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.

Polybutadiene latexes made in emuIsifier-free emulsion polymerization with diameters ranging from 50 to 300 nm of both unimodal and bimodal particles size distributions were analyzed by the line-start (LIST) method in a Brookhaven Disk Centrifuge Photosedimentometer (DCP). A special spin fluid was designed to be able to sediment polybutadiene since the density of the polymer is 0.89 gcm-a and is thus less dense than its suspending medimn. Density and viscosity gradients were created simply by adding five different mixtures of ethanol, water, and emulsifier in density sequence to the spinning disk. Coagulation problems caused by diluting polybutadiene latices with ethanol were overcome by using nonionic Triton X-100 surfactant. Good agreement in the average particle size and distribution as well as polydispersity between transmission electron microscope and disk centrifuge data was accomplished. The analysis time for polybutadiene latex particle sizing thus was reduced from several days to approximately 1 h.

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

SUMMARY:The kinetics of the emulsion polymerization of butadiene at 60.C in Smith-Ewart interval 111 were investigated using gamma radiation to produce initiating species. The aim of this work was to determine radical desorption rate coefficients through monitoring non-steady state kinetics. The acquired data of the average number of radicals per particle (ri) were in good agreement with data obtained from chemically initiated experiments. It was shown that the approach to steady state is kinetically determined and that thermal background initiation is negligible in the butadiene emulsion polymerization. While radical desorption rate coefficient data was determined with high uncertainty, the results are consistent with the transfer diffusion mechanism of radical exit.

The four important factors that determine the rate of emulsion polymerization are the propagation rate coefficient, the latex-particle concentration, the monomer concentration in the latex particles, and the free-radical concentration in the latex particles. Both theoretical considerations and experimental evidence suggest that the important factors that may reduce the rate of emulsion polymerization at high conversion are the propagation rate coefficient and the monomer concentration in the latex particles, and not the free-radical concentration in the latex particle, if the initiator is not depleted. Various approaches for increasing the rate of emulsion polymerization at high conversion are suggested.