Cationic polymerization of thietanes initiated by trityl salts: Mechanism of the initiation reaction

The initiation reaction of the cationic polymerization of 3,3-dimethylthietane by trityl hexafluoroantimonate occurs by a fast and quantitative direct addition of the trityl cation to the sulfide function thus producing the corresponding polymer with tritylthio head-groups. In contrast with this mechanism, the reaction of the trityl salt with the non-polymerizable sulfides tetrahydrothiophene and diethyl sulfide, leads to a slow hydride abstraction from the sulfide. With these sulfides an equilibrium [carbenium ion + sulfide ? sulfonium ion] is also observed. At 25°C the association constants were found to be 2,27 and 0,096 1 · mol^?1 for tetrahydrothiophene and diethyl sulfide, respectively (in CD₂Cl₂). The initiation mechanism by sulfonium ion formation is explained by the high reactivity of the formed four-membered ring trityl sulfonium ion which disturbs the carbenium ion-sulfonium ion equilibrium and ultimately leads to quantitative initiation.     

Cationic polymerization of anethole by triphenylmethyl hexafluoroantimonate

Cationic polymerizations of anethole in methylene dichloride initiated by triphenylmethyl hexafluoroantimonate were studied by dilatometry and UV-visible spectroscopy at temperatures between 0 and 35°C. Initiator consumption continued throughout polymerization, reaching 85–95%. Monomer conversion ranged from 75 to 98%. Reaction half-lives were on the order of 15 to 60 min at 25°C. Polymer molecular weights were bimodal with some evidence of branching and cross-linking. The principal termination reaction appeared to involve formation of a chromophore absorbing at 501 nm. Rate constants on the order of 10^?3 dm³·mol^?1·s^?1 for initiation and 1 dm³·mol^?1·s^?1 for propagation at 25°C were estimated.     

Stereocontrol in the free radical polymerization of N-vinylcarbazole

The free radical polymerization of N-vinylcarbazole (VK) was investigated in hexane, nonane, toluene, HMPT, DMF, DMAC, and DMSO. Isotactic propagation was found to be energetically favoured by between 1500 and 4600 J · mol^?1 for all the solvents investigated, although the entropy difference between syndiotactic and isotactic propagation strongly favoured syndiotactic propagation by between 11 and 20 J · mol^?1K^?1. Variation in the steric microstructure between an isotactic mole fraction of 0,25 and 0,47 was obtained by varying the polymerization temperature and solvent. The activation enthalpy differences between isotactic and syndiotactic propagation (ΔH≠_s/i ? ΔH≠_i/s), for the free radical polymerization of methyl methacrylate (MMA) and VK were compared and the steric configurations involved in the propagation process considered. It was concluded that electronic interaction between radical and monomer appeared to determine the energetic preference of the growing chain for isotactic or syndiotactic propagation in free radical propagation. This energetic preference was correlated with the e-factor in the Q ? e scale. Data for the free radical polymerization of VK, MMA, and vinyl trifluoroacetate showed that solvent polarity has a weak influence on the energetic preference of the growing chain for isotactic or syndiotactic propagation. This energetic preference was correlated with the π* solvent polarity scale.     

Cationic polymerization of formaldehyde

Cationic polymerization of monomeric formaldehyde has been studied at low temperature. It was found that high molecular weight polyoxymethylene with the reduced viscosity higher than 1.0 dl/g was readily obtained when the polymerization of formaldehyde was carried out in saturated hydrocarbons in the presence of LEWIS acids, BRÖNSTED acids, I₂, ICl ICl₃, IBr, sulfur dioxide and binary systems of sulfur dioxide with these compounds at considerably low temperature. The degree of polymerization was effected by the polymerization conditions, especially solvent, polymerization temperature and addition of chain transfer agents such as water, formic acid, methanol or methyl formate.     

Cationic polymerization of aldehydes

Cationic polymerizations of formaldehyde, higher aliphatic aldehydes and haloaldehydes are discussed. The polymerization is initiated by Lewis and Brønsted acids but is dominated by the fact that in almost all cases the polymers precipitate and crystallize from the solution. As a consequence the polymerizations become diffusion controlled sometimes even at low conversions. This phenomenon is not limited to cationic polymerization but additionally the growing ends are readily occluded and cationic initiators are adsorbed on the precipitated polymer. The morphology and the degree of swelling of the precipitated polymer plays an important role upon the rate of polymerization and conversion of monomer to polymer. As expected, polymerizations of aliphatic aldehydes are more readily carried out by cationic initiators than the polymerization of haloaldehydes; this may be caused by the difference in the initiation mechanism.     

