Self-aggregation of amphiphilic poly(asparagine) grafted with poly(caprolactone) in aqueous solution

Abstract

Novel amphiphilic graft copolymers composed of poly(asparagines) (PAsn) as a hydrophilic backbone and poly(caprolactone) (PCL) as a hydrophobic segment were successfully synthesized. A grafting reaction was carried out in three steps. The first step was the tosylating reaction of PCL-diol with p-toluenesulfonylchloride (Tscl). Subsequently, the PCL-OTs was then reacted with Hexamethylenediamine (HMD) to obtain amine terminated PCL. Finally, amine terminated PCL was reacted with poly(succinimide) (PSI) and followed by aminolysis to poly(aspargine)-graft-poly(caprolactone) (PAsn-g-PCL). The grafting reaction was confirmed by $^1H$, $^13C-NMR$, FT-IR spectroscopy. Resulting copolymers were characterized by various analytical techniques including elemental analyzer, differential scanning calorimeter and X-ray diffractometry. These results obtained form WXRD data could prove that the PAsn-g-PCL graft copolymer contained two kinds of structure, crystalline structure of PCL and amorphous structure of PAsn. The graft copolymers formed self-aggregates by a precipitation and dialysis method. As the DS of PCL increased, self-aggregates became more compact and the size of primary aggregates was reduced. CAC decreased and hydrophobicity of hydrophobic microdomains increased dramatically with increasing DS. Self-aggregates showed a bimodal size distribution originated from the self-aggregation and kinetically controlled particle aggregation, although the small primary aggregates was predominant. Spherical and dispersed aggregates of about 20 nm in diameter were observed by a transmission electron microscope (TEM). From a degradation test, PCL domain of self-aggregates in aqueous solution swelled in water and then degraded by a simple hydrolytic degradation due to scission of ester linkage. The polymer of lower DS shows the slightly lower polarization value than that of higher DS, indicating that PCL groups in the hydrophobic core are loosely formed. SANS results indicates th...