Stimuli-responsive reversible physical networks. I. Synthesis and physical network properties of amphiphilic block and random copolymers with long alkyl chains by living cationic polymerization

The living cationic polymerization of octadecyl vinyl ether (ODVE) was achieved with an 1-(isobutoxy)ethyl acetate [CH3CH(OiBu)OCOCH3]/EtAlCl2 initiating system in hexane in the presence of an added weak Lewis base at 30 degreesC. In contrast to conventional polymers, poly(octadecyl vinyl ether) underwent upper-critical-solution-temperature-type phase separation in various solvents, such as hexane, toluene, CH2Cl2 and tetrahydrofuran, because of the crystallization of octadecyl chains. Amphiphilic block and random copolymers with crystallizable substituents of ODVE and 2-methoxyethyl vinyl ether (MOVE) were synthesized via living cationic polymerization under similar conditions. Aqueous solutions of the copolymers yielded physical gels upon cooling because of strong interactions between ODVE units, regardless of the copolymer structure. The product gels, however, exhibited different viscoelastic properties: A 20 wt % solution of a block copolymer (400/20 MOVE/ODVE) became a soft physical gel that behaved like a typical gel, whereas the corresponding random copolymer gave a transparent but stiff gel with a certain relaxation time. Differential scanning calorimetry analysis confirmed that the crystalline-amorphous transition of the octadecyl chains was a key step for inducing such physical gelation. (C) 2005 Wiley Periodicals, Inc.