Homogeneous Zinc(II) Catalysis in Accelerated Vulcanization IV. The Mechanism of Cross-Link (De)Sulfuration

1999 ◽  
Vol 72 (1) ◽  
pp. 43-54 ◽  
Author(s):  
P. J. Nieuwenhuizen ◽  
J. M. van Veen ◽  
J. G. Haasnoot ◽  
J. Reedijk
Keyword(s):  
Activation Energy ◽  
Double Bond ◽  
Elemental Sulfur ◽  
Double Bond Position ◽  
Double Bonds ◽  
Cross Link ◽  
Reaction Stage ◽  
Double Bond Isomerization ◽  
Cross Links ◽  
Stage Modeling

Abstract This paper reports a detailed re-examination of the mechanism of zinc-mediated cross-link desulfuration by means of Reaction-Stage Modeling. Heating the model cross-link bis(2,3-dimethyl-2-buten- 1-yl)disulfide in the presence of catalytic amounts of bis(diethyl-dithiocarbamato)zinc(II), ZDEC, results in slow but definite formation of monosulfides displaying retention and isomerization of the double-bond position. The formation of monosulfides with retained double bonds implies that cross-link desulfuration occurs according to SNi reactions; whereas, the formation of isomerized monosulfides points to the operation of SNi′- and/or thione-like desulfuration reactions. Importantly, these observations constitute the first evidence for zinc accelerators acting as homogeneous catalysts in desulfuration. The activation energy for cross-link desulfuration of SNi reaction was established to be 86(16) kJ mol−1. In addition, ZDEC is reported to sulfurate model cross-links. The required sulfur atoms may originate either from desulfuration of other sulfides, or from elemental sulfur added to the mixture. Just as for cross-link desulfuration, zinc-mediated sulfuration was found to occur both with and without double-bond isomerization.

Homogeneous Zinc(II) Catalysis in Accelerated Vulcanization I. Reaction-Stage Modeling and Cross-Link Formation

1998 ◽  
Vol 71 (4) ◽  
pp. 750-765 ◽  
Author(s):  
Peter J. Nieuwenhuizen ◽  
Sandjai Timal ◽  
Jeroen M. van Veen ◽  
Jaap G. Haasnoot ◽  
Jan Reedijk
Keyword(s):  
Detailed Knowledge ◽  
Cross Link ◽  
Model Compounds ◽  
Zinc Compounds ◽  
Reaction Stage ◽  
Novel Approach ◽  
Cross Links ◽  
Link Formation ◽  
Stage Modeling

Abstract This paper reports a novel approach for the study of the mechanism of accelerated vulcanization, namely, Reaction-Stage Modeling (RSM). By carefully studying the reactivity of relevant model compounds under selected conditions, detailed knowledge about a particular reaction stage of vulcanization can be obtained. Background, experimental details and synthesis of model compounds are described. An RSM study after cross-link formation in the thiuram- and dithiocarbamate type vulcanization has been performed, and the role of zinc compounds herein was investigated. In contrast to earlier studies, it has appeared that at 140 °C, cross-links form from cross-link precursors solely via disproportionation. Allylic substitution was not observed. Zinc compounds act as catalysts for disproportionation, but especially ZDMC can be regarded as an efficient, soluble molecular turntable for sulfur atoms.

Chlorination of Natural Rubber in Solution with Gaseous Chlorine

1953 ◽  
Vol 26 (4) ◽  
pp. 902-911 ◽  
Author(s):  
C. S. Ramakrishnan ◽  
D. Raghunath ◽  
J. B. Pande
Keyword(s):  
Double Bond ◽  
Natural Rubber ◽  
Cyclization Reaction ◽  
Double Bonds ◽  
Chlorine Atoms ◽  
Reaction Stage ◽  
Stage 1 ◽  
Additive Reaction

Abstract The chlorination of rubber solutions by gaseous chlorine was followed by isolating the partially chlorinated products and preparing their ozonides. The ozonides were hydrolyzed, and the acids and aldehydes formed on hydrolysis were determined. By a comparison with the amounts of acids and aldehydes obtained from ozonides of unreacted rubber, the amount of residual isoprenic double bonds present was found. The loss of double bonds attending the introduction of chlorine atoms into the molecule of rubber indicates four definite stages in chlorination : (1) initial substitutive attack by chlorine, with concomitant cyclization, resulting in a loss of one double bond between two isoprenic units, (2) substitution, (3) additive reaction, and (4) essentially substitution. Chlorination of aged rubber solutions differs from the above in that the cyclization reaction (stage 1) seems to be absent.

