Kinetic Analysis of A2 + AB2 Polymerization Approach

The kinetic model of the co‐polycondensation with A₂ and AB₂ type monomers is developed and the analytical expressions of the various molecular parameters of the products are derived rigorously. The monomer feed ratio (α) of A₂ to AB₂ significantly affects the molecular parameters and the critical condition of gelation. Gelation can be avoided if α is > . At the critical state, the degree of branching decreases firstly and reaches its minimum value at about α≈0.22. Then, it increases with increasing α‐value. In comparison with experimental results, non‐equal reactivity of the active groups should be considered.

     

A Convenient A2 + B3 Approach to Hyperbranched Poly(arylene oxindole)s

Summary: We developed a facile approach to hyperbranched polymers by applying a superelectrophilic reaction within an A₂ + B₃ strategy. A significant reactivity difference between the intermediate and the starting material was utilized to avoid gelation in the A₂ + B₃ polymerization. A number of hyperbranched poly(arylene oxindole)s were achieved in a one‐step polymerization and characterized by NMR spectroscopy and gel permeation chromatography. Moreover, further modifications at the interior and exterior of the resulting polymers were explored as well.

Structure of the hyperbranched polymers produced using the A₂ + B₃ approach.     

An Advanced Kinetic Model of Hydrolytic Polymerization of ε‐Caprolactam Including the Amide Interchange Reactions

By proposing a recognized kinetic mechanism of the amide interchange reaction, where aminolysis plays the controlling role, a deterministic model dealing with the kinetic behavior of the hydrolytic polymerization of ε‐caprolactam including the amide interchange reactions can be established. The rate equations in terms of moments were derived by applying the Z‐transform technique and solved by the Runge–Kutta method. It was found that the presence of the amide interchange reaction led to a shorter equilibration time than otherwise would be the case. The results from this new model were found to be quite consistent with the experimental data reported by Tai.     

Molecular dynamics and metadynamics simulations of [2 + 2] photocycloaddition

Triplet state mechanism of [2 + 2] photocycloaddition forming a cyclobutane ring from two ethylenes is investigated in the context of photocatalysis. High‐level ab initio calculations are combined with ab initio adiabatic molecular dynamics and ab initio metadynamics for rare events modeling. In a photocatalytic scheme, a reactant reaches the triplet state either via intersystem crossing (ISC) or triplet sensitization. The model system adopts a biradical structure, which represents energy intersection with the ground state. The system either completes cyclization or undergoes fragmentation into two olefinic units. The potential and free energy surfaces of the cyclobutane/ethylenes system are mapped with multireference approaches describing possible reaction pathways. To obtain a full picture of a double bond photoreactivity, ab initio adiabatic dynamical calculations were used to estimate reaction yields and to model the effects of excess energy. The potential use of density functional theory based approaches for [2 + 2] photocycloaddition was investigated for future simulations and design of realistic photocatalytic systems. Dynamical aspects of [2 + 2] photocycloaddition via a triplet state manifold are investigated by combining ab initio multireference methods and ab initio molecular dynamics and metadynamics. The reaction pathways are studied for a model system of two ethylenes forming a cyclobutane ring to provide a basis for further studies on design of photocatalytic systems.     

Mechanisms of DABCO‐ and DMAP‐catalyzed [2 + 4] cycloaddition reactions of methylallenoate with methyleneindolonone: A DFT study

