A. V. Plutalova

Controlled synthesis of multiblock copolymers by pseudoliving radical polymerization via the reversible addition-fragmentation chain-transfer mechanism

With the use of two classes of reversible addition-fragmentation chain-transfer agents—dithiobenzoates and trithiocarbonates—multiblock copolymers based on styrene and n-butyl acrylate, which are the best-studied monomers in these processes, are synthesized. It is shown that the polymers containing dithiobenzoate and trithiocarbonate groups are highly efficient for the synthesis of block copolymers, which is independent of the number of stages at which the polymeric RAFT agents are used in polymerization: In all cases, the polymeric RAFT agent is fully consumed in the polymerization of the “alien” monomer. The mechanism governing chain formation during the synthesis of multiblock copolymers, that is, the character of monomer insertion into the polymer chain, via one or both ends, is studied. It is found that the order of monomer loading determines the ratio of chains growing through one or two ends. The thermal stability of amphiphilic multiblock copolymers, their solubility in various solvents, and self-organizing ability are investigated.

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.

Pseudoliving radical polymerization of methyl methacrylate in the presence of S,S′-bis(methyl-2-isobutyrate) trithiocarbonate

The polymerization of MMA mediated by symmetric trithiocarbonate as a reversible addition-fragmentation chain-transfer agent is studied. It is shown that the process proceeds according to the two-stage pseudoliving radical mechanism. The polymeric reversible addition-fragmentation chain-transfer agent is more efficient than its low-molecular-mass analog. The use of the polymeric reversible addition-fragmentation chain-transfer agent makes it possible to synthesize narrowly dispersed homopolymers of MMA and related copolymers with a controllable molecular mass. Both chain-transfer agents have practically no effect on the initial rate of copolymerization but allow weakening or even suppression of the gel effect at high conversions.

Thermal stability of styrene/n-butyl acrylate RAFT-based copolymers

Copolymers of styrene and n-butyl acrylate of various architectures (ABA and BAB block copolymers, random copolymers) are synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization using a symmetrical bifunctional trithiocarbonate, namely, dibenzyl trithiocarbonate. These bulk copolymers are subjected to thermolysis in the temperature range from 100 to 200 °C under an inert atmosphere (in the absence of air) and analyzed by size exclusion chromatography and UV-vis spectroscopy. Additionally, the treated polymers are used in the chain extension of styrene polymerization to verify their functionality as RAFT agents. The correlation between the thermal stability of copolymers and their chemical structure, i.e., the chemical nature of the monomer unit linked with the trithiocarbonate group, is discovered.

Mechanism of free-radical copolymerization of styrene with acrylonitrile and methacrylonitrile mediated by 2,2,6,6-tetramethyl-1-piperidinyloxy

By the examples of chemically similar styrene-acrylonitrile and styrene-methacrylonitrile systems, substantially different mechanisms of TEMPO-mediated copolymerization have been theoretically predicted and experimentally verified. Living copolymer-TEMPO adducts capable of thermal decomposition and polymerization reinitiation via the pseudoliving mechanism predominantly form in the first system. The reaction proceeds under the steady-state regime and is characterized by the living growth of M n of the copolymer with conversion. In the second system, the probabilities of formation of living and dead macromolecules are equal. Therefore, the living growth of M n of the copolymer ceases and the reaction passes to the conventional copolymerization regime.

Homophase and heterophase polymerizations of butyl acrylate mediated by poly(acrylic acid) as a reversible addition–fragmentation chain-transfer agent

The radical polymerization of n-butyl acrylate in organic, aqueous, and water–alcohol media in the presence of poly(acrylic acid) containing a trithiocarbonate group within the chain is studied for the first time. It is shown that in nonselective solvents (1,4-dioxane and DMF) poly(acrylic acid) serves as a reversible addition–fragmentation chain-transfer agent and the triblock copolymer poly(acrylic acid)–block–poly(n-butyl acrylate)-–block-poly(acrylic acid) is formed. In aqueous and aqueous–organic media (under conditions of emulsion, dispersion, and miniemulsion polymerizations as well as polymerization-induced selfassembly), the block copolymer being formed additionally serves as a stabilizer of polymer–monomer particles. The sizes of these particles and the molecular-mass characteristics of the resulting polymers may be controlled via variation in the concentration ratio of the components. It is found that, during polymerization in aqueous media, there is the formation of spherical polymer particles that preserve their morphology in thin films prepared via precipitation of the synthesized dispersion.

