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First Emulsion Polymerization of Styrene with Sodium Borohydride: Evidence of the Generation of Radical Intermediates by Sodium Borohydride in H2O Youngchan Jang,* Dai Seung Choi, and Jong Geun Kim Kumho Petrochemical R & D Center, Korea Kumho Petrochemical Company, Limited, P.O. Box 64, Yuseong, Taejon 305-600, South Korea Received January 29, 2004. In Final Form: May 19, 2004 Emulsion polymerization of styrene with sodium borohydride (NaBH4) in an aqueous sodium dodecyl sulfate (SDS) solution was successfully accomplished for the first time. Polystyrene with a high molecular weight (Mw > 2 000 000) and a broad molecular weight distribution (MWD ≈ 3.5) was obtained in a conversion of less than 30%. Several pieces of evidence that the polymerization proceeded through radical intermediates were observed. Variations in the concentration of NaBH4 showed a critical range in said concentration, i.e., a borderline that determined whether the main reaction was directed to either a polymerization or a competed reaction with variations in the NaBH4 level. Kinetic studies on the emulsion polymerization of styrene with NaBH4 performed at 50, 55, and 60 °C showed that the initiator had an approximately 50-min induction period. A plot of -ln(1 - X), where X is the fractional conversion, as a function of time resulted in a linear relationship, showing that the present initiator system followed first-order kinetics with respect to monomer concentration. The Arrhenius plot between ln k vs 1/T gave a good linear relation, and the overall activation energy was observed to be about 37.5 kcal/mol. The employment of CH3I with NaBH4 significantly increased conversion (>95%) and provided polystyrene with a well-controlled Mw and MWD ( [NaBH4], the radical species (•Q) acted as an active initiator for polymerization. A new •Q was produced continually through the reaction of NaBH4 with O2. On the other hand, when [O2] < [NaBH4], •Q was seriously destroyed by NaBH4, shown in eq 2, even though the reaction between NaBH4 and O2 produced a new •Q. In the case of the latter, an increase in the concentration of NaBH4 accelerated the reaction in eq 2 and reduced the level of the active species (•Q), resulting in lower conversion. This obviously showed why an increase in the concentration of NaBH4 lowered conversion. The Mw of polystyrene also decreased with an
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Figure 2. Effects of variation in the concentration of SDS on conversion: [NaBH4] ) 1.06 mmol; [styrene] ) 9.09 g; deionized H2O ) 50 mL; polymerization temperature ) 50 °C; polymerization time ) 3 h.
increase in the level of NaBH4. This showed that the chain termination reaction between the growing polymer chain and the hydrogen radical generated from BH4- occurred more frequently within the concentration range of NaBH4.
In an effort to see if this trend was present in the other concentration ranges of NaBH4, a series of polymerizations was performed in which the concentration of NaBH4 was kept below 0.32 mmol (Figure 1b). Surprisingly, conversion increased with an increase in the concentration of NaBH4 from 0.053 to 0.32 mmol. A quantitative conversion was finally obtained when the concentration of NaBH4 was 0.32 mmol. This clearly revealed that the increase in the concentration of NaBH4 was favorable to polymerization (eq 3) when the NaBH4 concentration was below the specified level.17 The Mw of polystyrene also increased with an increase in the level of NaBH4, showing that the chain termination reaction did not effectively occur within the concentration range of NaBH4. Evidently there was a critical range in the concentration of NaBH4 which was a borderline18 that determined whether the main reaction was directed to either eq 2 or eq 3 with variations in the level of NaBH4. Variations in the concentration of SDS at a constant level of NaBH4 affected the conversion as well (Figure 2). The conversion increased with increasing the concentration of SDS.19 (17) This was presumably the case when [O2] > [NaBH4], where NaBH4 mainly reacted with O2 to produce the radical species (•Q). (18) As mentioned above, the critical range in the concentration of NaBH4 should be related to the level of O2, although it was difficult to represent the borderline for the mole ratio of the two due to the lack of information on the level of O2. (19) Encyclopedia of Polymer Science and Technology; Mark, H. F., Gaylord, N. G., Bikales, N. M., Eds.; Interscience Publishers: New York, London, Sydney, 1966; Vol. 5 (Plastics, Resins, Rubbers and Fibers), pp 802-809.
Figure 3. Plots of (a) polymerization time versus conversion and (b) polymerization time versus -ln(1 - X) in the emulsion polymerization of styrene with NaBH4 in an aqueous SDS solution at 60 (9), 55 (2), and 50 °C (b): [NaBH4] ) 1.06 mmol; [SDS] ) 1.06 mmol; [styrene] ) 9.09 g; deionized H2O ) 50 mL.
