Electro-selective interconversion of living cationic and radical polymerizations

Orchestrating conflicting polymerization mechanisms in a single polymerization process through one external stimulus is a prerequisite to achieve in-situ selective synthesis of different monomers. Here we report an electrochemically controlled mechanism transformation that enables selective activation of living cationic or radical polymerization via an alternating voltage and dual electrocatalysts. Using identical mixed-monomer condition, a variety of desired block copolymer structures, including diblock, multiblock, random, and tapered copolymers can be obtained by simply varying the periods or phases of the alternating potential. Moreover, merging this electro-interconverted polymerization with a flow-chemistry technique can streamline preparation of functional polymer materials with complex multiblock structure. This study would offer a new vision on large-scale electrochemical synthesis of sequence-defined polymers.     

Activated monomer propagation in cationic polymerizations

New mechanism of cationic polymerization of cyclic ethers and acetals consisting in the activated (e.g. protonated) monomer addition to the electrically neutral macromolecule e.g.: is discussed. It is shown, that when the -OH in the tail-group is more nucleophilic than monomer itself this mechanism has chances to dominate over the traditional mechanism of propagation, involving tertiary oxonium ion. Kinetics of the polymerization process, elementary reactions, competition with the tertiary oxonium ion growth are discussed. Examples are given, based on the work on ethylene oxide and epichlorohydrin polymerization, showing how to apply the theory of the activated monomer in order to avoid side reactions, namely cyclization and disproportionation of the chains.     

On the propagation rate constants of cationic polymerizations

The concepts employed to explain polymerizations by ionizing radiations are used for a critical examination of the concepts involved in interpreting the kinetics of chemically initiated cationic polymerizations. It is explained how the interactions of the propagating carbenium ions with the solvent, monomer, and anion can result in the formation of up to six distinct unpaired species and several kinds of ion pairs; therefore, the consumption of the monomer can be governed simultaneously by many rate constants. Only one of these can have any general theoretical use, and suggestions are made for how it can be measured. For the first time, it is shown that the ion‐pairing process must involve a ligand displacement and so resembles the amination of the Ag⁺ ion, for example, in an aqueous solution by NH₃, rather than an association of inert ions of unchanging identity. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 2537–2544, 2002     

The kinetics of slow-initiated living polymerizations

Equations permitting derivation of initiation and propagation rate constants in non-ideal living polymerizations are presented. Earlier formulations are compared, simplified and tested against experimental data for the polymerizations of n-vinyl carbazole and n-butyl cyanoacrylate. Novel cases having more complex initiation processes (in the polymerization of cyanoacrylates by amines) are formulated and tested.     

Modification of photoinitiated cationic epoxide polymerizations by sulfides

The addition of sulfides has a marked effect on the rates of onium salt induced photoinitiated cationic ring‐opening polymerizations of epoxide monomers. Various behaviors have been observed that depend on the structure of the sulfide. Dialkyl sulfides strongly inhibit the photopolymerizations of these monomers, whereas diaryl sulfides have a retarding effect on the photopolymerizations. Real‐time infrared spectroscopy and optical pyrometry have been employed as analytical methods to probe the kinetic effects of the addition of a variety of sulfides on cationic epoxide ring‐opening photopolymerizations. A mechanism is proposed that involves the formation of sulfonium salts as intermediates. The observations made in this study have important implications for cationic photopolymerizations in general and for photoinitiated cationic ring‐opening polymerizations of epoxides in particular. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2504–2519, 2005     

Mechanisms and kinetics of living radical polymerizations

The polymerization rates and activation processes of several variants of living radical polymerization (LRP) are discussed on the basis of recent experimental and theoretical results. Because of bimolecular termination, which is inevitable in LRP as well as in conventional radical polymerization, the time‐conversion curves of LRP have several characteristic features depending on the experimental conditions, such as the presence or absence of conventional initiation. Despite the presence of termination (and initiation, in some cases), polymers obtained by LRP can have a low polydispersity, provided that the number of terminated chains is small compared to the number of potentially active chains. A large rate constant of activation, k_act, is another fundamental requisite for low polydispersities. Systematic experimental investigation into k_act has clarified the exact mechanisms of activation in several LRP systems. The magnitudes of k_act was found to largely differ from system to system.     

