L. B. Krentsel

Synthesis of norbornene–cyclooctene copolymers by the cross-metathesis of polynorbornene with polyoctenamer

Copolymers of norbornene and cyclooctene were synthesized for the first time by the cross-metathesis of polynorbornene with polyoctenamer. This strategy made it possible to use the 1st generation Grubbs catalyst, which exhibits low activity toward copolymerization of those monomers. Statistical multiblock copolymers with average block lengths varying from 200 to 2 units were obtained.

Alkaline hydrolysis of polyacrylonitrile, 2. On the product swelling

The swelling capacities of alkali hydrolyzed polyacrylonitrile in water Q w and in 0.9 wt.-% NaCL aqueous solution Q s were measured. Using these data the fraction of crosslinked units is estimated to vary from 0.8 to 1.6% for different samples. The analysis of Q s dependence on Q w is proposed as a method to estimate the degree of polymer network ionization α. The α value is found to be approximately 0.36 for all samples at the degree of neutralization 0.49-0.70. This indicates that a significant part of counterions does not affect the swelling, being bound by polyions. The influence of the conditions of hydrolysis (polymer/alkali and water/ethanol ratios) on the swelling properties of the product and the nature of crosslinks are discussed. Interchain interaction of an amidine group and an acrylonitrile unit resulting in a formation of β-diketone-like structure is proposed to explain the crosslinking. Kinetic measurements were carried out to estimate the ratio of the initial rate constants for the polyacrylonitrile and polyacrylamide alkaline hydrolysis which was found to be of order 10 -1 .

Thermal fractionation of vinyl acetate-vinyl alcohol copolymers

Thermal fractionation via the method of successive self-nucleation and annealing was used for the first time to study the crystallinity of vinyl acetate-vinyl alcohol copolymers with different random distributions of chain units. The lamella-thickness distribution was calculated through the Gibbs-Thomson equation. It was shown that, for all samples, the minimum lamella thickness is the same and corresponds to a block of no less than 15 vinyl alcohol units. On the basis of these data and with the use of the computer simulation of the polymer-analogous reaction via the Monte Carlo method, the block-length distribution in the crystalline phase was found. It was shown through a comparison of the lamella-thickness and block-length distributions that the maximum lamella thickness increases with the block length and vinyl alcohol content in the copolymer. In crystallites, blocks with lengths exceeding the maximum lamella thickness comprise a significant fraction. Thus, it is probable that these blocks form folds. The dependences of melting temperatures of crystalline lamellas on their thicknesses, as well as the dependences of the melting temperatures of copolymers not subjected to thermal fractionation on the chain-structure parameters, are adequately described by the Flory crystallization theory.

Cross-metathesis of polynorbornene with polyoctenamer: a kinetic study

Summary The cross-metathesis of polynorbornene and polyoctenamer in d-chloroform mediated by the 1st generation Grubbs’ catalyst Cl2(PCy3)2Ru=CHPh is studied by monitoring the kinetics of carbene transformation and evolution of the dyad composition of polymer chains with in situ 1H and ex situ 13C NMR spectroscopy. The results are interpreted in terms of a simple kinetic two-stage model. At the first stage of the reaction all Ru-benzylidene carbenes are transformed into Ru-polyoctenamers within an hour, while the polymer molar mass is considerably decreased. The second stage actually including interpolymeric reactions proceeds much slower and takes one day or more to achieve a random copolymer of norbornene and cyclooctene. Its rate is limited by the interaction of polyoctenamer-bound carbenes with polynorbornene units, which is hampered, presumably due to steric reasons. Polynorbornene-bound carbenes are detected in very low concentrations throughout the whole process thus indicating their higher reactivity, as compared with the polyoctenamer-bound ones. Macroscopic homogeneity of the reacting media is proved by dynamic light scattering from solutions containing the polymer mixture and its components. In general, the studied process can be considered as a new way to unsaturated multiblock statistical copolymers. Their structure can be controlled by the amount of catalyst, mixture composition, and reaction time. It is remarkable that this goal can be achieved with a catalyst that is not suitable for ring-opening metathesis copolymerization of norbornene and cis-cyclooctene because of their substantially different monomer reactivities.

Interchain exchange and interdiffusion in blends of poly(ethylene terephthalate) and poly(ethylene naphthalate)

The interchain exchange and interdiffusion in blends of poly(ethylene terephthalate) and poly(ethylene naphthalene-2,6-dicarboxylate) are investigated with reprecipitated commercial samples (M η ∼ 104) and samples containing no polycondensation catalyst (M η ∼ 103) synthesized in the course of this study. The kinetics of multiblock copolymer formation and gradual reduction of the mean block length in quasi-homogeneous blends were shown to fit a simple theoretical model of a second-order reaction. The increase of the reaction-rate constants on the transition from commercial samples to synthesized ones revealed a significant role of chain ends in interchain exchange. The detected activation energy of the interchange in the absence of catalysts (97 kJ/mol) was noticeably less than that previously reported for the polymer pair under study (120–170 kJ/mol). The obtained data were applied for analysing the interdiffusion between melts of the same polymers accompanied by the interchain exchange. By means of the microinterference method, the interdiffusion in the synthesized samples was shown to be much faster than that in the reprecipitated commercial samples, a result that may be due to the better compatibility of the initial polyesters as their molecular mass decreased. In later stages of the process in both systems, the interpenetration of components was slower than that predicted by Fick’s law, owing to formation of copolymer species that diminished the thermodynamical factor of mixing.