The mechanisms and stereoselectivities of the [2 + 4] cycloaddition reaction of methylallenoate R1 with methyleneindolonone R2 catalyzed by DABCO (Equation 1) and DMAP (Equation 2) organocatalysts have been examined with density functional theory (M06‐2X) calculations. Several possible reaction pathways (paths 1a, 1b, and 1c for Equation 1 and paths 2a and 2b for Equation 2) were located and compared. The results of our study reveal that for both reactions, three reaction stages have been characterized: nucleophilic addition of the catalyst ( DABCO or DMAP ) to R1 (Stage I ), addition of the other reactant R2 (Stage II ), intramolecular cycloaddition and liberation of the catalyst ( DABCO or DMAP ) afforded the final product (Stage III ). For the DABCO ‐catalyzed cycloaddition, we predict that path 1a leading to P(E) is the most energy favorable pathway among the three possible pathways. The carbon–carbon bond formation step is the rate‐determining step (ΔG ^?=23.6 kcal/mol). With DMAP catalyst, the same reaction gave P(Z) as the major product. The barrier for the rate‐determining step (addition of R1 to DMAP ) is 25.8 kcal/mol. The calculated results are in agreement with the experimental findings. Moreover, for both reactions, the analysis of global reactivity indexes has been carried out to examine the role of catalyst. The present study should provide a general mechanistic framework for the rational design of this kind of reactions.     

Facile Synthesis of Spirooxindole‐Cyclohexenes via Phosphine‐Catalyzed [4 + 2] Annulation of α‐Substituted Allenoates

A phosphine‐catalyzed [4 + 2] annulation of α ‐substituted allenoate with exocyclic alkene moiety of oxindoles or indan‐1,3‐diones has been developed. Thus, under the catalysis of PPh₃ (20 mol%), a series of spirooxindole‐ or spiroindan‐1,3‐dione‐cyclohexenes have been obtained in moderate to excellent yields and regioselectivity from the annulations of α ‐methyl allenoates with 3‐methyleneoxindoles or 2‐methyleneindan‐1,3‐diones. This method offers an easy access to structurally novel spirocyclohexenes.     

Understanding the reactivity and regioselectivity of [3 + 2] cycloaddition reactions between substituted nitrile oxides and methyl acrylate. A molecular electron density theory study

The [3 + 2] cycloaddition (32CA) reactions of three nitrile oxides (NOs) (R‐CNO; R = Ph, CO₂Me, and Br) with methyl acrylate (MA) have been theoretically studied within the molecular electron density theory. Topological analysis of the electron localization function of these NOs permits to establish that they will participate in zw‐type 32CA reactions. Analysis of the conceptual DFT indices indicates that these zw‐type 32CA reactions will have a low polar character as a consequence of the relatively low electrophilic character of MA and the low nucleophilic character of NOs, in agreement with the global electron density transfer computed at the corresponding TSs. The activation enthalpies associated with these 32CA reactions range from 8.2 to 12.7 kcal·mol^?1. The presence of the bromide atom provokes the larger acceleration. While the 32CA reaction involving the CO₂Me substituted NO is highly ortho regioselective, the other two reactions are poorly ortho regioselective. A bonding evolution theory study of the more favorable ortho regiosiomeric channel associated with the 32CA reaction involving the Br substituted NO indicates that this reaction is associated to a nonconcerted two‐stage one‐step mechanism, in which the activation energy is mainly related to the initial rupture of the C? N triple bond of the NO.     

Theoretical investigation toward organophosphine‐catalyzed [3 + 3] annulation of Morita–Baylis–Hillman carbonates with azomethine imines: Mechanism,origin of stereoselectivity,and role of catalyst

A computational study on the detailed mechanism and stereoselectivity of the chiral phosphine‐catalyzed C(sp²)? H activation/[3 + 3] annulation between Morita–Baylis–Hillman (MBH) carbonates and C,N‐cyclic azomethine imines has been performed. Generally, the catalytic cycle consists of two stages, that is, C(sp²)? H activation companied by the dissociation of the t‐BuO group forming phosphonium enolate, and [3 + 3] cycloaddition process followed by regeneration of the catalyst. The calculated results indicate that C(sp²)? H activation is rate‐determining while [3 + 3] cycloaddition is stereoselectivity‐determining. Furthermore, the advantageous hydrogen bond interactions and less steric hindrance in the RR configurational C? C bond forming transition states should be responsible for the favorability of RR‐configured product among the four possible products. The special role of the organocatalyst was also identified by natural bond orbital (NBO) and global reactivity index (GRI) analyses. The mechanistic insights obtained in the present study should be useful for understanding the novel organocatalytic C(sp²)? H activation and cycloaddition cascade reaction of MBH carbonates, and thus provide valuable clues on rational design of efficient organocatalysts for the C(sp²)? H activation/functionalizations.     

