Polysilylenel and polyethylene have homocatenated skeletons composed of silicon and carbon atoms, respectively. It has been shown experimentally that the skeleton absorption energies due to a-u* transitions in both the silicon and carbon catenates become lower as the number of atoms in the catenates increases; poly-(dimethylsilylene) (PDMS) shows absorption at around 4.28 eV (290 nm) while hexamethyldisilane absorbs around 6.53 eV (190 nm), polyethylene absorbs near 8.27 eV (150 nm), and ethane absorbs near 9.19 eV (135 nmh2 These bathochromic shifts are qualitatively explained by a-conjugation, which is the interaction between a-orbitals along the chain a x k 3 s 4 These u-orbital interactions depend on the helical conformation: as the helical angle of the polysilylene decreases, the a-orbital interaction becomes smaller, following the blue shift of ab~orption.~ For example, the lowest absorption peak energy of poly( dihexylsilylene) (PDHS) is observed to shift from 3.33 eV (372 nm) to 3.92 eV (316 nm) as a result of a conformational tran~ition.~ o-Conjugation in the homocatenates has also been investigated theoretically. One of the authors (H.T.) and his co-worker have calculated the following effective hole masses at the top of the valence bands of trans-planar polysilylene and trans-planar polyethylene: 0.13mo for polysilylene and 0.21mo for polyethylene, where mo is the mass of a free electron4 These results indicate that a-electrons are delocalized in homocatenates such as -SiSiSi-and -CCC-because the effective hole masses of these polymers are smaller than that of a free electron. The question still remains, however, both experimentally and theoretically, as to whether o-electrons delocalize in heterocatenates such as -SiSiC-.To clarify this, we prepared a periodic polycarbosilane,6 poly[l,1,2,2-tetramethyldisilylenemethylenel(1), and compared its absorption spectrum to those of two model compounds 2 and 3.7 The observed bathochromic shifts for 1 and 2 relative to 3 indicate the effect of a-conjugation along the skeleton. This is further supported by the small value of the calculated effective hole mass in the trans-planar parent polycarbosilane, poly-(disilylenemethylene) (4). 1 2 Me -Si-%-Me I I Me Me \ A H k / n 3 4 1996,28, 4733-4735 1~-N M R 13 4733 , 0 . 4 0 . 2 0 -0 . 2 -0 . 4 -0.6 ppm v s TMS * I$ 13C-NMR S 0 -5 ppm "S TMS I O * z9Si-NMR , 1 0 0 -1 0 -2 0 -3 0 ppm vs T M S Figure 1. lH-, I3C-, and 29Si-NMR spectra of 1 in CDC13 at 20 "C. Asterisks indicate the tetramethylsilane internal signals.Polycarbosilane 1 was synthesized by Wurtz coupling with crown ether.8 A total of 5.0 g (25 mmol) of bis-(chlorodimethylsilyl)methaneg was added dropwise to 50 mL of toluene including 1.4 g (61 mmol) of dispersed sodium and 0.6 g (2.7 mmol) of 15-crown-5. After 8 h of reflux, the reaction mixture was filtered, and the filtrate was poured dropwise into the excess ethanol. The resulting polymer was purified twice by reprecipitation from toluene solution with ethanol, giving 138 mg of 1 (yield: 4.1%). The molec...