Symposium SM10—Progress in Green Chemistry Approaches for Sustainable Polymer Materials

Terpenes are naturally occuring unsaturated hydrocarbon compounds produced predominantly by plants. Their unsaturation in combination with their rich structural diversity make them interesting building block candidates for biosourced polymer development. While direct homopolymerization of terpenes is a well-known approach to more complex polyolefin analogues, formation of polyester or polycarbonate macrostructures that incorporate terpene monomers is still at its infancy. In this regard, development of novel catalysts that can address the copolymerization of terpene oxides with either cyclic anhydrides (to form polyesters) or CO₂ (to provide polycarbonates) is of high current interest. Our group has been interested in the use of renewable compounds for heterocyclic and polymer synthesis, and initially our focus was on the copolymerization of D-limonene and CO₂. The resultant polycarbonates demonstrated a versatile synthetic behavior allowing to fine-tune functionality, thermal behavior and application potential such as increasing the hydrophobicity. At a later stage, we developed polyesters from a range of terpene oxides (including those on limonene, menthene and carene) illustrating a further diversity of structurally useful polymer backbones that allowed to fine-tune the thermal properties over an extensive glass transition temperature range (50-235 deg). In a more recent embodiment of our work, we have used beta-elemene (a terpene with three distinct double bonds) as a precursor towards functional polymer structures by copolymerization with cyclic anhydrides. This has allowed us to devise polyesters with two built-in functional groups that can be addressed and transformed individually giving impetus to materials with tunable mechanical and thermal features that are not easily feasible through conventional polymerization processes. Key to the development of all these poly-carbonates and poly-esters has been the enabling power of aluminium- and iron-based aminotriphenolate complexes that have priviliged catalytic reactivity in the activation of sterically congested terpene oxide monomers. In this presentation, both the importance of catalyst design and the potential of the functional terpene-derived polymers will be discussed in detail.

Thermosets, and epoxy resins in particular, constitute an important class of materials on the global polymer market that are valued in plethora of applications requiring superior mechanical properties as well as thermal and chemical stability. As downside, however, they can hardly be recycled and reprocessed due to their permanent cross-linked architecture. In response to this concern, adaptable covalent networks featuring dynamic cross-linking based on associative exchange reactions, also called vitrimers, emerged some years ago.[1-4] Above a certain temperature, the kinetic of the exchange reactions increases significantly, the topology of the network changes and the material can be reshaped or healed. In the case of epoxy-acid vitrimers, the relaxation of the network relies on transesterification exchanges promoted by catalysts such as zinc carboxylates, organotin compounds, triphenylphosphine, etc.[1-4] Despite the recognized activity of the latter, the search for alternative non-toxic catalysts with high efficiency, low leaching tendency as well as heat and air sensitivity is still relevant.
As will be discussed in the communication, driven by the attractive properties and catalytic activity of ionic liquids (ILs),[5-6] we designed epoxy-acid resins in the presence of ILs for the first time and produced unprecedented imidazolium-catalyzed epoxy vitrimers.[7] In addition to outstanding features such as chemical and thermal stability, low vapor pressure, high conductivity, imidazolium salts are known as very efficient homogeneous and heterogeneous catalysts especially in transesterification reactions.[5-6] In practice, a series of cross-linked epoxy-acid networks were synthesized from diepoxy and diacid compounds cured in the presence of imidazolium acetate catalysts obtained via a single-step and atom economical Radziszewski multicomponent reaction. The impact of the imidazolium on the course of the curing reaction, the structure of networks and the mechanical properties, will be discussed. The vitrimer-like properties of these IL-containing networks, and so the ability of the imidazolium additives to catalyze the transesterification exchange reactions, will also be demonstrated via stress relaxation experiments. The effect of the nature and concentration of the imidazolium derivatives on the network dynamics will also be discussed. Finally, it will be shown that such imidazolium-containing resins can be recycled several times while preserving their excellent mechanical properties.
In addition to demonstrate the capacity of ILs to play the role of catalysts in the field of adaptable networks, this contribution also emphasizes that a low amount of imidazolium within ester networks is likely to induce some relaxation. Moreover, given the great variety of ILs and the large set of reactions they can catalyze, this approach should be applicable to adaptable networks governed by other types of crosslinking bonds and could contribute to the recycling of various polymer resins in the future.

[1] D. Montarnal, M. Capelot, F. Tournilhac, L. Leibler, Science, 2011, 334, 965-968.
[2] M. Capelot, M.M. Unterlass, F. Tournilhac, L. Leibler, ACS Macro Lett., 2012, 1(7), 789-792.
[3] T. Liu, B. Zhao, J. Zhang, Polymer, 2020, 194, 122392.
[4] M. Guerre, C. Taplan, J. M. Winne and F. Du Prez, Chem. Sci., 2020, 11, 4855–4870
[5] Z.I. Ishak, N.A. Sairi, Y. Alias, M.K.T. Aroua, R. Yusoff, Catal. Rev, 2017, 59(1), 44-93.
[6] P. Stiernet, A. Aqil, X. Zhu, A. Debuigne, ACS Macro Lett., 2020, 9(1), 134-139.
[7] P. G. Falireas, J.-M. Thomassin, A. Debuigne, Eur. Polym. J., 2020, in press.

