Lithium polybutadiene chain structure

The structure of the live lithium chain ends is a matter of controversy and will be discussed in a later section. After the lithium-polybutadiene is terminated with protic material, the isolated poly butadiene polymer exhibits a mixed microstructure (—35% cis-1,4, 54% trans-1,4, and -11% 1,2). 

Most of the data referred to above were obtained in earlier work, and were based on infrared spectroscopy. In recent years, more reliable data were obtained by means of NMR spectroscopy, using both and resonances (9-14). Some of these investigations suggested that, aside from the dramatic effects of polar solvents on the chain structure in organolithium systems, there were some subtle effects even in non-polar media, e.g., caused by initiator concentration and type of non-polar solvent. Sinn and coworkers ( ), for example, used infrared spectroscopy to show an effect of butyl lithium concentration on the chain structure of polyisoprene and polybutadiene. Hence an extensive study was carried out recently (, M) on the influence of reaction parameters on the chain structure of polybutadiene and polyisoprene prepared in non-polar media. 

Effect of Initiator Concentration and Solvents on Chain Structure of Lithium Polybutadiene... 

Unpublished work of Halasa and co-workers 31) on the study of live chain ends using 13C-NMR as a probe into their structures has led to some interesting new findings. These workers studied the lithium live ends of polybutadiene in the presence of a new polar modifier, dipiperi-dylethane (DPE). This modifier forms a complex with n-BuLi which initiates the polymerization of 1,3-butadiene to give polymer having 100% 1,2 microstructure. 

As may be seen from Fig. 3, there are no resonance peaks at 120-128 ppm characteristic of 1,4-microstructure in polybutadiene polymer. However, on addition of methanol to the chain live ends, resonance peaks at 120-128 ppm appear in ratios of 60% trans-1,4, 14% m-1,4, and 26% 1,2. This suggests that the protonation of the chain live ends with methanol is an independent reaction and does not relate to the actual structure of the propagating species. It may be said that the structure of the allylic lithium of polybutadiene (DP > 1) is postulated to exist in the 1,2-form (13). Yet hydrolysis of 13 gives mixed 1,4- and 1,2-microstructures. 

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