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References
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For other examples of stereospecific radical polymerization, see the references (No. 2) cited in ref 3.
We became aware that the effects of reaction conditions on the stereochemistry of TrMA polymerization had been reported focusing on the solvent effect in a non-refereed periodical in Japanese language with English abstract after we completed the present study.7 However, in the report, the authors did not give any clear explanations for the temperature and [M]0 effects and interpreted the solvent effect in terms of solubility parameter of the polymer and the solvents; in addition, the highest mm content achieved in the report was 88%. Our work was performed independently from the aforementioned research.
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Influence of monomer concentration on stereospecificity has been reported for the coordination polymerization of propylene and explained in terms of isomerization (epimerization) of the growing end of a polymer chain. See: V. Buisco and R. Cipullo, J. Am. Chem. Soc., 116, 9392 (1994).
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The anionically obtained optically active poly(TrMA) exhibits an excellent chiral recognition ability to many classes of racemic compounds: Y. Okamoto, S. Honda, I. Okamoto, H. Yuki, S. Murata, R. Noyori, and H. Takaya, J. Am. Chem. Soc., 103, 6971 (1981).
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We reported the mm contents of poly(PDBSMA)s obtained under the conditions identical to those of run 8, 9, and 10 to be 97%, 98%, and 93%, respectively,3 but in the repeated experiments we found the polymers with higher isotacticity are obtained with a high reproducibility. The lower mm contents in the previous study can be ascribed to contaminants in the monomer such as methacrylic acid. Evidentially, the copolymerization of PDBSMA with 2 mol% of methacrylic acid under the reaction conditions of run 10 gave a polymer having a triad tacticity of mm/mr/rr=95.8/2.9/1.3 and the copolymerization of TrMA with a 1 mol% of methacrylic acid under the reaction conditions of run 3 resulted in a triad tacticity of mm/mr/rr=88.0/9.0/3.0.
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Depolymerization could also lead to the thremodynamically more stable radical having a higher meso addition possibility.
It has been reported that electron spin resonance spectra of TrMA polymerization systems at the temperatures ranging form −90°C to 30°C using photoinitiation suggests the existence of two conformers of radicals though the relation between the two and our radical models is currently unknown. See: M. Kamachi, Adv. Polym. Sci., 82, 209 (1987).
M. Kamachi, Y. Kuwae, S.-i. Nozakura, K. Hatada, and H. Yuki, Polym. J., 13, 919 (1981).
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Nakano, T., Matsuda, A. & Okamoto, Y. Pronounced Effects of Temperature and Monomer Concentration on Isotactic Specificity of Triphenylmethyl Methacrylate Polymerization through Free Radical Mechanism. Thermodynamic versus Kinetic Control of Propagation Stereochemistry. Polym J 28, 556–558 (1996). https://doi.org/10.1295/polymj.28.556
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