Molecular weight distribution of commercial PVC

The molecular weight distribution (MWD) of commercial suspension grade poly(vinyl chloride) (PVC) resins with K values from 50 to 93 and mass grade WC resins with K values from 58 to 68 has been determined by size exclusion chromatography (SEC), using literature Mark-Houwink coefficients. The MWD is characterized by the number average molecular weight (M-n), the weight average molecular weight (M-w) and the polydispersity (M-w/M-n). Our results for M-w are consistent with recently published data, but we find different results for M-n and consequently for M-w/M-n. The polydispersity of PVC increases with increasing K value. This effect can be explained by two mechanisms. The first mechanism is a reduced terminating reaction rate between two growing polymer chains (disproportionation) at:higher molecular weight owing to the reduced mobility of the polymer chains. The second mechanism is long-chain branching of molecules with high molecular weight which lets the molecules grow at two ends. For two examples graphs of the measured MWD are compared with the theoretically expected MWD.