Yu. V. Kostina

Controlled Synthesis of Polyacrylonitrile via Reversible Addition-Fragmentation Chain-Transfer Pseudoliving Radical Polymerization and Its Thermal Behavior

The polymerization of acrylonitrile mediated by various trithiocarbonates as reversible addition-fragmentation chain-transfer agents is studied. It is shown that, when polymerization is performed in DMSO, a narrowly dispersed PAN with a controlled molecular mass can be prepared. The pseudoliving radical polymerization of acrylonitrile is conducted for the first time via the reversible addition-fragmentation chaintransfer mechanism in carbon dioxide at an increased pressure. The structure of the polymers is investigated via NMR and IR spectroscopy. As shown by DSC and IR pyrolysis, the thermal behavior of PAN is determined by its molecular mass, the width of the molecular-mass distribution, and the conditions of synthesis. The incorporation of functional groups of the reversible addition-fragmentation chain-transfer agent into a macromolecule changes the structure of the polyconjugated system and makes it possible to control the conditions of its formation.

PIM-1/MIL-101 hybrid composite membrane material: Transport properties and free volume

A hybrid composite membrane material based on the PIM-1 polymer of intrinsic microporosity and MIL-101 nanoparticles of mesoporous chromium terephthalate has been prepared and tested. Fourier-transform IR spectroscopy has revealed the presence of interaction between the polymer matrix and the nanoparticles introduced into it. The addition of MIL-101 nanoparticles leads to an increase in the permeability and diffusion coefficients for gases (He, O2, N2, CO2) compared to the original polymer. Using positron annihilation lifetime spectroscopy (PALS), it has been shown that these changes result from an increase in the free volume in the polymer composite material.

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.

Sorption and catalytic effects upon the dehydration of methanol on perfluorinated copolymer-containing sulfo groups

The peculiarities of methanol transformation on the surface of perfluorinated copolymer, fluoroplastic, containing sulfo groups were studied. At temperatures of 70–120°C, the polymer was established to have high initial activity during the conversion of methanol into dimethyl ether; this reaction was, however, accompanied by strong water sorption that leads to a rapid drop in the activity. The sorption/desorption processes were characterized by temperature effects such as heating during sorption and cooling during desorption. The structure of the polymer in the salt (sodium cations) and acid (hydrolyzed polymer) forms was studied by IR spectroscopy. H3O+SO3− ionic pairs were found to be present in the acid form.

Effect of chloroform on the structure and gas-separation properties of poly(ether imides)

The FTIR spectra of poly(ether imide) films prepared from their chloroform solutions were recorded in a wide temperature interval. The cast films were shown to contain residual solvent. This residual solvent existed in films as unbound chloroform that may be removed by heating to 60–70°C and as bound chloroform that is involved in complex formation with polymers and may be removed by heating at temperatures close to their glass transition temperatures (180°C). Quantum-chemical calculations were performed for structures that model fragments and monomer units of poly(ether imides), as well as their complexes with chloroform. Chloroform was shown to be capable of preferential binding with an oxygen atom in a Ph-O-Ph′ fragment via hydrogen bonds. In this case, the conformational set of poly(ether imide) chains is changed. The above evidence is invoked to explain changes in transport characteristics with time for poly(ether imide) films cast from chloroform solutions.

Effect of structure and conformational composition on the transport behavior of poly(ether imides)

With the example of two isostructural poly(ether imides), the effects of isopropylidene and hexafluoroisopropylidene groups and hydrogen bonding with chloroform on changes in geometry and energy parameters of polymer chains have been analyzed. The relationship between changes in the geometry characteristics of polymer chains and the gas-separation behavior of films cast from these poly(ether imides) has been established. It has been shown that an increase in permeability is related to a rise in rotation barriers and, consequently, to enhancement of chain rigidity, while the improvement of gas-separation selectivity is associated with a reduction in the amount of isoenergy conformers and an increase in the conformational uniformity of polymer chains. As was detected by FTIR spectroscopy and confirmed by quantum-chemical calculations, the conformational nonuniformity and the small-scale mobility of poly(ether imide) chains are possible only in the Ph-O-Ph′ fragment. The data obtained are applicable for estimation of the role of molecular mobility of polymer chains in the mechanism of gas or penetrant transfer through the polymer film and for predicting the transport behavior of polymers from this family. With the example of two other poly(ether imides), the prediction is confirmed by the experimental data on gas separation.

Viscosity of polyacrylonitrile solutions: The effect of the molecular weight

The copolymer of acrylonitrile with methyl acrylate and itaconic acid (93: 5.7: 1.3) is synthesized via free-radical copolymerization. For solutions of the initial copolymer and its 12 fractions in dimethyl sulfoxide, viscosities are measured in a wide shear-stress range. The viscosity behavior of 10% solutions is examined in more detail, and the rheological studies of several fractions are performed in the concentration interval from 5 to 20%. All solutions exhibit weak non-Newtonian behavior. This makes it possible to determine the zero-shear viscosity. The dependences of this viscosity on the number-average and weight-average molecular masses for the equiconcentrated solutions obey an exponential law with the same exponent, equal to 2.3. The macromolecules of copolymers are inclined toward association. For low-molecular-mass fractions, this effect is the most pronounced.

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.

Effects of synthesis conditions and the mechanism of homopolymerization of acrylonitrile on the thermal behavior of the resulting polymer

The effect of the conditions of polyacrylonitrile synthesis (nature of the solvent, homogeneity of the reaction medium, and type of initiation) on the conditions of formation of the polyconjugation system during the pyrolysis of polyacrylonitrile is studied. The controlled synthesis of polyacrylonitrile via radiation polymerization mediated by dibenzyl trithiocarbonate as a reversible addition-fragmentation chain-transfer agent at 25 and 80°C is performed for the first time. It is shown that the type of initiation in reversible addition-fragmentation chain-transfer polymerization (material initiation or radiolysis) strongly affects the structure of the formed polyconjugation system.

Study of the mechanism of the thermochemical reaction of polyimides with hydroxyl groups via vibrational-spectroscopy and quantum-chemistry methods

The thermochemical transformation of polyimides with one hydroxyl group in a diamine fragment that leads to changes in the polymer structure is studied. It is found that in an inert atmosphere in the temperature range 400–450°C, an intramolecular thermochemical reaction accompanied by evolution of carbon dioxide occurs. The FTIR spectra of all studied polyimide films prepared from different solvents and annealed in an inert atmosphere in the range 25–450°C show similar changes that do not agree with the structures of the reaction products (polybenzoxazole and polydehydrocyclobutabenzene) described in the literature. Possible reaction paths are calculated via quantum-chemistry methods. A mechanism of the intramolecular thermochemical transformation and the related structure of the formed polymer (aromatic lactam) are proposed.