Controlled radical copolymerization of styrene with acrylic acid and tert-butyl acrylate under conditions of reversible addition-fragmentation chain transfer: Control of the chain microstructure

The copolymerization of styrene and acrylic acid and the copolymerization of styrene and tert-butyl acrylate that are mediated by benzyl dithiobenzoate and dibenzyl trithiocarbonate as reversible addition-fragmentation chain-transfer agents are studied for the first time. It is shown that the copolymerization of these monomer pairs mediated by reversible addition-fragmentation chain-transfer agents occurs via the pseudoliving radical mechanism and is characterized by a linear increase in the number-average molecular weight with an increase in the total conversion of the monomers and by the formation of narrowly dispersed copolymers. Variation in the conditions of the radical copolymerization of styrene and acrylic acid (in the type of reversible addition-fragmentation chain-transfer agent and the addition of a polar solvent) and of the radical copolymerization of styrene and tert-butyl acrylate (in the composition of the monomer mixture) makes it possible to control the chain microstructures of the copolymers.

Controlled copolymerization of acrylonitrile in bulk via the reversible addition-fragmentation chain-transfer mechanism

The pseudoliving radical binary copolymerization of acrylonitrile with methyl acrylate, styrene, n-butyl acrylate, and tert-butyl acrylate in bulk in the presence of the reversible addition-fragmentation chain-transfer agent dibenzyl trithiocarbonate is performed for the first time. The addition of trithiocarbonate makes it possible to prepare a narrowly dispersed visually optically transparent copolymer in a wide range of monomer-feed compositions even at limiting conversions. Conditions for the synthesis of acrylonitrile copolymers with controlled molecular masses and narrow molecular-mass distributions are ascertained. In the above copolymers, the trithiocarbonate group is shown to be located within the chain.

Specific features of the copolymerization of acrylonitrile and acrylamide in the presence of low-molecular-mass and polymeric trithiocarbonates and properties of the obtained copolymers

Regularities of the formation of acrylonitrile-acrylamide copolymers obtained from initial monomer feeds containing 1–50 wt % acrylamide in DMSO solutions with the participation of low-molecular-mass and polymeric trithiocarbonates as reversible addition-fragmentation chain transfer agents are studied for the first time. It is shown that the copolymerization in the presence of low-molecular-mass trithiocarbonates proceeds via a pseudo-living mechanism. The synthesized copolymers prove to be inefficient as reversible addition-fragmentation chain transfer agents, a result that leads to products with bimodal molecular-mass distributions. The rheological characteristics of solutions, as well as the thermal behavior of the copolymers obtained in the absence and in the presence of reversible addition-fragmentation chain transfer agents, are studied. The effect of the synthesis conditions on the properties of the synthesized copolymers is discussed.

Synthesis of amphiphilic copolymers based on acrylic acid, fluoroalkyl acrylates and n-butyl acrylate in organic, aqueous–organic, and aqueous media via RAFT polymerization

Amphiphilic narrow dispersed copolymers of acrylic acid and fluoroalkyl acrylates are synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization using dibenzyl trithiocarbonate in DMF solution. These copolymers and polyacrylic acid containing the trithiocarbonate group within the polymeric chain are used for the synthesis of triblock copolymers in organic, aqueous–organic or aqueous media by chain extension with n-butyl acrylate or its mixture with fluoroalkyl acrylates. Both dispersion polymerization and emulsifier-free emulsion polymerization lead to self-assembling of triblock copolymers in the core–shell particles; the former route provides the formation of microstructured polymer films with a percolated structure.