Kinetic simulations for cyclization of α,ω‐telechelic polymers

In this work, we conducted kinetic simulations to examine the effects of various experimental conditions on cyclization during the feed of α,ω‐telechelic polymers into a reaction mixture. The simulations showed that the interplay between the feed rate and rate coefficients for cyclization and multiblock formation were the dominant and controlling parameters. The simulations were in good agreement with previously published results on cyclization of α,ω‐telechelic polystyrene with different molecular weights by the Cu‐catalyzed azide/alkyne cycloaddition (CuAAC) reaction. They also showed that high dilution was not a necessary condition for cyclization and that high percentages of monocyclic could be rapidly produced in solutions that are more concentrated. Previously reported work demonstrated that cyclic polystyrene could be prepared in less than 9 min at 25 °C using the CuAAC “click” reaction. These simulations allow for optimization and better experimental design, leading to the possibility of large scale production of cyclic polymers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

New global potential energy surface of the MgH2 system and dynamics studies of the reaction H + MgH → Mg + H2

A new global potential energy surface for the ground state of MgH₂ was constructed using the permutation invariant polynomial neural network method. About 70 000 ab initio energy points were calculated via the multi‐reference configuration interaction method method with aug‐cc‐pVTZ and aug‐cc‐pVQZ basis sets, and these points were used to construct the potential energy surface (PES). To avoid basis set superposition error, the basis set was extrapolated to the complete basis set limit using the two point energy extrapolation formula. The root mean square error of the present PES is only 8.85 meV. Initial state (v = 0, j = 0) dynamics studies were performed using the time‐dependent wave packet method with a second‐order split operator for the total angular momentum J up to a value of 50. Furthermore, the reaction probability, integral cross section, and thermal rate constant are reported and compared with available theoretical studies.     

Investigation of Cationic Polymerization of β‐Pinene Using Calorimetric Measurements

An experimental investigation of the kinetics of cationic polymerization of β‐pinene was performed using two different initiator systems under two different operating conditions (shot additions of initiator, and continuous feeding of monomer). The experiments were done using calorimetric measurements under isoperibolic conditions. The heat of polymerization of β‐pinene was found to be ?30.6 kcal · mol^?1. A simple kinetic model was tentatively proposed, and the model fit reasonably well to the different experimental runs. Different values of the fitting parameters were obtained for runs carried out under different conditions, which can probably be ascribed to the presence of adventitious impurities in the commercial‐grade monomer used.

     

Dynamical properties of S(3P) + HD reaction on 13A″state and their quantum wavepacket calculation

The time‐dependent wavepacket method is used to study the reaction dynamics of S(³P) + HD (v = 0, 1, 2) on the adiabatic 1³A″ potential energy surface constructed by Han and coworkers [J. Chem. Phys. 2012, 136, 094308]. The reaction probabilities and integral cross sections as a function of collision energy are obtained and discussed. The results calculated by using the CC and the CS approximation have been compared, which suggests that for this direct abstraction reaction, the cheaper CS approximation calculation is valid enough in the quantum calculation. The investigation also shows that the reaction probabilities and integral cross sections tend to increase with collision energy. By analyzing the v‐dependent behavior of the integral cross sections, the significant effect of the vibrational excitation of HD is found. Also found in the calculation is a significant resonance feature in the reaction probabilities versus collision energy. © 2014 Wiley Periodicals, Inc.     

