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Synthesis and characterization of diblock copolythiophenes with chiral and achiral side-chain substituent, respectively.

Side-chain liquid crystal polymers (LCP) with tailored azobenzene moieties are widely studied due to the easy photomodulation of their properties (birefringence, non-linear optical properties and chiroptical properties) making them suitable for several applications as materials for holographic data recording, non-linear optics, artificial muscles or molecular switches.[1-3] 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 [4]. Here, we present the synthesis by ATRP and the characterization of monodisperse azopolymers of well-controlled structure (linear and three-arms star shaped) and different average molecular weights. The photomechanical effects (i.e. the dependence of volume and density) exhibited upon trans-to-cis and cis-to-trans photoisomerization of the azobenzene mesogenic groups have been investigated by ellipsometry and related to molecular weight, with particular attention to important parameters for potential applications such as the relative variation of total volume, response time, stability and reproducibility to achieving optomechanic material. In similar way, brush polymers containing photosensitive mesogenic side chains have been directly synthesized on glass substrates by a grafting method involving polymerization of a methacrylic azobenzenic monomer under ATRP conditions. Brush polymer surfaces, with respect to traditional films, are resistant to solvents and to high temperatures due to the covalent bonds between chain and substrate, which stabilizes the film morphology. The peculiar characteristics of polymeric brushes within the possibility of a fine control on the structure, make them very attractive for application as command surfaces. Furthermore, the presence of a chiral group of one absolute configuration in the side chain can lead to induction of surprising supramolecular chirality in liquid crystal state. The chiroptical properties of thin films of the azopolymers were investigated before and after irradiation with right (R)- and left (L)- circularly polarized light (CP) by circular dichroism (CD). All the irradiated polymeric films displayed amplified CD signals, with the largest level of supramolecular photoinduced chirality exhibited by linear polymers. The switching of the chiroptical proprieties make these materials suitable to use as chiroptical switches and of potential interest in nanoscale technologies for all-optical manipulation of informations.

The induction of chirality in achiral liquid crystalline azoaromatic polymers with circularly polarized light (CPL) has been studied on well-defined macromolecular derivatives with different molecular structures, synthesized by atom transfer radical polymerization (ATRP). Two methacrylic homopolymers have been investigated: a linear and a three arms star polymer. Both the polymeric samples are featured by low polydispersity and similar average polymerization degree, so that structure-properties correlations are made possible. Irradiation with CPL of one handed rotation sense of casted films and a planar cell of these polymers in the glassy nematic phase allowed to ascertain by UV-Vis and CD spectroscopy the formation of a supramolecular chiral structure with high chirality induced into the material. The results are interpreted in terms of the structural features of the polymers.

Side-chain liquid crystal polymers (LCP) with tailored azobenzene moieties are widely studied due to the easy photomodulation of their properties (birefringence, non-linear optical properties and chiroptical properties) making them suitable for several applications as materials for holographic data recording, non-linear optics, artificial muscles or molecular switches.[1-5] The presence of a chiral group of one absolute configuration in the side chain can lead to induction of supramolecular chirality both in amorphous and liquid crystal state. Here, we present the synthesis by ATRP and the characterization of monodisperse chiral azopolymers of well-controlled structure (linear and three-arms star shaped) and different average molecular weights. These new azopolymers include L-lactic residues in the side chain, which can transmit their chirality to a supramolecular organization of the liquid crystal phase. The nature of the liquid crystal phases of the synthesized polymers has been characterized by powder X-ray diffraction (XRD) and polarized optical microscopy (POM). The chiroptical properties of thin films of the azopolymers were investigated before and after irradiation with right (R)- and left (L)- circularly polarized light (CP) by circular dichroism (CD). All the irradiated polymeric films displayed amplified CD signals, with the largest level of supramolecular photoinduced chirality exhibited by linear polymers. [1] T. Ikeda, O. Tsutsumi, Science 1995, 268, 1873. [2] S. Xie, A. Natansohn, P. Rochon, Chem. Mater. 1993, 5, 403. [3] T. Verbiest, M. Kauranen, A. Persoons, J. Mater. Chem. 1999, 9, 2005. [4] R.M. Tejedor, M. Millaruelo, L. Oriol, J.L. Serrano, R. Alcalà, F.J. Rodrìguez, B. Villacampa, J. Mater. Chem., 2006, 16, 1674. [5] G. Iftime, F.L. Labarthet, A. Natansohn, P. Rochon, J. Am. Chem. Soc., 2000, 122, 12646.