Funding: F.R.S.-FNRS, PDR, VITRIPILS project (ID 9236)

Degradability as a material function is characterized by its kinetics and the degradation time period depending on the specific system environment. (Co)Polyesters like PLGA (poly[rac-lactide-co-glycolide]) or PCL (poly(ε-caprolactone)) degrade in aqueous environments via a hydrolysis reaction, which can be catalyzed by enzymes. In case of these two polymers, water molecules diffuse into the polymer in order to reach the ester bonds in the bulk. The degradation depends on the crystallinity and starts in the amorphous region of the polymer. In these regions, polymers are degraded by chain cuts that are occurring either randomly between all repeating units of the polymer (random chain cut mechanism) or preferentially at the end of the polymer chains (chain end cut mechanism). If the generated fragments are small enough, they dissolve in water and diffuse away, which leads to polymer mass loss. We recently showed with a Monte Carlo simulation that the direction of the diffusion of water molecules or small fragments through a semi-crystalline polymer is not influenced by the internal morphology of its crystalline regions and can be described with a random walk behavior through the amorphous regions of the polymer. [1] Here, we present an equation for the degradation of polymer chains for the random chain cut and the end chain cut mechanism depending on the initial number of repeating units, the initial number of chain ends and the length of diffusive fragments. The degradation function of molecular mass consists of three regions, which are characterized by the current molecular weight of the polymer. It is further investigated, how the type of the chain cut mechanism changes the degradation behavior and additional modifications, e.g. the effect of increasing water sorption and/or autocatalysis, are discussed.
Based on the analytical models, which are verified with a Monte Carlo model, we suggest new ways to present and evaluate experimental polymer degradation data that are more precise than the currently established first order assumptions. We demonstrate this by evaluating experimental data and discuss how the model helps to advance degradable material design.

[1] Hoffmann, F., Machatschek, R., & Lendlein, A. (2020). Understanding the impact of crystal lamellae organization on small molecule diffusion using a Monte Carlo approach. MRS Advances, 5(52-53), 2737-2749.

Foams are versatile materials encountered in our daily life for a wide variety of uses such as cushioning, thermal and acoustic insulation or medical applications. The combination of the mixed properties between a continuous matrix and gas cells and the diversity of pore structures represent a powerful tool for the design of new materials. Among the different polymer foam fabrication processes, the use of supercritical CO₂ has been one of the most investigated in the past decade. Nevertheless, the design of crosslinked polymer foams with high foaming ratio still remains a challenge. Various crosslinking processes mainly based on heating, irradiation with the addition of an external agent have been applied after foaming but remain difficult to perform due to mass transfer issues of the crosslinking agent. When crosslinking occurs before foaming, it dramatically limits the material expansion.

In order to overcome these drawbacks, the present work aims taking advantage of the thermoreversible Diels-Alder cycloaddition to elaborate foams of poly(ε-caprolactone) (PCL) covalent networks. Based on this reaction, we considered to induce cross-linking after the foam expansion by playing on the thermal equilibrium of the thermoreversible Diels-Alder cycloaddition. Therefore, low molar mass star-shape PCL end-capped by furan or maleimide were impregnated with CO₂ under supercritical conditions and then foamed under appropriate control of the pressure and temperature. The resulting foam possesses a much higher volume expansion than a pre-crosslinked sample foamed in the same conditions, thanks to the low crosslinking ratio during foaming. These foams exhibit also improved thermal stability thanks to its chemical crosslinking as compared to non-crosslinked PCL foams. Interestingly, these foams possess shape memory properties due to the semi-crystallinity of the PCL. Thermal stability and shape memory properties were evaluated by dynamic mechanical analysis in both tensile and compression testing with controlled force mode, stress and temperature ramps. Since significant maleimide/furan adduct cycloreversion can be achieved at high temperature, system reversibility and recyclability have also been attested.

This foaming process proves itself very interesting by the formation of highly physically expanded and recyclable crosslinked foams from a non-initially foamable material. This contribution aims at reporting a new concept that can be used for the preparation of highly expanded and crosslinked polymers foams from any semi-crystalline polymers.

The establishment of fiber reinforced polymers (FRPs) as lightweight structural components is driven by the need for strong, stable materials in stressful environments. Mainly fashioned from glass or carbon reinforcing fibers impregnated with polymer matrices, these composites can significantly reduce fuel consumption in aircrafts, lowering carbon emissions. Nevertheless, they are currently derived from depletable, greenhouse gas (GHG) emitting fossil fuel sources. FRPs present an opportunity to aid the effort of mitigating climate change, a global phenomenon attributed to the accumulation of atmospheric GHGs and associated with negative, irreparable impacts on ecosystems and humanity. Polymer synthesis using alternative, renewable monomer feedstocks would allow for regenerative material origins while providing a pathway to GHG reductions associated with virgin supplies.
One of the most common polymer matrices in aerospace composites is phenolic resin, a material used in secondary structural components for its low flammability and high temperature resistance. Due to strict material specifications within the aerospace industry, the phenol and formaldehyde monomers used to synthesize phenolics must be identical to current supplies. Therefore, the foundational requirements in decarbonizing phenolic’s supply chain are scaling up benchtop methods or leveraging currently industrialized processes that produce pure monomers which are also renewably sourced. Current efforts include thermochemical conversion of biomass and fermentation of sugars for phenol and oxidation of bio-methanol for formaldehyde. Another benefit of creating pure supplies is the opportunity to directly substitute feedstocks taking advantage of downstream infrastructure.
Establishing the sustainability of renewable polymer feedstocks requires a balance of whole system perspectives which include social, technical, economic, environmental, and political aspects. This work will assess material criticality and resiliency of feedstocks for organic materials through metrics such as the magnitude and location of material reserves, material substitutability, and corporate importance from a whole systems perspective.

