Formation of Anomalous Structures in PVC and Their Influence on Thermal Stability. I. Endgroup Structures and Labile Chlorine Substituted by Phenol

Abstract

Abstract The thermal stability and structural details were compared for fractions of S-PVC and a series of “U-PVC” prepared at sub-saturation conditions. Molecular weights and long chain branching were obtained by GPC and viscometry. “Labile chlorine” was determined by phenolysis, double bonds by bromination, and endgroup structure by NMR. The rate of thermal dehydrochlorination was determined in nitrogen at 190°C. In S-PVC there are few tertiary chlorine, and phenolysis therefore measures allylic chlorine. By NMR it was found that ∼CH2-CH[dbnd]CH[sbnd]CH2Cl is the main unsaturated structure. Phenolysis and bromination therefore mainly give a measure of this group. The main saturated chain end is ∼CHCl[sbnd]CH2Cl. These groups account for 80% of the endgoups and are the result of the mechanism of chain transfer to monomer: head-to-head addition followed by 1,2-Cl migration and Cl· elimination. They have only a weak influence on the thermal stability, however. U-PVC, on the other hand, shows a strong relation between phenolysis and thermal stability. By NMR it was shown that the relative content of 1-chloro-2-olefin structures is considerably lower than in S-PVC although the total unsaturation and the relative amount of 1,2-dichloro ends are similar. 1,3-Dichloro ends were also found. The observed structural differences cannot possibly account for the lower thermal stability of U-PVC. They are, however, an indication of changes in the reaction pattern due to the monomer starvation. Apparently, other, more labile structures are also formed at sub-saturation conditions.