Cationic polymerization of isobutylene proceeding by redox initiation

The cationic polymerization of isobutylene with weak Lewis acids, such as BBr₃ and FeBr₃, has been studied. These halides are unable to induce individually the polymerization in nonpolar solvents, such as heptane or toluene, or they only initiate the polymerization to low conversions in polar media, such as dichloromethane. The complex compounds, formed by interaction of both Lewis acids, initiate a fast polymerization of isobutylene, which proceeds up to high conversions. The coinitiation effect of HBr in the mixture with BBr₃ and FeBr₃ was also investigated. The hypothetic formation of efficient initiators and the initiation mechanism of olefin polymerization are discussed on the basis of the ionogenic action of both Lewis acids.     

Cationic polymerization of tetrahydrofuran initiated by multifunctional initiators

A series of multifunctional cationic initiators was synthesized from 1,5,9-cyclododecatriene and from methylbenzenes by the Wohl-Ziegler reaction with N-bromosuccinimid. The initiators were treated with silver perchlorate to prepare star-shaped polytetrahydrofurans with three to six arms.     

Cationic polymerization of cyclic thioacetals by triethyloxonium tetrafluoroborate

Polymerization of 1-oxa-3-thiacyclopentane with catalytic amounts of triethyloxonium tetrafluoroborate was studied. The initiation reaction was a fast and quantitative formation of ethylsulfonium salt of monomer. The propagation reaction was a slow and quite complex reaction with sulfonium transfer reactions. The reaction mechanism is discussed.     

Cationic polymerization of cyclic sulfides

The mechanism of the cationic polymerization of cyclic sulfides, initiated with triethyloxonium salts is reported. Differences due to changing the counter-ion from BF to SbCl are discussed. Initiation consists of the alkylation of monomer forming a cyclic sulfonium ion which is the active species in the propagating step. A generally occurring termination reaction is the formation of non-strained (linear or cyclic) sulfonium ions. In the case of epithiopropane (propylene sulfide) these can slowly re-initiate the polymerization by an intramolecular reaction forming the 3-membered cyclic sulfonium salt. This re-initiation is not possible with the thietanes because of the greater difficulty to form 4-membered cyclic sulfonium salts. When SbCl is the counter-ion, an additional, more drastic termination can occur most probably by reaction of the growing chain with the counter-ion, thereby forming an alkyl chloride and SbCl₅. After the polymerization of propylene sulfide with BF as the counter-ion, the polymer degrades to low molecular weight oligomers, predominantly the cyclic tetramer. This degradation is a back-biting reaction occuring via sulfonium salts. With SbCl as the anion, the sulfonium salts are destroyed by the termination reaction and degradation does not occur. It was possible to follow the concentration of the growing species (4-membered cyclic sulfonium salt) during the polymerization of 3,3-dimethylthietane by means of 300 MHz NMR spectroscopy. By measuring the rate constants of propagation for different initiator concentrations or in the presence of different amounts of an indifferent electrolyte, it was possible to calculate separate rate constants for propagation via free ions (k) and via ion-pairs (k). The ratio k/k was about 60 when BF was the anion and 35 when it was SbCl.     

The electroinitiated polymerization of N-vinylcarbazole with zinc bromide,II. Kinetics of polymerization

The electroinitiated polymerization of N-vinylcarbazole (VCZ) in acetone solvent at 25°C with zinc bromide, as catalyst, and platinum electrodes was investigated in detail. The polymerization rate was found to be directly dependent on current strength, zinc bromide concentration and the initial water content of the solvent. The molecular weights of the formed polymers (1000–6000) are independent of current strength, zinc bromide concentration and degree of conversion of monomer to polymer, but are affected by water, monomer concentration, and by the type of electrodes used. The chemical composition of the polymer is invariant with current strength, zinc bromide concentration, degree of conversion, and electrode material but does vary slightly with the water content of acetone.     

