Preparation and Solution Properties of Tadpole-Shaped Double Hydrophilic Diblock Copolymers
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Abstract
Well-defined thermo-responsive tadpole-shaped double hydrophilic diblock copolymer (PEG45-b-cPNIPAM) was synthesized through the combination of atom transfer radical polymerization (ATRP) and intramolecular azide-alkynyl click chemistry reaction, where PEG was poly(ethylene glycol) and PNIPAM was poly(N-isopropylacrylamide). Firstly, PEG45(-alkynyl)-Br bearing an alkynyl end group and a bromo side group was synthesized as a macro-initiator to initiate the polymerization of N-isopropylacrylamide (NIPAM). Then, after the preparation of the thermo-responsive linear double hydrophilic diblock polymer, PEG45(-alkynyl)-b-lPNIPAM-Cl, which had an alkynyl and a chloro group at the terminal of PEG and PNIPAM block respectively, the chloro group was modified into azide group by the substitution reaction with sodium azide. Finally, via intramolecular azide-alkynyl click chemistry, PEG45-b-cPNIPAM was obtained, containing a cyclic PNIPAM “head” and a linear PEG “tail”. The structures of PEG45-b-cPNIPAM with different PNIPAM block lengths were characterized by Nuclear Magnetic Resonance (1H-NMR), Gel Permeation Chromatography (GPC), Fourier Transform Infrared Spectroscopy (FT-IR) and Laser Light Scattering (LLS) measurements. GPC results indicated that due to the smaller mean square radius of gyration, PEG45-b-cPNIPAM possessed lower molecular weight than its corresponding control linear diblock copolymer with the same length of PEG and PNIPAM block. Moreover, LLS results showed that the solution properties of the polymers were greatly affected by their topological structure. Upon being heated above the phase transition temperature, both the linear and tadpole-shaped diblock copolymer formed aggregates in aqueous solution due to the collapse of the PNIPAM blocks. However, the hydrodynamic radius of the aggregates obtained from PEG-b-cPNIPAM was much smaller than that obtained from linear diblock copolymers with the same block length for lack of inter-chain entanglement. Besides, concerning the tadpole-shaped PEG45-b-cPNIPAM110 with longer PNIAPM block length, after the rapid phase transition, the collapsed polymer chains in the aggregates could further undergo conformation adjustment, thus resulting in the decrease of aggregates size with the increasing solution temperature.
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