31,607 research outputs found
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Post-polymerization modification by direct C-H functionalization
Post-polymerization modification of polymers is an important tool for accessing macromolecular materials with desired functional groups and tailored properties. Such strategy may become the only route to a target polymer when the availability or reactivity of the corresponding monomer is not suited for direct polymerization.
Most post-polymerization modification processes are based on transforming functional groups that are pre-installed in the side chains or chain-ends of a polymer. Despite the excellent efficiency and versatility, they are limited to certain backbone structures and often require additional synthetic effort for the synthesis of the corresponding pre-functionalized monomers. More specifically, they are useful only when the pre-functionalized monomers can be readily prepared and incorporated to a polymer by direct polymerization. In contrast, direct functionalization of C-H bonds along the polymer backbone offers a markedly different strategy for the synthesis of functional polymers. Despite the inert nature, the ubiquity of the C-H bonds and their tunable reactivity make them ideal targets for selective chemical modification.
In this dissertation, it is first demonstrated that poly(vinyl ester)s and poly(vinyl ether-co-vinyl ester) can be readily prepared via a ruthenium catalyzed C–H oxyfunctionalization of the corresponding poly(vinyl ether)s under mild conditions. The method can be further applied for the synthesis of high molecular weight poly(propenyl ester)s which cannot be obtained using other methods. In addition the method allows poly(isopropenyl ester) to be synthesized without the use of extremely high pressures.
Using a similar strategy poly(ethylene glycol-co-glycolic acid) can be prepared by the ruthenium-catalyzed oxidation of poly(ethylene glycol) (PEG). A new process has been developed so that the transformation will cause little chain degradation. The presence of the hydrolytically labile ester groups in the PEG backbone renders the copolymer biodegradable, which may allow the PEG of higher molecular weight to be used in biomedical applications without the concerns of bioaccumulation of PEG into various organs.
Lastly, it is demonstrated that azido-functionalized, isotactic polypropylene can be prepared via the direct C–H azidation of a commercially available polymer using a stable azidoiodinane. The azidated PP can further undergo copper-catalyzed azide-alkyne cycloaddition with alkyne terminated polymer to obtain PP-based graft copolymers. It is expected that the ability to incorporate versatile functional groups, such as azides, into common polyolefin feedstocks should expand their applications and potentially enable the realization of new classes of materials.Chemistr
Symmetry Energy Effects on the Mixed Hadron-Quark Phase at High Baryon Density
The phase transition of hadronic to quark matter at high baryon and isospin
density is analyzed. Relativistic mean field models are used to describe
hadronic matter, and the MIT bag model is adopted for quark matter. The
boundaries of the mixed phase and the related critical points for symmetric and
asymmetric matter are obtained. Due to the different symmetry term in the two
phases, isospin effects appear to be rather significant. With increasing
isospin asymmetry the binodal transition line of the (T,\rho_B) diagram is
lowered to a region accessible through heavy ion collisions in the energy range
of the new planned facilities, e.g. the FAIR/NICA projects. Some observable
effects are suggested, in particular an "Isospin Distillation" mechanism with a
more isospin asymmetric quark phase, to be seen in charged meson yield ratios,
and an onset of quark number scaling of the meson/baryon elliptic flows. The
presented isospin effects on the mixed phase appear to be robust with respect
to even large variations of the poorly known symmetry term at high baryon
density in the hadron phase. The dependence of the results on a suitable
treatment of isospin contributions in effective QCD Lagrangian approaches, at
the level of explicit isovector parts and/or quark condensates, is finally
discussed.Comment: 14 two column pages, 14 figures, new results with other hadron EoS.
Accepted for publication in Phys.Rev.
