William H. Starnes

Structural and mechanistic aspects of the thermal degradation of poly(vinyl chloride)

Abstract A critical review of the title subject supports the following major conclusions. Thermal dehydrochlorination of poly(vinyl chloride) (PVC) begins with internal allylic chloride and tertiary chloride structural defects formed during polymerization. The tertiary chloride is associated with 2,4-dichloro-n-butyl, 1,3-di(2-chloroethyl), and chlorinated long branches. Mechanisms for the formation of all of the labile defects are well established. ‘Carbonylallyl’ structures and certain isotactic conformers of ordinary monomer units are unimportant as initiators of thermal dehydrochlorination. Both the initiation and the subsequent formation of conjugated polyene sequences occur via carbenium chloride ion pairs or by a closely related concerted four-center quasi-ionic route. Six-center concerted processes, pathways involving free radicals, and other mechanistic schemes suggested recently are not involved in polyene elongation. However, during thermal degradation, ordinary monomer units are converted into internal allylic chloride defects by a mechanism that may include the abstraction of hydrogen by triplet cation diradicals derived from polyene intermediates. Cyclization reactions seem likely to contribute to the termination of polyene growth. When PVC is thermolyzed in blends with other polymers, unusual kinetic phenomena are detected that remain to be fully explained.

Anomalous behavior of molybdenum oxide as a fire retardant for polyvinyl chloride

Abstract The widespread application of polyvinyl chloride (PVC) as wire and cable insulation in the communication and electrical industries is due in part to its inherent fire retardant properties, which are improved by the addition of antimony oxide. This addition also increases somewhat the already high rate of smoke formation. Molybdenum trioxide, used either with or instead of Sb 2 O 3 , has been reported to be a smoke suppressant as well as a flame retardant. This is corroborated by laboratory-scale measurements reported here. However, in a large-scale test under high enthalpy input conditions, MoO 3 appeared to be ineffective in limiting the flame spread. A detailed investigation of the pyrolysis of PVC by thermogravimetric analysis and mass spectroscopy showed that the addition of MoO 3 results in the reduction of the temperature at which HCl is evolved, an increase in the temperature at which the organic pyrolyzate is produced, and a change in compostion of this pyrolyzate from aromatic to aliphatic. The underlying chemical mechanism is explained by the action of MoO 3 as a Lewis acid: 1. 1. The dehydrochlorination of PVC is catalyzed and occurs at a lower temperature; 2. 2. This process yields trans polyenes, either directly or via rapid isomerization of cis polyenes; 3. 3. The trans polyenes, being unable to cyclize and split off benzene, are stable to higher temperatures, at which a different mechanism obtains to give aliphatic products. The ineffectiveness of MoO 3 is ascribed to the difference in flame characteristics between aliphatic and aromatic fuels. The disagreement between laboratory and large-scale tests is attributed to the higher temperature at which the aliphatic pyrolyzate appears, which is reached only under conditions of higher enthalpy input that prevail in the large-scale tests.

Mechanism of Poly(Vinyl Chloride) Fire Retardance by Molybdenum(VI) Oxide. Further Evidence in Favor of the Lewis Acid Theory

Many recent investigations have been concerned with the mechanism of action of MoO3 as a smoke-suppressant and fire-retardant additive for poly(vinyl chloride) (PVC).1–14 These studies have shown that Mo03 functions within the polymer matrix1–14 and retards the production of the volatile aromatics (especially benzene) that are the principal fuel and source of smoke under low-enthalpy-input conditions.1–4,6–8,13,14 However, no consensus of opinion has been reached with regard to the nature of the chemical reactions that cause the aromatics suppression.

New insights into the flame-retardance chemistry of poly(vinyl chloride)

Summary Figure 3 is a flow chart of our present conception of the salient processes in the pyrolysis and combustion of PVC. From our theoretical and experimental investigations which we have summarized in this paper, we are convinced that many of the major reactions can occur by mechanisms which are ionic rather than radical in nature, and that these reactions are therefore particularly susceptible to modification by Lewis or Bronsted acid catalysts. Figure 4 shows the effects such catalysts would have on the major pathways in the mechanism. All the data on the effects observed with molybdenum trioxide or ferric oxide 5 additives, for example, are consistent with this point of view, and we believe that they strongly support our present theory.

SOME ASPECTS OF CONCERTED REACTIONS IN ORGANOMETALLIC SYSTEMS

After a considerable amount of effort, there now exists a good understanding of concerted reactions in organic chemistry; however this is not yet the case in the area of organometallic chemistry, where the problem is greatly complicated by the presence of the metal, even more so by the widely differing natures of the various metals. The present report deals with results from our laboratory in which we have tried to bring some understanding to several reactions in which transition metals are involved. In organic chemistry, concerted reactions can be placed into two large categories: (a) reactions involving internal reorganization and (b) intermolecular reaction. These can be depicted as follows:

2,4,6-Tri-t-butylresorcinol and its diketo-tautomer from thermolysis of a 3-hydroxy-4-vinylcyclobut-2-enone

The products of the thermal rearrangement of 3-hydroxy-4-(3,3-dimethyl-trans-but-1-enyl)-2,4-di-t-butylcyclobut-2-enone are 2,4,6-tri-t-butylresorcinol and its diketo-tautomer.

A cyclobutenone from photolysis of a cyclohexa-2,5-dienone

A new type of photoproduct, a vinylcyclobutenone, has been isolated from the photolysis of a cyclohexa-2,5-dienone.