Conformational flexibility of partially hydrogenated rings in the repeating units of rigid-chain heterocyclic polymers

The conformational flexibility of partially hydrogenated rings in naphthylimide, naphthalic anhydride, and some of their derivatives used as model compounds for the repeating unit in rigid-chain heterocyclic polymers, was studied by the semiempirical quantum-chemical AM1 method. The partially hydrogenated rings in all of these compounds possess conformational flexibility. Transition from a planar equilibrium form to a distorted sofa conformation with a torsion angle of ±20° results in an increase in the energy of less than 1 kcal mol⁻¹. The high flexibility of partially hydrogenated rings, along with other factors, can cause finite length of the Kuhn segment in rigid-chain heterocyclic polymers. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya. No. 1, pp. 64–66, January 1997.     

Conformational flexibility of six-membered dihydrocycles

The conformational flexibility of six-membered 1,4- and 1,2-dihydrocycles caused by a flattened shape of the minimum on the potential energy surface is discussed. The effect of the conformational flexibility of the rings on the physicochemical characteristics of compounds is considered. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2095–2105, December, 1997.     

Effect of electronic interactions between double bonds on the conformational flexibility of 1,4-cyclohexadiene

The electronic structure of 1,4-cyclohexadiene for various angles between the double bond planes has been calculated by the AM1 method. The effects of through-bond and through-space interactions, which result in flattening and unflattening, respectively, are oppositely directed. These effects are specified for various bending angles. In addition to steric factors, electronic factors affect the conformational flexibility of 1,4-cyclohexadiene.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1676–1677, September, 1994.     

Unperturbed dimensions of cyclochain polymers

The conformational parameters of three polyoxadiazoles were calculated by a direct computer simulation assuming free rotation about virtual bonds. The results obtained proved to be close to those derived from viscosity and sedimentation experiments.     

Conformational flexibility and aromaticity of azanaphthalenes

Conformational flexibility of naphthalene and its different mono-and diaza analogues has been studied by MP2/6-31G(d, p) quantum chemical computations. It is shown that all molecules possess a considerable conformational flexibility. The out-of-plane deformation energy of both a bicyclic π-system on the whole and separate rings directly depends on the degree of aromaticity of the conjugated system. Effects of the amount and arrangement of nitrogen atoms on conformational features and aromaticity of the molecules considered were analyzed.     

A CoMFA analysis with conformational propensity: An attempt to analyze the SAR of a set of molecules with different conformational flexibility using a 3D-QSAR method

CoMFA analysis, a widely used 3D-QSAR method, has limitations to handle a set of SAR data containing diverse conformational flexibility since it does not explicitly include the conformational entropic effects into the analysis. Here, we present an attempt to incorporate the conformational entropy effects of a molecule into a 3D-QSAR analysis. Our attempt is based on the assumption that the conformational entropic loss of a ligand upon making a ligand-receptor complex is small if the ligand in an unbound state has a conformational propensity to adopt an active conformation in a complex state. For a QSAR analysis, this assumption was interpreted as follows: a potent ligand should have a higher conformational propensity to adopt an `active-conformation'-like structure in an unbound state than an inactive one. The conformational propensity value was defined as the populational ratio, N<sub>active</sub>/N<sub>stable</sub>, of the number of energetically stable conformers, N<sub>stable</sub>, to the number of `active-conformation'-like structures, N_active. The latter number was calculated by counting the number of conformers that satisfied the structural parameters deduced from the active conformation. A set of SAR data of imidazoleglycerol phosphate dehydratase inhibitors containing 20 molecules with different conformational flexibility was used as a training set for developing a 3D structure-activity relationship by a CoMFA analysis with the conformational propensity value. This resulted in a cross-validated squared correlation coefficient of the CoMFA model with the conformational propensity value (R ² _cross = 0.640) higher than that of the standard CoMFA model (R ² _cross = 0.431). Then we evaluated the quality of the CoMFA models by predicting the inhibitory activity for a new molecule.     

