Submitted to: Polymers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/30/2012
Publication Date: 2/6/2012
Citation: Wyatt, V.T., Strahan, G.D. 2012. Degree of branching in hyperbranched poly(glycerol-co-diacid)s synthesized in toluene. Polymers Journal. 4(1):396-407.
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: Hyperbranched polymers were synthesized by using a Lewis acid (dibutyltin(IV)oxide) to catalyze the polycondensation of glycerol with either succinic acid (n (aliphatic chain length)=2), glutaric acid (n=3) or azelaic acid (n=7) in toluene. These are the first examples of diacid-glycerol hyperbranched polymers produced in a non-polar solvent system. Rate of esterification (water production) was followed with a Dean-Stark apparatus at the minimum oil bath temperature (135 degrees C) required for esterification to proceed. The ability to dissolve the resulting polymers in various solvents indicated that the crosslinking of polymer chains was avoided or negligible. However, while keeping all other reaction conditions the same, increasing the reaction temperature to 155-160 degrees C resulted in crosslinked hydrogels that were insoluble in all solvents. 1H NMR and 13NMR spectrometry (1-dimensional and 2-dimensional) was used to determine the degrees of branching for the hyperbranched polymers. Contrary to studies performed in other systems that suggest that degree of branching and rate of reaction decrease with increasing aliphatic chain length of the diacid, it was determined that the glutaric acid-derived polymers gave the highest degree of branching (28.8+/-2.5%, 70.6+/-1.7%) over the 24 hr reaction period followed by the succinic acid-derived polymers (16.9+/-1.8%, 36.2+/-1.2%) and the azelaic acid-derived polymers (7.1+/-1.0%, 14.3+/-0.7%) when 1:1 or 2:1 (diacid:glycerol) molar ratios were used, respectively. Water production (rate of reaction) for each diacid-glycerol correlated with increased degrees of branching. Removing the Dean-Stark apparatus resulted in a 61.2% decrease in degree of branching for succinic acid-derived polymers and a 41.4% decrease in the degree of branching for the glutaric acid-derived polymers suggesting that hydrolysis of the ester bond is faster than esterification of the secondary alcohol on glycerol.