Many traditional applications of polymers have relied on their high long-term stability. For example, it is desired that plastic packaging retains its structure and properties for months or years, while the plastic used in joint replacements should survive for decades in the human body. However, there are growing concerns about the impact of plastic pollution on humans and the environment. It is predicted that almost 12,000 metric tons of plastic will have accumulated in landfills and the environment by 2050. Furthermore, for many applications of polymers in therapeutics and regenerative medicine, it is desired that polymers break down, rather than accumulate in the body. Therefore, interest in developing degradable polymers has been growing. Significant progress has been made in the preparation and application of polyesters and polysaccharides. However, these polymers degrade gradually in many different environments, sometimes more slowly or more rapidly than desired, which hinders their widespread application. To gain a new level of control over when and where polymers degrade, we have been developing “self-immolative polymers” (SIPs). These polymers are designed to be stable during their use, but then to degrade (depolymerize) rapidly when triggered by a stimulus. Their mechanism of degradation involves a reaction cascade such that a single stimulus event is sufficient to degrade an entire polymer chain. In addition, the stimulus to which they respond can be easily changed by switching a single capping molecule at the end of the polymer. This presentation will describe our research on polyglyoxylates. Polyglyoxylates were synthesized by anionic-like polymerization mechanisms. A diverse library of end-caps was developed, allowing depolymerization to be triggered by stimuli including light, heat, pH changes, reducing agents, and reactive oxygen species. The properties of the polyglyoxylates were tuned by incorporating different pendent groups, either through polymerization of different glyoxylate monomers or through post-polymerization transesterification reactions. Furthermore, amidation reactions were used to convert readily accessible poly(ethyl glyoxylate) to various polyglyoxylamides, which exhibit very different properties than the polyglyoxylates. The function of these degradable polymers was demonstrated through formation of polymer particles, micelles, and vesicles that were capable of loading and releasing molecules on demand. In addition, polyglyoxylates were applied as coatings to release agricultural chemicals in response to stimuli. Overall, this work demonstrates the versatility and applicability of self-immolative polymers across a range of fields.

Recent years have witnessed an increasing demand on renewable resource-derived polymers owing to increasing environmental concern and restricted availability of petrochemical re-sources. Thus, a great deal of attention was paid to renewable resources-derived polymers and to thermosetting materials especially, since they are crosslinked polymers and thus cannot be recycled. Also, most of thermosetting materials contain aromatic monomers, able to confer high mechanical and thermal properties to the network. Therefore, the access to biobased, non-harmful, and available aromatic monomers is one of the main challenges of the years to come. Starting from phenols available in large volumes from renewable resources, our team designed platforms of chemicals usable for the synthesis of various polymers. Hence, we studied, tanins, lignin-derived vanillin, eugenol or cardanol. Various aromatic building blocks bearing polymerizable functions were synthesized: epoxy, amine, acid, carbonate, alcohol, (meth)acrylates... (Figure 1). These bio-based aromatic monomers can potentially lead to numerous polymers. The substitution of bisphenol A was studied in epoxy thermosets. Materials were prepared from the biobased epoxy monomers obtained from vanillin. Their thermo-mechanical properties were investigated and the effect of the monomer structure was discussed. High Tg phenol-free phenolic thermosets have been synthesized. Phenol and formaldehyde free phenolic thermosets were also prepared with high thermal stability. The properties of the materials prepared were found to be comparable to the current industrial references, indicating a potential replacement of fossil resources. The tunability of the final properties was achieved through the choice of monomer and through a well-controlled oligomerization reaction of these monomers.

Motivated by the recent drive to replace petrochemical plastics for more renewable source-based materials, our efforts focus on designing and characterizing a next generation biomolecular-based polymer for solution assembly applications. Sugar-derived poly(D-glucose carbonate) (PGC) molecules are synthesized, and their solution chain dynamics and assembly behavior are analyzed in the production of nanoparticles. Unlike conventional vinyl-based polymers with flexible coil-like chains, the PGC system is characterized by its entire backbone composed of semiflexible, hydrophobic glucose monomers where its chain rigidity and local amphiphilicity created by the backbone compared to added side chains are expected to significantly impact both local and global solution chain behavior. First, with a PGC-containing amphiphilic diblock copolymer system, we explored kinetically controlled assembly pathways by variation of solvent composition that lead to the hierarchical assembly of ribbon-like fibers with features that do not follow the traditional BCP micellar-like packing. We show that while the stiffness of the PGC backbone impacts the local BCP chain conformation and chain packing within the assembled nanostructure, the backbone hydrophobicity drives the unidirectional hierarchical fibril growth by the formation of soft patchy precursor particles. Second, to understand how the chain behavior affects the final assembly morphology, the hydrophilic block equivalent homopolymers were studied using small-angle neutron scattering techniques to obtain polymer chain solution properties for a better understanding of the PGC block copolymer chains in similar solvent conditions. These results suggest polymers with unconventional backbone chemistries, frequently found in natural carbohydrate-based molecules, can shed light on the effects of polymer backbone rigidity and hydrophobicity on the assembly pathway and final assembled structures while also presenting opportunities to better understand other bio-inspired green chemistries-based molecules can organize and assemble in various environment conditions.

