1,721,094 research outputs found
On the Connection between Microstructure and Kinetics of (Pseudo)copolymerization
No full textThe connection between microstructure and kinetics in the case of irreversible growth of long (pseudo)copolymers has been discussed, extended and established. The theoretical results have been cast in simple matrix equations which enable one to compute the probability to find any given sequence in terms of ratios of kinetic constants, rather than in terms of stochastic parameters. As an application, the case of regioregular polymerization of vinyl monomers by C1-symmetric coordination catalysts has been considered. In addition to propagation, first order in monomer concentration, chain and site epimerization, both zeroth-order in monomer concentration, have been taken into account. The resulting equations have been tested against a set of literature data obtained by five different catalysts, each operated at five different concentrations, resulting in the evaluation of ratios of kinetic constants. For all catalysts the quantitative analysis of the kinetics confirms that the contribution of chain epimerization is small or negligible in the concentration range examined
On the diatropic perimeter of iterated altan-molecules
No full textAb initio calculations of the current density induced by an external magnetic field in altan-[10,5]coronene 3 (or altan2-[10]annulene) indicate the onset of a diatropic current on the outer loop (the "perimeter"), thus in contrast to the intentional design of altan-molecules. Spectral decomposition of the current shows that the failure of the altanisation design can be expected whenever the perimeter of the altan-molecule is of nearly the same size as that of the parent molecule. Equal lengths of these two perimeters always occur in iterated altan-molecules, which contain a cycloacenic fragment. Like longer polyacenes and cycloacenes, altan2-[10]annulene 3 and altan-kekulene 2 are predicted to have an open-shell singlet ground state
Establishing the connection between microstructural data and Kinetic constants of (pseudo)copolymerization
No full text
On the definition of the atomic charge. Relationship between 13C NMR chemical shifts, dipole moments, and charges in saturated hydrocarbons
No full textThe problem of defining a reliable quantum mechanical charge by
comparison with one-electron properties is analyzed, and it is stressed that properties
involving the virtual space are not suited to that end. Particular attention is devoted
to the relationship between charges and chemical shifts for the case of saturated
hydrocarbons. A simple explanation of the Grant and Paul a effect is suggested, which
can also account for the modified population analysis proposed by Flisz ́ar. Moreover the
vexata quaestio of the direction of the C—H bond dipole moment has been reexamined.
The awkward theoretical prediction Cq—Hy. can be reconciled with the one based on
experimental data and electronegativities Cy—Hq. if one considers that the former is
determined by an hydridization contribution to the dipole moment, which tend to cancel
in a summation over all the bonds formed by each atom
On the microstructural analysis of (pseudo)copolymers
No full textThe main theoretical problems encountered
in the microstructural analysis of (pseudo)copolymers
have been discussed. The theoretical framework for the
application of Markovian models in this field has been
established by arguments of information theory. As indicators
of microstructural regularity, which are independent
from models of polymerization kinetics, block lengths
and redundancy estimates have been presented and discussed.
Redundancy estimates are tightly connected to the
informational entropy and can be obtained with minor
effort from experimental data. Literature microstructural
data of ethylene-propylene copolymers and polypropylenes
have been examined in order to give practical examples
of the computation of the indicators, which are found
to easily describe the main features of the samples examined
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