Cationic polymerization and copolymerization of vinylferrocene

Vinylferrocene was shown to undergo readily cationic polymerization by FRIEDEL-CRAFTS and other cationic catalysts. The polymers obtained possessed rather low molecular weights, the highest being 3500. In spite of the high reactivity of the ferrocene nucleus towards electrophilic substitution, no mode of propagation other than vinyl addition was detected when the infrared spectra of polymers prepared by cationic and free radical reactions were compared. The electronic spectra of polyvinylferrocene and its oxidized form were similar to those of ferrocene and ferricinium ion, respectively. In the cationie copolymerization with styrene, the latter was incorporated into the polymer only when the styrene content in the monomer feed exceeded 90%. The copolymerization with vinyl isobutyl ether (M₂) by BF₃OEt₂ in toluene at 0°C gave the reactivity ratios: r₁ = 0.1 ± 0.1, r₂ = 9.7 ± 1.0. The high cationic reactivity of vinylferrocene is consistent with the stability of the α-ferrocenylearbonium ion inferred from solvolysis and other reactions.     

The polymerization of acenaphthylene by carbocation salts in nitrobenzene

Acenaphthylene was polymerized in nitrobenzene at 25°C by NO₂SbF₆, C₂H₅COSbF₆, C₆H₅COPF₆, and (C₆H₅)₂CHSbF₆. With monomer so rigorously purified that it contained only ca. 0,01 mol-% of inhibiting impurities the reaction pattern indicated propagation by unpaired cations, and a k = 23,3 ± 2 dm³·mol^?1·s^?1 (the first for this monomer) was obtained. There are at least two chain-breaking reactions of which one is the proton transfer to monomer which has K = 2,3 dm³·mol^?1·s^?1. The kinetic irregularities and colours which develop at high conversions are probably due to the formation of non-propagating allylic cations from unsaturated end-groups.     

Growth of Tahyna virus at low temperatures.

Replication of two Tahyna virus strains in the Aedes albopictus mosquito cell line was studied in the temperature range from 6 to 28 degrees C. The virus grew in this temperature range; its replication rate was related to the temperature of incubation. At lower temperatures the virus titres increased more slowly and did not reach as high maximum values as at higher temperatures. Short increase in incubation temperature resulted in a short increase in the titre of virus previously incubated at 10 degrees C, but not of virus previously incubated at 15 degrees C. At 10 and 15 degrees C, the released virus was demonstrated for more than 300 days past infection (p.i.).     

Cationic polymerization of glycidol. Polymer structure and polymerization mechanism

Glycidol (2-epoxy-1-propanol) was polymerized under the action of Lewis acids (BF₃OEt_2,, SnCl₄) and protonic acids (CF₃COOH, CF₃SO₃H). Polymers with number-average molecular weights (M?_n) varying from 2500 to 6000 g/mol were obtained. The structure of obtained polymers was characterized by ¹³C and ²⁹Si NMR spectroscopy, and the amount of 1-3 units, 1-4 units and chain branches was determined. The structure of the polymers was related to the polymerization mechanism, especially to the contribution of activated monomer mechanism in chain growth.     

Severe ethanol stress inhibits yeast proteasome activity at moderate temperatures but not at low temperatures

Since yeast research under laboratory conditions is usually conducted at 25–30°C (moderate temperature range), most of the findings on yeast physiology are based on analyses in this temperature range. Due to inefficiencies in cultivation and analysis, insufficient information is available on yeast physiology in the low-temperature range, although alcoholic beverage production is often conducted at relatively low temperatures (around 15°C). Recently, we reported that severe ethanol stress (10% v/v) inhibits proteasomal proteolysis in yeast cells under laboratory conditions at 28°C. In this study, proteasomal proteolysis at a low temperature (15°C) was evaluated using cycloheximide chase analysis of a short-lived protein (Gic2-3HA), an auxin-inducible degron system (Paf1-AID*-6FLAG), and Spe1-3HA, which is degraded ubiquitin-independently by the proteasome. At 15°C, proteasomal proteolysis was not inhibited under severe ethanol stress, and sufficient proteasomal activity was maintained. These results provide novel insights into the effects of low temperature and ethanol on yeast physiology.     

Molecular weight distribution studies, 4 Kinetics of the gamma-ray induced free radical polymerization of vinyl acetate at low temperatures

The kinetics of the gamma-radiation initiated free radical polymerization of vinyl acetate were studied in bulk over a temperature range of ?60°C to 0°C. Four radiation dose rates of 0,83, 0,33, 0,26, and 0,079 Mrad were used. The overall rate of polymerization was found to vary with the 0,46 to 0,53 power of the radiation intensity, indicating a free-radical polymerization mechanism. The overall activation energy was found to tie between 16 and 18 kj/mol. The values of k_trm/k_p and (k_tc + k_td)k_p² were evaluated from the overall polymerization rates and the number-average molecular weights. The values of the radiation chemical yield G_VAC were found to lie between 2,4 and 4,1.