Polymeric materials modulating regenerative processes in vivo need to fulfill multiple requirements that include handling, macroscopic performance after implantation, guiding of cellular behavior, as well as degradation creating space for new tissue to form. Adressing the individual requirements by macromolecular chemistry is likely to lead to highly complex polymer systems. In addition, biomaterials with novel chemistry face substantial hurdles in the regulatory process. For translation, use of components already established in the clinic would be highly beneficial, but requires the realization of multifunctionality [1] by material design. Electrospun polymer meshes are of interest as they allow a multiscale design by adjustment of fiber diameter and fiber organization in the mesh (orientation, density) in addition to the macroscopic shape. Furthermore, the porous structure allows facilitates cell migration. Polylactide (PLA) is – due to the histocompatibility with different tissues and its hydrolytic degradability - commonly applied in biomedicine. The rate of crystallization of PLA is however quite slow, and as crystallinity of the polymer matrix is of importance for its mechanical behavior, control over the crystallization would be advantageous. Interestingly, mixtures of isotactic Poly(L-lactide) (PLLA) and Poly(D-lactide) (PDLA) form stereocrystallites [2] that are formed faster than PLA homocrystallites, and moreover have higher thermal, mechanical, and hydrolytic resistance. Furthermore, lactide is considered a green polymer.
Here, we hypothesized that by forming coaxial nanofibers of PLLA and PDLA, it is possible to initiate the formation of stereocrystallites at the interface of the two layers and therefore have a temporal and spatial control over the crystallization of the matrix. In fact, electrospinning of coaxial fibers consisting of a lower M_w (= 47 kg mol^-1) PLLA core and a higher M_w (= 147 kg mol^-1) PDLA shell resulted in amorphous fibers. Annealing of the fibers above the glass transition temperature T_g led to stereocrystallization as shown by Wide Angle X-Ray Scattering. DSC studies could be used to quantify crystallization, but does not represent the situation after spinning as crystallization also happened during the heating phase. The crystallization led to changes in the mechanical behavior as determined in tensile tests at room and temperature in the dry state and under physiological conditions. Interestingly, the stereocrystal containing materials had lower Young’s moduli E (9±1 MPa) compared to the as-spun materials (46±8 MPa), and were also softer than PDLA or PLLA, which can be rationalized by localization of the crystals and relaxation effects. In a second step, gelatin was added to the PDLA phase in order to generate fibers that display adhesion sites for cells on the surface, which was shown by labelling of gelatin on the fiber surface. Gelatin did not negatively influence the (stereo)crystallization of PLA and led to a slight reduction of the T_g. The overall morphology can therefore be understood as semiinterpenetrating polymer network with physical netpoints (crystallites). Altogether, a material system with crystallization controlled by design as well as with cell adhesion sites could be formed in one step based on components established in the clinics.
1: A. Lendlein, R.. Trask, Multifunct. Mater. 2018, 1, 010201.
2. V. Izraylit,P. Hommes-Schattmann, A.T. Neffe, O.E. Gould, A. Lendlein, Eur. Polymer J. 2020, 137, 109916.

Polyphosphoesters have recently emerged as new materials for biomedical applications thanks to their biocompatibility and biodegradability properties that can be finely tuned by varying their functional side-chains linked to a pentavalent phosphorus. The preparation of polyphosphoesters frequently relies on the ring-opening polymerization of 5-membered cyclic phosphate monomers (CPMs), which are generally synthetized through a 3-step procedure. Starting from bulk chemicals the preparation of such monomers by a batch process is time-consuming, difficult to scale-up and involves safety concerns such as the uncontrolled generation of HCl by-product and the handling of sensitive and corrosive intermediates. To this end, we developed a semi-continuous flow platform allowing the direct preparation of CPMs starting from PCl₃ and a 1,2-diol derivative without isolation of the intermediates. The first step involved the preparation of cyclic chlorophosphite derivatives by reacting a neat diol with a highly concentrated organic solution of PCl₃ at room temperature with an associated residence time of 1 minute in a compact coil reactor. This procedure allowed to produce a wide range of novel cyclic chlorophosphites starting from various diols in moderate to high yields and with a daily productivity of up to ~500 g. The scope was also extended to thioalcohol, dithiol, aminoalcohol and diamine derivatives and the process was eventually adapted to a base-involving procedure allowing the conversion of more demanding substrates such as highly hindered pinacol-type diols. The next oxidation step toward cyclic chlorophosphates was directly concatenated with the upstream production of chlorophosphites in a single continuous flow system. A high pressure and an improved mixing capacity allowed to perform the oxidation using 4 equivalents of molecular oxygen in only 21 seconds of residence time at 65 °C toward a quantitative conversion while batch procedures usually require tens of hours or days of reaction time. A final functionalization of chlorophosphates toward various CPMs was subsequently integrated in a semi-continuous flow platform where the effluent coming from the oxidation step is directly reacted in a batch reactor. The system allowed to produce the CPMs directly from PCl₃ in an extremely shortened time by a single process without purification and isolation of the sensitive intermediates while mitigating the hazards linked the corrosive derivatives produced. Among the novel monomers prepared, a bifunctional derivative was successfully copolymerized by ring-opening polymerization allowing the introduction of a typical alkoxy pendant group linked to a pentavalent phosphorus and an additional convertible chloromethyl function directly linked to a main-chain carbon. The introduction of such functionalizations opens new opportunities for the preparation of novel materials with various properties which could be time-dependent through the quick (P-linked group) or slower (C-linked group) degradation of the polyphosphoester linkages.

