tests were performed on the cured polymer. The final char yield at 1000 °C was 39 ± 1 % with a Tgof 79 ± 1 °C based on the tan delta.
Utilizing carbon dioxide as a polymer feedstock is an ongoing challenge for the field of polymer synthesis. In this talk, we will describe the catalytic conversion of carbon dioxide and 1,3-butadiene to a degradable polymer structure. The microstructure of the material arises from an unexpected combination of ring-opening and conjugate addition mechanisms. The procedure, isolation, and characterization of the material will be described and how the structure was ultimately assigned. Potential mechanisms of propagation will be provided along with the degradation properties of the materials. The discovery expands the possibilities of structure that are accessible from olefins and CO2 feedstocks.
Homogeneous polymer network is highly attractive due to its synthetic ease, high purity and straightforward mechanical property. We are interested in Diels-Alder (DA) chemistry for macromolecular syntheses due to its simplicity, scalability, and absence of catalysts. However, most of the common DA reactants are consisted of two components, an electron-rich diene and an electron-poor dienophile. As a result, there are associated challenges such as stoichiometry, solubility, and long synthetic steps. We propose that a highly reactive single component will resolve these issues. Cyclopentadiene (CPD) is the most promising candidate due to its unique property of serving as both diene and dienophile to form dicyclopentadiene (DCPD). Furthermore, its dimer's known retro-DA at 180 °C is ideal for selective depolymerization. Intriguingly, not many efforts have been put into exploring this potential compound, partly due to the historically significant challenge of handling its reactivity, dating back to Staudinger and Stille. Wudl later controlled the instability of the CPD monomer by synthesizing a protected cyclic DCPD monomer, which was then cracked in situ at high heat. A single-component polymer network was synthesized with trimer formation as the crosslinking point. However, the specific cyclic design of the monomer and trimer crosslinking have limited the syntheses of new DCPD functional materials significantly. We are inspired that with our strategy to gain access to a variety of pure CPD derivatives, new DCPD materials can be obtained. Specifically, we utilized our previously reported norbornadiene-tetrazine cascade reactions to synthesize and isolate a pure 4-arm CPD monomer. A new highly crosslinked DCPD polymer network was successfully synthesized. The material is tough and has fast shape-recovery with Tg of 22 °C. The storage modulus is 1.4 GPa before Tg and 5.0 MPa after Tg. The material is ideal for shape-memory effect and is selectively depolymerized at 150 °C with proper additives.
RAFT dispersion polymerization of a prototypical methacrylic monomer, methyl methacrylate (MMA), is performed in mineral oil using various poly(lauryl methacrylate) (PLMA) precursors prepared with a trithiocarbonate-based RAFT agent. GPC analysis indicated reasonably narrow molecular weight distributions (Mw/Mn ≤ 1.39) for all diblock copolymers, with 1H NMR studies indicating high MMA conversions (≥95%) for all syntheses. An efficient one-pot synthesis protocol enabled high blocking efficiencies to be achieved when targeting higher PMMA DPs. However, the relatively high glass transition temperature (Tg) of the corresponding core-forming PMMA block unexpectedly constrains the evolution in copolymer morphology during polymerization-induced self-assembly (PISA). More specifically, well-defined PLMA22–PMMAx spheres (x = 19–39) and relatively short worms (x = 69–97) can be obtained at 90 °C when using a PLMA22 precursor but targeting higher x values (x ≥ 108) invariably leads to colloidally unstable aggregates of spheres, rather than long worms or vesicles. Interestingly, similar constraints were observed when targeting higher solids, when using n-dodecane instead of mineral oil, or when employing an alternative steric stabilizer block. Raising the PISA synthesis temperature from 90 to 115 °C (i.e., from below to above the Tg of the final PMMA block) does not alleviate this unexpected problem. Moreover, only spherical nanoparticles can be obtained at 115 °C when targeting PMMA DPs between 50 and 400 with the same PLMA22 precursor. This suggests that nanoparticle formation may occur by a chain expulsion/insertion mechanism at this relatively high reaction temperature. PLMA22–PMMAx nanoparticles were characterized in terms of their particle size and morphology using dynamic light scattering (DLS), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). DLS and TEM studies of a 0.1% w/w dispersion of PLMA22–PMMA69 short worms indicated an irreversible worm-to-sphere transition on heating from 20 to 150 °C. Oscillatory rheology and TEM studies indicated that this thermal transition was only partially reversible for a 20% w/w dispersion of PLMA22–PMMA69 short worms.
