Regularity of Deuteration in Linear Polyethylene Prepared by Saturation of Polycyclopentene over Homogeneous Catalysts

When isotopically labeling polymer chains for small-angle neutron scattering (SANS), it is highly desirable to achieve even intra- and interchain distributions of deuterium (D), such that scattering centers are uniformly placed along and among the chains. A common approach to introduce D is to catalytically saturate an unsaturated precursor polymer with D2. Heterogeneous catalysts often induce net H/D exchange between the polymer and D2 gas, yielding excess D on the polymer which is nonuniformly distributed; however, the homogeneous Wilkinson's catalyst [tris(triphenylphosphine)rhodium(I) chloride] has been shown to yield statistically uniform labeling. Here, 13C NMR spectroscopy is employed to determine both the deuteration level (DL) and regularity of deuteration in partially deuterated polyethylene (dPE) synthesized by ring-opening metathesis polymerization of cyclopentene followed by deuteration over either Wilkinson's catalyst or an alternative homogeneous catalyst, carbonylchlorohydridotris(triphenylphosphine)ruthenium(II) (Ru-H). Both catalysts produce deuterated methylenes other than the vicinal -CDH-CDH- pair expected from regular deuteration, as a consequence of β-elimination events prior to saturation; under typical saturation conditions, β-elimination is more prevalent with Ru-H. Compared with a DL of 20% expected for ideal regular deuteration, DL values determined by 13C NMR peak integration are 20.1% for Wilkinson's and 21.9% for Ru-H, indicating significant net H/D exchange over Ru-H. However, SANS from both dPEs shows no angular dependence in the q-range relevant to single-chain dimensions, demonstrating that the deuterium distribution is statistically uniform along and among polymer chains.