EXPERIMENTAL INVESTIGATION OF VINYL-CHLORIDE POLYMERIZATION AT HIGH CONVERSION - POLYMER MICROSTRUCTURE AND THERMAL-STABILITY AND THEIR RELATIONSHIP TO POLYMERIZATION CONDITIONS

Suspension poly(vinyl chloride) (PVC) samples with various monomer con versions were synthesized using batch and semi-batch polymerization pr ocesses at different temperatures. The concentration of tertiary chlor ines in the PVC was determined using C-13 nuclear magnetic resonance, and the concentration of internal double bonds was analysed by ozonoly sis. The dehydrochlorination rate of the PVC was measured at 190-degre es-C by the conductimetric method under nitrogen environment. It was f ound that the concentration of tertiary chlorines increases significan tly with increase in monomer conversion after the critical conversion at which the reactor pressure starts to fall. At the same conversion l evel, it increases with polymerization temperature. The concentration of allylic chlorines is much lower than that of tertiary chlorines. A conversion dependence of allylic chlorines was not found for the prese nt PVC samples. However, the concentration of allylic chlorines increa ses with polymerization temperature. Excellent correlation between the dehydrochlorination rate and the concentration of tertiary chlorines was found based on the present experimental data, but no significant r elationship was found between dehydrochlorination rate and concentrati on of allylic chlorines. Therefore, tertiary chlorines in PVC chains a re probably the defect structure that is most responsible for the redu ced thermal stability of PVC. The mechanisms of formation of internal defect structures and the effects of polymerization conditions on the concentration of the defect structures are discussed in some detail ba sed on diffusion-controlled free-radical polymerization theory, and th e monomer concentration effects are further confirmed by the results u sing a semi-batch polymerization process. The concentration of tertiar y chlorines can be minimized by semi-batch operation at or near the sa turation pressure, providing PVC of higher thermal stability.