Effect of 2,2,6,6-tetramethyl-1-piperidinyloxy nitroxide substituent on kinetic parameters of pseudoliving polymerization

The main kinetic and thermodynamic parameters of the pseudoliving radical polymerization of styrene mediated by 4-linoleamido-2,2,6,6-tetramethyl-1-piperidinyloxy have been studied. It has been shown that the introduction of the said substituent into nitroxide leads to a marked reduction in the rate constant of reinitiation that is compensated for by the simultaneous reduction in the rate constant of reversible termination. As a result, the rate of pseudoliving polymerization, the rate of molecular mass growth, and the polydispersity of the polymer appear to be practically the same for processes mediated by both unsubstituted and substituted nitroxides.     

Controlled free-radical azeotropic copolymerization of styrene and n-butyl acrylate in the presence of tert-butyl dithiobenzoate

The free-radical azeotropic bulk copolymerization of styrene and n-butyl acrylate at 90°C mediated by tert-butyl dithiobenzoate and copoly(strene—n-butyl acrylate) dithiobenzoate as reversible chain-transfer agents has been studied. It has been shown that low-and high-molecular mass chain-transfer agents allow one to efficiently control the molecular-mass characteristics of the copolymers. For all studied systems, the molecular mass linearly increases with conversion, and the copolymers are characterized by low polydispersity indexes. When polystyryl dithiobenzoate and poly(butyl acrylate) dithiobenzoate are used as polymer reversible chain-transfer agents in the azeotropic copolymerization of styrene and n-butyl acrylate, the diblock copolymers with the controlled block lengths are prepared. As evidenced by ESR studies, radical intermediates are formed in the course of the azeotropic copolymerization of styrene and n-butyl acrylate mediated by tert-butyl dithiobenzoate and the copolymer reversible chain-transfer agent; the kinetics of formation of these intermediates has been investigated. It has been demonstrated that the rate of the azeotropic copolymerization mediated by low-and high-molecular-mass reversible chain-transfer agents decreases with an increase in their concentration. The possible causes of this phenomenon are discussed.     

Controlled Free-Radical Copolymerization of N-vinyl succinimide and n-Butyl acrylate via a Reversible Addition–Fragmentation Chain Transfer (RAFT) Technique

Summary : Copolymerization of N-vinyl succinimide and n-butyl acrylate in the presence of dibenzyl trithiocarbonate as a reversible addition-fragmentation chain transfer agent was investigated. The linear dependence of molecular mass on conversion and low values of polydispersity index confirmed pseudo-living mechanism of the process. For the first time the soluble copolymers of N-vinyl succinimide and n-butyl acrylate with high composition homogeneity have been synthesized by copolymerization in bulk. The copolymerization kinetics was studied by NMR ¹H spectroscopy; the reactivity ratios were determined: r_VSI = 0.11, r_BA = 2.54. The copolymer microstructure was estimated; it was shown that in conditions of RAFT polymerization gradient copolymers enriched with BA on the tails of the macromolecule and with VSI in the middle can be obtained. The method of elimination of trithiocarbonate fragment by the reaction with an excess of AIBN was proposed leading to formation of the simplest gradient structure of N-vinyl succinimide – n-butyl acrylate copolymer.     

Polymerization of Methyl Methacrylate in the Presence of Boroxyl Radicals. Synthesis of Block Copolymers

The kinetics of the radical polymerization of methyl methacrylate under the action of the amine–tripropylborane–oxygen system is studied at different borane : oxygen ratios in two modes: at direct contact between the polymerization mixture and oxygen and under the postpolymerization conditions, i.e., when, after brief contact, oxygen is removed from the system by vacuum pumping. It is shown that reversible-deactivation postpolymerization becomes possible when organoborane is in a twofold excess over oxygen. There were block copolymers of methyl methacrylate with styrene and acrylonitrile characterized under these conditions.     

Controlled radical polymerization of N-vinylpyrrolidone and N-vinylsuccinimide under the conditions of reversible chain transfer by the addition-fragmentation mechanism

Regular trends in controlled radical polymerization of N-vinylpyrrolidone and N-vinylsuccinimide by the mechanism of reversible chain transfer in the presence of a series of dithiobenzoates and trithiocarbonates were studied. The possibility of preparing soluble poly-N-vinylsuccinimide in concentrated solutions using benzyl benzodithioate as reversible chain-transfer agent was demonstrated.     

