Beyond Stereoselectivity,Switchable Catalysis: Some of the Last Frontier Challenges in Ring‐Opening Polymerization of Cyclic Esters

Metal‐based catalysts and initiators have played a pivotal role in the ring‐opening polymerization (ROP) of cyclic esters, thanks to their high activity and remarkable ability to control precisely the architectures of the resulting polyesters in terms of molar mass, dispersity, microstructure, or tacticity. Today, after two decades of extensive research, the field is slowly reaching maturity. However, several challenges remain, while original concepts have emerged around new types or new applications of catalysis. This Review is not intended to comprehensively cover all of these aspects. Rather, it provides a personal overview of the very recent progress achieved in some selected, important aspects of ROP catalysis—stereocontrol and switchable catalysis. Hence, the first part addresses the development of new metal‐based catalysts for the isoselective ROP of racemic lactide towards stereoblock copolymers, and the use of syndioselective ROP metal catalysts to control the monomer sequence in copolymers. A second part covers the development of ROP catalysts—primarily metal‐based catalysts, but also organocatalysts—that can be externally regulated by the use of chemical or photo stimuli to switch them between two states with different catalytic abilities. Current challenges and opportunities are highlighted.     

Convergence Theory for Preconditioned Eigenvalue Solvers in a Nutshell

Preconditioned iterative methods for numerical solution of large matrix eigenvalue problems are increasingly gaining importance in various application areas, ranging from material sciences to data mining. Some of them, e.g., those using multilevel preconditioning for elliptic differential operators or graph Laplacian eigenvalue problems, exhibit almost optimal complexity in practice; i.e., their computational costs to calculate a fixed number of eigenvalues and eigenvectors grow linearly with the matrix problem size. Theoretical justification of their optimality requires convergence rate bounds that do not deteriorate with the increase of the problem size. Such bounds were pioneered by E. D’yakonov over three decades ago, but to date only a handful have been derived, mostly for symmetric eigenvalue problems. Just a few of known bounds are sharp. One of them is proved in doi: 10.1016/S0024-3795(01)00461-X for the simplest preconditioned eigensolver with a fixed step size. The original proof has been greatly simplified and shortened in doi: 10.1137/080727567 by using a gradient flow integration approach. In the present work, we give an even more succinct proof, using novel ideas based on Karush–Kuhn–Tucker theory and nonlinear programming.     

Hamiltonian for arbitrary energy states of molecules

A way how a general rovibronic Hamiltonian for a polyatomic molecule can be obtained using generalized coordinates and introducing a “well” for the potential function of nuclear oscillations has been shown. Eigenfunctions for individual types of motion have a very simple form, which makes it possible to analyze both low-and high-energy states of molecules without changing the solution algorithm and to form an energy matrix without changing the basis set.     

Hyperbolic BMOA classes

Let φ be a holomorphic mapping between complex unit balls. We obtain several characterizations of those φ for which the Möbius-invariant BMOA condition holds with respect to the Bergman metric.     

Simulation of photochemical reactions of butadiene and calculation of the quantum yields

The photochemical reactions of butadiene-1,3 have been simulated. The calculated quantum yields of the products quantitatively agree with experimental data (deviation from the experimental data in on average less than 30%). The efficacy of the method in predicting photochemical processes of various types (isomerization, decomposition) occurring simultaneously via different pathways has been shown. It has been confirmed that the quantum beat frequency of resonating states can be used as a parameter of the method and, as such, allows obtainable quantitative estimates to be refined.     

Estimation of the contributions of absorption bands for overtones and composite frequencies in infrared spectra of brominated and oxygen-containing organic compounds

Intensities of fundamental, overtone, and composite absorption bands for 27 brominated hydrocarbons and 20 oxygen-containing organic compounds are calculated in an anharmonic approach. The first and second derivatives of the electric dipole moment of the molecule with respect to normal coordinates are determined using ab initio quantum-chemical MP2/6-31G(1d) calculations. For the studied compounds, the average contributions of overtones and composite frequencies to absorption in the region 100–4000 cm⁻¹ is 4.8% for brominated hydrocarbons and 3.2% for oxygen-containing compounds. The major part of the contribution of overtones and composite frequencies falls into the regions (mainly from 1600 to 2800 cm⁻¹) where fundamental transitions are observed rarely. The calculation performed well describe the positions of maxima and the intensities of fundamental, overtone, and composite absorption bands and can be used for the standardless spectrochemical analysis of compounds by their overtone spectra.     

Simulation of photochemical processes and calculation of the quantum yields of isomerization reactions of substituted dienes

The possibility of construction of a semiempirical method for simulation of photochemical processes and calculation of the quantum yields of reactions has been studied. The practicability of the approach was exemplified with the photochemical transformations of some diene compounds into their cyclic isomers, namely, 2,4-dimethylpentadiene-1,3 → trimethylcyclobutene, 2,3-dimethylbutadiene-1,3 → dimethylcyclobutene, pentadiene-1,3 → 3-methylcyclobutene, cis-butadiene-1,3 → cyclobutene, and 2-methylbutadiene-1,3 → 1-methylcyclobutene. The calculated quantum yields of the reactions are in qualitative and quantitative agreement with experimental data.     

A method for simulation of photochemical processes and calculation of quantum yields of reactions

The possibility of construction of a semiempirical method for simulation of photochemical processes and calculation of quantum yields of reactions has been studied. The practicability of the approach was demonstrated for the o-xylene → m-xylene, m-xylene → p-xylene, m-xylene → o-xylene, and o-diethylbenzene → m-diethylbenzene photoisomerization reactions as an example. The calculated quantum yields of the reactions are in qualitative agreement with experimental data.     

Electropolymerizable 2,2'-Carboranyldithiophenes. Structure-Property Investigations of the Corresponding Conducting Polymer Films by Electrochemistry, UV-Visible Spectroscopy and Conducting Probe Atomic Force Microscopy

Carborane-functionalized conducting polymer films have been electrogenerated in dichloromethane from the anodic oxidation of ortho- (1), meta- (3) and para-carborane (4) isomers linked to two 2-thienyl units. The corresponding electrochemical response was characterized by a broad reversible redox system corresponding to the p-doping/undoping of the polythiophene backbone, the formal potential of which increased in the order poly(1) < poly(3) < poly(4), from ca. 0.50 to 1.15 V vs Ag/Ag(+) 10(-2) M. From further UV-visible spectroscopy analysis, the optical band gap was estimated at 1.8, 2.0 and 2.2 eV for poly(1), poly(3) and poly(4), respectively. The more conjugated and electroconductive character of poly(1) is ascribed to a more planar conformation of the conjugated backbone resulting from an intramolecular β-β' cyclization reaction in the monomer, consequently yielding a fused conjugated polymer. Molecular modeling calculations using the DFT method support this hypothesis. The surface topography and maps of the conductive domains of the electropolymerized films were evaluated by conducting probe AFM. The three polymers exhibit fairly similar morphological characteristics and a surface roughness of ~2 nm. Current-voltage (I-V) characteristics of conducting AFM tip-carborane polymer-ITO junctions showed that poly(1) had the highest conductivity.