Stimuli-responsive reversible physical networks. II. Design and properties of homogeneous physical networks consisting of periodic copolymers synthesized by living cationic polymerization

The design and precision synthesis of physical networks consisting of copolymers with crystallizable pendant groups are described in this work. Amphiphilic periodic, statistical, and gradient copolymers consisting of octadecyl vinyl ether (ODVE) units were synthesized via living cationic polymerization. The synthesis involved the copolymerization of ODVE and 2-methoxyethyl vinyl ether (hydrophilic) with an 1-(isobutoxy)ethyl acetate ECH3CH(OiBu)OCOCH3]/Et1.5AlCl1.5 initiating system in the presence of a weak Lewis base to yield copolymers with very narrow molecular weight distributions (weight-average molecular weight/number-average molecular weight similar or equal to 1.2). All aqueous solutions of the copolymers behaved as a viscous liquid above 50 degrees C. When cooled below 25 degrees C, the solutions turned into transparent, transient physical gels (exhibiting terminal flow), regardless of the sequence distribution. Viscoelastic studies showed that a periodic copolymer gave a hard gel that was more brittle than the gels obtained from the corresponding statistical and gradient copolymers. This difference and the differences in the relaxation time and relaxation mode distribution of the copolymer gels were consistent with the sequence distributions of ODVE in the respective copolymers. These results indicate that the mechanical properties of a physical network can be controlled by the primary polymer structures. (c) 2005 Wiley Periodicals, Inc.