SEEDED EMULSION POLYMERIZATION OF BUTADIENE .1. THE PROPAGATION RATE COEFFICIENT

The kinetics of the emulsifier free seeded polymerization of butadiene at 60-degrees-C in Smith-Ewart interval Ill was investigated using sodium peroxodisulfate as initiator. The aim of this work was to measure the propagation rate coefficient (k(p)) of butadiene at 60-degrees-C in emulsion polymerization. Model-independent techniques to measure propagation rate coefficients like spatially intermitted polymerizations1-3 (''rotating sector'' and ''laser flash'' photolysis) have not been used for butadiene so far since network formation prevents acquiring the necessary data from GPC. All experiments were conducted in the presence of tert-dodecylmercaptan, as is usual in industrial practice. The fractional conversion was based on gravimetrically calibrated on-line densitometry and was found to be highly accurate. By analogy with the well-known Ugelstad plots, the product of the propagation rate coefficient (k(p)) and the average number of radicals per particle (nBAR) versus seed latex particle diameter clearly shows Smith-Ewart caseI and case II regimes. From a constancy in values of k(p)nBAR (case II regime) in this plot, a value for k(p) could be calculated that was 3 times larger than the current literature value. It was found that negligible ''thermal background initiation'' is present in the butadiene system. Two linear regions in polymerization rate are observed in interval III. Model calculations are presented in excellent agreement with the experimental data. From these calculations a value of the rate coefficient for transfer to monomer, k(tr) could be estimated.