Kinetic resolution of racemic {alpha}-olefins with ansa-zirconocene polymerization catalysts: Enantiomorphic site vs. chain end control

Copolymerization of racemic {alpha}-olefins with ethylene and propylene was carried out in the presence of enantiopure C1-symmetric ansa metallocene, {1,2-(SiMe2)2({eta}5-C5H-3,5-(CHMe2)2)({eta}5-C5H3)}ZrCl2 to probe the effect of the polymer chain end on enantioselection for the R- or S-{alpha}-olefin during the kinetic resolution by polymerization catalysis. Copolymerizations with ethylene revealed that the polymer chain end is an important factor in the enantioselection of the reaction and that for homopolymerization, chain end control generally works cooperatively with enantiomorphic site control. Results from propylene copolymerizations suggested that chain end control arising from a methyl group at the beta carbon along the main chain can drastically affect selectivity, but its importance as a stereo-directing element depends on the identity of the olefin

Social, Spacing, and Cooperative Behavior of the Collared Peccary, Tayassu tajacu

Social behavior of the collared peccary was studied on the lower, eastern slopes of the Mazatzal Mountains, Arizona. The social unit in this species is a cohesive herd, in which small inter-individual distances are maintained. Two conspicuous acts, one olfactory and one auditory, functioned to maintain close spacing. Social interactions were brief but tended to synchronize the activities of animals and also to bring them closer together. Amicable and neutral actions occurred far more frequently than agonistic interactions. Most agonistic behavior did not involve physical contact. Cooperative nursing, predator defense, and feeding occurred; all adults were tolerant of young, and males showed little overt competition over estrous females. The absence of sexual dimorphism, the 1:1 sex ratio within social groups, and the small, precocial litters in this species suggest an evolutionary history of pronounced sociality. Kin selection possibly was important in the evolution of social behavior in peccaries

Enhancing space transportation: The NASA program to develop electric propulsion

The NASA Office of Aeronautics, Exploration, and Technology (OAET) supports a research and technology (R and T) program in electric propulsion to provide the basis for increased performance and life of electric thruster systems which can have a major impact on space system performance, including orbital transfer, stationkeeping, and planetary exploration. The program is oriented toward providing high-performance options that will be applicable to a broad range of near-term and far-term missions and vehicles. The program, which is being conducted through the Jet Propulsion Laboratory (JPL) and Lewis Research Center (LeRC) includes research on resistojet, arcjets, ion engines, magnetoplasmadynamic (MPD) thrusters, and electrodeless thrusters. Planning is also under way for nuclear electric propulsion (NEP) as part of the Space Exploration Initiative (SEI)

Catalyst Site Epimerization during the Kinetic Resolution of Chiral α-Olefins by Polymerization

A new enantiopure C1-symmetric olefin polymerization precatalyst, (1,2-SiMe_2)_2{η^5-C_5H_2-4-((S)-CHEtCMe_3)}{η^5-C_5H-3,5-(CHMe_2)_2}ZrCl_2, (S)-2, was synthesized, and its use for the kinetic resolution of 3-methyl-substituted racemic α-olefins was investigated. Upon activation with methyl aluminoxane (MAO), selectivity factors for most olefins were greater when (S)-2 was used as the catalyst as compared to its previously reported methylneopentyl analogue, (1,2-SiMe_2)_2{η^5-C_5H_2-4-((S)-CHMeCCMe_3)}{η^5-C_5H-3,5-(CHMe_2)_2}ZrCl_2, (S)-1. Pentad analysis of polypropylene produced by the two catalysts at various propylene concentrations indicates that (S)-2 undergoes more efficient site epimerization (polymeryl chain swinging prior to subsequent monomer enchainment) at intermediate propylene concentrations compared to (S)-1. At high and low propylene concentrations, however, the two catalysts behave similarly. On the other hand, polymerization of 3,5,5-trimethyl-1-hexene at different olefin concentrations and temperatures illustrated that selectivity differences between the two catalysts are likely not a consequence of inefficient site epimerization for (S)-1

Learn to earn: enabling coordination within a ride-hailing fleet

Accepted manuscrip

Synchronization of Energy Consumption By Human Societies Throughout the Holocene

We conduct a global comparison of the consumption of energy by human populations throughout the Holocene and statistically quantify coincident changes in the consumption of energy over space and time—an ecological phenomenon known as synchrony. When populations synchronize, adverse changes in ecosystems and social systems may cascade from society to society. Thus, to develop policies that favor the sustained use of resources, we must understand the processes that cause the synchrony of human populations. To date, it is not clear whether human societies display long-term synchrony or, if they do, the potential causes. Our analysis begins to fill this knowledge gap by quantifying the long-term synchrony of human societies, and we hypothesize that the synchrony of human populations results from (i) the creation of social ties that couple populations over smaller scales and (ii) much larger scale, globally convergent trajectories of cultural evolution toward more energy-consuming political economies with higher carrying capacities. Our results suggest that the process of globalization is a natural consequence of evolutionary trajectories that increase the carrying capacities of human societies