Living radical and cationic polymerizations in water and organic media

This lecture will focus two living polymerizations that can be carried out in water as well as in organic media. The first one is transition metal-catalyzed living radical polymerization, for which Ru(II) and other transition metals play a critical role to control the process; for aqueous systems, Ru(II) and Fe(II) half metallocene complexes are useful. The second is cationic polymerization with water-tolerant Lewis acids as catalysts, including rare earth triflates and boron trifluoride for selected monomers. Discussion will be directed to the design of initiating systems, search of versatile catalysts, and precision synthesis of new polymers.     

A theoretical approach to the thermochemistry of radical and anionic polymerizations of various unsaturated monomers

In this work, we have calculated the thermodynamic parameters of the first steps of the free radical and anionic polymerizations of various unsaturated monomers, using ab initio methods of quantum chemistry. The enthalpies and entropies of polymerization were estimated assuming that they correspond to those of the model reaction A  B(p) + HABAH(p′) → HABABAH(p′) where p and p′ stand for the physical state of the considered species. The enthalpies of polymerization were rationalized using the equation ΔH = −ΔΣ N_ABE_AB + SE(A  B) + SE(HABAH) − SE(HABABAH) where N_AB is the number of A  B bonds, E_AB the corresponding bond energy, − ΔΣ N_ABE_AB the variation of the sum of the bond energy terms, and SE(X) the thermodynamic stabilization energy of compound X. The preferential mode of polymerization of each monomer was derived from the enthalpies of the initiation and initial propagation steps of the two types of polymerization. Thus, we were able to make some comments concerning the feasibility of the polymerization of the monomers under consideration.     

The role of the triplet state in the photosensitization of cationic polymerizations by anthracene

The photosensitization mechanism for cationic polymerizations initiated by diaryliodonium salts photosensitized by anthracene was investigated using fluorescence and phosphorescence spectroscopy. In situ photosensitizer fluorescence measurements confirmed that the photosensitization reaction proceeds by an electron transfer process. Transient phosphorescence studies demonstrated that electron transfer occurred from the triplet excited state of anthracene to the initiator, with an intrinsic kinetic rate constant of 2 × 10⁸ L/mol s. Further evidence for the role of the triplet state was provided by an observed seven-fold decrease in the polymerization rate upon addition of a triplet state quencher. Finally, numerical solution of the photophysical kinetic equations indicated that the triplet state concentration was approximately three orders of magnitude higher than that of the singlet state, and that 94-96% of the active cationic centers are produced by reaction of the initiator with the triplet state. These results indicate that the electron transfer occurs primarily from the triplet state of anthracene, with the singlet state providing only a minor contribution to the photosensitization reaction. © 1995 John Wiley & Sons, Inc.     

Titanium-based lewis acids for living cationic polymerizations of vinyl ethers and styrene: Control of lewis acidity in design of initiating systems

This brief account discusses the development of HCl/TiCl_4-n(OR)_n (n = 1–4), the titanium-based new initiating systems for living cationic polymerizations of vinyl ethers and styrene. The focus of this development is controlling the Lewis acidity of the metal halide components [TiCl_4-n(OR)_n] or “activators” in relation to the structure of the monomers. Thus, for vinyl ethers, relatively mild Lewis acids such as TiCl(OiPr)₃ and TiCl₂(OiPr)₂ are effective, whereas for styrene, a stronger Lewis acid such as TiCl₃(OiPr) is employed along with an added salt (nBu₄N⁺Cl⁻). In both cases, living polymers of controlled molecular weights can be obtained in methylene chloride solvent at −15°C.     

A novel approach to study the isothermal behaviour of substances after 0.01–1 s from the chemical process initiation

New modifications of a contact heater are suggested for investigating thermal decomposition processes, as well as for determining kinetic characteristics of substances under conditions of intense heating to a fixed constant temperature.

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Zusammenfassung Zur Untersuchung von thermischen Zersetzungsprozessen und der Bestimmung der Reaktionskinetik von Substanzen bei intensivem Erhitzen auf vorgegebene konstante Temperaturen wird eine neue Abart von Kontakterhitzern beschrieben.