Kinetic Studies. To obtain a better understanding of the emulsion polymerization of styrene with NaBH4, kinetic studies were carried out at three different temperatures: 50, 55, and 60 °C. The results are shown in Figure 3. The time conversion curves clearly showed that the initiator system had an approximately 50-min induction period (Figure 3a)), which was another evidence that the polymerization proceeded through a radical mechanism with the help of oxygen. It is a typical phenomenon that a radical polymerization assisted by oxygen has a significant induction period.20-23 Plots of -ln(1 - X), where X is the fractional conversion, as a function of time resulted in linear relationships (Figure 3b)), revealing that the present initiator system follows first-order kinetics with respect to monomer concentration. From the slope of these lines, the polymerization rate constant (k) at each temperature was determined. The Arrhenius plot of ln k versus 1/T gave a good linear relation (Figure 4), and the overall activation energy for the polymerization of styrene was observed to be about 37.5 kcal/mol. This value is quite close to that obtained with a macroinitiator, (20) Ehrlich, P.; Pittilo, R. N. J. Polym. Sci. 1960, 43, 389. (21) Grimsby, F. N.; Gillard, E. R. Ind. Eng. Chem. 1958, 50, 1048. (22) Shoenemann, K. Chem. Eng. Sci. 1963, 18, 565. (23) Tatsukami, Y.; Takahashi, T.; Yoshioka, H. Makromol. Chem. 1980, 181, 1107.
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Figure 4. Arrhenius plot for the polymerization of styrene with an initiator of NaBH4 in aqueous SDS solution at 50-60 °C: [NaBH4] ) 1.06 mmol; [SDS] ) 1.06 mmol; [styrene] ) 9.09 g; deionized H2O ) 50 mL.
4-(4′-vinylphenylmethoxy)-2,2,6,6-tetramethyl-1-piperidynyloxy (STEMPO), in the polymerization of styrene, where the value was found to be about 39.7 kcal/mol.24 Study of the Effects of Alkyl Iodide on Polymerization. The observed evidence of the presence of radical intermediates led us to modify the initiator system to see if the in situ generation of more active radical intermediates could improve conversion. The effect of CH3I on polymerization is shown in entries 7-9 (Table 1). Employment of CH3I significantly enhanced conversion and provided polystyrene with a narrower MWD below 2.30. The methyl radical25 formed by the reaction of CH3I with the borane radical anion (BH3•-) would probably cause a significant enhancement in conversion and a narrower MWD, possibly due to its higher activity and better homogeneity than those of the radical intermediate (•Q). (24) The significance of the present initiator system is that the polymerization of styrene can be performed with easily accessible initiator, NaBH4, to obtain similar activity to that of the polymerization performed with the macroinitiator, which is not as readily available as NaBH4. For details, see: Tao, Y.; He, J.; Wang, Z.; Pan, J.; Jiang, H.; Chen, S.; Yang, Y. Macromolecules 2001, 34, 4742. (25) As the benzyl radical (ArCH2•) was formed by the reaction of ArCH2Br with the borane radical anion (BH3•-) (Scheme 1), the methyl radical would be formed by the analogous reaction when using CH3I and NaBH4 for the polymerization. The methyl radical would subsequently act as an initiator, although some side reactions to form both CH4 and C2H6 could not be excluded. At this stage, both the methyl radical and the radical intermediate •Q were expected to compete with each other as initiator, but the observation of a higher conversion and a narrower MWD upon employment of CH3I showed that the polymerization was mainly induced by the methyl radical.
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Figure 5. Effects of variation in the size of alkyl group in alkyl iodide on conversion in the emulsion polymerization of styrene with NaBH4 in an aqueous SDS solution at 50 °C: polymerization time ) 3 h; deionized H2O ) 50 mL; styrene ) 9.09 g; [NaBH4] ) 1.06 mmol; [NaBH4]:[SDS]:[RI] ) 1:1:1.
An incremental increase in the level of CH3I provided polystyrene with a lower Mw, showing that the extra CH3I acted as a chain transfer agent. The effect of the variations in the size of the alkyl group on conversion is shown in Figure 5. The conversion decreased with an increase in the size of the alkyl group in alkyl iodide, apparently due to the steric effect of alkyl radical as an initiator. Ultimately, the conversion became similar to that of the polymerization performed in the absence of alkyl iodide when the alkyl group contained more than six carbon atoms. Use of allyl iodide and benzyl bromide rather decreased conversions to below 40%, evidently due to the higher stability of allyl and benzyl radicals. Conclusion Emulsion polymerization of styrene with NaBH4 in an aqueous SDS solution was accomplished for the first time in this study. Several pieces of evidence that the polymerization proceeded through radical intermediates were observed. Oxygen was proposed to play a role, in combination with NaBH4, for the generation of the active radical species. Further studies on the mechanistic aspects and polymerizations of other polar olefin monomers with this initiator are underway. LA040012O