Lignin is an abundant, under-utilized, and cheaply available bioresource. It is a phenolic macromolecule which substantially differs depending on the plant species and the type of isolation process. In this presentation, we describe a green approach to make making chemically reactive lignins and lignin-containing composite materials. These composite materials were shown to be applicable as growing media for hydroponics. Our reaction scheme was to couple lignin with a bifunctional linker molecule comprising of an aromatic amine group and a protected vinyl sulfone group. The chemical process begins with the conversion of the aniline-amine group of this linker to an electrophilic diazonium salt to react with the electron-rich naphthalene rings of the lignin. The other functional end of the linker was then deprotected under basic conditions to yield the vinyl sulfone for forming covalent bonds to the hydroxyl groups of substrates via the Michael reaction. The only byproducts are harmless inorganic salts. Unlike common composite fabrication processed that require organic solvent, the process is performed in water and thus is eco-friendly.
The increase in hemp farming in the U.S. in recent years has resulted in a large supply of hemp fibers. We demonstrated the application of our green approach to fabricate hemp fiber composites for hydropnics. The lignin inside hemp fibers was functionalized by employing a bifunctional linkers to make chemically reactive hemp. The anchored vinyl sulfonate group of the linker was then used to cross-link hemp fibers with polyvinyl alcohol (PVA) to yield hemp fiber composites. These hemp composites were analyzed to have a Young’s modulus of 5.4 × 10^-3 GPa but were also found durable. Other properties of this composite such as water retention capacity values, C/N ratios, and salinity were evaluated to discern their suitability for hydroponic applications. The hemp fiber composites were applied as hydroponic growing media to grow model plant systems. Our results demonstrated that the hemp composites can be as effective growing media as other commercially available ones.

The recycling of carbon dioxide (CO₂) by transforming this waste into value has become a major goal in contemporary science. Strategies are emerging to turn this renewable carbon feedstock into valuable engineering plastics while diversifying renewable resources for the sustainable production of consumer materials.¹ Novel routes to efficiently turn CO₂ into polymers are expected to accelerate and facilitate the transition from existing fossil-based to future generations of more sustainable materials while trying to meet the requirements of a circular economy for consumer plastics.
In the first part of this talk, we will discuss how CO₂ can be converted into monomers² that are then involved in step-growth copolymerizations with diamines to produce non-isocyanate polyurethanes (NIPUs)³, greener variants of conventional polyurethanes that are commonly prepared by the toxic isocyanate chemistry. These monomers can be easily and quantitatively produced at the multi-kg scale under solvent-free conditions in our lab. We will show how they can be exploited to design some representative high performance NIPUs materials. We will first discuss how adhesives and coatings can be produced under solvent-free conditions, with adhesion performances on various substrates (aluminum, wood, stainless steel, glass) that compete to those of commercial products, provided that appropriate curing is applied.⁴ We will then report on an innovative robust and solvent-free process for the construction of flexible or rigid self-blown NIPU foams that offers the first realistic alternative to the traditional isocyanate route.⁵ In this process, the CO₂-based monomer is not only exploited to construct the NIPU matrix but also for its self-blowing. If time permits, we will also show that NIPUs hydrogels can be produced in water at room temperature without any catalyst with impressive short gel times.⁶ These hydrogels can be easily reinforced by introducing natural polymers or clay in the formulations prior to curing. All these technologies can be easily scaled-up and are highly versatile, opening new opportunities in the design of more sustainable materials while valorizing CO₂ as a renewable carbon feedstock.
In the second part of the talk, we will describe an innovative approach for the facile preparation of new regioregular functional NIPUs (e.g. poly(oxo-urethane)s and poly(oxazolidone)s) at room temperature by using a novel family of monomers prepared by the CO₂ chemistry.⁷ The special reactivity of these monomers will be discussed, and if time permits, we will show how they can be exploited for the facile construction of other relevant polymers (e.g. polycarbonates, sulfur-containing polymers) under mild operating conditions.^7,8

References.
1. Detrembleur et al. Chem. Soc. Rev. 2019, 48, 4466.
2. Detrembleur et al. Catal. Sci. Technol. 2017, 7, 2651.
3. Cramail et al. Chem. Rev. 2015, 115, 12407.
4. Detrembleur et al. ACS Sustainable Chem. & Eng 2018, 6, 14936; Polym. Chem. 2018, 9, 2650; Polym. Chem. 2017, 8, 5897.
5. Detrembleur et al. Angew. Chem. Int. Ed. 2020, 59, 17033 ; patent application EP3760664(A1)
6. Detrembleur et al. ACS Sustainable Chem. & Eng 2019, 7, 12601; Macromol. Rapid Commun. 2020, 2000482.
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8. Detrembleur et al. Angew. Chem. Int. Ed. 2019, 58, 11768; J. Mater. Chem. A. 2019, 7, 9844; ACS Appl. Polym. Mater. 2020, 2, 922.