     

Emulsion polymerizations. I. A novel approach to the evaluation of the molecular weight distributions

A simple, probabilistic approach describing the number distribution of the molecular weight (MWD) of polymers formed by emulsion polymerization is presented. The radical populations of the reacting system are formally split into infinite, quickly convergent series of particularly tagged radical populations. These populations are characterized by the nature and the number of the state transitions of the associated latex particles. For each kind of single radical or radical pairs a distribution function is then defined. These functions can be specified on the basis of α_i and Q_i, the growth probabilities, within a latex particle in state i, of a single radical and a radical pair respectively. The overall MWDs are then given by summation over all the distributions of the radical populations and over all the allowed states. Only one set of ordinary differential equations (the Smith–Ewart equations) are involved in the mathematical formulation, single distribution functions being obtained by solving simple exponential-type integrals. In this paper, analytic solutions are presented for the zero-one, zero-one-two and general systems. Analogies and differences between our approach and previously reported treatments are critically discussed.     

A chemical systems approach to evolution

A novel approach to the evolution of organisms is given. It asserts that biological evolution was consequential upon geochemical, environment, change and was inevitable not accidental.     

A finite-difference approach to the electron correlation problem

A variant of the transcorrelated method of Boys and Handy employing finite differences is presented. It is based upon the following two properties of the transcorrelated Hamiltonian operator C^?1HC: (1) C^?1HC possesses an energy eigenvalue spectrum which is identical to that associated with H itself; and (2) if \documentclass{article}\pagestyle{empty}\begin{document}$$ C \equiv \begin{array}{*{20}c} \pi & {e^{r_{ij} /2} } \\ {i > j} & {} \\ \end{array} $$\end{document} then C^?1HC is free of the singularities of H at the points where the interelectron separation r_ij is zero. A bivariational principle for approximating the eigenvalues and the left and right eigenfunctions of C^?1HC is introduced and the resulting set of coupled integro-differential equations are solved in finite-difference form by means of a coupled self-consistent field, Newton Raphson algorithm. As a preliminary test of the method, a calculation of the ground-state energy of the helium atom is presented.     

A combinatorial approach to the electron correlation problem

Starting from a path-integral formulation of quantum statistical mechanics expressed in a space of Slater determinants, we develop a method for the Monte Carlo evaluation of the energy of a correlated electronic system. The path-integral expression for the partition function is written as a contracted sum over graphs. A graph is a set of distinct connected determinants on which paths can be represented. The weight of a graph is given by the sum over exponentially large numbers of paths which visit the vertices of the graph. We show that these weights are analytically computable using combinatorial techniques, and they turn out to be sufficiently well behaved to allow stable Monte Carlo simulations in which graphs are stochastically sampled according to a Metropolis algorithm. In the present formulation, graphs of up to four vertices have been included. In a Hartree-Fock basis, this allows for paths which include up to sixfold excitations relative to the Hartree-Fock determinant. As an illustration, we have studied the dissociation curve of the N(2) molecule in a VDZ basis, which allows comparison with full configuration-interaction calculations.     

A new approach to the chemical synthesis of keto-proteins

To increase the versatility of protein-conjugation, an orthogonal protection strategy is described, which enables the efficient synthesis of keto-proteins bearing a reactive ketone functionality using Boc, Fmoc, and chemical ligation methodologies. A 1,3-dithiolane group was used to protect the ketone function of levulinate- and pyruvate-derivatized peptides during solid-phase synthesis, acidolytic cleavage, and purification. When required, the 1,3-dithiolane group could be cleanly removed using aqueous silver or mercuric solutions to regenerate the reactive keto-protein at ambient temperature. The liberated keto-protein was chemoselectively conjugated in situ to an aminooxy-derivatized monodisperse polymer.     

Recent developments in living cationic polymerization

Scope and limitation of the vinyl ether polymerization initiated by NR₄ClO₄(KClO₄;LiClO₄)/CH₃CHI-OR was discussed. Besides isobutyl vinyl ether (IBVE), N-vinylcarbazole (NVC) and 2-chloroethyl vinyl ether (CEVE) were initiated by NR₄ClO₄/CH₃CHI-OR. These polymerizations exhibited the characteristics of a living polymerization. However, in order to observe narrow molar mass distribution NVC was initiated with CH₃CHI-OR, without salts. Block copolymers were synthesized by the method of sequential monomer addition (NVC, IBVE, CEVE). The PCEVE segment was modified by nucleophilic substitution, which allowed the synthesis of amphiphilic block copolymers.