Kinetics and mechanism of vinyl chloride polymerization: Effects of additives on polymerization rate, molecular weight and defect concentration in the polymer

Poly(vinyl chloride), or PVC, has various defects which limit its processing temperature by lowering thermal stability. Possible structural defects in PVC are short and long chain branches, chloroallyl groups, end groups and head-to-head structures. Some structural defects connect with an active chloride, which easily loses HCl at elevated temperatures. In this thesis, a comprehensive study was performed on the vinyl chloride polymerization in the presence of small amount of organic additives. These additives were weakly basic compounds, high dipole carbonyl compounds, ether compounds and some heteroraromatics. The initial polymerization rates and the molecular weights of the resulting polymers increased in the presence of weakly 'basic' compounds such as dimethyl terephthalate (DMT), ethylene carbonate (EC), gamma-butyrolactone (GBL), tributylphosphine oxide (TBPO) and trimethyl-1,3,5-benzene tricarboxylate (TMB). A kinetic model was developed for vinyl chloride polymerization in the presence of these weakly basic additives, using the hypothesis that a hydrogen-bond complex formed between an additive and the terminal hydrogen of the propagating radical, and that the major termination reaction was between a small radical (possibly Cl• or combined Cl•) and the propagating radical. The kinetic model successfully explained the increase of the polymerization rate and the resulting polymer molecular weights. An optimal additive concentration exists to get maximum molecular weight increase and possibly reducing structural defects of the resulting polymers.; Various methods were applied to evaluate the additive effects on the resulting polymer structures. Differential Scanning Calorimetry (DSC) was used to study the crystallization behavior of the resulting polymers. The thermal stability of the resulting polymers was evaluated by dynamic Thermogravimetric Analysis (TGA) and dehydrochlorination. A 2-parameter model was developed to describe the initial dehydrochlorination of the polymers at 170-200°C with rapid removal of HCl. The additive effect on the thermal stability of the resulting polymers was evaluated. 1D 600 MHz 1H NMR spectroscopy, combined with 2D correlation via homonuclear scalar coupling (COSY), pulsed-field gradient heteronuclear multiple quantum coherence (gHMQC) and heteronuclear multiple-bond correlation (gHMBC) NMR spectroscopy was used to identify and quantify the structural defects of the resulting polymers. Some additives were found to decrease the labile structure concentrations and thus increase the thermal stability of the resulting polymers. A linear correlation relationship was found between the rate of dehydrochlorination and the concentration of labile structural defects in the resulting polymers.