Azobenzene-Containing Linear-Dendritic Diblock Copolymers by Click Chemistry: Synthesis, Characterization, Morphological Study, and Photoinduction of Optical Anisotropy

作者：del Barrio Jesus; Oriol Luis^*; Alcala Rafael; Sanchez Carlos

来源：Macromolecules, 2009, 42(15): 5752-5760.

DOI：10.1021/ma9003133

We report here the synthesis of a series of new photoaddressable linear-dendritic diblock copolymers composed of poly(ethylene glycol) (PEG) and the first four generations of dendritic aliphatic polyesters based on 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) functionalized at the periphery with mesogenic and photochromic cyanazobenzene units. The dendritic block was synthesized through a double-stage convergent approach starting from the 2-azidoethyl ester or bis-MPA. Subsequent functionalization of the periphery allowed us to obtain liquid crystalline azodendrons bearing an azide group at the focal point. Finally, the block copolymers (BCs) were obtained by Huisgen's 1,3-dipolar cycloaddition (click chemistry) between the azo-dendrons and an alkyne-functionalizcd PEG. The chemical structure and low polydispersity of the BCs were checked by nuclear magnetic resonance spectroscopy (NMR), Fourier transform infrared spectroscopy (FT-IR), matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), and gel permeation chromatography (GPC). Liquid crystalline properties were analyzed by differential scanning calorimetry (DSC) and polarized-light optical microscopy (POM). The azodendrons showed a smectic phase over a broad temperature range. Mesomorphism was also detected for the BCs except for the BC hearing two cyanoazobenzene units. X-ray diffraction (XRD) experiments demonstrated the lamellar nanosegregation of the diblock copolymers. The lamellar nature of the morphology was also confirmed by transmission electron microscopy (TEM) measurements. Birefringence was induced using 488 nm linearly polarized light on thin films of the BC bearing 16 cyanoazobenzene units under different irradiation conditions. Stable values of the in-plane order parameter up to 0.56 were obtained for this material.

• 出版日期2009-8-11