Lignin Biopolymers in the Age of Controlled Polymerization

Polymers made from natural biomass are gaining interest due to the rising environmental concerns and depletion of petrochemical resources. Lignin isolated from lignocellulosic biomass is the second most abundant natural polymer next to cellulose. The paper pulp process produces industrial lignin as a byproduct that is mostly used for energy and has less significant utility in materials applications. High abundance, rich chemical functionalities, CO2 neutrality, reinforcing properties, antioxidant and UV blocking abilities, as well as environmental friendliness, make lignin an interesting substrate for materials and chemical development. However, poor processability, low reactivity, and intrinsic structural heterogeneity limit lignins' polymeric applications in high-performance advanced materials. With the advent of controlled polymerization methods such as ATRP, RAFT, and ADMET, there has been a great interest in academia and industry to make value-added polymeric materials from lignin. This review focuses on recent investigations that utilize controlled polymerization methods to generate novel lignin-based polymeric materials. Polymers developed from lignin-based monomers, various polymer grafting technologies, copolymer properties, and their applications are discussed.

Renewable Terpene Derivative as a Biosourced Elastomeric Building Block in the Design of Functional Acrylic Copolymers

In order to move away from traditional petrochemical-based polymer materials, it is imperative that new monomer systems be sought out based on renewable resources. In this work, the synthesis of a functional terpene-containing acrylate monomer (tetrahydrogeraniol acrylate, THGA) is reported. This monomer was polymerized in toluene and bulk via free-radical polymerizations, achieving high conversion and molecular weights up to 278 kg·mol^-1. The synthesized poly(THGA) shows a relatively low T_g (-46 °C), making it useful as a replacement for low T_g acrylic monomers, such as the widely used n-butyl acrylate. RAFT polymerization in toluene ([M]₀ = 3.6 mol·L^-1) allowed for the well-controlled polymerization of THGA with degrees of polymerization (DP _n) from 25 to 500, achieving narrow molecular weight distributions ( D̵ ≈ 1.2) even up to high conversions. At lower monomer concentrations ([M]₀ = 1.8 mol·L^-1), some evidence of intramolecular chain transfer to polymer was seen by the detection of branching (arising from propagation of midchain radicals) and terminal double bonds (arising from β-scission of midchain radicals). Poly(THGA) was subsequently utilized for the synthesis of poly(THGA)- b-poly(styrene)- b-poly(THGA) and poly(styrene)- b-poly(THGA)- b-poly(styrene) triblock copolymers, demonstrating its potential as a component of thermoplastic elastomers. The phase separation and mechanical properties of the resulting triblock copolymer were studied by atomic force microscopy and rheology.

Copolycarbonates Based on a Bicyclic Diol Derived from Citric Acid and Flexible 1,4-Cyclohexanedimethanol: From Synthesis to Properties

Octahydro-2,5-pentalenediol (OPD), is a compelling citric acid-based bicyclic diol with excellent rigidity and thermal stability. Herein, a series of copolycarbonates (co-PCs) were synthesized, starting from OPD, 1,4-cyclohexanedimethanol (CHDM), and diphenyl carbonate (DPC). All polycarbonates are amorphous with glass transition temperatures increased when increasing the content in OPD units. Dynamic mechanical analysis (DMA) revealed the sub T_g β-relaxations at low temperatures originating from the CHDM conformational transition, indicative of the possibility of impact-resistance. Morphological analysis of the fracture surfaces revealed the toughening mechanism under tensile was shear yielding of the matrix triggered by internal cavitation. The incorporation of OPD steadily increased the Young's modulus, from 482 to 757 MPa, with the OPD fraction increased from 0 to 30 mol %. As the OPD content further increased, a "ductile-to-brittle" transition occurred due to the low number-average molecular weight (M_n) and the low entangled strand density (high entanglement molecular weight).

