PHOTOCHROMIC CHIRAL BLOCK COPOLYMERS NANOSEGREGATES OBTAINED BY ATOM TRANSFER RADICAL POLYMERIZATION (ATRP)

The possibility given by Atom Transfer Radical Polymerization (ATRP) of obtaining macromolecules with controlled molecular mass and low polydispersity is an important tool which has been adopted to produce polymethacrylic esters functionalized with various chemical moieties. In particular, the method has been reported for the polymerization of methacrylic esters to yield linear liquid crystalline polymers containing azoaromatic moieties in the side chain. In the present communication the ATRP procedure has been applied to the preparation of star shaped block copolymers of controlled mass and composition starting from a reactive central core of C3 symmetry, used as polymerization site of optically active methacrylic monomers functionalized with photochromic and liquid crystalline moieties. In this way, the first polymerisation step affords star shaped homopolymers which can be used, after purification, as macroinitiators for the second monomer. This allows to obtain monodispersed di-block copolymers with star shaped architecture, characterized by chain sections of different length, stiffness, thermal and spectroscopic properties. These characteristics can be easly manipulated just by choosing the sequence and the duration of the first and second polymerization step. DSC and XRD analysis show that the block copolymers behave as the sum of the two separated homopolymers. Thus, a dispersion at nanoscale level of one phase (the core block) into a continuous one (the shell block), resulting in a liquid crystalline phase dispersed into a continuous amorphous phase, or vice versa, can be obtained. This enables to produce materials possessing nanosegregated domains characterized by different responsive properties to external stimuli such as light, heat etc. These materials in our opinion could be very interesting for several applications, such as holographic materials, chiroptical switches etc. Due to the different absorption spectra of the different chromophores we can manipulate in a selective way only one phase using light of the appropriate wavelength. For example, it could be possible to write an hologram on the LC phase and use the amorphous chiral phase as a chirooptical switch. It could be also possible to use the optomechanical characteristic of the LC phase and exploit the amorphous phase as a physical cure of the system, thus achieving an optomechanic material.