The kinetics of the emulsifier free seeded polymerization of butadiene at 60 O C in SmithEwart interval I11 was investigated using sodium peroxodisulfate as initiator. The aim of this work was to measure the propagation rate coefficient (k,) of butadiene at 60 "C in emulsion polymerization. Modelindependent techniques to measure propagation rate coefficients like spatially intermitted polymerizationsl-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 tertdodecylmercaptan, as is usual in industrial practice. The fractional conversion was b d on gravimetrically calibrated on-line densitometry and was found to be highly accurate. By analogy with the well-known Ugeletad plots, the product of the propagation rate coefficient (k,) and the average number of radicale per particle (e) versus seed latex particle diameter clearly shows Smith-Ewart case I and case I1 regimes. From a constancy in values of kpR (case I1 regime) in this plot, a value for k, 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 111. 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,,, could be estimated.

The kinetics of the emulsion polymerization of butadiene at 60 "C in Smith-Ewart interval I11 were investigated using sodium peroxodisulfate (persulfate) as initiator. The aim of this work was to elucidate and model the persulfate initiator efficiency. Monomer conversion was based on gravimetrically calibrated on-line densitometry. Plots of the product of the propagation rate coefficient, k,, and the average number of radicals per particle, a, versus initiator concentration showed minimal dependence on the initiator concentration if tert-dodecyl mercaptan (TDM) was present during polymerization. These results could not be modeled using normal emulsion polymerization theory. The model used in the absence of TDM fits two separate sets of data, a versus particle diameter and a versus initiator concentration. In the absence of TDM, a depends strongly on the initiator concentration. The significant effect of TDM is ascribed to a decrease in the radical desorption. A possible effect on the radical absorption is refuted on the basis of theory and experiment. IntroductionImportant papers on the butadiene emulsion polymerization have been published as a result of the synthetic rubber program' by Kolthoff et aL2 and by Morton et aZ

SUMMARYThe observation that b,utadiene emulsion polymerizations in the presence of fatty acid emulsifiers need minimal amounts of tertiary or n-dodecanethiols to polymerize at a reasonable rate is often referred to as the promoting effect of mercaptans and is evaluated in this paper.Experimental evidence is presented which shows that fatty acid emulsifiers can actively reduce the average number of radicals per particle. In this paper it will be shown that three components are necessary for retardation of the rate of polymerization in the absence of dodecanethiol to occur: I ) Only diene monomer polymerizations show retardation. 2) Only peroxodisulfate-initiated polymerizations show retardation. 3) Retardation of the rate of polymerization only occurs in the presence of fatty acid emulsifiers. These three components are combined for the first time in a reaction scheme which is an extension of a reaction scheme proposed by Kolthoff in 195 1. Experimental evidence justifies the suggestion that reactions between a fatty acid radical and butadiene play an important role in the promoting effect.

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