Formation of anomalous structures in PVC and their influence on the thermal stability: 2. Branch structures and tertiary chlorine

Branch structures were determined in fractions of a commercial suspension of PVC (S-PVC) and experimental PVC samples obtained at subsaturation conditions (U-PVC). The analyses were performed with ¹³C n.m.r. spectroscopy at 50.31 MHz after reductive dehalogenation with tributyltinhydride. With increasing monomer starvation U-PVC was found to have an increasing amount of butyl and long chain branches (LCB). A polymer prepared at 55°C and 59% of the saturation pressure of vinylchloride had 3.4 butyl branches and 2.0 LCB per 1000 monomer units. In the S-PVC series the total content of these two structures varied between 0.5 and 1.0 per 1000 monomer units. By using tributyltindeuteride as reducing agent the structure of the butyl branches could be determined as ～CHClCH₂CCl(CH₂CHClCH₂CH₂Cl)CH₂CHCl～. A major part of the LCB points also contained tertiary chlorine. The formation of LCB is suggested as occurring after abstraction of hydrogen from the polymer chain by macroradicals and chlorine atoms. The latter will lead to LCB points with tertiary hydrogen and internal double bonds. The rate of dehydrochlorination at 190°C in nitrogen could be related to the amount of tertiary chlorine (correction coefficient=0.97). It was assumed that tertiary chlorine is the most important labile structure in PVC.