Polyfluoroolefins such as polyvinylidene fluoride (PVDF) and its copolymers are an important class of semi-crystalline high added-value polymers, owing to their uncommon properties of chemical and ageing resistance as well as their superior electroactivity. The preparation of well-defined polyfluoroolefin-based architectures such as block copolymers is not easy due to the peculiar reactivity of the fluorinated monomers. Recent studies of the RAFT polymerization of VDF showed how this chemistry could be harnessed to synthesize a large range of original polymer architectures such as block copolymers (BCP) for example. Amphiphilic PVDF-based block copolymers are still very rare. This communication will present recent advances in the macromolecular engineering of VDF-based materials allowing the preparation of well-defined PVDF-based architectures. It will also show how these semicrystalline BCP can self-assemble in solution to form unprecedented higher-order morphologies via crystallization-driven self-assembly.
Elemental sulfur is a waste product of fossil fuel production that has accumulated annually in megaton quantities. The discovery of inverse vulcanization, pioneered by Pyun in 2013, unlocked a new class of material. Plant-derived terpenoids are widely used for pharmacological and biological activities. Terpenoids and terpenoid derivatives are also possible surrogates to take the place of petrochemical olefins. In the current research, it was discovered that the formation of crosslinks between the elemental sulfur and the olefinic units of terpenoid with heat treatment leads to remeltable materials with structural characteristics competitive with those of commercial goods.
Portland cement production accounts for CO2 emission at a level similar to that of all global road transportation. Carbon-negative cement products are therefore essential to humanity’s quest to supplant traditional technologies with more sustainably-sourced alternatives. In this contribution, carbon-negative polymer cements were prepared in a 100% atom-economical reaction between plant-derived terpenoid alcohols (citronellol, geraniol, or farnesol) and elemental sulfur. The influence of component ratio on thermal and mechanical properties of composites were determined by di-unsaturated geraniol and tri-unsaturated farnesol , GerSx, FarSx (x = wt % sulfur) respectively.
The sulfur incorporated into composite was characterized by using DSC measurements. The 1H NMR spectrometric measurements revealed unexpected cyclization of terpenoids in the composite. The high mechanical strengths of the terpenoid-sulfur composite are the accountable factors for the use of building materials. Compressive strength measurements were undertaken to assess the viability of terpenoid-sulfur composites as structural elements. Chemical resistance of the material is another significant attribute that was assessed for these composites. The noticeable improved compressive strengths were discovered for both high olefins contain composites (80 -85% of terpenoids).
Control over macromolecular architecture and chemical functionality allows for tuning polymer material properties, such as mechanical performance and ionic conduction. Here, we present an effective post-polymerization functionalization method to synthesize grafted poly(butadiene) (PBD) via allylic bromination and subsequent atom transfer radical polymerization (ATRP) grafting. The highlight of this work is that the graft density and graft length can be tuned by adjusting the amount of bromine functionalized to the polymer backbone and by altering monomer to macroinitiator ratios, respectively. Furthermore, the use of ATRP allows for tailoring polymer graft chemistry, which is critical for uses in different applications such as polymer electrolyte membranes. Additionally, functionalizing block copolymers using the same allylic bromination and ATRP grafting, it is possible to control material nanostructure, as confirmed with small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The work presented here demonstrates a novel and tunable post-polymerization functionalization method that opens new avenues for converting polydienes.