Distortions in quasi-one-dimensional polyconjugated systems

The Hückel and Lennard-Jones approximation is used to show that structural distortions with bond-length alternation in the chains are impossible because of the high rigidity of the core in a conjugated system for any long polyconjugated system of polyacene type that can be represented as polyene chains linked together.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 23, No. 3, pp. 350–352, May–June, 1987.     

A study of the mechanism of photocrosslinking polymers by quinones

Photocrosslinking of poly(methyl methacrylate) (PMMA) was studied in the solid state in the presence of various quinones. For the study of photocrosslinking mechanism, a PMMA film containing p-benzoquinone (Q) was irradiated with UV light (λ > 370nm) and then purified by reprecipitation. It was found that the reprecipitated polymer has quinone-type moieties, besides the hydroquinone-type moieties, chemically bound to the polymer chain. The reprecipitated polymer film also crosslinking efficiency was higher than that of PMMA film containing Q. It was concluded that the formation of quinone-type structure during irradiation played an important role in the photocrossing of PMMA containing Q.     

Correlation of certain physicochemical parameters of polyconjugated systems

Journal of Structural Chemistry -     

Preparation and characterization of multilayered structures with polyconjugated systems

Soluble poly(3-alkylthiophenes) can be conveniently tailored by the introduction of polar groups in the side chain with the aim of obtaining structures able to give multilayered films with the Langmuir Blodgett technique. The films, uniform in thickness, show orientation of the polyenic backbone in the dipping direction as detected by UV-visible measurements in polarised light. These materials can be conveniently used in electronic device preparation.     

Self‐assembly of mono‐ and multilayers of polyconjugated conducting polymers

To obtain organised mono‐ and multilayers of policonjugated polymers or oligomers by self assembly techniques has recently attracted much attention. This is due to the simplicity of the method and to the fact that no sophisticated apparatus is required. In the present feature the state of the art in this field is discussed including our own research results. A survey on self assembly of thiols, disulfides, and alkylsilanes on gold or glass surfaces is presented as well as self assembly of ferrocenylalkyl‐substituted heterocycle adsorbates on ITO surfaces. Carboxyalkyl bithiophenes and terthiophenes, in which the oligothiophene tail is either perpendicularly or linearly linked to the tethering carboxyalkyl chain form stable monolayers on ITO electrodes and are anodically oxidised to produce polymer layers or to couple with oligothiophenes in solution, depending on the structure. As for multilayers, an introductory survey on the self assembly technique usually employed is given. Several examples of multilayers constituted of electroconductive polymers are discussed. Finally our recent results concerning different multilayers from polythiophenes bearing anionic and cationic moieties are discussed.     

Photocrosslinking of polymers bearing epoxy or epithio groups by quinones in the solid state

Photocrosslinking of poly(glycidyl methacrylate) (PGMA) and a copolymer of 2,3-epithiopropyl methacrylate and methyl methacrylate [P(ETMA-co-MMA)] was studied in the solid state in the presence of various quinones. The efficiency of photocrosslinking was strongly dependent upon the structures of quinones and the kinds of polymers. For example, the alkyl-substituted quinones such as 2-ethyl-p-benzoquinone (EQ), 2-tert-butyl-p-benzoquinone (tBQ) and 2,5-di-tert-butyl-p-benzoquinone (2,5-dtBQ) did not induce photocrosslinking of poly(methyl methacrylate) (PMMA), whereas they acted as efficient photocrosslinking agents in PGMA and P(ETMA-co-MMA). The formation of charge transfer complexes did not play a principal role in this effective photocrosslinking, because the order of the photocrosslinking efficiency [EQ > tBQ > 2,5-dtBQ > p-benzoquinone (Q)] was not in agreement with that of the magnitude of the electron affinities of quinones, i.e., Q > EQ > tBQ > 2,5-dtBQ. Photopolymerization of propylene sulfide (PS) in the presence of tBQ or Q was also investigated. The presence of tBQ induced polymerization of PS upon UV irradiation. From these results, it was deduced that the photocrosslinking of P(ETMA-co-MMA) film containing tBQ proceeds via a cationic polymerization of epithio groups. A similar mechanism should be applicable to the photocrosslinking of PGMA film containing tBQ.     

