Reversible Morphological Evolution of Responsive Giant Vesicles to Nanospheres by the Self-Assembly of Crystalline-b-Coil Polyphosphazene Block Copolymers

The preparation of long-term-stable giant unilamellar vesicles (GUVs, diameter 1000nm) and large vesicles (diameter 500nm) by self-assembly in THF of the crystalline-b-coil polyphosphazene block copolymers [N=P(OCH2CF3)(2)](n)-b-[N=PMePh](m) (4a: n=30, m=20; 4b: n=90, m=20; 4c: n=200, m=85), which combine crystalline [N=P(OCH2CF3)(2)] and amorphous [N=PMePh] blocks, both of which are flexible, is reported. SEM, TEM, and wide-angle X-ray scattering experiments demonstrated that the stability of these GUVs is induced by crystallization of the [N=P(OCH2CF3)(2)] blocks at the capsule wall of the GUVS, with the [N=PMePh] blocks at the corona. Higher degrees of crystallinity of the capsule wall are found in the bigger vesicles, which suggests that the crystallinity of the [N=P(OCH2CF3)(2)] block facilitates the formation of large vesicles. The GUVs are responsive to strong acids (HOTf) and, after selective protonation of the [N=PMePh] block, they undergo a morphological evolution to smaller spherical micelles in which the core and corona roles have been inverted. This morphological evolution is totally reversible by neutralization with a base (NEt3), which regenerates the original GUVs. The monitoring of this process by dynamic light scattering allowed a mechanism to to be proposed for this reversible morphological evolution in which the block copolymer 4a and its protonated form 4a(+) are intermediates. This opens a route to the design of reversibly responsive polymeric systems in organic solvents. This is the first reversibly responsive vesicle system to operate in organic media.