“…Model-compound reactions in the presence of MoO 3 led to rationalizations for these observations. , Molybdenum(VI)-promoted alkene isomerization and alkylation dominated at 200 °C. However, at higher temperatures, cationic cracking reactions also occurred, causing fragmentation of the models and their alkene products.…”
mentioning
confidence: 91%
“…Many of those that appear to function primarily in the condensed phase are transition-metal compounds, particularly oxides and chlorides. Molybdenum(VI) oxide has received particular scrutiny with respect to its mode of smoke suppression. ,,− On the basis of evidence from isotopic labeling studies and the behavior of small-molecule PVC models, a series of Lewis acid promoted pathways has been proposed for the thermal decomposition of PVC in the presence of MoO 3 or MoO 2 Cl 2 (a product formed from MoO 3 and HCl). , These routes are summarized in Scheme . 1 …”
mentioning
confidence: 99%
“…In 1981 Lattimer and Kroenke 5 suggested that reductive coupling might account for the smoke-suppression activity of MoO 3 in PVC. This possibility was shown to be unimportant, however, by later work which revealed that MoO 3 behaves primarily as a Lewis acid coupling agent. − There are a number of reasons why this additive is an unlikely reductive coupling promoter. As was noted already, reductive coupling should inhibit PVC dehydrochlorination.…”
Several types of additives that contain transition
metals can promote the cross-linking of
poly(vinyl chloride) (PVC) by a mechanism that apparently involves
reductive coupling of the polymer
chains. In solid PVC, the cross-linking occurs at 200 °C, and
model-compound experiments show that it
can be ascribed to the preferential reductive coupling of allylic
chloride structures when the coupling
agent is Cu(0). However, the concurrent coupling of other
chloride moieties has not been entirely ruled
out. The evidence for reductive coupling consists of rapid gel
formation accompanied by substantial
reductions (or minor changes) in the rates of total mass loss (as
determined by thermogravimetric analysis),
CC formation (as observed by Fourier transform IR spectroscopy), and
HCl evolution (as determined
by acid−base titrimetry). Additives that promote the coupling
process are sources of a zero- or low-valent metal upon pyrolysis. These additives include a number of
transition-metal carbonyls, divalent
formates or oxalates of the late transition metals, simple Cu(I)
halides, and various complexes of Cu(I)
containing phosphites or other ligands. Since the reductive
coupling agents tend to have low acidities,
they are not expected to promote the cationic cracking of char.
Thus they are potentially attractive as
replacements for the PVC smoke suppressants that stimulate
cross-linking by acting as Lewis acids.
“…Model-compound reactions in the presence of MoO 3 led to rationalizations for these observations. , Molybdenum(VI)-promoted alkene isomerization and alkylation dominated at 200 °C. However, at higher temperatures, cationic cracking reactions also occurred, causing fragmentation of the models and their alkene products.…”
mentioning
confidence: 91%
“…Many of those that appear to function primarily in the condensed phase are transition-metal compounds, particularly oxides and chlorides. Molybdenum(VI) oxide has received particular scrutiny with respect to its mode of smoke suppression. ,,− On the basis of evidence from isotopic labeling studies and the behavior of small-molecule PVC models, a series of Lewis acid promoted pathways has been proposed for the thermal decomposition of PVC in the presence of MoO 3 or MoO 2 Cl 2 (a product formed from MoO 3 and HCl). , These routes are summarized in Scheme . 1 …”
mentioning
confidence: 99%
“…In 1981 Lattimer and Kroenke 5 suggested that reductive coupling might account for the smoke-suppression activity of MoO 3 in PVC. This possibility was shown to be unimportant, however, by later work which revealed that MoO 3 behaves primarily as a Lewis acid coupling agent. − There are a number of reasons why this additive is an unlikely reductive coupling promoter. As was noted already, reductive coupling should inhibit PVC dehydrochlorination.…”
Several types of additives that contain transition
metals can promote the cross-linking of
poly(vinyl chloride) (PVC) by a mechanism that apparently involves
reductive coupling of the polymer
chains. In solid PVC, the cross-linking occurs at 200 °C, and
model-compound experiments show that it
can be ascribed to the preferential reductive coupling of allylic
chloride structures when the coupling
agent is Cu(0). However, the concurrent coupling of other
chloride moieties has not been entirely ruled
out. The evidence for reductive coupling consists of rapid gel
formation accompanied by substantial
reductions (or minor changes) in the rates of total mass loss (as
determined by thermogravimetric analysis),
CC formation (as observed by Fourier transform IR spectroscopy), and
HCl evolution (as determined
by acid−base titrimetry). Additives that promote the coupling
process are sources of a zero- or low-valent metal upon pyrolysis. These additives include a number of
transition-metal carbonyls, divalent
formates or oxalates of the late transition metals, simple Cu(I)
halides, and various complexes of Cu(I)
containing phosphites or other ligands. Since the reductive
coupling agents tend to have low acidities,
they are not expected to promote the cationic cracking of char.
Thus they are potentially attractive as
replacements for the PVC smoke suppressants that stimulate
cross-linking by acting as Lewis acids.
“…The additives that cause crosslinking act within the polymer matrix, and in the case of poly(vinyl chloride) (PVC), many of them are known to function there as actual or incipient Lewis-acid catalysts, in keeping with a mechanism first suggested some 30 years ago [1]. This mechanism involves several reactions that generate crosslinks [1][2][3], one of which is the Friedel-Crafts alkylation of alkene linkages that result from PVC thermolysis (Eq. 1).…”
Section: Introductionmentioning
confidence: 99%
“…1). However, strong Lewis acids suffer from a major disadvantage at the very high temperatures reached in large fires, which is their tendency to promote the cationic cracking of the crosslinked char into volatile aliphatic fragments [2] that are superlative fuels [2,4]. In an attempt to solve the problem of char cracking, less acidic additives that act as reductive crosslinking agents were developed at the College of William and Mary.…”
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