Infrared Study of Some Structural Changes in Natural Rubber during Vulcanization

1958 ◽  
Vol 31 (4) ◽  
pp. 719-736 ◽  
Author(s):  
Frederic J. Linnig ◽  
James E. Stewart
Keyword(s):  
Double Bond ◽  
Gamma Rays ◽  
Structural Changes ◽  
Elemental Sulfur ◽  
Carboxyl Groups ◽  
Double Bonds ◽  
Infrared Study ◽  
Tetramethylthiuram Disulfide ◽  
Vulcanized Rubber ◽  
Double Bond Shift

Abstract A knowledge of the structure of vulcanized rubber is essential to the interpretation of vulcanization and oxidation studies and the physical properties of the material. In the present work an infrared study has been made of structures resulting from a number of different methods of vulcanization. Sulfur vulcanizates show the presence of a shifted double bond, originally observed by Sheppard and Sutherland. The presence of conjugated double bonds is also indicated. Accelerators such as tetramethylthiuram disulfide and zinc dibutyl dithiocarbamate increase the rate of the double-bond shift and reduce the amount of conjugated double bonds. Neither the double-bond shift nor conjugation is observed as a result of vulcanization with tetramethylthiuram disulfide alone, hydrogen sulfide and sulfur dioxide (Peachey process), a peroxide, or gamma rays. These result in a possible decrease in carbonyl structures, and in the case of the last three, possible increased absorption due to OH and ionized carboxyl groups. Apparently, the double-bond shift and conjugation are primarily phenomena related to the use of elemental sulfur. The other vulcanization systems studied evidently involve different mechanisms. An implication of the present work is that there may be a relationship between the reported ease of oxidation of sulfur vulcanizates, accelerated vulcanizates, and sulfurless vulcanizates (tetramethylthiuram disulfide alone), which decreases in the order named, and the probable amount of conjugation in the compound, which decreases in the same order.

MNDO CI study of the photoisomerization of pentadieniminium

1990 ◽  
Vol 55 (12) ◽  
pp. 2874-2879 ◽  
Author(s):  
Peter Ertl
Keyword(s):  
Schiff Base ◽  
Double Bond ◽  
Configuration Interaction ◽  
Mndo Method ◽  
Double Bonds

Photoisomerization mechanism in model retinal-like protonated Schiff base pentadieniminium was investigated by using MNDO method with configuration interaction. Isomerizations around various double bonds were studied and twisted biradical geometries in S0 and S1 states were optimized. Photoisomerization proceeds exclusively around the central double bond where the twisted S1 state is strongly stabilized and the S0-S1 gap is minimal.

Mechanisms of Double-Bond Isomerization Reactions of n-Butene on Different Lewis Acids

ACS Catalysis ◽  
2021 ◽  
pp. 11293-11304
Author(s):  
Fengjiao Yi ◽  
Peng He ◽  
Huimin Chen ◽  
Yurong He ◽  
Zhichao Tao ◽  
...  
Keyword(s):  
Double Bond ◽  
Lewis Acids ◽  
Double Bond Isomerization

scholarly journals Linear Dynamic Viscoelasticity of Dual Cross-Link Poly(Vinyl Alcohol) Hydrogel with Determined Borate Ion Concentration

Gels ◽  
2021 ◽  
Vol 7 (2) ◽  
pp. 71
Author(s):  
Takuro Taniguchi ◽  
Kenji Urayama
Keyword(s):  
Vinyl Alcohol ◽  
Ion Concentration ◽  
Poly Vinyl Alcohol ◽  
Relaxation Dynamics ◽  
Cross Link ◽  
Dynamic Viscoelasticity ◽  
Linear Dynamic ◽  
Cross Links ◽  
Pva Gel ◽  
Pva Hydrogel

We investigated the linear dynamic viscoelasticity of dual cross-link (DC) poly(vinyl alcohol) (PVA) (DC-PVA) hydrogels with permanent and transient cross-links. The concentrations of incorporated borate ions to form transient cross-links in the DC-PVA hydrogels (CBIN) were determined by the azomethine-H method. The dynamic viscoelasticity of the DC-PVA hydrogel cannot be described by a simple sum of the dynamic viscoelasticity of the PVA gel with the same permanent cross-link concentration and the PVA aqueous solution with the same borate ion concentration (CB = CBIN) as in the DC-PVA gel. The DC-PVA hydrogel exhibited a considerably higher relaxation strength, indicating that the introduction of permanent cross-links into temporary networks increases the number of viscoelastic chains with finite relaxation times. In contrast, the relaxation frequency (ωc) (given by the frequency at the maximum of loss modulus) for the DC-PVA hydrogel was slightly lower but comparable to that for a dilute PVA solution with the same CB. This signifies that the relaxation dynamics of the DC-PVA hydrogels is essentially governed by the lifetime of an interchain transient cross-link (di-diol complex of boron). The effect of permanent cross-linking on the relaxation dynamics was observed in the finite broadening of the relaxation-time distribution in the long time region.