Fast pyrolysis bio-oil from lignocellulosic biomass for the development of bio-based cyanate esters and cross-linked networks

Fast pyrolysis of pine wood was carried out to yield a liquid bio-oil mixture that was separated into organic and aqueous phases. The organic phase (ORG-bio-oil) was characterized by gas chromatography–mass spectroscopy, 31P-nuclear magnetic resonance spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. It was further used as a raw material for producing a mixture of biphenolic compounds (ORG-biphenol). ORG-bio-oil, ORG-biphenol, and bisphenol-A were reacted with cyanogen bromide to yield cyanate ester monomers. Cyanate esters were characterized using FTIR spectroscopy and were thermally cross-linked to develop thermoset materials. Thermomechanical properties of cross-linked cyanate esters were assessed using dynamic mechanical analysis and compared with those of cross-linked bisphenol-A-based cyanate ester. ORG-biphenol cyanate ester was observed to have a superior glass transition temperature (350–380°C) as compared to bisphenol-A cyanate ester (190–220°C). Cyanate esters derived from bio-oil have the potential to be a sustainable alternative to the bisphenol-A-derived analog.

A novel bio-based AB2 monomer for preparing hyperbranched polyamides derived from levulinic acid and furfurylamine

A new AB₂ type bio-based monomer (FDA-E) with two amino functional groups and one ester functional group was prepared from renewable levulinic acid and furfurylamine using a three-step reaction.

Naturally-Derived Amphiphilic Polystyrenes Prepared by Aqueous Controlled/Living Cationic Polymerization and Copolymerization of Vinylguaiacol with R⁻OH/BF₃·OEt₂

In this study, we investigated direct-controlled/living cationic polymerization and copolymerization of 4-vinylguaiacol (4VG), i.e., 4-hydroxy-3-methoxystyrene, which can be derived from naturally-occurring ferulic acid, to develop novel bio-based amphiphilic polystyrenes with phenol functions. The controlled/living cationic polymerization of 4VG was achieved using the R⁻OH/BF₃·OEt₂ initiating system, which is effective for the controlled/living polymerization of petroleum-derived 4-vinylphenol in the presence of a large amount of water via reversible activation of terminal C⁻OH bond catalyzed by BF₃·OEt₂, to result in the polymers with controlled molecular weights and narrow molecular weight distributions. The random or block copolymerization of 4VG was also examined using p-methoxystyrene (pMOS) as a comonomer with an aqueous initiating system to tune the amphiphilic nature of the 4VG-derived phenolic polymers. The obtained polymer can be expected not only to be used as a novel styrenic bio-based polymer but also as a material with amphiphilic nature for some applications.

Modification of Poly(ethylene 2,5-furandicarboxylate) with Biobased 1,5-Pentanediol: Significantly Toughened Copolyesters Retaining High Tensile Strength and O2 Barrier Property

Poly(ethylene 2,5-furandicarboxylate) (PEF) is a biobased polyester characterized by high gas barrier properties as well as high tensile modulus and strength, but poor toughness. Toughening PEF without sacrificing its modulus, strength and gas barrier performance is a great challenge for PEF modification. In this study, high molecular weight random poly(ethylene- co-1,5-pentylene 2,5-furandicarboxylate)s (PEPeFs) were synthesized via melt copolycondensation of 2,5-furandicarboxylic acid (FDCA), ethylene glycol (EG) and 1,5-pentanediol (PeDO), a cheap, biobased and commercially available odd-carbon comonomer. The synthesized PEPeFs were characterized and assessed with intrinsic viscosity, ATR-FTIR, ¹H NMR, DSC, TGA and tensile, impact and O₂ permeation test. Mayo-Lewis equation with "reactivity ratio" of 3.78 for PeDO and 0.75 for EG could be used as an empirical equation to correlate the copolyester composition (ϕ_PeF) with monomer composition. PEPeFs proved nearly amorphous copolyesters having excellent thermal stability. Brittle-ductile transition was achieved at ϕ_PeF as low as 9 mol %. Increasing ϕ_PeF led to increase in elongation at break and notch impact strength and decrease in T_g, O₂ barrier performance and tensile modulus and strength. However, in comparison with PEF, PEF-rich PEPeFs (ϕ_PeF 9-47%) not only showed greatly improved elongation at break (29-265% vs 4%) and enhanced impact strength (2.2-3.9 kJ/m²) but also retained very high Young's modulus (2.8-3.3 vs 3.3 GPa) and yielding strength (72-83 vs 82 MPa). Particularly, when compared with bottle-grade PET, PE₈₂Pe₁₈F possesses equal T_g (ca. 75 °C) and comparable elongation at break (ca. 115%), but greatly improved yielding strength (83 MPa) and O₂ gas barrier property (4.8 times). As modified PEF materials possessing superior thermo-mechanical and O₂ gas barrier properties, these integrally biobased copolyesters may find practical applications in eco-packaging and other fields.