Influence of steric and electronic effects of substituents on the molecular structures and conformational flexibility of 1,8-naphthalenedicarboximides

A series of 1,8-naphthalenedicarboximide derivatives containing substituents of different steric and electronic nature were studied by X-ray diffraction analysis.Ab initio quantumchemical calculations in the HF/3–21G approximation demonstrated the high conformational flexibility of the imide tetrahydro ring in the molecules of these compounds. The electronic nature of the substituents has no effect on the geometry and conformational flexibility of naphthalenedicarboximides due to weak conjugation between the imide and naphthalene fragments in the molecules. However, the steric effects of the bulky substituents noticeably affect the equilibrium geometry of the imide ring by increasing its conformational flexibility. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 68–73, January, 2000.     

Effect of benzannelation on the conformational flexibility of the rings in oxo, imino, and methylene derivatives of cyclohexa-1,4-diene

The effect of benzannelation on the equilibrium conformation and flexibility of the dihydrocycle in cyclohexa-1,4-dienone,para-quinone, and their imino and methylene analogs was studied by the semiempirical quantum-chemical AM1 method. The equilibrium conformations of the carbonyl derivatives are planar. In the imino- and methylene-substituted analogs, the dihydrocycle adopts a boat conformation due to repulsions between substituted analogs, the dihydrocycle adopts a boat conformation due to repulsions between the hydrogen atoms at the exocyclic double bond and in theperi positions of the benzene rings. Annelation of cyclohexa-2,5-dien-1-one andpara-quinone with benzene rings at the C=C double bonds causes an increase in the conformational flexibility of the partially hydrogenated ring owing to an increase in the bending strain in the first compound and a decrease in the conjugation between the carbonyl groups and the remaining part of the molecule in the second compound. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 388–390, March, 1998.     

Influence of steric and electronic effects of substituents on the molecular structures and conformational flexibility of 1,8-naphthalenedicarboximides

A series of 1,8-naphthalenedicarboximide derivatives containing substituents of different steric and electronic nature were studied by X-ray diffraction analysis.Ab initio quantumchemical calculations in the HF/3–21G approximation demonstrated the high conformational flexibility of the imide tetrahydro ring in the molecules of these compounds. The electronic nature of the substituents has no effect on the geometry and conformational flexibility of naphthalenedicarboximides due to weak conjugation between the imide and naphthalene fragments in the molecules. However, the steric effects of the bulky substituents noticeably affect the equilibrium geometry of the imide ring by increasing its conformational flexibility. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 68–73, January, 2000.     

Electronic structure and conformational flexibility of 1,4-dihydroazines and their 4-ylide derivatives

The conformational flexibility of 1,4-dihydropyridine, 1,4-dihydropyrimidine, 1,4-dihydropyridazine, 1,4-dihydro-1,3,5-triazine, and their 4-oxo, imino, and methylene derivatives was studied by the semiempirical quantum-chemical AM1 method. It was demonstrated that the replacement of the methylene group in the dihydroazine ring by the exocyclic double bond results only in an increase in the rigidity of the heterocycle rather than leading to a loss of its conformational flexibility. It was suggested that nonplanar conformations of rings in ylide derivatives are stabilized by the nonaromatic cyclic -system. Introduction of the exocyclic double bond does not cause a substantial change in the -electronic structure of the heterocycle. The aromaticity indicesI ₆ andI ₆() were calculated. The correlation between these indices and a change in the energy upon bending of the heterocycle was established.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No 8 pp 1938–1941, August, 1996.     

On the flexibility of beta-peptides

The full conformational space was explored for an achiral and two chiral beta-peptide models: namely For-beta-Ala-NH2, For-beta-Abu-NH2, and For-beta-Aib-NH2. Stability and conformational properties of all three model systems were computed at different levels of theory: RHF/3-21G, B3LYP/6-311++G(d,p)//RHF/3-21G, B3LYP/6-311++G(d,p), MP2//B3LYP/6-311++G(d,p), CCSD//B3LYP/6-311++G(d,p), and CCSD(T)//B3LYP/6-311++G(d,p). In addition, ab initio E = E(phi, micro, psi) potential energy hypersurfaces of all three models were determined, and their topologies were analyzed to determine the inherent flexibility properties of these beta-peptide models. Fewer points were found and assigned than expected on the basis of Multidimensional Conformational Analysis (MDCA). Furthermore, it has been demonstrated, that the four-dimensional surface, E = E(phi, mu, psi), can be reduced into a three-dimensional one: E = E[phi, f(phi), psi]. This reduction of dimensionality of freedom of motion suggests that beta-peptides are less flexible than one would have thought. This agrees with experimental data published on the conformational properties of peptides composed of beta-amino acid residues.     