Based on the bulk chemical 1,4-butynediol, readily available epoxides and carbon dioxide, a new series of unsubstituted exovinylene carbonates were synthesised. Chemoselective additions of diamines or diols to these cyclic carbonates allow the regiocontrolled synthesis of new functionalised polyurethanes and polycarbonates under mild conditions. This route to polyurethanes avoids the use of toxic isocyanates.
For the monomer synthesis, we also developed a straight-forward approach to separate the catalyst from the exo-vinylene carbonate (EVC) products of the carboxylative cyclisation of primary propargylic alcohols. The liquid-liquid phase synthesis utilises lipophilic silver(I) carboxylate salts and N,N-dioctyl modified phosphine (FatPhos) ligands to enhance catalyst solubility in non-polar solvents. Following simple liquid-liquid phase separation, EVCs relevant as monomers for poly(hydroxyurethane) and poly(carbonate) synthesis were isolated in the non-miscible polar phase in good yields and the catalyst was used in recycle experiments.

References:
1] Saumya Dabral, Ulrike Licht, Peter Rudolf, Gerard Bollmann, A. Stephen K. Hashmi, Thomas Schaub, Green Chem. 2020, 22, 1553-1558.
2] Chloe Johnson, Saumya Dabral, Peter Rudolf, Ulrike Licht, A. Stephen K. Hashmi, Thomas Schaub, ChemCatChem, 2020, DOI: 10.1002/cctc.202001551
3] Saumya Dabral, Bilguun Bayarmagnai, Marko Hermsen, Jasmin Schiessl, Verena Mormul, A. Stephen K. Hashmi, Thomas Schaub, Org. Lett. 2019, 21, 1422-1425.

Nano- and microparticles are used as carriers for the controlled delivery of an active ingredient with a high potential for applications for, among others, the health care and personal care industry. Particles made up of aliphatic polylactones, typically, polylactide, polyglycolide and polycaprolactone are very well-known since many years. Aliphatic polyphosphoesters less popular even though they this class of polymers has witnessed a renewed interest. This communication aims at comparing the potential of aliphatic polylactones and aliphatic polyphophoesters with a special attention paid on particles, based on the recent developments carried out in the Center of Education and Research of Macromolecules of the University of Liège.
Ring-opening polymerization of 5-membered cyclic polyphosphoesters is very efficient for the synthesis of high molar mass aliphatic polyphosphoesters. As far as cyclic 5-memberd phosphates are concerned, their extremely high sensitivity to hydrolysis remains a strong limitation at the time being. In this context, efficient processes allowing the synthesis of a wide range of aliphatic polyphosphoesters, functionalized or not, will be reported based on: (1) flow technologies, (2) organocatalysis and (3) chemical modification of the polymers. Advantages and drawbacks of aliphatic polyphosphoesters compared to polyesters will be detailed with a special attention on both the synthesis of these polymers and their properties, to assess their potential for future applications.
Particles made up of biodegradable poly(phospho)esters will be shown to be prepared by the self-assembly of amphiphilic polymers or by nanoprecipitation in batch or in flow reactors. Depending on the preparation process and the chemical structure of the polyesters, the size and size-dispersity of particles as well as the loading content, loading efficiency and release rate of a model active ingredient will be discussed.

Acknowlegements: Philippe Lecomte is research associate by the belgian FNRS. The authors thanks all the partners of the IN-Flow project carried out under the Interreg V-A Euregio Meuse-Rhine Programme, with €2.1 million from the European Regional Development Fund (ERDF). By investing EU funds in Interreg projects, the European Union invests directly in economic development, innovation, territorial development, social inclusion, and education in the Euregio Meuse-Rhine.

The ecosystem of 3D printing to fabricate objects from polymeric nanocomposite materials is important for wide use and rapid scale development towards high-performance materials. Other materials based on metallic, ceramic, hydrogels, and other non-metallic do not have as wide a range of properties leading to new structure-composition-property relationships. Nanomaterials together with sustainable biobased polymers can be used to enhance performance in thermoplastics, thermosets, and elastomers and their thermo-mechanical properties. In this talk, we emphasize the use of sustainable cellulosic and non-cellulosic additives to improve the performance of composite polymer materials. This has been demonstrated in the choices of 3D printing methodologies (FDM, SLA, SLS, VSP) which can make use of blended or formulated compositions of these sustainable filler materials. By incorporating nanomaterials such as nanocellulose, nanosilica, nanoclays with 3D printing new designs of toughened materials on commodity polymers, the value chain of biobased materials can be extended to higher values. 4D Printing will be demonstrated based on stimuli-responsive properties as reported.