Controlled Radical Copolymerization of Cinnamic Derivatives as Renewable Vinyl Monomers with Both Acrylic and Styrenic Substituents: Reactivity, Regioselectivity, Properties, and Functions

A series of cinnamic monomers, which can be derived from naturally occurring phenylpropanoids, were radically copolymerized with vinyl monomers such as methyl acrylate (MA) and styrene (St). Although the monomer reactivity ratios were close to zero for all the cinnamic monomers, such as methyl cinnamate (CAMe), cinnamic acid (CA), N-isopropyl cinnamide (CNIPAm), cinnamaldehyde (CAld), and cinnamonitrile (CN), they were incorporated into the copolymers and significantly increased the glass transition temperatures despite the relatively low incorporation rates of up to 40 mol % due to their rigid 1,2-disubstituted structures. The regioselectivity of the radical copolymerization of CAMe was evaluated on the basis of the results of ruthenium-catalyzed atom transfer radical additions as model reactions. The obtained products suggest that the radicals of MA and St predominantly attack the vinyl carbon of the carbonyl side of CAMe and that the propagation of CAMe mainly occurs via the styrenic radical. The ruthenium-catalyzed living radical polymerization, nitroxide-mediated polymerization (NMP), and reversible addition-fragmentation chain transfer (RAFT) polymerization provided the copolymers with controlled molecular weights, narrow molecular weight distributions, and controlled comonomer compositions. The copolymers of N-isopropylacrylamide (NIPAM) and CNIPAm prepared via RAFT copolymerization showed thermoresponsivity with a lower critical solution temperature (LCST) that could be tuned by altering the comonomer incorporation and a higher LCST than the copolymers of NIPAM and St, which possessed similar molecular weights and similar NIPAM contents, due to the additional N-isopropylamide groups in the CNIPAm units compared to the St units.

Towards Sustainable High-Performance Thermoplastics: Synthesis, Characterization, and Enzymatic Hydrolysis of Bisguaiacol-Based Polyesters

The utilization of wood-derived building blocks (xylochemicals) to replace fossil-based precursors is an attractive research subject of modern polymer science. Here, we demonstrate that bisguaiacol (BG), a lignin-derived bisphenol analogue, can be used to prepare biobased polyesters with remarkable thermal properties. BG was treated with different activated diacids to investigate the effect of co-monomer structures on the physical properties of the products. Namely, derivatives of adipic acid, succinic acid, and 2,5-furandicarboxylic acid were used. Moreover, a terephthalic acid derivative was used for comparison purposes. The products were characterized by ¹ H NMR spectroscopy, attenuated total reflectance FTIR spectroscopy, gel-permeation chromatography, thermogravimetric analysis, and differential scanning calorimetry to assess their structural and thermal properties in detail. The polymers showed glass-transition temperatures ranging up to 160 °C and thermal stabilities in excess of 300 °C. Furthermore, the susceptibility of the polyester to enzymatic hydrolysis was investigated to assess the potential for further surface functionalization and/or recycling and biodegradation. Indeed, hydrolysis with two different enzymes from the bacteria Thermobifida cellulosilytica led to the release of monomers, as quantified by HPLC. The results of this study indicate that our new polyesters represent promising renewable and biodegradable alternatives to petroleum-based polyesters currently employed in the plastics industry, specifically for applications in which high-temperature stability is essential to ensure overall system integrity.