Hydrophilicity Impact upon Physical Properties of the Environmentally Friendly Poly(3-hydroxybutyrate) Blends: Modification Via Blending

We have integrated scientific research of polymer blends on the base of poly-3-hydroxybutyrate (PHB and its copolymers) with bench testing in blend preparation by both solvent casting and melt extrusion. As a second component, we have used traditional synthetic macromolecules with various hydrophilicity degree and hence with different morphologies and physical behavior. Besides, variation of polymer hydrophilicity permits to control both the service characteristic and the rate of (bio)degradation operating in the presence of water. Therefore, a substantial part of our work is devoted to water transport in the parent PHB and its blends. Combining the morphology knowledge (SEM, WAXS, FTIR tecynique), transport characteristics (permeability cells and McBain spring microbalance), and mechanical testing, we propose that blending of the parent biodegradable polymer with synthetic macromolecules is a perspective tool to design novel materials with improved characteristics. Both the water transport coefficients and the mechanical characteristics are essentially sensitive to structure and morphology of the blends. Hydrophilicity variation in the order LDPE < SPEU < PVA at blending with PHB shows that the morphology transformation in immicsible or partly miscible blends shifted along the PHB concentration scale as LDPE (at ∼16 wt% PHB) < PVA (∼30 wt% PHB) < SPEU (∼50 wt% PHB) Our instrumental monitoring the structural hierarchy of parent polymers and their blends as well as , simultaneously, the study of transport processes, their modeling, and computer simulating open up the way to understanding the precepts of polymer operation in corrosive and bioactive media.     

Decrease in the Ignition Threshold of Low‐Aspect Laser Targets Using Moderate Profiling of the Pulse Power

It is shown that the ignition of low-aspect targets (aspect ratio lower than 25) can be realized using moderate profiling of the pulse, which consists of two stages with a linear power rise in each of them. Because the low‐aspect targets are more stable than targets with a high aspect ratio, they can be used in ignition experiments in systems with direct irradiation.     

Organoboranes-atmospheric oxygen systems as unconventional initiators of radical polymerization

This paper provides the first generalization and analysis of our results and published data about a new type of pseudoliving process: radical polymerization mediated by boroxyl radicals generated through the oxidation of organoboranes by atmospheric oxygen. The kinetics and mechanism of polymerization of methyl methacrylate and vinyl acetate in the presence of ammonia-organoboranes and boracyclanes are studied. Methods of practical use of polymerization initiated by organoborane-oxygen systems for macromolecular design are discussed.     

An unusual mechanism of polymerization of MMA initiated by ammonia-triisobutyl borane and atmospheric oxygen

The kinetics of polymerization of methyl methacrylate in vacuum initiated by ammonia-triisobutyl borane (iso-Bu₃B · NH₃) oxidized in air is studied. It is shown that the rate of reaction shows the first order with respect to the monomer concentration and a variable order with respect to the initiator concentration; the process is characterized by a low activation energy. It is demonstrated that polymerization proceeds according to a two-step mechanism. The mechanism of polymerization initiation and reinitiation is investigated via ESR spectroscopy, and it is found that one primary radical generated during initiation can form up to 200 substantial chains during the subsequent chain-transfer process.     

Controlled synthesis of styrene and methyl methacrylate copolymers in the presence of reversible addition-fragmentation chain-transfer agents

Molecular-mass characteristics of styrene-methyl methacrylate copolymers formed via the reversible addition-fragmentation chain transfer copolymerization mediated by dithiobenzoates have been studied. Low-molecular-mass reversible-addition fragmentation chain-transfer agents active in the homopolymerization of both monomers and in the homopolymerization of only one of the monomers (styrene) can be used for the controlled synthesis of narrow dispersed copolymers. Conditions for the synthesis of narrow dispersed block copolymers with the desired structure and molecular mass of the blocks have been found. The polymer reversible addition fragmentation chain-transfer agent determines the composition and molecular mass of the first block. The structure of the second block is defined by the composition of the monomer mixture, and the molecular-mass characteristics are set by the concentration of the agent and the conversion of monomers.