The rates of H/D exchange have been measured between (a) the activated olefins methyl methacrylate-d(5) and styrene-d(8), and (b) the Cr hydrides (eta(5)-C(5)Ph(5))Cr(CO)(3)H (2a), (eta(5)-C(5)Me(5))Cr(CO)(3)H (2b), and (eta(5)-C(5)H(5))Cr(CO)(3)H (2c). With a large excess of the deuterated olefin the first exchange goes to completion before subsequent exchanges begin, at a rate first order in olefin and in hydride. (Hydrogenation is insignificant except with styrene and CpCr(CO)(3)H; in most cases, the radicals arising from the first H. transfer are too hindered to abstract another H. .) Statistical corrections give the rate constants k(reinit) for H. transfer to the olefin from the hydride. With MMA, k(reinit) decreases substantially as the steric bulk of the hydride increases; with styrene, the steric bulk of the hydride has little effect. At longer times, the reaction of MMA or styrene with 2a gives the corresponding metalloradical 1a as termination depletes the concentration of the methyl isobutyryl radical 3 or the alpha-methylbenzyl radical 4; computer simulation of [1a] as f(t) gives an estimate of k(tr), the rate constant for H. transfer from 3 or 4 back to Cr. These rate constants imply a DeltaG (50 degrees C) of +11 kcal/mol for H. transfer from 2a to MMA, and a DeltaG (50 degrees C) of +10 kcal/mol for H. transfer from 2a to styrene. The CH(3)CN pK(a) of 2a, 11.7, implies a BDE for its Cr-H bond of 59.6 kcal/mol, and DFT calculations give 58.2 kcal/mol for the Cr-H bond in 2c. In combination the kinetic DeltaG values, the experimental BDE for 2a, and the calculated DeltaS values for H. transfer imply a C-H BDE of 45.6 kcal/mol for the methyl isobutyryl radical 3 (close to the DFT-calculated 49.5 kcal/mol), and a C-H BDE of 47.9 kcal/mol for the alpha-methylbenzyl radical 4 (close to the DFT-calculated 49.9 kcal/mol). A solvent cage model suggests 46.1 kcal/mol as the C-H BDE for the chain-carrying radical in MMA polymerization.
A new multicenter ethylene polymerization process is described whereby two different single-site catalysts, one competent for producing vinyl-terminated oligomers or macromonomers and one competent for producing high-molecular weight ethylene-alpha-olefin copolymers, are held in close spatial proximity via ion-pairing with a dianionic binuclear bis-borate cocatalyst. Ethylene polymerizations mediated by stoichiometrically appropriate quantities of Me2Si(tBuN)(eta5-3-ethylindenyl)ZrMe2 and Me2Si(tBuN)(eta5-C5Me4)TiMe2 activated by the bis-borate cocatalyst [Ph3C+]2[1,4-(C6F5)3BC6F4B(C6F5)3-2] yield a more homogeneous polyethylene product when compared to control polymerizations using the mononuclear activator [Ph3C+][B(C6F5)4-]. The bulk and spectroscopic properties of the polymer produced using the binuclear activator are consistent with highly branched polyethylene.
Today, polyurethanes
are effectively not recycled and are made
principally from nonrenewable, fossil-fuel-derived resources. This
study provides the first high-resolution material flow analysis of
polyurethane flows through the U.S. economy, tracking back to fossil
fuels and covering polyurethane-relevant raw materials, trade, production,
manufacturing, uses, historical stocks, and waste management. According
to our analysis, in 2016, 2900 thousand tonnes (kt) of polyurethane
were produced in the United States and 920 kt were imported for consumption,
2000 kt entered the postconsumer waste streams, and 390 kt were recycled
and returned to the market in the form of carpet underlayment. The
domestic production of polyurethane consumed 1100 kt of crude oil
and 1100 kt of natural gas. With the developed polyurethane flow map,
we point out the limitation of the existing mechanical recycling methods
and identify that glycolysis, a chemical recycling method, can be
used to recycle the main components of postconsumer polyurethane waste.
We also explore how targeting biobased pathways could influence the
supply chain and downstream markets of polyurethane and reduce the
consumption of fossil fuels and the exposure to toxic precursors in
polyurethane production.
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