Formation of quaternary stereogenic centers by NHC-Cu-catalyzed asymmetric conjugate addition reactions with Grignard reagents on polyconjugated cyclic enones

The copper-catalyzed conjugate addition of various Grignard reagents to polyconjugated enones (dienone and enynone derivatives) is reported. The catalyst system, composed of copper triflate and an NHC ligand, led to the unusual selective formation of the 1,4-addition products. This reaction allows for the creation of all-carbon chiral quaternary centers with enantiomeric excesses up to 99%. The remaining unsaturation on the 1,4 adducts give access to valuable synthetic transformations.     

Mechanism of inhibiting action of quinones in oxidizing polymers and model compounds

Quinones (Q) do not affect the liquid-phase oxidation of hydrocarbons and retard the solid-phase oxidation of polymers terminating chains in reactions with alkyl macroradicals. This difference is the result of specific influence of a polymer matrix on the kinetics of free radical reactions. Quinones were found to terminate chains in oxidizing polypropylene containing hydroperoxide groups by the reaction with hydroperoxyl radicals. This is the result of peroxide group's decomposition with hydrogen peroxide production and chain termination in the following reactions: The equilibrium reaction between hydroquinone (QH₂) and quinonimine (QI) was evidenced to proceed as a chain reaction with chain propagation in reactions of •QH with QI and HQI• with QH₂. Analysis of the reactions of quinone with phenols (ArOH) proved that it can be important as an additional way of chain termination in oxidizing polymers and hydrocarbons at elevated temperatures.     

Micro-composite systems by in-situ formation of liquid crystal polymers in commodity polymers

In-situ formation of polyimide was carried out in solution of copolymers derived from styrene and 4-vinylpyridine. The in-situ formed polyamic acid formed a strong hydrogen-bonding with pyridine moiety of the copolymers, resulting in homogeneous solutions. Cast films obtained from the solutions were clear and transparent without phase separations, and mechanical properties of the films were much stronger than those of films from the original copolymers. In-situ polymerization of acrylamide in poly(styrene) was carried out by anionic polymerization to form Nylon 3 which was dispersed in poly(styrene) as fine particles, and mechanical properties of the poly (styrene) were greatly improved.     

Single two-electron transfers vs successive one-electron transfers in polyconjugated systems illustrated by the electrochemical oxidation and reduction of carotenoids.

Examination of cyclic voltammetric responses reveals that inversion of the standard potentials of the first and second electron transfers occurs in the oxidation of beta-carotene and 15,15'-didehydro-beta-carotene (but not in their reduction) as well as in the reduction of canthaxanthin (but not in its oxidation). The factors that control potential inversion in these systems, and more generally in symmetrical molecules containing conjugated long chains, are investigated by quantum chemical calculations. Two main interconnected effects emerge. One is the localization of the charges in the di-ion toward the ends of the molecule at a large distance from one another, thus minimizing Coulombic repulsion. The same effect favors the solvation of the di-ion providing additional stabilization. In contrast, the charge in the ion radical is delocalized over the whole molecular framework, thus disfavoring its stabilization by interaction with the solvent. The combination of the two solvation effects allows potential inversion to occur as opposed to the case where the two electrophores are linked by a saturated bridge where potential inversion cannot occur. Localization of the charges in the di-ion, and thus potential inversion, is favored by the presence of electron-accepting terminal groups for reductions (as the two carbonyl groups in canthaxanthin) and of hole-accepting terminal groups for oxidations (as in beta-carotene).     

Formation of dehydrogenated derivatives upon the oxidative dehydrogenation of adamantanes

Russian Chemical Bulletin -     

Theory of steric stabilization of colloid systems by grafted polymers

An analytical mean-field theory of steric stabilization of dispersions by non-charged polymer chains grafted at one end onto the surface of colloid particles has been developed. The chains were assumed to be grafted densly, so that the neighboring coils overlap and form a single stabilizing layer. In the case of low-molecular-weight dispersing medium and relatively loose stabilizing layers the shape of the potential curve of interparticle interaction is determined by the value of is the Hamaker constant and Γ is the surface concentration of stabilizing polymer) and the quality of the dispersing medium as a solvent for polymer chains. Depending on the value of γ the system may exhibit the stability of both thermodynamic ( α<1 ) and kinetic ( α> 1 ) character. When the molecules of the dispersing medium are short polymer chains the increase in their length leads to a drastic decrease in the stability range of the system and to its destabilization.