STAR-SHAPED PHOTOCHROMIC CHIRAL LIQUID CRYSTALLINE BLOCK COPOLYMERS NANOSEGREGATES OBTAINED BY ATRP

Introduction 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 moieties1. 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 chain2. Result 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 particular, three different photochromic monomers (Figure 1) have been used in the present work.. Fig. 1. Structure of the monomers 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. The homopolymers of M6A possess two liquid crystalline phases (smectic A and nematic), those of (S)-ML6A show only one liquid crystalline TGB phase, while the homopolymers of (S)-MAP-C are amorphous. DSC analysis shows that the block copolymers behave as the sum of the two separated homopolymers: they display two Tg and endothermic peaks assigned to the liquid crystalline transitions with no difference in the transition temperature between copolymers and the related homopolymers. The block copolymers also exhibit the same XRD reflections observed in the corresponding liquid crystalline 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 (Figure 2). Fig. 2. Ideal representation of NANOSEGREGATED LC block copolymers. In black is represented the core block, in grey the shell block. 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 (Figure 3) we can manipulate in a selective way only one phase using light of the appropriate wavelength. Fig. 3. UV-vis spectra of Star(M6A) (___) and Star[(S)-MAP-C] (…..) For example, it could be possible to write an hologram on the LC phase and use the amorphous 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. Reference 1 K. Matyjaszewski, J. Xia. Chem. Rev. 101 2921 (2000). 2 X. He, D. Yan, Y. Mai. Eur. Polym. J., 40 1759 (2004).