Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/8/2011
Publication Date: 8/8/2011
Citation: Wyatt, V.T. 2011. The Lewis-acid-catalyzed synthesis of hyperbranched poly(glycerol-diacid)s in toluene. Journal of the American Oil Chemists' Society. 89(2):313-319. Interpretive Summary: Recent legislative energy initiatives have promoted increases in production of renewable fuels such as biodiesel, a diesel fuel alternative that is produced from agricultural fats and oils. However, increased production of biodiesel is accompanied by increased production of glycerol, the co-product from the process used to produce biodiesel. Accordingly, there is a need to find new uses for glycerol. Work performed in our laboratory has previously demonstrated the potential of using free glycerol to make pre-polymers that can be further reacted to produce highly branched polymers. Glycerol-based polymers are of interest for potential uses as cosmetics, food additives, surfactants, and lubricants. The development of new markets for glycerol also would have a significant impact on the economics of biodiesel production, since increased credit for this co-product would decrease overall production costs of biodiesel and make it more commercially competitive with petroleum-derived diesel. Since these polymers have the potential to be used in a variety of applications, it is important to know which conditions give the desired degree of branching and crosslinking. In this study, we have changed the reaction medium and demonstrated the ability to obtain branched polymers with or without crosslinks. While crosslinked polymers have many applications (i.e. hydrogels and rubbers), they are sometimes undesirable because they cannot be dissolved in solvents which make it difficult to further process them into other materials.
Technical Abstract: The first examples of monomeric glycerol-derived hyperbranched polyesters produced in a non-polar solvent system are reported here. The polymers were made by the Lewis acid (dibutyltin(IV)oxide)-catalyzed polycondensation of glycerol with either succinic acid (n (aliphatic chain length)=2), glutaric acid (n=3) or azelaic acid (n=7) in toluene. On average, poly(glycerol-diacid)s were recovered in yields of 88.3% based on weights of the starting materials and the final products. The polyesters were characterized by gel permeation chromatography (GPC) to determine molecular weight (Mn), polydispersity index (PDI), and degree of polymerization (D.P.). Incomplete conversion of reactants to products was observed for 10h polyesterifications of glutaric acid and glycerol (2:1 molar ratio) performed at 155oC when the standard (neat) synthetic protocol was employed. However, yield was improved and gelation avoided when esterifications were performed in quasi-melt solutions with toluene at 135oC. Expanding on the success of those findings, it was determined that gelation could be avoided for each of the three diacids studied when reaction conditions were varied. When all other conditions were held constant, degree of polymerization increased with decreasing aliphatic chain length of the diacid. However, amounts of water accumulated in a Dean-Stark apparatus during condensation did not show a correlation to the degree of polymerization. Karl Fischer titrations determined that poly(glycerol-diacid)s retained 2.64+0.035% and 1.70+0.040% water in the final products when 1:1 and 2:1 (diacid:glycerol) molar ratios were used, respectively.