Structure and rheology of polymers with arbitrary flexibility: A numerical approach

A numerical approach, based on the configurational distribution function of a polymer chain in flow, has been used to calculate the zero-shear rheological properties. Starting from a bead-spring representation of the chain, the stiffness is introduced by repulsive springs between next-nearest neighbors. The connection to models based on the bending equation and their limitation is discussed. To obtain a correct model of a semiflexible chain, an inhomogeneous spring constant has to be used. Calculations have been carried out for the free draining case, and a simple relation between the intrinsic viscosity, the translational diffusion coefficient and the persistence length for arbitrary solvent conditions is proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1995–2003, 1998     

Thermogravimetric analysis of polymers with indanic structure

The thermal stability of polycarbonates, polythiocarbonates, and polyesters derived from a diphenol with an indanic structure, was studied and compared with the corresponding polymers derived from a diphenol with the same number of carbon atoms but no forming the indanic ring, showing that polymers with an indanic structure degrade at lower temperature due the rigidity of the indanic ring respect to those with an aliphatic structure. Also, the kinetic parameters of the thermal decomposition were determined by using the Arrhenius relationship and a computer program. In the considered range, all the polymers studied degraded in a single stage and theE values increase when the bulk of the side group increases and the rigidity decreases.

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Zusammenfassung Die stabilität gegen Warme von Polycarbonaten, Polythiocarbonaten und polyestern gewinnen aus einem Diphenol mit einer indan Struktur wird studiert und vergleicht mit denjenigen von analogen Polymeren gewinnen auseinem Diphenol mit der gleichen nummer von Kohlenstoffatomen ohne Bildung von einem Indanring. Diese Versuche zeigen, daß die Polymere mit einer Indanstruktur bei tiefer Temperatur ais diejenige mit einer alifatischen Struktur degradieren auf Grund der Streng des Indanringes. Die kinetische Parametern der Zersetzung gegen Warme der Polymeren wurden durch die Arrhenius Gleichung und ein Komputerprogram in dem angesehenden Bereich bestimmt. Alle Polymere durch einen Schritt zerfallen. Die E-Werte zunehmen, wenn die große der Seitegruppe zunimmt und die Strenge abnimmt.

     

Synthesis of biodegradable conjugated polymers with controlled backbone flexibility

A series of flexible, highly bright fluorescent poly(p‐phenyleneethynylene)s (PPEs) was prepared by employing a disulfide containing nonconjugated monomer at various ratios under Sonogashira reaction conditions. PPEs with flexible linkers exhibited fluorescence properties comparable to those of a fully conjugated PPE when less than 50% of flexible monomers were incorporated into the backbone. To evaluate the self‐assembly properties of PPEs, a series of conjugated polymer nanoparticles (CPNs) was fabricated by treating PPEs with organic acids followed by dialysis. CPNs containing linkers exhibited different complexation behavior with polysaccharides, warranting further investigation into how flexibility and biodegradability of CPNs influence their cellular interaction and entry. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 1403–1412     

Are kinetic parameters of non-isothermal thermogravimetric degradation of polymers unequivocal?

Due to the complex character of the thermal degradation of polymers as a solid-gas chain reaction, an unequivocal kinetic characteirzation is possible only for stationary states of both radical concentration and reaction mechanism. These conditions are hardly realizable in non-isothermal thermogravimetry. Additional the weight losses are depedent on the volatility of the reaction products. That is not always certain in polymer degradation. As a consequence the deduced ‘kinetic parameters’ are not unequivocal. They are conversion and heating rate dependent and may be influenced by sample shape and size. Thus the ‘kinetic parameters’ are in fact from the point of view of mathematics the fitting parameters of a ‘rate equation’ like relation, specific for the used reaction conditions only. From the point of view of chemical kinetics they are neither attributable to a determined reaction mechanism nor can they be used for predictions. Dedicated to the 70^th Anniversary of Dr. Jo Flynn     