Radical‐initiated polymerization of β‐methyl‐α‐methylene‐γ‐butyrolactone

β‐Methyl‐α‐methylene‐γ‐butyrolactone (MMBL) was synthesized and then was polymerized in an N,N‐dimethylformamide (DMF) solution with 2,2‐azobisisobutyronitrile (AIBN) initiation. The homopolymer of MMBL was soluble in DMF and acetonitrile. MMBL was homopolymerized without competing depolymerization from 50 to 70 °C. The rate of polymerization (R_p) for MMBL followed the kinetic expression R_p = [AIBN]^0.54[MMBL]^1.04. The overall activation energy was calculated to be 86.9 kJ/mol, k_p/k_t^1/2 was equal to 0.050 (where k_p is the rate constant for propagation and k_t is the rate constant for termination), and the rate of initiation was 2.17 × 10^?8 mol L^?1 s^?1. The free energy of activation, the activation enthalpy, and the activation entropy were 106.0, 84.1, and 0.0658 kJ mol^?1, respectively, for homopolymerization. The initiation efficiency was approximately 1. Styrene and MMBL were copolymerized in DMF solutions at 60 °C with AIBN as the initiator. The reactivity ratios (r₁ = 0.22 and r₂ = 0.73) for this copolymerization were calculated with the Kelen–Tudos method. The general reactivity parameter Q and the polarity parameter e for MMBL were calculated to be 1.54 and 0.55, respectively. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1759–1777, 2003     

Hyperbranched polyimides prepared by ideal A2+B3 polymerization,non-ideal A2+B3 polymerization and AB2 self-polymerization

In this paper, hyperbranched polyimides having the same repeating unit were synthesized by employing ideal A₂+B₃ polymerization, non-ideal A₂+B₃ polymerization and AB₂ self-polymerization methods. The polymerization behavior, polymer properties were compared for three methods. Hyperbranched polyimides by ideal A₂+B₃ polymerization, non-ideal A₂+B₃ polymerization and AB₂ self-polymerization methods show apparent difference in many physical properties, such as inherent viscosity, glass transition temperature, and film formation behavior etc. The hyperbranched polymers by the non-ideal A₂+B₃ polymerization are suitable for smooth, flexible and self-standing film preparation, which provides useful information for hyperbranched polymers toward self-standing materials.     

Mechanical properties and thermal analysis of low‐density polyethylene + polypropylene blends with dialkyl peroxide

Polypropylene + low density polyethylene (PP + LDPE) blends involving 0, 25, 50, 75 and 100 wt% of PP with dialkyl peroxide (DAP) were prepared by melt blending in a single‐screw extruder. The effects of adding dialkyl peroxide on mechanical and thermal properties of PP + LDPE blends have been studied. It was found that at lower concentrations of peroxide (e.g., 0–0.08 wt% of dialkyl peroxide) LDPE component is cross‐linked and Polypropylene (PP) is degraded in all compositions of PP + LDPE blends. Mechanical properties (Tensile strength at break, at yield and elongation at break), Melt flow index (MFI), hardness, Scanning Electron Microscope (SEM) and thermal analyses (DSC) of these blends were examined. Because of serious degradation or cross‐linking the mechanical properties and the crystallinty (%) of those products were decreased as a result of increasing peroxide content. Copyright © 2009 John Wiley & Sons, Ltd.     

One‐pot inimer promoted ROCP synthesis of branched copolyesters using α‐hydroxy‐γ‐butyrolactone as the branching reagent

An array of branched poly(?‐caprolactone)s was successfully synthesized using an one‐pot inimer promoted ring‐opening multibranching copolymerization (ROCP) reaction. The biorenewable, commercially available yet unexploited comonomer and initiator 2‐hydroxy‐γ‐butyrolactone was chosen as the inimer to extend the use of 5‐membered lactones to branched structures and simultaneously avoiding the typical tedious work involved in the inimer preparation. Reactions were carried out both in bulk and in solution using stannous octoate (Sn(Oct)₂) as the catalyst. Polymerizations with inimer equivalents varying from 0.01 to 0.2 were conducted which resulted in polymers with a degree of branching ranging from 0.049 to 0.124. Detailed ROCP kinetics of different inimer systems were compared to illustrate the branch formation mechanism. The resulting polymer structures were confirmed by ¹H, ¹³C, and ₁H‐₁₃C HSQC NMR and SEC (RI detector and triple detectors). The thermal properties of polymers with different degree of branching were investigated by DSC, confirming the branch formation. Through this work, we have extended the current use of the non‐homopolymerizable γ‐butyrolactone to the branched polymers and thoroughly examined its behaviors in ROCP. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1908–1918     

