Semicrystalline polymers of low glass transition temperature, such as polyethylene (PE), can be either brittle or ductile depending on their content of intercrystallite stress transmitterssuch as tie molecules (TMs), chains that directly bridge the intercrystalline amorphous layer. TM content will increase with increasing molecular weight (M) or with the fraction of high-M chains in a disperse polymer and with decreasing intercrystallite repeat spacing d, which can be manipulated through thermal history and the incorporation of comonomer. The present work examines the failure mode of model narrow-distribution linear PEs (LPEs) of high crystallinity, where d is varied through crystallization history (either quenching or slowly cooling), and ethylene-butene copolymers (hydrogenated polybutadienes (hPBs)) of moderate crystallinity, where d is limited by the short-branch content. For each series (LPEs with different thermal histories and quenched hPBs), a rather sharp brittle-to-ductile transition (BDT) is observed with increasing M, at a value M BDT. However, across the three series, the value of M BDT does not depend solely on the value of d; indeed, a higher M is required to achieve ductility in quenched samples of hPB than in LPE, despite the much lower values of d for hPB. Consequently, the calculated value of TM fraction at the BDT increases strongly as crystallinity decreases, by a factor of ∼50 from slow-cooled LPE to quenched hPB. This strong dependence is explained by considering the influence of TMs on the brittle fracture stress (σb), with the BDT occurring when there are sufficient TMs for σb to exceed the yield stress (σ y ), which is strongly dependent on crystallinity but independent of TM content.
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.
Macromonomers bearing a functional group exclusively at one end are often synthesized by terminating a living polymerization with a suitable agent, at the cost of one initiator molecule per chain. An attractive alternative is to use chain transfer to install the functional endgroup. Herein, polycyclopentene (PCP) bearing a single styryl endgroup is synthesized by ring-opening metathesis polymerization with a well-defined Mo-based (Schrock) initiator, by employing divinylbenzene as a chain-transfer agent. High regioselectivity of the chain-transfer step, with minimal secondary metathesis of double bonds in the PCP backbone, is confirmed by mass spectral endgroup analysis. A styryl-functional PCP macromonomer copolymerizes effectively with styrene in a free-radical process (styrene reactivity ratio r S ≈ 1.7), producing a comb copolymer with a polystyrene backbone and PCP “teeth”.
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