Porous poly(ionic liquid)s (PILs) combine the intrinsic features of ionic liquids (ILs) with properties of polymers and porous structures notably enlarged surface area, high ionic density, spatial structuration, and the tunability of their properties via counterions exchange. Over the years, they have become essential materials for many applications, i.e., gas separation, CO2 capture, energy storage, polymer electrolyte in Li-batteries, sensors and catalysis.^[1] Amongst them, porous poly(imidazolium)s (PIMs) are of great interest for cutting edge applications, especially for heterogenous catalysis. Their chemical composition catalyzes a wide range of reactions while their macrostructure allows easy recyclability without the need of advanced purification methods. However, structuring PIMs into porous materials is not trivial and requires multistep approaches based on the polymerization of preformed imidazolium monomers^[2] or by post-modification of porous organic networks. Therefore, the search for simpler and straightforward synthesis of porous PILs is highly relevant. In this respect, the emergence of multicomponent reactions (MCR), in which three or more compounds react and form a product containing essentially all atoms of the starting reagents, offers new opportunities.^[3] This communication aims at reporting a straightforward and atom economical approach for designing interconnected macroporous imidazolium-based networks. The latter combines for the first time the aqueous mediated Radziszewski MCR^[4] under high internal phase emulsion (HIPE) conditions. The catalytic activity and recyclability of these materials are illustrated for transesterification and decarboxylation reactions.^[5]
[1] S. Zhang, K. Dokko, M. Watanabe, Chem. Sci. 2015, 6, 3684–3691.
[2] K. Mathieu, C. Jérôme, A. Debuigne, Polym. Chem. 2018, 9, 428–437.
[3] R. Kakuchi, Angew. Chem. Int. Ed. 2013, 53, 46–48.
[4] S. Saxer, C. Marestin, R. Mercier, J. Dupuy, Polym. Chem. 2018, 9, 1927–1933.
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Siloxane based polymeric materials are widely used all over the world due to their chemical, mechanical, and thermal stability, and low-toxicity. Despite their usefulness, a critical issue has been the ability to recycle them effectively. The present methods used to recycle silicone-based materials, such as PDMS, involve high temperatures/pressures, and complicated setups. To address this issue, we have established an efficient room temperature technique for depolymerization of silicone-based polymers and resins in the presence of low catalytic amounts of fluoride in specific high swell organic solvents. The products primarily contain cyclic siloxane units (D4, D5, D6) as verified by GCMS and ²⁹Si NMR. Nearly any silicone resin can be depolymerized quite rapidly using our methods. Silicone rich systems result in the best conversions and the highest quantity of identifiable cyclics, while complex resins resulted in complicated products alongside discernable cyclics. We have also repolymerized the products from this process to reform silicones via acid, base and fluoride catalysis. Our discovery has the potential for large scale industrial processing due to the use of mild conditions and solvent recycling ability.

The global need to devise strategies that can provide applicable solutions to recycling plastics is clear and urgent. It is estimated that a total of 8300 million metric tonnes of plastics have been produced to date. Incredibly, more than 40 years after the launch of the recycling symbol, less than 10% of plastics that have been manufactured are recycled; the rest are either poured into landfill, burned to produce CO2 and other harmful gases or, worst of all, discarded into the environment. There are several potential approaches to solving the plastic problem, all of which have merit and will address a part of the problem in a different way. To ensure success, all approaches need to be followed. One of the best known is the creation of a circular economy, in which used plastic is recycled chemically to yield virgin grade materials. Our work has focussed on new methods to catalytically recycle polymers as well as develop new materials with interesting properties that can be fully recycled by chemical means. Efficency of the chemistry used is an important aspect to ensure that the energy requirements remain low and waste is limited. To this end, we have focussed on applying highly efficient click chemistries to create new materials, including those from sustainably sourced starting materials, that can be assembled and disassembled on demand. This presentation will cover some of the latest developments from our laboratory in these areas.

Besides the hydrolysis rate of the ester bonds, the degradation behavior of polymers is also influenced by their crystallinity.¹ The tight assembly of polymer chains restricts the access of water/ions to cleavable chemical bonds. In bulk, it is often very difficult to quantify this effect. Degradation-induced crystallization can promote an increase in crystallinity during degradation, which cannot be distinguished from an increase in crystallinity simple because of faster degradation of amorphous segments. Polymers like oligo(ε-caprolactone) (OCL) exhibit hydrolytic degradation times even at acidic pH in the range of several days to months. During that time period other ageing phenomena can occur as well. In contrast, the degradation of OCL in ultrathin films takes only a few hours. Here, we hypothesize that we can determine the degradation rate of OCL segments in amorphous and crystalline domains via this approach, which can further be utilized to tailor the degradation of ultrathin OCL films in acidic environments.

We apply the Langmuir technique to generate ultrathin OCL films at the air-liquid interface with precisely adjusted semi-crystalline morphology, including crystal size, number density, thickness, and melting temperature.² Partially crystalline or highly crystalline films of hydroxy and methacrylate end capped linear oligomers are prepared by crystallization at different film areas and surface pressures. Both semi-crystalline films and amorphous films are hydrolytically degraded at pH ~1.5 both at room- and at elevated temperature, either at constant surface pressure at the air-water interface or as transferred films on silicon surfaces.
By comparing the data for highly crystalline with amorphous films using appropriate quantitative models, we are able to extract the degradation rate constants of OCL segments in amorphous and crystalline domains. The comparison between the polymers with two different end-groups enables further insights whether the chain packing at the crystal surfaces affects free volume within crystalline regions, thereby increasing the permeation of proteolytic ions for faster hydrolytic degradation. Amorphous ultrathin films of OCL diol degraded completely at pH 1.2 within ~16 h. Yet, highly crystalline ultrathin films of OCL diol degraded only by ~10 % after 14 h at the same pH, even at an elevated temperature of 46 °C.

The present work demonstrates the option to control the hydrolytic degradation of OCL in ultrathin films via crystallization. Besides for determination of degradation rate constants, thin layers with controlled crystallinity can be further investigated as degradable coatings. These coatings are thermoswitchable and the difference in degradation behavior between crystalline/amorphous films suggest a thermally induced acceleration of degradation.