Carbazole has attracted considerable interest as building block in material science for its well known hole-transporting and electroluminescence properties [1]. Since the discovery of the photoconductive character of poly(N-vinylcarbazole) (PVK), carbazole containing polymers [2] have attracted much attention because of their unique features. The use of these material in advanced micro- and nanotechnologies spreads in many different applications such as photoconductive polymers, electroluminescent devices, programmable optical interconnection, data storage, chemical photoreceptors, NLO, surface relief gratings, blue emitting materials, holographic memory. On the other hand, an intense interest has currently arisen toward investigations dealing with the amplification of chirality of polymeric materials, in solution as well as in the solid state [3]. Two novel optically active polymethacrylates bearing in the side chain a chiral group of one prevailing absolute configuration linked in 3 position to the carbazole chromophore, deriving from the chiral monomers (S)-(−)-methacryloyl-2-oxy-N-[3-(9-ethylcarbazole)]-pyrrolidine [(S)-(−)-MECP] and (S)-(−)-methacryloyl-2-oxy-N-[3-(9-ethylcarbazole)]-succinimide [(S)-(+)-MECSI] have been prepared and characterized with the aim to improve the conformational order of the macromolecules and investigate the influence of the intrinsic macromolecular chirality on their photoconductive and photorefractive properties. Spectroscopic, thermal and chiroptical characterizations indicate the occurrence of dipolar interactions between side chain moieties and the presence of an ordered chiral conformations, at least for chain segments of the macromolecules, that influence their photoconductive properties. [1] J.V. Grazuleviciusa, P. Strohrieglb, J. Pielichowskic, K. Pielichowski, Prog. Polym. Sci. 28 (2003) 1297–1353 [2] K. Meerholz, B. Volodin, B. Sandalphon, N. Kippelen, N. Peyghambarian, Nature 1994, 371, 497 [3] M.M. Green, N.C. Peterson, T. Sato, A. Teramoto, R. Cook, S. Lifson, Science, 1995, 268, 1860

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.

ATRP è uno dei metodi di maggior successo per la sintesi di macromolecole con peso molecolare e distribuzione dei pesi molecolari controllati. Risulta essere infatti un metodo molto versatile, essendo appopriato per la polimerizzazione di una vasta gamma di monomeri, non sensibili nei confronti di diversi gruppi funzionali e tollerante verso impurità presenti nel solvente ed anche reagenti, inclusa l’acqua. Sono stati polimerizzati con successo con questo metodo numerosi esteri metacrilici, per esempio il n-butil metacrilato [1,2], esteri metacrilici fluorurati [3,4] e 1-fonossicarbonil etil metacrilato [5], come anche polimeri liquido cristallini contenenti un gruppo azoaromatico in catena laterale [6,8]. In questo lavoro, la tecnica ATRP è stata utilizzata con successo per la preparazione di una serie di polimeri LC con conformazione a stella che consistono in 3 catene lineari legate a un corpo centrale di natura aromatica. Variando la durata del processo di polimerizzazione, questo metodo ci ha consentito di ottenere macromolecole aventi diverse lunghezze di catena con valori di polidispersità sufficientemente bassi che sono state caratterizzate mediante le comuni tecniche spettroscopiche e termiche. Riferimenti [1] V. Percek, B. Barboiu, H. J. Kim. J. Am. Chem. Soc., 120 (1998) 305. [2] K. Ibrahim, B. Lofgren, J. Seppala. Eur. Polym. J., 39 (2003) 939. [3] J. Xia, T. Johonson, S. Gaynor, K. Matyjaszewski, J. DeSimone. Macromolecules, 32 (1999) 4802. [4] Z. Zhang, S. Ying, Z. Shi. Polymer, 40 (1999) 5439. [5] K. Demirelli, A. Kurt, M. Coskun. Eur. Polym. J., 40 (2004) 451. [6] X. He, H. Zhang, X. Wang. Polym. J., 34 (2002) 523. [7] X. Wang, H. Zhang, G. Zhong, X. Wang. Polymer, 45 (2004) 3637. [8] X. He, D. Yan, Y. Mai. Eur. Polym. J., 40 (2004) 1759.

Photoinitiating systems bearing the photoactive moiety in the side chain of a polymeric backbone display interesting properties when compared to the related low molecular weight counterparts as far as their activity in the radical crosslinking of acrylic monomers is concerned. We report here the behaviour of polymethacrylic photoinitiators characterized by the presence in the side chain of the camphorquinone moiety, a photosensitive group active in the presence of tertiary amine synergist in the radical photocrosslinking of acrylic monomers upon activation with visible light.