L. A. Smirnova

Dependence of copolymer composition upon molecular weight in homogeneous radical copolymerization

Abstract In the early stages of homogeneous radical copolymerizations of many vinyl monomers, deviation from the Mayo-Lewis classical scheme has been found. The copolymer composition depends on molecular weight, the samples being nonhomogeneous in composition. The phenomenon appears in a kinetic sense as the dependence of copolymer composition upon the concentration of a radical initiator or of a chain transfer agent. As shown by equilibrium dialysis and light scattering, the fact that the composition depends upon the molecular weight is explained by preferential sorption of one of the monomers by polymer chains, the quantity depending upon the molecular weight of copolymer.

Graft polymerization of acrylamide on chitosan: Copolymer structure and properties

Graft copolymers have been prepared through the free-radical polymerization of acrylamide in water-acetic acid solutions of chitosan in the presence of ammonium persulfate as an initiator. The effect of temperature, pH of a medium, and concentrations of initiator, chitosan, and acrylamide on the grafting kinetics and efficiency has been established. The local concentration of acrylamide in the reaction zone increases due to intermolecular hydrogen bonding between polysaccharide and monomer molecules. Copolymer films are characterized by better mechanical properties compared to those of polysaccharide. Hydrogels with good viscoelastic properties have been obtained when grafting was performed in the presence of N,N-methylenebisacrylamide as a crosslinking agent.

The effect of monomer preferential solvation in radical copolymerization: Reactivity ratios and compositional distribution

The effects of preferential solvation of monomers by polymer coils in homogeneous radical copolymerization at low conversion (5–7%) have been studied for styrene-acrylonitrile, styrenemethacrylic acid, styrene-acrylamide, styrene-acrylic acid, styrene-butyl methacrylate, vinyl acetate-2-methyl-5-vinyl-pyridine and vinyl acetate-2-vinyl pyridine. In the simplest cases, the copolymer composition and microstructure can be described by the Mayo-Lewis model with the apparent or effective monomer reactivity ratios: r2 = r20γ, r1 = r10γ, where r10, r20 are ideal reactivity ratios defined by the monomer structure, γ is the monomer distribution coefficient. The most fundamental effect of preferential solvation is the relation between the copolymer composition and molecular weight caused by the dependence of preferential solvation coefficient γ upon the length of a growing chain. This dependence leads to two previously unknown types of chemical heterogeneity of low-conversion copolymers, viz. intra- and intermolecular gradient chemical heterogeneity.

Thermodynamics of mixing of some vinyl monomers. On the possibility of predicting deviations from the classical scheme of copolymerization

Abstract For six pairs of vinyl monomers (vinyl acetate/N-vinylpyrrolidone, acrylonitrile/styrene, styrene/methacrylic acid, vinyl acetate/styrene, styrene/butyl methacrylate, methyl methacrylate/styrene) the equilibrium liquid-vapour has been studied in the range 298–343 K and enthalpies of mixing have been measured at 298 and 318 K. Total and excess thermodynamic functions of mixing have been calculated. It has been shown that systems non-ideal from the view of the equilibrium liquid-vapour also display deviations from the classical scheme of copolymerization. A criterion (excess change in Gibbs function of mixing ΔGE), predicting deviation from the classical scheme of copolymerization on the basis of the thermodynamic properties of monomer mixtures, has been suggested; its applicability is shown for the system styrene/butyl methacrylate. Correlation of ΔGE values and quantitative aspects of deviation from the Mayo-Goldfinger scheme has been found.

Hydrodynamic and molecular characteristics of graft copolymers of chitosan with acrylamide

The conformational properties of macromolecules of chitosan and its copolymers with acrylamide in a mixed solvent 0.33 M CH3COOH + 0.3 M NaCl have been investigated by means of translation diffusion and viscometry. The copolymer macromolecules in a solvent suppressing polyelectrolyte effects possess a higher intracoil density (ρav = 0.010 g/cm3) than the chitosan macromolecules (ρav = 0.006 g/cm3), even though the hydrodynamic radius R h of chitosan is smaller by a factor of ∼1.5.

Radical degradation of chitosan under the action of a redox system

Degradation of chitosan in solution under the action of redox radical initiators was studied with the aim to prepare chemically oligomers of natural polysaccharides. The influence of temperature on the rate and extent of the process was examined. The optimal ratios of the substrate to initiator and of the reductant to oxidant in the initiating system were determined.

Degradation of Chitosan in Solutions in the Presence of a Redox System

The degradation of chitosan in aqueous solutions of acetic and hydrochloric acids initiated by the hydrogen peroxide-ascorbic acid redox system has been studied. The nature of acid strongly affects the degree of degradation and the molecular mass of oligochitosan—the reaction product. The relationship between the molecular mass of oligochitosan and the concentration of acetic acid in the solvent is described by a curve with a minimum. If acetic acid is present in a 5-fold molar excess with respect to chitosan units, the starting solution behaves as a Newtonian liquid. It has been demonstrated that, exactly in this solution, oligochitosan with the minimum amount of units is formed.

The effect of catalytic amounts of thiols on the compositions of unsaturated acid copolymers

Addition of catalytic amounts of thio-compounds result in changes in the copolymerization constants of acrylic or methacrylic acids with vinyl monomers, such as methyl methacrylate, styrene, or vinyl acetate. The UV spectra of methacrylic acid with mercapto thiolates contain the 310 and 340 nm charge transfer lines. The composition of the 1:1 complex has been determined by dielectric titration, and the UV spectroscopy by the Scott method was used to find the equilibrium constant of the complex (Ke = 0·27 ± 0·03). The catalytic quantities of mercapto thiolates have been found to affect also the composition of the 2-methyl-5-vinylpyridine copolymer with vinyl acetate or styrene.

Potential bootstrap effect and phase separation of reaction solutions

Abstract The phase separation in monomer–copolymer solutions was studied for styrene–acrylonitrile, styrene–butyl methacrylate and styrene–methyl methacrylate systems in order to find conditions to stimulate a bootstrap effect in radical copolymerization. There was phase separation at Tsep

Effect of silicon, germanium and tin compounds on copolymerization of unsaturated acids☆

Abstract A study was made of the effect of silicon, germanium and tin compounds on copolymerization of methacrylic and acrylic acids with vinyl monomers. It was established that chlorides and trisubstituted hydrides of silicon, germanium and tin result in a reduction of the content of acid units in the copolymer, the effect markedly depending on the type of substituent and the main element of the modifier of the system. A distinctive feature of new modifiers is the action on copolymerization in catalytic amounts not exceeding 1% in the monomer mixture. It was shown that charge transfer complexes are formed between unsaturated acids and compounds of elements of group IVB. An assumption was put forward that the variation in copolymer composition is the consequence of interaction of the modifier with the intermediate complex, as a result of which the level of resonance stabilization is lowered.