LOW-VALENT METALS AS REDUCTIVE CROSS-LINKING AGENTS - A NEW STRATEGY FOR SMOKE SUPPRESSION OF POLY(VINYL CHLORIDE)

Several types of additives that contain transition metals can promote the cross-linking of poly(vinyl chloride) (PVC) by a mechanism that ap parently involves reductive coupling of the polymer chains. In solid P VC, the cross-linking occurs at 200 degrees C, and model-compound expe riments show that it can be ascribed to the preferential reductive cou pling of allylic chloride structures when the coupling agent is Cu(O). However, the concurrent coupling of other chloride moieties has not b een entirely ruled out. The evidence for reductive coupling consists o f rapid gel formation accompanied by, substantial reductions (or minor changes) in the rates of total mass loss (as determined by thermograv imetric analysis), C-C formation (as observed by Fourier transform IR spectroscopy), and HCl evolution (as determined by acid-base titrimetr y). Additives that promote the coupling process are sources of a zero- or low-valent metal upon pyrolysis. These additives include a number o f transition-metal carbonyls, divalent formates or oxalates of the lat e transition metals, simple Cu(I) halides, and various complexes of Cu (I) containing phosphites or other ligands. Since the reductive coupli ng agents tend to have low acidities, they are not expected to promote the cationic cracking of char. Thus they are potentially attractive a s replacements for the PVC smoke suppressants that stimulate cross-lin king by acting as Lewis acids.