Increasing environmental and social awareness arising from the use of oil as main source of raw materials, has awaken the interest of many scientists to explore new synthetic pathways for polymer production. These new approaches embrace the use of alternative feedstock and sustainable synthetic routes aiming for enhancing biodegradability and recyclability. This shift towards more benign strategies is considered positive from different perspectives. Through the exploitation of bio-based feedstock it is possible to minimize the carbon foot print at the same time as new tools for material formation are provided. The use of selective catalysts reduces the formation of side-products, minimizes or avoids the use of solvents and diminish energy consumption. At the same time, new polymeric structures can be formed in terms of functionality and architectures.
The work herein presented have focused on the development of sustainable synthetic routes for the formation of bio-based polymeric materials with targeted properties. In order to afford this, two bio-sources have been explored for the retrieval of different epoxy-fatty acids, i.e. outer birch bark and vegetable oils. In order to be able to tailor the final material properties and thus be able to target specific applications, we have relied on the selectivity of lipases to preserve the epoxy-functionalities during synthesis. Through the design of specific polymer architectures, e.g. telechelic oligomers, and branched or linear macromers, different degrees of functionality could be prepared. By covalently reacting the epoxy groups through different polymerization techniques, polymer networks were achieved. The proposed synthetic approach resulted in polymeric materials with wide variety of properties ranging from functional networks, to high Tg materials and adhesives, prepared all from renewable sources. This also proved the benefits of the use of lipases in synthesis of polymers for material applications.
The use of enzymes as selective catalysts for processing renewable monomers into added value polymers and materials has received increased attention during the last decade. In the present work Candida antarctica lipase B (CalB) was used as catalyst in one-pot routes to synthesise multifunctional oligoester resins based on an epoxy-functional omega-hydroxy-fatty acid (EFA) extracted from birch bark. The chemoselective enzymatic process resulted in three different EFA-based telechelic oligomers with targeted molecular weights; containing maleimide, methacrylate or oxetane as end-groups, respectively. The enzyme catalysed synthesis of the maleimide and the oxetane telechelic oligomers reached full conversion of monomers (>95%) after 2 h. In the case of methacrylate functional oligomer the EFA monomer reached full conversion (>98%) after 2 h but the integration of the methacrylate moiety took more than 10 h. This was due to a rate limiting reaction path using ethylene glycol dimethacrylate as substrate. The oligomer products were characterised by NMR, MALDI-TOF-MS and SEC.
A series of epoxy-functional telechelic oligomers containing oxetane end groups have been synthesized. The precursor monomer, extracted from outer Birch bark, was first polymerized through enzyme-catalyzed esterification to form oligomers having epoxy and/or oxetane groups in the structures. The oligoesters were subsequently crosslinked through cationic polymerization either by epoxy or oxetane homopolymerization or copolymerization when both functionalities were present. A study of the polymerizations of the resins was performed "in situ" using real-time Fourier transform infrared spectroscopy revealing a preferred copolymerization when compared with the homopolymerization. By tailoring the different structures, it was possible to control the final mechanical properties of the networks.
The use of monomers based on natural materials as a future supply of raw materials has gained more interest in the last decade. Sources ranging from wood to plant oils and algae are exploited as alternatives to traditional fossil-based resources for the synthesis of polymeric materials. The use of these raw materials is not only of interest because of its abundance, but also in terms of price, durability, and/or biodegradability. In the present study, a series of resins utilizing a monomer derived from birch bark is prepared. The thermoset resins are formed by reacting an epoxy-functional omega-hydroxy fatty acid with methacrylate monomers using enzyme catalysis to form multifunctional resins via a one-pot synthesis. The derived oligomers are crosslinked through different polymerization routes to produce thermosets with different properties and/or functionalities. This approach allows natural-based resins with tuned functionalities and mechanical and thermal properties to be obtained.