Characterization of Poly(1-phenyl-1-silabutane) Tacticity via 1 H/ 13 C/ 29 Si Triple-Resonance 3D-NMR Minghui Chai, Takeshi Saito, Zhengjie Pi, Claire Tessier, and Peter L. Rinaldi* Department of Chemistry, The University of Akron, Akron, Ohio 44325-3601 Received October 17, 1996 Revised Manuscript Received January 9, 1997 Since the 1980s, poly(carbosilanes) have been the subject of many studies, most often directed toward their use as precursors to silicon carbide. 1 More re- cently, investigations dealing with the thermal, me- chanical, and liquid crystal properties of poly(carbosi- lane)s have been published. 2 In order to understand the properties of silicon-containing polymers, it is important to characterize their structures. NMR has been a powerful technique for the structure analysis of polymers. This paper demonstrates the combined use of 1 H/ 13 C/ 29 Si triple-resonance 3D-NMR methods and pulse field gradients (PFG) for selective detection of signals from 1 H atoms coupled to both 13 C and 29 Si at low natural abundance. The information which is obtained from 3D-NMR allows resolution and assign- ment of the resonances from the mm, mr/rm, and rr triad sequences in poly(1-phenyl-1-silabutane) (PPSB) without resorting to the preparation of stereoregular polymer with known relative configuration. Similar 1 H/ 13 C/ 15 N and 1 H/ 13 C/ 31 P triple-resonance 3D-NMR techniques have been enormously useful for characterization of biopolymers such as proteins and polynucleotides; 3 however, the biological experiments are usually performed in conjunction with uniform 13 C and 15 N isotopic labeling. In polymer chemistry, when isotopic labeling is possible, it is often very difficult and expensive. Recent work 4,5 has shown that triple- resonance 3D-NMR techniques can be tremendously useful for characterizing polymer structure even without labeling. Those reports involved the use of 1 H (100% natural abundance), 13 C (1.1% natural abundance), and a third nucleus which is present in 100% abundance (e.g., 19 F 4 or 31 P 5 ). Performance of 1 H/ 13 C/ 29 Si triple-resonance NMR (natural abundance of 29 Si ) 4.7%) is extremely challenging because it requires selective detection of the 1 H- 13 C- 29 Si spin systems which are present in only 0.05% of the molecules, while suppressing the signals from the remaining 99.95% of the molecules. Neverthe- less, with modern instrumentation and a stable instru- ment environment, such experiments are possible and can produce very useful data. The new poly(carbosilane) [PhSiH(CH 2 CH 2 CH 2 )] n (PPSB) was prepared from PhSi(allyl)ClH in a two-step sequence. 6 Self-hydrosilation of PhSi(allyl)ClH using Karstedt’s catalyst 7 gave [PhSiCl(CH 2 CH 2 CH 2 )] n . Re- duction with LiAlH 4 followed by a nonaqueous workup (to avoid hydrolysis of the Si-H bonds) afforded PPSB in high yield. The 1D-NMR spectra of PPSB are shown in Figure 1. The aliphatic region of the 1 H spectrum (Figure 1a) exhibits two broad multiplets at 0.9 and 1.6 ppm from the methylene protons R and  to Si, respec- tively; stereoisomeric effects are not evident. In the aliphatic region of the 13 C spectrum (Figure 1b), each methylene carbon exhibits two resonances resulting from racemic (r) and meso (m) diad configurations in the polymer chain. The 29 Si spectrum (Figure 1c) shows three types of resonances that can be attributed to rr, rm/mr, and mm triad stereosequences. Usually, it is possible to distinguish between mm or rr and mr/rm triad stereosequences by the standard NMR experi- ments based on the higher probability of forming rm/ mr sequences. However, it is usually difficult in a random polymer to distinguish between mm and rr sequences. When the polymer is not stereoregular, or its chiral centers are so far apart that the stereochemical influence on shift is small, it becomes difficult to differentiate between stereosequences. The pulse sequence used to collect the 3D-NMR spectrum is shown in Figure 2. Details of the sequence have previously been described. 5 The experiment in- volves sequential INEPT-type 8 transfers from 1 H to 13 C to 29 Si to 13 C and finally back to 1 H, using 1 J CH and 2 J CSi , 9 as illustrated on the structure in Figure 2. The last two gradient pulses are applied on the basis of the ratios of the 1 H and 13 C resonance frequencies in order Figure 1. One-dimensional NMR spectra of PPSB: (a) expansion of the methylene region from the 600 MHz 1 H spectrum; (b) expansion of the methylene region from the 150 MHz 13 C spectrum; (c) expansion showing the main-chain repeat unit resonances from the 119 MHz 29 Si spectrum. 1240 Macromolecules 1997, 30, 1240-1242 S0024-9297(96)01534-3 CCC: $14.00 © 1997 American Chemical Society