Synthesis and characterization of a comb- like polyacetylene copolymer A. Bolognesi, M. Catellani, S. Destri and W. Porzio Consiglio Nazionale Delle Richerche, Istituto di Chimica Delle Macromolecole, 20133 Milano, Via E. Bassini 15/,4, Italy (Received 18 July 1985; revised 11 October 1985) Graft copolymers were synthesized via lithiation of a polybutadiene backbone, transmetallation with titanium alkoxides and subsequent polymerization of acetylene. The products obtained were characterized by ultra-violet/visible spectroscopy, infra-red spectroscopy, wide-angle X-ray scattering and transmission electron microscopy. The 12 doped copolymers exhibit interesting electrical conductivity. The proposed percolation model accounts for the properties of these materials. (Keywords: synthesis; polyaeetylene; polybutadiene; graft copolymers; morphology; conductivity) INTRODUCTION Recently some efforts have been devoted to the synthesis of copolymers and blends of polyacetylene (PA) with the aim of obtaining a material having good electrical properties and improved air stability by comparison with the homopolymer 1. Moreover the synthesis of a PA copolymer can lead, in principle, to a soluble material which can be studied in more detail with the conventional spectroscopic techniques, namely ultra-violet/visible (u.v.-vis.), Raman, infra-red (i.r.) and nuclear magnetic resonance (n.m.r.). Three different synthetic approaches have been reported for the formation of PA copolymers 1-a. In this work we report on another synthetic route to obtain a comb-like copolymer made up of PA side chains grafted onto polybutadiene (PB) matrix and on its characterization by i.r., u.v.-vis., wide-angle X-ray scattering (WAXS) and transmission electron microscopy (TEM) techniques. EXPERIMENTAL A solution of cis-l,4-polybutadiene was prepared by dissolving 180mg of polymer in 150ml of dry cyclohexane. In a typical run only 5% of repeating units of butadiene in the cis-l,4-polybutadiene were lithiated, in the allylic positions, with the corresponding amount of sec-butyllithium (s-BuLi)4. Tetramethylethylenediamine (TMEDA) was added just after the addition of s-BuLi in a 1:1 ratio with respect to it. The solution was stirred for 2 h at room temperature, its colour changing from yellow to dark orange. Subsequently a given amount of Ti(OBu h was added to the stirred solution. The dark orange solution change its colour to dark yellow-green. Acetylene was bubbled into this solution for 4 min at 41 h-1 flux. The colour of the solution immediately became dark blue. The polymerization was stopped by bubbling nitrogen instead of acetylene. All the operations were performed in an inert atmosphere, s-BuLi was a commercial product; TMEDA was distilled prior to use and stored over molecular sieves. 0032-3861/86/071128-03503.00 © 1986Butterworth& Co. (Publishers) Ltd. 1128 POLYMER,1986, Vol 27, July Cyclohexane was distilled over Na/K alloy in an inert atmosphere, cis-l,4-Polybutadiene was a commercial product (Janssen) (M, 200000). I.r. spectra were performed on a Perkin-Elmer 457 instrument; u.v.-vis. spectra were recorded on a Perkin-Elmer 920instrument. WAXS experiments were carried out both on a Weissenberg camera and on a powder diffractometer (Siemens D-500). TEM and electron diffraction (ED) observations were performed on a Jeol 200B instrument. For the X-ray, ED and TEM characterizations and d.c. measurements see ref. 5. RESULTS AND DISCUSSION This synthetic route can be summarized as follows: Step I Reaction of s-BuLl with cis-l,4-polybutadiene in dry cyclohexane. The presence of TMEDA favours the formation of carbanions in the allylic positions of the polymeric matrix 4. Step 2 Exchange reaction between Ti(OBu)4 and the polymeric lithiated compound to obtain a Ti42 bond on the polymer backbone. Meanwhile the Ti(IV) atoms bonded to the polymer are reduced to Ti(III) species 6. Step 3 Polymerization of acetylene leading to a comb- like polymer which colours the solution dark blue. The scheme of these reactions is reported in Figure 1 while our main results are summarized in Table 1. A long period of catalyst ageing does not produce acetylene polymerization. Probably the active centres are not stable in this system for such a long period. In a series of runs (e.g. run 2) in which the 1 :1 Li/Ti ratio was used, the precipitation of copolymer as a gel was observed. At present we cannot give an explanation of this phenomenon. The reaction between Li-polymer and Ti(OR)4 leads to substitution of at least one of the butoxy groups of Ti(OBu)~ with a polymer chain and, subsequently to a reduction of Ti(IV) to Ti(III). One can suppose that a larger amount of Ti(OR h with respect to s-BuLl can favour the formation of crosslinked polymer. In fact when acetylene polymerizes on these matrices, the