Influence of collision energy and reagent vibrational excitation on the dynamics of the reaction H + LiH

We present a detailed quasiclassical trajectory (QCT) study of the dynamics corresponding to the reaction H + LiH proceeding via depletion and H‐exchange paths on a new potential energy surface of the electronic ground state. The effects of collision energy and reagent initial vibrational excitation on the reaction probability and cross sections are studied over a wide range of collision energies. The QCT‐calculated reaction probability and cross sections are in good agreement with previous time‐dependent wave packet results. More importantly, we found that the vibrational excitation of LiH molecule inhibits the LiH depletion reaction, whereas it promotes the H‐exchange reaction. In addition, the differential cross sections calculated for the depletion reaction at different collision energies and excitation states indicate a strong forward scattering of the product molecule H₂. © 2013 Wiley Periodicals, Inc.     

A rare occurrence of a β‐turn in an amyloid βA4 peptide

The fragment β(25–35) of the amyloid β‐peptide, like its parent βA4, has shown neurotrophic and late neurotoxic activities in cultured cells. The 3D structure of this important peptide was examined by ¹H and ¹³C 2D‐NMR and MD simulations in DMSO‐d₆ and water. The NMR parameters of chemical shift, ³J(N,Hα) coupling constants, temperature coefficients of NH chemical shifts and the pattern of intra and inter‐residue NOEs were used to deduce the structures. In DMSO‐d₆, the peptide was found to take up a type I β‐turn around the C‐terminal residues Ile⁸–Gly⁹–Leu¹⁰–Met¹¹, whereas in water at pH 5.5, it adopts a random coil conformation. This is only the second report of a β‐turn in the β‐amyloid class of peptides. The solution structures generated using restrained molecular dynamics were refined by MARDIGRAS to an R factor of 0.33 in the case of DMSO‐d₆ and to 0.56 for water. Copyright © 2002 John Wiley & Sons, Ltd.     

α‐methylene‐γ‐butyrolactones: versatile skin bioactive natural products

Natural products containing an α‐methylene‐γ‐butyrolactone moiety, mainly of the sesquiterpene type, are widely observed in plants, which upon coming into contact with skin, will induce major skin toxicological side effects or phytodermatitis. Indeed two main dermatological pathologies have been associated with a skin exposure to molecules containing an α‐methylene‐γ‐butyrolactone moiety: allergic contact dermatitis (ACD) and chronic actinic dermatitis (CAD). ACD is an immunologically based disease resulting from modifications of epidermal proteins by sensitizers or haptens. Indeed, α‐methylene‐γ‐butyrolactones are highly electrophilic structures that can act as Michael acceptors towards nucleophilic residues of proteins. Cysteine and lysine are the most modified residues leading, in the case of enantiomerically pure lactones, to the formation of diastereomeric adducts. This chemical enantioselectivity induces an enantiospecificity of the allergic reaction, i.e., an individual sensitized to one enantiomer will not develop clinical symptoms when exposed to the other enantiomer and vice versa. Sesquiterpene lactones have been also associated with another pathology that involves UV irradiation and DNA modifications. Interestingly, it was found that α‐methylene‐γ‐butyrolactones, in addition to their electrophilic properties, were highly photoreactive molecules able to react with thymine/thymidine to form [2 + 2] photoadducts in very high yields. In all cases a syn regioselectivity was observed, probably associated with the polarization of the exomethylenic bond. This high photoreactivity of α‐methylene‐γ‐butyrolactones towards thymidine could be an explanation of the progressive evolution of allergic contact dermatitis towards chronic actinic dermatitis. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 000–000; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200900013