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Tm: melting temperature.
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The Tm value of homochiral PLLA is 162 °C: N. Yui, in “Polymeric Materials Encyclopedia,” J. C. Salamone, Ed., CRC Press, Boca Raton, FL, 1996, Vol 10, p 7947. The PLLA (Mn 12400) that we synthesized from LLA (Aldrich) using Al(OiPr)3 in toluene at 70 °C indicated its Tm at 164 °C.
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The structure of 3b is expected to be square-pyramidal or trigonal-bipyramidal with some distortion as those of the related complexes (Ref 5a–d), and the most stable conformation of the simplistic complex of 3a by DFT(B3LYP/6-31G*) calculations (Ref 5e) also supported it. The structure of the complex 3b can not be C2-symmetrical in solution, because the two methyl-groups of the backbone are not equivalent in 1H and 13C NMR (see Experimental). If the conformation of 3b is rigid, four peaks for two methylene-protons (CH2 × 2) of the backbone should appear in 1H NMR, and two peaks for two methylene-carbons (CH2 × 2) should appear in 13C NMR. As shown in Figure 2, two methylenes are unambiguously identical in 1H and 13C NMR at 27 °C, and they indicate that the conformation of 3b is flexible.
We also isolated the isopropoxide complex using 2-propanol instead of benzyl alcohol. In the 1H NMR spectrum, both the methine and methyl protons of isopropoxide also appeared as broad peaks at 4.33 and 1.31 ppm, respectively.
In this paper, the stereoselectivity are indicated in two forms, Pmeso and Piso. Both of them are the probability of the same chiral sense linkage (meso/iso) on the basis of the stereoselective mechanism on the catalyst. Even in the same polymer, Pmeso and Piso give different numbers as shown. Pmeso should be used for evaluation in the chain-end control mechanism (our system) to afford the stereoblock polymer (PLLA–PDLA)n. Piso is on the basis of the site control mechanism, in which each polymer molecule should be optically active either PLLA or PDLA. As Coates et al. demonstrated, 5c,d the isotactic poly(rac-LA)’s via the site control mechanism are not the mixture of PLLA and PDLA but stereoblock (PLLA–PDLA)n’s when the polymer exchanges occur after insertion of the mismatched monomer on the catalyst. As a matter of fact, the polymer exchanges also occurred in Feijen’s system. 6b We dared to calculate two forms Pmeso and Piso to compare our system (chain-end control) with Feijen’s (site control in the presence of polymer exchanges) because both cases afford the same (PLLA–PDLA)n. An unambiguous simple evaluation of isotacticity of the polymer molecules without mechanistic considerations (and assumptions) could be done by measurement of Tm.
J. E. Kasperczyk, Macromolecules, 28, 3937 (1995): Pmeso is the probability of meso linkages in the chain-end control mechanism.
The monomer conversion reached 80% after 2 h.
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Ishii, R., Nomura, N. & Kondo, T. Stereoselective Bulk Polymerization of Racemic Lactide for Stereoblock Poly(racemic lactide) Using an Achiral Aluminum Complex. Polym J 36, 261–264 (2004). https://doi.org/10.1295/polymj.36.261
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DOI: https://doi.org/10.1295/polymj.36.261