Vinyl Chloride Suspension Polymerization Mediated By Nitroxide Free Radicals

Radical polymerization is one of the major procedures for preparing poly(vinyl chloride)(PVC). It is known that the main drawback of radical polymerization is that the kinetic parameters of the process and the molecular weight characteristics of the resulting polymers are difficult to control because of the high reactivity of radical species. One of the ways to solve the problem is based on the new concept of controlled/living radical polymerization. By performing controlled/living radical polymerization, it is feasible to prepare polymers of definite structure and molecular weight and narrower distribution. Until recently, the main methods of controlled/living radical polymerization are degenerative transfer free radical polymerization, iniferter polymerization, nitroxide mediated polymerization, atom transfer radical polymerization and reversible addition and fragmentation chain transfer. The most feasible one for PVC is nitroxide mediated polymerization.The initiator and dispersant were selected. Then the suspension polymerization of vinyl chloride (VC) was carried out at40～70℃in the composite dispersant system composed of poly(vinyl alcohol) and hydroxypropyl methylcellulose, with nitroxide D as the source of nitroxide radicals and peroxide A as initiator. The effects of the concentration of peroxide A, the concentration of nitroxide D and polymerization temperature on the polymerization kinetics, molecular weight and polydispersity index (PDI) of PVC were investigated in detail. The kinetics of VC suspension polymerization was monitored by gravimetric method while the molecular weight and its distribution of PVC synthesized were determined by gel permeation chromatography (GPC). The results showed that the chain propagation in VC polymerization could be efficiently controlled by adding suitable amount of nitroxide D and peroxide A at a reasonable temperature. The optimal molar ratio of VC, peroxide A and nitroxide D was1000:7:1. As the polymerization temperature increased, the polymerization rate of VC first increased and then decreased, which is different from the mechanism of common radical polymerization. Simultaneously, the molecular weight of synthesized PVC first increased and then decreased, while the PDI first decreased and then increased. The optimal polymerization temperature was50℃. At this temperature, the PDI of PVC was1.7～2.0and did not change significantly in the range of conversion3-50%, which was considerably lower than that in common radical polymerization. However, with the conversion increasing further, the PDI of PVC increased. A kinetic model for polymerization rate of PVC was established based on the hypothesis of equal reactivity, long chain assumption and under the quasi steady state. The initiation efficiency of initiator peroxide A at each temperature was obtained by fitting method with a kinetic model of vinyl chloride suspension polymerization with peroxide mixture as initiator. The spin trapping rate constant (kst), dormant species deactivation rate constant (kdea) and dormant species activation rate constant (ka) were estimated by fitting method. The effect of temperature on kst, kdea and ka were also investigated. It was in fairly good agreement with the experimental data.