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

Abstract

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 D 2 . Heterogeneous catalysts often induce net H/D exchange between the polymer and D 2 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, 13 C 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 13 C 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.

Minimum Molecular Weight and Tie Molecule Content for Ductility in Polyethylenes of Varying Crystallinity

“…The hydrogenated polymers were recovered by hot filtration, precipitated into methanol, and dried under vacuum overnight. Previous studies have repeatedly shown that hydrogenation over Pd/CaCO 3 under these conditions is not accompanied by chain rearrangements. ,− …”

“…The polymer solution was transferred to a 2 L Parr stainless steel reactor, and Pd/CaCO 3 was added at a 2:1 weight ratio of catalyst (including support) to the polymer. The reactor was charged with 400 psi of H 2 at room temperature and stirred at 130 °C for PCP or 100 °C for PB for 24 h, by which point saturation had reached more than 99.9% for PCP as determined by 1 H NMR spectroscopy 40 and more than 99% for PB as determined by FTIR spectroscopy. 36 The hydrogenated polymers were recovered by hot filtration, precipitated into methanol, and dried under vacuum overnight.…”