Synthesis and self-assembly of amphiphilic star-block copolymers consisting of polyethylene and poly(ethylene glycol) segments

摘要

We report on the synthesis and self-assembly in water of well-defined amphiphilic star-block copolymers with a linear crystalline polyethylene (PE) segment and two or three poly(ethylene glycol) (PEG) segments as the building blocks. Initially, alkynyl-terminated PE (PE-~≡) is synthesized via esterification of pentynoic acid with hydroxyl-terminated PE, which is prepared using chain shuttling ethylene polymerization with 2,6-bis[1-(2,6- dimethylphenyl) imino ethyl] pyridine iron (II) dichloride/methylaluminoxane/ diethyl zinc and subsequent in situ oxidation with oxygen. Then diazido- and triazido-terminated PE (PE-(N_3)_2 and PE-(N _3)_3) are obtained by the click reactions between PE-~≡ and coupling agents containing triazido or tetraazido, respectively. Finally, the three-arm and four-arm star-block copolymers, PE-b-(PEG) _2 and PE-b-(PEG)_3, are prepared by click reactions between PE-(N_3)_2 or PE-(N_3)_3 and alkynyl-terminated PEG. The self-assembly of the resultant amphiphilic star-block copolymers in water was investigated by dynamic light scattering, transmission electron microscopy, and atomic force microscopy. It is found that, in water, a solvent selectively good for PEG blocks; these star-block copolymer chains could self-assemble to form platelet-like micelles with insoluble PE blocks as crystalline core and soluble PEG blocks as shell. The confined crystallization of PE blocks in self-assembled structure formed in aqueous solution is investigated by differential scanning calorimetry.