Acylation of Pullulan by Ring-Opening of Lactones

In this paper, we report on the production of modified pullulan derivatives that have been produced in solution by ring-opening ε-caprolactone and [ l ]-lactide using a tin octanoate [Sn(Oct) 2 ] catalyst system in DMSO. These derivatives designated as hexanoates ( 1 and 2 ) and dilactates ( 4 − 7 ) have been characterized and have varying degrees of substitution (DS). 1 and 2 were characterized by interesting thermal properties as well as crystalline morphology. From DSC analysis, 2, unlike the parent polysaccharide, appears to melt at 114 °C. The crystalline morphology of this polymer was characterized by X-ray powder diffraction and d spacings were calculated. This pullulan derivative possessed new crystallographic reflections not seen in either pullulan or poly (ε-caprolactone). The pullulan-6 dilactate samples ( 4 − 7 ), produced had DS values ranging from 0.1 to 0.75. Depending upon the DS, these samples were soluble in a wide range of organic solvents including acetone and methanol. Although these materials did not exhibit any observable melting transitions from inspection of DSC, glass transition temperatures were detected for 5 [DS = 0.21], 6 [DS = 0.44], and 7 [DS = 0.80], at 130, 120, and 90 °C, respectively. As confirmed by 13 C NMR, the ring-opening process for all of these pullulan derivatives appears to be taking place through an acyl−oxygen cleavage, producing an ester linkage to the backbone with a hydroxy terminus.