Renewable polymer is now indispensable for sustainable developments of modern materials. Vinyl polymers constitute one of the largest families among various synthetic polymers and are generally prepared from petroleum-derived vinyl monomers. Nature also produces a variety of vinyl compounds with unique structures, which are often observed in terpenoids and phenylpropanoids. We found that these natural vinyl compounds can be classified into several families such as non-polar olefins, styrenes, and acrylates in a similar way to petroleum-derived vinyl monomers depending on the substituents attached to the vinyl groups. This suggests that they can be polymerized by designing suitable catalysts and initiating systems for the monomers. In addition, their unique natural structures will give specific properties and functions to the resulting bio-based polymers, which cannot be easily attained by polymerization of common simple vinyl monomers derived from petroleum.
From this viewpoint, we have been investigating polymerization of various unique bio-based vinyl compounds, which can be directly obtained from plants or indirectly derived from natural compounds via simple transformation reactions. In particular, we have been focusing on the controlled polymerizations because the controlled polymer structures will further bestow special properties and functions to the bio-based polymers.
In this talk, I will discuss our recent results on controlled radical and cationic polymerizations of bio-based vinyl monomers derived from terpenoids and phenylpropanoids for high-performance and functional sustainable polymeric materials.

The biodegradable polymer, poly(lactic acid) (PLA), has become a material of high industrial value. However, the low toughness and low heat distortion temperature of PLA still limits its industrial use. Since controlling crystallinity is a convenient strategy to improve the heat distortion temperature, many studies have been conducted on PLA crystal engineering. The use of crystallization additives could accelerate the nucleation and growth behavior of PLA, but they should have environmentally friendly properties. Herein, natural polyphenols were employed to improve the crystallinity of PLA, which have strong antioxidant properties. Natural polyphenols can be obtained from various fruits and plants, and they are environmentally friendly and harmless to humans. Polyphenols were used as nucleants for the heterogeneous nucleation of PLA chains by adding in solutions for film casting. Polyphenols were prepared with controlling their sizes in two ways, a top-down ball milling method and a bottom-up precipitation method. PLA was dissolved into a volatile solvent at room temperature, and the biopolymer films were made by a solvent-casting technique. The cast film of solution was partially contacted with a hot plate and experienced directional evaporation under one-way air flow. As a nucleation agent, the existence of polyphenols improved the crystallinity of the PLA films. The polyphenols prepared by the ball milling and precipitation methods have smaller particle sizes than the corresponding raw polyphenols. The degree of PLA crystallinity reflects the size of the polyphenol particles: The smaller the particle size, the larger the surface area, and more nuclei could result. The one-way airflow at a proper temperature produced controlled moving of crystallization front, and as a result, uniform PLA films having polyphenol nuclei were produced. It is expected that natural polyphenols, which have biocompatibility and biodegradability, could serve as crystallization seeds in making PLA films by controlling the crystallinity of PLA, and this novel way to control the crystallinity of PLA could offer new solutions for the future replacement of plastics.

Flame-retardant materials are widely used in various applications. Most of the flame-retardant materials contain Br and Cl that are reported to form toxins. Montmorillonite-clay/cellulose nanocomposite, a new green functional flame retardant material, shows excellent mechanical properties, gas barrier properties, biodegradability, and flame retardant properties. When the multilayered montmorillonite-clay/cellulose composites are subjected to fire, montmorillonite forms protective surface barriers of intaking oxygen and volatiles, meanwhile, outside layers of dilative fire retardant composites form a char layer to protect further propagation and insulate the underlayer materials. Nano-cellulose is induced as for its high strength, and high crystalline stiffness to improve composites' mechanical properties, and it is considered as an excellent candidate in the use of thermal management with high thermal conductivity. This intrinsic high thermal conductivity of nano-fibrillated cellulose has brought a new concept for montmorillonite clay/cellulose flame retardant materials [1], reducing the highest temperature by modifying thermal degradation route with the help of high anisotropy ratio of thermal conductivity composites in in-plane direction to dissipate heat. However, the anisotropy of thermal conductivities of composites has not been fully investigated. In this work, we prepared CNF water suspension and clay suspension and mixed them for around five minutes. Next, clay-CNF co-suspension was filtered under vacuum conditions for several hours by using ultra-filtration membranes. The samples which obtained after filtration was finally dried in a dryer under 93 °C for 15 minutes[1]. we measured the anisotropy ratio of thermal conductivity as a function of montmorillonite-clay/cellulose content. We result shows that the cross-plane thermal conductivity depends on the clay content non-monotonically; as the clay content increases from 0% (pure cellulose), the thermal conductivity decreases until the content reaches 80% and then increases until 100%. The in-plane thermal conductivity also shows a non-monotonic dependence on the content; increasing until 50 % and then decreases. This gives rise to the largest anisotropy at clay content of 50 %, where in-plane thermal conductivity is 30 times larger than the cross-plane thermal conductivity. Furthermore, Raman spectroscopy was performed to investigate the orientation of cellulose nanofibrils in the composite, and the analysis reveals that the degree of alignment strongly depends on the clay content with the maximum achieved at 50 % content, revealing that the large anisotropy is originated from the alignment of the cellulose nanofibrils.
KEYWORDS: montmorillonite, nanocellulose, thermal conductivity, anisotropy ratio
[1] Medina L, Nishiyama Y, Daicho K, et al. Nanostructure and Properties of Nacre-Inspired Clay/Cellulose Nanocomposites—Synchrotron X-ray Scattering Analysis[J]. Macromolecules, 2019, 52(8): 3131-3140.

Two chemical transoformation reactions for polymer synthesis and degradation will be presented aiming at efficient utilization of renewable resources.

1. Polymer synthesis using CO2
Homopolymerization and copolymerization of 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one, made from butadiene and CO2 will be presented.

2. C-C and C-O bond cleavage
Catalyst design for hydrogenative cleavage of C-C and C-O bonds and its use to degrade beta-O-4 structure often found in lignin will be discussed.