Theoretical and experimental approaches to kinetics at the collapse transition of a homopolymer

Summary We discuss some recent theoretical studies of the kinetics of the collapse transition in homopolymers. An isolated polymer is modelled using computer simulation, and a time-dependent mean-field theory. The mean-field theory is analysed analytically for early stages, and for short polymers the equations are studied numerically. The results of simulation and theory are compared yielding, we argue, a consistent physical picture. Quantitative comparisons are not yet given, but seem relatively promising. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4– 1994.     

Metabelian groups with a single defining relation and the Magnus embedding

Translated from Matematicheskie Zametki, Vol. 47, No. 4, pp. 597–605, April, 1995.     

Characterisation of oxygen defects in calciumdifluoride

We study the aggregation of oxygen dipoles well dispersed in a CaF₂ crystal upon annealing at temperatures ranging from 370 to 420 K. The concentration of oxygen dipoles is monitored by measuring the intensity of the ionic thermocurrent peak as well as by absorption and luminescence spectroscopies. Results from three methods agree within experimental error and yield an activation energy of (1.2±0.1) eV for the diffusion of isolated oxygen centres in the crystal.     

Synthesis of 3-imino and 3-ylideno derivatives of 4-fluoro-5-polyfluoroalkyl-1,2-dithiolenes

Reactions of 4-fluoro-5-polyfluoroalkyl-1,2-dithiol-3-thiones with hydroxylamine and hydrazines occur with replacement of the thiocarbonyl by imino group affording oximes and hydrazones respectively. N-Alkyl-and N-aryl-3-imino-1,2-dithiolenes formed in reactions of 3-chlorothio-1,2-dithiolium salts with primary alkyl-or arylamines. 3-Chlorothio-1,2-dithiolium salts react with compounds possessing an active methylene group yielding 3-ylideno derivatives of 1,2-dithiolenes.     

Synthesis of 4-Benzylsulfonyl-5-polyfluoroalkyl-ν-triazoles from 1,1-Dihydropolyfluoroalkyl Benzyl Sulfones

The reaction of 1,1-dihydropolyfluoroalkyl benzyl sulfones with sodium azide or trimethylsilyl azide in the presence of tertiary amines gives ammonium salts of 4-benzylsulfonyl-5-polyfluoroalkyl--triazoles. The proposed reaction scheme includes the participation of catalytic amounts of azide ion in the cyclization of the formed intermediate vinyl azides to triazoles. X-ray analysis of 1,4-diazabicyclo[2.2.2]octane salt of 4-benzylsulfonyl-5-(-H-hexafluoropropyl)--triazole has been carried out.     

Ring-opening carbonyl–olefin metathesis of norbornenes

A computational and experimental study of the hydrazine-catalyzed ring-opening carbonyl–olefin metathesis of norbornenes is described. Detailed theoretical investigation of the energetic landscape for the full reaction pathway with six different hydrazines revealed several crucial aspects for the design of next-generation hydrazine catalysts. This study indicated that a [2.2.2]-bicyclic hydrazine should offer substantially increased reactivity versus the previously reported [2.2.1]-hydrazine due to a lowered activation barrier for the rate-determining cycloreversion step, a prediction which was verified experimentally. Optimized conditions for both cycloaddition and cycloreversion steps were identified, and a brief substrate scope study for each was conducted. A complication for catalysis was found to be the slow hydrolysis of the ring-opened hydrazonium intermediates, which were shown to suffer from a competitive and irreversible cycloaddition with a second equivalent of norbornene. This problem was overcome by the strategic incorporation of a bridgehead methyl group on the norbornene ring, leading to the first demonstrated catalytic carbonyl–olefin metathesis of norbornene rings.

A computational and experimental study has uncovered a second generation hydrazine that enables the catalytic ring-opening carbonyl–olefin metathesis of norbornenes.     

Reversible carboxylation of N-heterocyclic carbenes

Spectroscopic analysis, thermogravimetric analysis, and crossover experiments performed on a series of imidazolium carboxylates revealed carboxylation was reversible with N-aryl substituted adducts.     

Femtosecond excitation energy transport in triarylamine dendrimers

The search for a model that can be used to describe the optical excitation migration in dendrimers has attracted great attention. In most cases in a dendrimer the conjugation is disrupted at the branching point; however, the excitation is delocalized. The strength of interactions among neighboring chromophores plays a key role in determining the energy migration mechanism. Conversely, having many identical chromophores held tightly together in an ordered macromolecular architecture will allow for many dipoles to be accessible for optical excitation. Therefore, the relative orientation of dipoles will be important in determining the mechanism of energy migration. Here we report the synthesis and photo-physical investigation of triarylamine-based dendrimers. Two important synthetic steps were utilized in the synthesis. First, we employed diphenylmethyl protective groups on the amines to assist in deprotective hydrogenolysis of the larger structures. Second, highly active catalysts for formation of both di- and triarylamines that are based on a 1:1 ratio of P(t-Bu)3 and Pd(dba)2 improved reaction yields of the C-N bond formation and decreased reaction times The energy migration processes in the dendrimers were investigated utilizing ultrafast time-resolved fluorescence anisotropy measurements. The fluorescence anisotropy of all three dendrimers decayed to a residual value within approximately 100 fs. This fluorescence anisotropy decay showed a general trend in decreasing with increasing dendrimer generation. The residual anisotropy value also showed a gradual decrease with an increase in the dendrimer generation. This fast energy depolarization is discussed through a coherent excitonic mechanism among dipoles oriented in different directions. We believe that the formation of coherent domains leads to fast energy migration extending over a large part of the dendrimer.