Rosen-Zener Transition in a Nonlinear Two-Level System
We study Rosen-Zener transition (RZT) in a nonlinear two-level system in
which the level energies depend on the occupation of the levels, representing a
mean-field type of interaction between the particles. We find that the
nonlinearity could affect the quantum transition dramatically. At certain
nonlinearity the 100% population transfer between two levels is observed and
found to be robust over a very wide range of external parameters. On the other
hand, the quantum transition could be completely blocked by a strong
nonlinearity. In the sudden and adiabatic limits we have derived analytical
expressions for the transition probability. Numerical explorations are made for
a wide range of parameters of the general case. Possible applications of our
theory to Bose-Einstern Condensates (BECs) are discussed.Comment: 8 pages, 8 figure
Landau-Zener Tunnelling in a Nonlinear Three-level System
We present a comprehensive analysis of the Landau-Zener tunnelling of a
nonlinear three-level system in a linearly sweeping external field. We find the
presence of nonzero tunnelling probability in the adiabatic limit (i.e., very
slowly sweeping field) even for the situation that the nonlinear term is very
small and the energy levels keep the same topological structure as that of
linear case. In particular, the tunnelling is irregular with showing an
unresolved sensitivity on the sweeping rate. For the case of fast-sweeping
fields, we derive an analytic expression for the tunnelling probability with
stationary phase approximation and show that the nonlinearity can dramatically
influence the tunnelling probability when the nonlinear "internal field"
resonate with the external field. We also discuss the asymmetry of the
tunnelling probability induced by the nonlinearity. Physics behind the above
phenomena is revealed and possible application of our model to triple-well
trapped Bose-Einstein condensate is discussed.Comment: 8 pages, 8 figure
Application of density dependent parametrization models to asymmetric nuclear matter
Density dependent parametrization models of the nucleon-meson effective
couplings, including the isovector scalar \delta-field, are applied to
asymmetric nuclear matter. The nuclear equation of state and the neutron star
properties are studied in an effective Lagrangian density approach, using the
relativistic mean field hadron theory. It is known that the introduction of a
\delta-meson in the constant coupling scheme leads to an increase of the
symmetry energy at high density and so to larger neutron star masses, in a pure
nucleon-lepton scheme. We use here a more microscopic density dependent model
of the nucleon-meson couplings to study the properties of neutron star matter
and to re-examine the \delta-field effects in asymmetric nuclear matter. Our
calculations show that, due to the increase of the effective \delta coupling at
high density, with density dependent couplings the neutron star masses in fact
can be even reduced.Comment: 5 pages, 4 figure
Primate Primordial Germ Cells Acquire Transplantation Potential by Carnegie Stage 23.
Primordial germ cells (PGCs) are the earliest embryonic progenitors in the germline. Correct formation of PGCs is critical to reproductive health as an adult. Recent work has shown that primate PGCs can be differentiated from pluripotent stem cells; however, a bioassay that supports their identity as transplantable germ cells has not been reported. Here, we adopted a xenotransplantation assay by transplanting single-cell suspensions of human and nonhuman primate embryonic Macaca mulatta (rhesus macaque) testes containing PGCs into the seminiferous tubules of adult busulfan-treated nude mice. We discovered that both human and nonhuman primate embryonic testis are xenotransplantable, generating colonies while not generating tumors. Taken together, this work provides two critical references (molecular and functional) for defining transplantable primate PGCs. These results provide a blueprint for differentiating pluripotent stem cells to transplantable PGC-like cells in a species that is amenable to transplantation and fertility studies
Shoshonitic enclaves in the high Sr/Y Nyemo pluton, southern Tibet: Implications for Oligocene magma mixing and the onset of extension of the southern Lhasa terrane
Post-collisional potassic and high Sr/Y magmatism in the Lhasa terrane provides critical constraints on the timing and mechanism of subduction of Indian lithosphere and its role in the uplift of the Tibetan Plateau. Here, we report whole-rock geochemistry, mineral geochemistry, zircon U Pb ages, and in situ zircon Hf isotope ratios for the Nyemo pluton, a representative example of such magmatism. The Nyemo pluton is composed of high Sr/Y host rocks and coeval shoshonitic mafic microgranular enclaves (MMEs). Whole-rock compositions of the host rocks and MMEs form linear trends in Harker diagrams, consistent with modification of both end-members by magma mixing. Although the main high Sr/Y phase of the pluton formed by partial melting of the lower crust of the thickened Lhasa terrane, the MMEs display abnormally enriched light rare earth elements, low whole-rock ε_(Nd)(t) and low zircon ε_(Hf)(t) that suggest derivation from low degree melting of hydrous and enriched mantle. Based on the occurrence of shoshonitic magma and high La/Yb and high Sr/Y with adakitic affinity host rocks around 30 Ma, the Nyemo pluton is best explained as a record of onset of extension that resulted from convective removal of the mantle lithosphere beneath Tibet in the Oligocene
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Computerized adaptive testing and short form development for child and adolescent oral health patient-reported outcomes measurement.
ObjectivesTo develop computerized adaptive testing (CAT) and short forms of self-report oral health measures that are predictive of both the children's oral health status index (COHSI) and the children's oral health referral recommendation (COHRR) scales, for children and adolescents, ages 8-17.Material and methodsUsing final item calibration parameters (discrimination and difficulty parameters) from the item response theory analysis, we performed post hoc CAT simulation. Items most frequently administered in the simulation were incorporated for possible inclusion in final oral health assessment toolkits, to select the best performing eight items for COHSI and COHRR.ResultsTwo previously identified unidimensional sets of self-report items consisting of 19 items for the COHSI and 22 items for the COHRR were administered through CAT resulting in eight-item short forms for both the COHSI and COHRR. Correlations between the simulated CAT scores and the full item bank representing the latent trait are r = .94 for COHSI and r = .96 for COHRR, respectively, which demonstrated high reliability of the CAT and short form.ConclusionsUsing established rigorous measurement development standards, the CAT and corresponding eight-item short form items for COHSI and COHRR were developed to assess the oral health status of children and adolescents, ages 8-17. These measures demonstrated good psychometric properties and can have clinical utility in oral health screening and evaluation and clinical referral recommendations
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