Chain statistics in vinyl acetate-vinyl alcohol multiblock copolymers

The kinetics and structure of products of the alkaline hydrolysis of poly(vinyl acetate) in an acetone-water mixture are studied via 1H NMR spectroscopy. The reaction is considerably accelerated and, according to the dynamic light-scattering data, is accompanied by polymer-coil expansion. The model of the effect of neighboring units is shown to be inapplicable to description of the general kinetics of the process; however, it may be successfully used for the quantitative interpretation of the experimental dependences of the triad composition of macromolecules on conversion. Thus, the conformational factors affect the reactivity of vinyl acetate groups, regardless of the nature of their nearest chain neighbors. This circumstance makes it possible to use the neighbor-effect model to describe the unit distribution over chains of the reaction product, vinyl acetate-vinyl alcohol copolymer, and thus to obtain information necessary for studying the relationship between the chain structure and properties of statistical multiblock copolymers.

Regulation of the degree of blockiness of the norbornene–cyclooctene copolymer synthesized via the cross-metathesis reaction

The kinetics of the polymer cross-metathesis reaction in a mixture of polynorbornene and polycyclooctene catalyzed by the Grubbs first-generation ruthenium catalyst at room temperature is studied. The structure of the reaction product, a multiblock copolymer of norbornene and cyclooctene, is determined by a number of factors typical for a mixture of polymers reacting with each other via the interchange reaction with the participation of terminal groups. The addition of 5 mol % catalyst transforms the mixture into an almost random copolymer over a day. At a lower content of the catalyst, the maximum conversion is reached in a mixture enriched with polynorbornene. The interchain exchange results in an increase in the fraction of trans C=C bonds in polycyclooctene and cyclooctene–norbornene copolymers to 80%.

The effect of IR radiation on structurization of butadiene-nitrile rubber

The accelerated aging of butadiene-nitrile rubber (NBR) induced by IR radiation is studied. At 100°C, intensive aging (the formation of imide and anhydride crosslinks) of NBR is observed in air and argon. The activity of aging under IR radiation is significantly higher than the heat aging. The IR radiation selectively activates nitrile groups without any chemical transformation in butadiene units of the rubber. On the basis of the data obtained, it is suggested that the use of IR radiation makes it possible to model processes occurring as a result of local temperature fluctuations during prolonged natural aging of NBR. Thus, the accelerated aging induced by IR irradiation at 100°C can be recommended as an effective method for estimating structural instability of NBR during storage.

Synthesis of new multiblock copolymers via cross-metathesis reaction of polytrimethylsilylnorbornene and polycyclooctene

The cross-metathesis reaction between polytrimethylsilylnorbornene and polycyclooctene is investigated for the first time. Using the Grubbs Ru-carbene complex of the first generation, the synthesis of random multiblock copolymers of trimethylsilylnorbornene and cyclooctene is carried out. The effect of the reaction conditions on the block length and thermal and crystalline properties of new copolymers is studied by NMR, GPC, and DSC methods. With the use of in situ 1Н NMR spectroscopy, the kinetics of the crossmetathesis reaction is investigated. As for unsubstituted polynorbornene (Polym. Sci., Ser. B 58, 292 (2016)), the catalyst interacts initially with polycyclooctene, giving a polymer carbene complex [Ru]=PCO. Further, this complex attacks the polytrimethylsilylnorbornene chain via cross reaction with formation of complex [Ru]=PNB-Si and a diblock copolymer of trimethylsilylnorbornene and cyclooctene as reaction products. The subsequent cross reactions form a multiblock copolymer with gradually increasing block length. Throughout the entire process, the concentration of [Ru]=PCO exceeds the concentration of [Ru]=PNB-Si by more than two orders of magnitude. Simultaneously, the total concentration of carbene complexes decreases in time through their decay. The reaction kinetics is satisfactorily described by a model proposed previously for interaction between polycyclooctene and polynorbornene.

The role of chain structure in the rheological behavior of vinyl acetate-vinyl alcohol copolymers

A comparative study of the viscoelastic properties of melts of vinyl acetate-vinyl alcohol copolymers with equimolar compositions characterized by different statistical distributions of chain units has been performed. It has been shown that the principle of temperature-frequency superposition is obeyed by copolymers close to a random copolymer, but is violated by copolymers with the block distribution of units. Unlike amorphous random copolymers, a multiblock copolymer is characterized by weak crystallinity, the absence of the relaxation flow state, and a more pronounced tendency to form interchain hydrogen bonds both between two hydroxyl groups and between hydroxyl and ester groups.