Selective Fragmentation of Biorefinery Corncob Lignin into p-Hydroxycinnamic Esters with a Supported Zinc Molybdate Catalyst

Lignin is the largest renewable resource of bioaromatics, and the catalytic fragmentation of lignin into phenolic monomers is increasingly recognized as an important starting point for lignin valorization. Herein, we report that ZnMoO₄ supported on MCM-41 can catalyze the fragmentation of biorefinery technical lignin, enzymatic mild acidolysis lignin, and native lignin derived from corncob to yield lignin oily products that contain 15-37.8 wt % phenolic monomers, in which the high selectivities towards methyl coumarate (1) and methyl ferulate (2) were obtained (up to 78 %). The effects of some key parameters such as the influence of the solvent, reaction temperature, time, H₂ pressure, and catalyst dosage were examined in view of activity and selectivity. The loss of Zn from the catalyst is discussed as the primary cause of deactivation, and the catalytic activity and selectivity can be well preserved in at least six runs by thermal calcination. The high selectivity to 1 and 2 leads to their easy separation and purification from lignin oily product to provide sustainable monomers for the preparation of functional polyether esters and polyesters.

Thermosetting Polymers from Lignin Model Compounds and Depolymerized Lignins

Lignin is the most abundant source of renewable ready-made aromatic chemicals for making sustainable polymers. However, the structural heterogeneity, high polydispersity, limited chemical functionality and solubility of most technical lignins makes them challenging to use in developing new bio-based polymers. Recently, greater focus has been given to developing polymers from low molecular weight lignin-based building blocks such as lignin monomers or lignin-derived bio-oils that can be obtained by chemical depolymerization of lignins. Lignin monomers or bio-oils have additional hydroxyl functionality, are more homogeneous and can lead to higher levels of lignin substitution for non-renewables in polymer formulations. These potential polymer feed stocks, however, present their own challenges in terms of production (i.e., yields and separation), pre-polymerization reactions and processability. This review provides an overview of recent developments on polymeric materials produced from lignin-based model compounds and depolymerized lignin bio-oils with a focus on thermosetting materials. Particular emphasis is given to epoxy resins, polyurethanes and phenol-formaldehyde resins as this is where the research shows the greatest overlap between the model compounds and bio-oils. The common goal of the research is the development of new economically viable strategies for using lignin as a replacement for petroleum-derived chemicals in aromatic-based polymers.

Bio-Based Transparent Conductive Film Consisting of Polyethylene Furanoate and Silver Nanowires for Flexible Optoelectronic Devices

Exploiting biomass has raised great interest as an alternative to the fossil resources for environmental protection. In this respect, polyethylene furanoate (PEF), one of the bio-based polyesters, thus reveals a great potential to replace the commonly used polyethylene terephthalate (PET) on account of its better mechanical, gas barrier, and thermal properties. Herein, a bio-based, flexible, conductive film is successfully developed by coupling a PEF plastic substrate with silver nanowires (Ag NWs). Besides the appealing advantage of renewable biomass, PEF also exhibits a good transparency around 90% in the visible wavelength range, and its constituent polar furan moiety is revealed to enable an intense interaction with Ag NWs to largely enhance the adhesion of Ag NWs grown above, as exemplified by the superior bending and peeling durability than the currently prevailing PET substrate. Finally, the efficiency of conductive PEF/Ag NWs film in fabricating efficient flexible organic thin-film transistor and organic photovoltaic (OPV) is demonstrated. The OPV device achieves a power conversion efficiency of 6.7%, which is superior to the device based on ITO/PEN device, manifesting the promising merit of the bio-based PEF for flexible electronic applications.

