A study of deuterium distribution in deuterated polyolefins by pyrolysis-photoionization mass spectrometry

The saturation of polydienes with deuterium produces labeled polyolefins th at, when studied with the identical hydrogenated polydienes, have proven qu ite useful for the understanding of many of the basic properties of these m aterials. However, until now the only way to determine the amount of deuter ium substitution on the labeled molecules has been to compare the densities of the hydrogenated and deuterated polymers, which gives no information on how the deuterium is distributed along the chains. The information is impo rtant, since it is clear from the density results that with some saturation catalysts (Pd) there must be some H-D exchange since there are more than t wo deuterium atoms per diene monomer on the labeled species, while for othe rs (Wilkinson's) there appears to be no exchange. In this work, pyrolysis-p hotionization mass spectrometry has been used to examine the structure of t he hydrogenated and deuterated polydienes in more detail. The deuterium dis tributions were determined using a modified Bernoullian model that takes in to account the first two D's that were added by the saturation event. The m ost revealing data come from the tetramer C4H8-xDx, which confirm the avera ge level of deuteration as measured by density. For polymers saturated with Pd catalysts, the pyrolysis-photionization mass spectra show that the deut erium was randomly distributed by the H-D exchange. Perdeuteriopolyisobutyl ene was also examined, as was its blend with hydrogenous polyisobutylene, w hich helped to confirm that exchange had occurred during the saturation of the polydienes using the catalyst rather than during the measurement step. The data for the polydiene deuterated using Wilkinson's catalyst confirmed that there was little, if any, exchange.