In this study, four new silicon-containing poly(ether-azomethine)s with linear structures were prepared using original silicon and biphenyl moiety-containing monomers: two diamines and two dialdehydes.
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.
Thanks
to chemical stabilization, aldehyde-assisted fractionation
(AAF) of lignocellulosic biomass has recently emerged as a powerful
tool for the production of largely uncondensed lignin. Depolymerization
of AAF lignin via ether cleavage provides aromatic monomers at near
theoretical yields based on ether cleavage and an oligomeric fraction
that remains largely unexploited despite its unique material properties.
Here, we present an in-depth analytical characterization of AAF oligomers
derived from hardwood and softwood in order to elucidate their molecular
structures. These bioaromatic oligomers surpass technical Kraft lignin
in terms of purity, solubility, and functionality and thus cannot
even be compared to this common feedstock directly for material production.
Instead, we performed comparative experiments with Kraft oligomers
of similar molecular weight (Mn ∼ 1000) obtained through solvent
extraction. These oligomers were then formulated into polyurethane
materials. Substantial differences in material properties were observed
depending on the amount of lignin, the botanical origin, and the biorefining
process (AAF vs Kraft), suggesting new design principles for lignin-derived
biopolymers with tailored properties. These results highlight the
surprising versatility of AAF oligomers towards the design of new
biomaterials and further demonstrate that AAF can enable the conversion
of all biomass fractions into value-added products.
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