Resorcinarene‐based ATRP initiators for star polymers

Two novel multifunctional initiators for atom transfer radical polymerization (ATRP) were synthesized by derivatization of tetraethylresorcinarene. The derivatization induced a change in the conformation of the resorcinarene ring, which was confirmed by NMR spectroscopy. The initiators were used in ATRP of tert‐butyl acrylate and methyl methacrylate, producing star polymers with controlled molar masses and low polydispersities. Instead of the expected star polymers with eight arms, polymers with four arms were obtained. Conformational studies on the initiators by rotating‐frame nuclear Overhauser and exchange spectroscopy NMR and molecular modeling suggested that of eight initiator functional groups on tetraethylresorcinarene, four are too close to each other to be able to initiate the chain growth. Starlike poly(tert‐butyl acrylate) macroinitiators were used further in the block copolymerization of methyl methacrylate. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4189–4201, 2004     

New aspects of the single phase aggregate model for oriented polymers

A reinvestigation of the single phase aggregate model (SPAM) of Ward applied to the elastic properties of uniaxially oriented polymers is presented. It is shown that the Voigt and the Reuss orientation average of elastic constants can be represented as a linear transformation in a 5-dimensional vector space. The corresponding transformation matrix can be diagonalized, showing that the eigenvalues of this transformation are the orientation parametersP ₀ =1,P ₂ andP ₄ . Taking advantage of this representation an easy test of applicability of the SPAM is proposed. Possibilities to evaluate the SPAM parameters are reviewed.     

Arm‐first synthesis of star polymers with polywedge arms using ring‐opening metathesis polymerization and bifunctional crosslinkers

This work presents a two‐step, one‐pot process to make star polymers with polywedge arms. In a one‐pot reaction, after the polywedge arms are synthesized, crosslinker species are added to the reaction, rapidly forming star polymers. Crosslinker species with different degrees of conformational freedom were designed and synthesized and their capacity to generate star polymers was evaluated. Mass conversions up to 92% and stars with up to 17 arms were synthesized with the most rigid crosslinker. The effects of arm molecular weight and molar ratio of crosslinker to arm on mass conversion and arms per star were explored further. Finally, the size‐molecular weight scaling relationship for polywedges with linear and star architectures was compared, corroborating theoretical results regarding star polymers with arms much larger than their core. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 732–740     

Ab initio calculations of intramolecular parameters for a class of arylamide polymers

Using DFT methods, we have determined intramolecular parameters for an important class of arylamide polymers displaying antimicrobial and anticoagulant inhibitory properties. A strong link has been established between these functions and the conformation that the polymers adopt in solution and at lipid bilayer interfaces. Thus, it is imperative for molecular dynamics simulations designed to probe the conformational behavior of these systems to accurately describe the torsional degrees of freedom. Standard force fields were shown to be deficient in this respect. Therefore, we have computed the relevant torsional energy profiles using a series of constrained geometry optimizations. We have also determined electrostatic parameters using our results in combination with standard RESP charge optimization. Force constants for bond and angle potentials were calculated by iteratively matching quantum and classical normal modes via a Monte Carlo scheme. The resulting new set of parameters accurately described the conformation and dynamical behavior of the arylamide polymers.     

Conformational flexibility of six-membered 1,2-dihydrocycles based on the results of AM1 calculations

The conformational flexibility of 2H-pyran, 1,2-dihydropyridine, 2H-thiopyran, their oxo, imino, and methylene derivatives, and the 5-oxo, imino, and methylene derivatives of 1,3-cyclohexadiene was studied by the semiempirical quantum-chemical AM1 method. All compounds have planar equilibrium conformations and similar electronic structures. The transition to a distorted sofa conformation with the =C-X-C(=Y)-C= torsion angle (X = CH₂, NH, O, or S; Y = H₂, O, NH, or CH₂) of ±20° increases the energy of the molecule by less than 1.5 kcal mol^–1. The reasons for the high conformational flexibility of these dihydrocycles were analyzed. The nonaromatic character of the cyclic -system is the factor that determines the conformational flexibility of the dihydrocycle in ylide derivatives.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2650–2652, November, 1996.