Novel and sustainable methods to develop greener polymers

This Thesis reports the development of green approaches towards the synthesis of novel polyesters. Generally, polyesters are synthesised using harsh conditions (such as high temperatures) and rely upon petrochemical-derived building blocks. By using biomass, more specifically plant oil derived sources, such as glycerol and vernolic acid (fatty acid, FA), we will explore a more renewable approach. In addition, scCO2 will be explored as a novel solvent and processing aid and also, immobilised enzymes will be used as biocatalysts. Chapter 1 has introduced the essential background of green chemistry and the importance of using biomass in the production of polymers. In addition, it also presents the fundamentals of polymer synthesis, with an emphasis on polyesters (linear and branched). A general introduction of enzymes and their use in polymer chemistry is given. Since the main aim of this thesis is to synthesise greener polyesters, the impact of using alternative solvents, such as scCO2, is well described, not only in polymerisations but also in the extraction of valuable compounds from biomass. Chapter 2 provides a detailed description of the high-pressure equipment, the standard operating procedures, analytical techniques and methods used to synthesis and characterise throughout this thesis. Chapter 3 explores the use of glycerol, which is a by-product from biodiesel production, to synthesise linear polyesters through polycondensation with bio-based succinic acid. Bulk polycondensations (120 – 140 ºC), with no catalyst and using stannous octanoate (Sn(Oct)2), as a non-selective metal catalyst produced branched poly(glycerol succinate) (PGLSA). By combining scCO2 and enzyme catalysis (Candida antarctica lipase B, CaLB), it was possible to decrease the temperature of this process to as low as 40 ºC, and produced linear PGLSA. Chapter 4 investigates the synthesis of water soluble glycerol-based surfactants. Using the methodology developed in Chapter 3, PGLSA was used as a main block for the synthesis of potential surfactants. Therefore, with specific molar ratios, PGLSA was functionalised with lauric acid or methoxy poly(ethylene glycol) (with a molecular weight of 350 Da), using scCO2 and CaLB. The evaluation of the performance as surfactants was assessed and it was demonstrated that linear PGLSA-based polymers are interesting candidates as surface-active agents. A comparison with commercial available surfactants is made. Chapter 5 exploits oil obtained from nature to develop greener polyesters. Ironweed is a prolifically growing natural plant in East Africa and is one of the few plants containing naturally occurring epoxy oils in its seeds – 40% of vernonia oil (VO) by weight of the seed. In this work, VO was extracted using scCO2, which offers several advantages, such as its environmental compatibility, its selectivity towards the lipid part of the seeds and allows the oil extraction at low temperatures (60 ºC). One of the main components of VO is vernolic acid, which accounts for 80 wt.% of the oil. The epoxy groups and double bonds from the triglyceride of VA offer great opportunities for modification. Therefore, VA was isolated from the oil and converted into new monomers for polyesters. Four different potential monomers were synthesised, a hydroxy and dihydroxy acids (13-hydroxyoctadeca-9,11-dienoic acid (HA) and 12,13-dihydroxyoctadec-9-enoic acid (DHA), respectively) a diacid (dodec-2-enedioic acid) and diol (dodec-2-ene-1,12-diol, (DED)). Chapter 6 investigates the synthesis of the vernolic acid-based polyesters from the monomers synthesised in Chapter 5, through polycondensation. The condensation of AB2 and A2 + B2 monomers is normally performed under harsh conditions (> 200 ºC). In this work, polycondensation reactions of HA (AB monomer), DHA (AB2 monomer) and DED (A2 monomer) with azelaic acid (AB2 commercially available monomer) were performed in bulk (at 100 – 140 ºC) and under supercritical conditions. Using scCO2 allowed the decrease of the reaction temperature (35 – 95 ºC) which is desirable from a sustainable perspective. Chapter 7 sums up the overall conclusions achieved from this work and summaries possible opportunities for future research in this field. In summary, the methodologies described in this Thesis, using biomass derived building blocks, such as glycerol and vernolic acid, and combining the use of scCO2 and CaLB (when possible), allowed the syntheses of greener polyesters. These approaches are an important step towards the development of the next generation of biorenewable and biodegradable polymers