Liquid-crystal polymerization, polyacetylene

K. Akagi, S. Katayama, H. Shirakawa, K. Araya, A. Mukoh, T. Narahara, Highly conductive polyacetylene film prepared by the liquid crystal polymerization method under magnetic field, Synth. Met., 17, 241-246 (1987). 

A method has been developed recently to eliminate the thermal process for producing polyacetylene films. This technique is a liquid-crystal polymerization... 

A method has been developed recently to eliminate the thermal process for producing polyacetylene films. This technique is a liquid-crystal polymerization method, which allows oriented films to be prepared directly through an anisotropic reaction field (68-70). The field is produced by macroscopically orientated nematic liquid crystals, which are used as the polymerization solvent for a Ziegler-Natta catalyst. This technique yields highly oriented films in the trans configuration. This process produces polyacetylene with good conductivity (10 s/cm) without the need for thermal treatment and stretching, which causes defects and breaks fibrils in films. 

The same authors 369,3701 also obtained similar results if the liquid crystal solvent was aligned by flow during the polymerization. They showed that the polymerization conditions lead to alignment of the fibrils within the polymer mass and of the chains within the fibrils polymers produced in this way could also be doped to a conductivity of 104 S cm-1 371). The morphology of polyacetylene produced by polymerization in a liquid crystal solvent, aligned both magnetically and by flow, has been studied by Montaner et al. 371). They show that the polymer film is made up of very long fibrils built from microfibrils. In one fibril, the orientation of microcrystalline domains with respect to the fibril axis is very well defined, whilst the orientation of the different fibrils in the sample spreads over 20°. 

As described in Section II.B.l above, doping causes a drastic change in the electrical properties of polyacetylene. The initial values of electrical conductivity were of the order of 10 S cm" for unoriented materials d24-i30 when doped by iodine and AsFs, were enhanced to the order of 10 S cm, which was obtained in the parallel direction of the doped films oriented by mechanical stretching 31 Improvements in polymerization methods and in the catalyst systems also enhanced the electrical conductivity. Highly oriented films prepared in liquid crystal solvents (Section II.A.l.d.iii) exhibited a conductivity higher than 10 S cm, as did also a well stretch-oriented film prepared by Ti(OBu)4-EtsAl dissolved in silicon oil and aged at 120°C. In further studies Naarmann and Theophilou and Tsukamoto and coworkers attained a conductivity of ca 10 S cm k... 

On the other hand, orientation is one of the most important factors to enhance the conductivity of one dimensional pi-conjugated polymers like polyacetylene. Several attempts were made to obtain an oriented polyacetylene [3-5]. Akagi and Shirakawa applied magnetic force into a polymerization system consisting of nematic liquid crystals and obtained a highly oriented polyacetylene. Kahlert and Leising synthesized an oriented polyacetylene film by pyrolyzing prepolymers. Ozaki polymerized acetylene on the surface of benzene crystals and prepared an oriented polyacetylene. 

K. Akagi, Helical polyacetylene asymmetric polymerization in a chiral liquid-crystal field. Chem. Rev. 109, 5354-5401 (2009)... 

Orientation of the polymer chains can be obtained by different methods. Highly oriented films of poly acetylene have been made by performing polymerization in a liquid crystal 11,2]. It has also been shown that it is possible to attain highly oriented films of polyacetylene 13] and poly(p-phenylene vinylene) [4] by stretching their precursors. Kaneto et al. 15] have oriented polythiophene by stretching a film, prepared electrochemically on In-Sn oxide (ITO)-coated poly(ethyleneterephthalate) films. The processability of the recently developed poly(3-alkylthiophenes) [6-8] makes this class of polymers suitable for stretching. Yoshino et al. [9] have shown that it is possible to orient poly(3-alkylthiophene), both as free-standing films and in a blend with an elastomer. 

Chiral Dopants and Chiral Nematic Liquid Crystals Acetylene Polymerization in Chiral Nematic Liquid Crystal Characterization of Helical Polyacetylene Film... 

In this chapter, I outlined recent developments of a new polymerization techniques using a asymmetric liquid crystal reaction field of the N -LC phase, focusing on helical polyacetylene with a super-hierarchical spiral structure (H-PA). Chiral binaphthyl axially chiral derivatives are added as a chiral dopant to nematic liquid crystal to produce the chiral N -LC phase. 

Oriented polyacetylene films have been synthesized by using a nematic liquid crystal as a polymerization solvent. The maximum value of parallel electrical conductivity of the films doped with AsFs was 11,000 S cm [17]. The use of an aged mixture of Ti(OC4Hc,)4-A1(C2H0. in silicone oil that has been mixed with various quantities of n-butyllithium, as the catalyst for the polymerization of acetylene, yields polyacetylene that is highly regular, compact, and crystalline, in well-defined parallel planes. When it is stretched and doped, the polyacetylene film shows a dramatic increase in conductivity to a value greater than lO S cm [18]. 

In their evaluation of the high-pressure, solid-state polymerization at room temperature, Aoki et al. [17] have found that both ci.r- and frans-polyacetylene are formed, as is the case for the catalytic systems. The crystal structure of solid acetylene as determined by Koski and Sandor [162] by neutron diffraction on C2D2 at liquid helium temperature, is the starting point for their discussion of the mechanism. 

---