POLARON MECHANISM IN THE THERMAL STABILIZATION OF POLY(VINYL CHLORIDE) .2. ACTION OF METAL CHLORIDES

By polaron theory, it is shown that the mechanism of dehydrochlorination (DHC) of PVC is determined by the interaction between the conjugated double bonds in the polymer and the metal or organometallic chlorides. Lewis acid compounds, such as metal chloride, oxidize the pi electrons of conjugated polyene sequences (CjPS) into polarons, which leads to accelerated chain DHC, By contrast, organotinchlorides can significantly delay DHC, The difference can be explained by the effect of the alkyl chains in the organotin compounds. It has been demonstrated that, by controlling the cis configuration of CjPS existing initially or developing in the polymer during the heat treatment, and by lowering the Lewis acidity of the organotin chlorides, the alkyl chains attached to the central metal atom contribute their stabilization effect by inhibiting the proliferation of polarons or by favoring the readdition reaction of HCl to conjugated double bonds and the cyclization of the polymer chain.