Double bond isomerization of ethyl linoleate and vegetable oils to conjugated derivatives over an LDH supported ruthenium catalyst

RSC Advances ◽  
2015 ◽  
Vol 5 (45) ◽  
pp. 36075-36082 ◽  
Author(s):  
Sivashunmugam Sankaranarayanan ◽  
Gobi Selvam ◽  
Kannan Srinivasan
Keyword(s):  
Double Bond ◽  
Vegetable Oils ◽  
Ethyl Linoleate ◽  
Ruthenium Catalyst ◽  
Reaction Conditions ◽  
Double Bond Isomerization

Isomerization of ethyl linoleate and vegetable oils to conjugated derivatives is achieved over an MgAl-LDH supported ruthenium catalyst under mild reaction conditions.

scholarly journals Microtubules and microtubule-associated proteins from the nematode Caenorhabditis elegans: periodic cross-links connect microtubules in vitro.

1986 ◽  
Vol 103 (1) ◽  
pp. 23-31 ◽  
Author(s):  
E J Aamodt ◽  
J G Culotti
Keyword(s):  
Caenorhabditis Elegans ◽  
Apparent Molecular Weight ◽  
Cross Link ◽  
Nematode Caenorhabditis Elegans ◽  
Microtubule Associated Proteins ◽  
C Elegans ◽  
Associated Proteins ◽  
Cross Links ◽  
The Cross

The nematode Caenorhabditis elegans should be an excellent model system in which to study the role of microtubules in mitosis, embryogenesis, morphogenesis, and nerve function. It may be studied by the use of biochemical, genetic, molecular biological, and cell biological approaches. We have purified microtubules and microtubule-associated proteins (MAPs) from C. elegans by the use of the anti-tumor drug taxol (Vallee, R. B., 1982, J. Cell Biol., 92:435-44). Approximately 0.2 mg of microtubules and 0.03 mg of MAPs were isolated from each gram of C. elegans. The C. elegans microtubules were smaller in diameter than bovine microtubules assembled in vitro in the same buffer. They contained primarily 9-11 protofilaments, while the bovine microtubules contained 13 protofilaments. The principal MAP had an apparent molecular weight of 32,000 and the minor MAPs were 30,000, 45,000, 47,000, 50,000, 57,000, and 100,000-110,000 mol wt as determined by SDS-gel electrophoresis. The microtubules were observed, by electron microscopy of negatively stained preparations, to be connected by stretches of highly periodic cross-links. The cross-links connected the adjacent protofilaments of aligned microtubules, and occurred at a frequency of one cross-link every 7.7 +/- 0.9 nm, or one cross-link per tubulin dimer along the protofilament. The cross-links were removed when the MAPs were extracted from the microtubules with 0.4 M NaCl. The cross-links then re-formed when the microtubules and the MAPs were recombined in a low salt buffer. These results strongly suggest that the cross-links are composed of MAPs.

9, 13-dicis-Rhodopsin and its one-photon-one-double-bond isomerization

Biochemistry ◽  
1988 ◽  
Vol 27 (17) ◽  
pp. 6495-6499 ◽  
Author(s):  
Yoshinori Shichida ◽  
Katsuki Nakamura ◽  
Toru Yoshizawa ◽  
Achla Trehan ◽  
Marlene Denny ◽  
...  
Keyword(s):  
Double Bond ◽  
Double Bond Isomerization

EXPERIMENTAL VALIDATION OF THE CHARLESBY AND HORIKX MODELS APPLIED TO DE-VULCANIZATION OF SULFUR AND PEROXIDE VULCANIZATES OF NR AND EPDM

2016 ◽  
Vol 89 (4) ◽  
pp. 671-688 ◽  
Author(s):  
M. A. L. Verbruggen ◽  
L. van der Does ◽  
W. K. Dierkes ◽  
J. W. M. Noordermeer
Keyword(s):  
Experimental Validation ◽  
Main Chain ◽  
Sol Gel ◽  
Chain Scission ◽  
Cross Linking ◽  
Cross Link ◽  
Practical Reality ◽  
Cross Links ◽  
The Cross ◽  
Theoretical Considerations

ABSTRACT The theoretical model developed by Charlesby to quantify the balance between cross-links creation of polymers and chain scission during radiation cross-linking and further modifications by Horikx to describe network breakdown from aging were merged to characterize the balance of both types of scission on the development of the sol content during de-vulcanization of rubber networks. There are, however, disturbing factors in these theoretical considerations vis-à-vis practical reality. Sulfur- and peroxide-cured NR and EPDM vulcanizates were de-vulcanized under conditions of selective cross-link and random main-chain scissions. Cross-link scission was obtained using thiol-amine reagents for selective cleavage of sulfur cross-links. Random main-chain scission was achieved by heating peroxide vulcanizates of NR with diphenyldisulfide, a method commonly employed for NR reclaiming. An important factor in the analyses of these experiments is the cross-linking index. Its value must be calculated using the sol fraction of the cross-linked network before de-vulcanization to obtain reliable results. The values for the cross-linking index calculated with sol-gel data before de-vulcanization appear to fit the experimentally determined modes of network scission during de-vulcanization very well. This study confirms that the treatment of de-vulcanization data with the merged Charlesby and Horikx models can be used satisfactorily to characterize the de-vulcanization of NR and EPDM vulcanizates.

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