Poly(ethylene adipate)

Polycondensation Poly(ethylene adipate) can be synthesized through a variety of methods. First, it could be formed from the polycondensation of dimethyl adipate and ethylene glycol mixed in equal amounts and subjected to increasing temperatures (100 °C, then 150 °C, and finally 180 °C) under nitrogen atmosphere. Methanol is released as a byproduct of this polycondensation reaction and must be distilled off. Lastly, a two-step reaction between adipic acid and ethylene glycol can be carried out. A polyesterification reaction is carried out first followed by polycondensation in the presence of a catalyst. Both of these steps are carried out at 190 °C or above. Many different catalysts can be used such as stannous chloride and tetraisopropyl orthotitanate. Generally, the PEA is then dissolved in a small amount of chloroform followed by precipitation out in methanol. Ring-opening polymerization An alternate and less frequently used method of synthesizing PEA is ring-opening polymerization. Cyclic can be mixed with di-n-butyltin in chloroform. This requires temperatures similar to melt condensation. == Properties ==

PEA has a density of 1.183 g/mL at 25 °C and it is soluble in benzene and tetrahydrofuran. PEA can come in a high molecular weight or low molecular weight variety, i.e.10,000 or 1,000 Da. Further properties can be broken down into the following categories. Mechanical properties In general, most aliphatic polyesters have poor mechanical properties and PEA is no exception. Little research has been done on the mechanical properties of pure PEA but one study found PEA to have a tensile modulus of 312.8 MPa, a tensile strength of 13.2 MPa, and an elongation at break of 362.1%. Crystallization properties PEA has been shown to be able to form both ring-banded and Maltese-cross (or ring-less) type spherulites. Ring-banded spherulites most notably form when crystallization is carried out between 27 °C and 34 °C whereas Maltese-cross spherulites form outside of those temperatures. The length of the crystal edges are given as follows: a = 0.547 nm, b = 0.724 nm, and c = 1.55 nm. The monoclinic angle, α, is equal to 113.5°. Electrical properties Conductivity of films made of PEA mixed with salts was found to exceed that of PEO4.5LiCF3SO3 and of poly(ethylene succinate)/LiBF4 suggesting it could be a practical candidate for use in lithium-ion batteries. Notably, PEA is used as a plasticizer and therefore amorphous flows occur at fairly low temperatures rendering it less plausible for use in electrical applications. Blends of PEA with polymers such as poly(vinyl acetate) showed improved mechanical properties at elevated temperatures. PEA is not miscible with low density polyethylene (LDPE). == Degradability ==

Biodegradability Aliphatic copolyesters are well known for their biodegradability by lipases and esterases as well as some strains of bacteria. PEA in particular is well degraded by hog liver esterase, Rh. delemar, Rh. arrhizus, P. cepacia, R. oryzae, and Aspergillus sp.'' An important property in the speed of degradation is the crystallinity of the polymer. Neat PEA has been shown to have a slightly lower degradation rate than copolymers due to a loss in crystallinity. PEA/poly(ethylene furanoate) (PEF) copolymers at high PEA concentrations were shown to degrade within 30 days while neat PEA had not fully degraded, however, mixtures approaching 50/50 mol% hardly degrade at all in the presence of lipases. Poly(ethylene adipate) urethane combined with small amounts of ligin can aid in preventing degradation by acting as an antioxidant. Additionally, the mechanical properties of the PEA urethane increased by ligin addition. This is thought to be due to the rigid nature of ligin which aids in reinforcing soft polymers such as PEA urethane. Ultrasonic degradation Using toluene as a solvent, the efficacy of degrading PEA through ultrasonic sound waves was examined. Degradation of a polymer chain occurs due to cavitation of the liquid leading to scission of chemical chains. In the case of PEA, degradation was not observed due to ultrasonic sound waves. This was determined to be likely due to PEA not having a high enough molar mass to warrant degradation via these means. A low molecular weight has been indicated as being necessary for the biodegradation of polymers. == Applications ==

Plasticizer Poly(ethylene adipate) can effectively be used as a plasticizer reducing the brittleness of other polymers. Adding PEA to PLLA was shown to reduce the brittleness of PLLA significantly more than (PBA), (PHA), and (PDEA) but reduced the mechanical strength. Microcapsules for drug delivery PEA microbeads intended for drug delivery can be made through water/oil/water double emulsion methods. By blending PEA with Poly-ε-caprolactone, beads can be given membrane porosity. Microbeads were placed into a variety of solutions including a synthetic stomach acid, pancreatin, Hank's buffer, and newborn calf serum. The degradation of the microcapsules and therefore the release of the drug was the greatest in newborn calf serum, followed by pancreatin, then synthetic stomach acid, and lastly Hank's buffer. The enhanced degradation in newborn calf serum and pancreatin was attributed to the presence of enzyme activity and that simple ester hydrolysis was able to be carried out. Additionally, an increase in pH is correlated with higher degradation rates. ==References==