Bright Side of Lignin Depolymerization: Toward New Platform Chemicals

Lignin, a major component of lignocellulose, is the largest source of aromatic building blocks on the planet and harbors great potential to serve as starting material for the production of biobased products. Despite the initial challenges associated with the robust and irregular structure of lignin, the valorization of this intriguing aromatic biopolymer has come a long way: recently, many creative strategies emerged that deliver defined products via catalytic or biocatalytic depolymerization in good yields. The purpose of this review is to provide insight into these novel approaches and the potential application of such emerging new structures for the synthesis of biobased polymers or pharmacologically active molecules. Existing strategies for functionalization or defunctionalization of lignin-based compounds are also summarized. Following the whole value chain from raw lignocellulose through depolymerization to application whenever possible, specific lignin-based compounds emerge that could be in the future considered as potential lignin-derived platform chemicals.

Reworkable Polyhydroxyurethane Films with Reversible Acetal Networks Obtained from Multifunctional Six-Membered Cyclic Carbonates

Multifunctional 6-membered cyclic carbonates (6-CCs) comprising acetal structures have been synthesized via phosgene-free routes and utilized for the fabrication of reworkable networked poly(acetal-hydroxyurethane) (PAHU) films. Dibenzoyl-protected di(trimethylolpropane) (DTMP) reacts with multifunctional aldehydes derived from nonexpensive alcohols to afford protected multifunctional DTMPs. After deprotection, the multifunctional DTMPs can react with diphenyl carbonate to efficiently form multifunctional 6-CCs. The polyaddition of the 6-CCs and diamines effectively proceeds in DMF to give networked PAHU films with good transparency and flexibility. These films possess the reworkability based on acid-catalyzed reversibility of acetal linkages. In particular, the film fabricated using large amounts of hexa-functional 6-CCs can reform reproducibly with maintaining to some degree its mechanical properties.

Vanillin derived a carbonate dialdehyde and a carbonate diol: novel platform monomers for sustainable polymers synthesis

Vanillin has been regarded as one of the important biomass-based platform chemicals for aromatic polymers synthesis. Herein, novel symmetric bis(4-formyl-2-methoxyphenyl)carbonate (BFMC) and bis(4-(hydroxymethyl)-2-methoxyphenyl)carbonate (BHMC) polymeric monomers have been synthesized in high yields using vanillin as a raw chemical, which have been submitted for polymer synthesis via well-established polymeric strategies. A new class of poly(carbonate ester)s oligomers with amide moieties in their side chain can be prepared by using the BFMC as one of monomers via the Passerini three compound reaction (3CR). A new class of poly(carbonate ester)s oligomers and poly(carbonate urethane)s can be prepared via reactions between BHMC with dicarboxylic acid chlorides and diisocyanates, respectively. Their structure have been confirmed by ¹H NMR, ¹³C NMR and FTIR, and the gel permeation chromatograph (GPC) analysis shows that the Mn of poly(carbonate ester)s oligomers ranges from 3100 to 7900 with PDI between 1.31 and 1.65, and the Mn of poly(carbonate urethane)s ranges from 16 400 to 24 400 with PDI ranging from 1.36 to 2.17. The DSC analysis shows that the poly(carbonate ester)s oligomers have relative low T_g ranging from 37.4 to 74.1 °C, and the poly(carbonate urethane)s have T_g ranging from 97.3 to 138.3 °C, mainly correlating to the structure of dicarboxylic acid chlorides and diisocyanates used.

Novel classes of lignin-derived poly(carbonate ester)s, poly(carbonate ester)s pending amide moiety oligomers, and poly(carbonate urethane)s have been designed and synthesized.