Abstract
Anionic polymerization of methyl bicyclobutane-1-carboxylate (MBCB) with tert-butyllithium (t-BuLi)/bis(2,6-di-tert-butylphenoxy)ethylaluminum [EtAl(ODBP)2] in toluene at −78 °C gives transtactic polymers (trans contents > 90%) with high yields, but with low initiator efficiency and broad molecular weight distribution. In order to obtain information on the initiation mechanism, oligomers of MBCB were prepared with t-BuLi/EtAl(ODBP)2 in toluene at −78 °C, and fractionated into dimer to pentamer by size exclusion chromatography. The dimer fraction was further subjected to recycling SEC and found to consist of two types of dimers of different chemical structures. 1H NMR analyses of the dimers revealed that one type of the dimer comprises of an initiating tert-butyl (t-Bu) group and two MBCB units while the other contains one t-Bu carbonyl group within the monomeric units in addition to the initial t-Bu group. Based on the NMR spectroscopic information on the oligomers, 1H NMR spectra of the polymer was investigated with the focus on the end-group structures, that confirms that the carbonyl addition of t-BuLi to MBCB occurred and the ketonic compound thus formed participates in the polymerization.
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The monomer activation through the coordination with Et(ODBP)2 has been observed in 13C NMR spectra measured in toluene-d8 at −78 °C; the chemical shift of the methine carbon at 3-position changed from 15.8 ppm to 28.2ppm upon addition of an equimolar amount of Et(ODBP)2, suggesting a decrease of electron density favorable for anionic polymerization, while the carbonyl carbon signal shifted from 172.2 to 181.5 ppm.
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Kawauchi, T., Nakamura, M., Kitayama, T. et al. Structural Analyses of Methyl Bicyclobutane-1-carboxylate Oligomers Formed with tert-Butyllithium/Aluminum Bisphenoxide and Mechanistic Aspect of the Polymerization. Polym J 37, 439–448 (2005). https://doi.org/10.1295/polymj.37.439
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DOI: https://doi.org/10.1295/polymj.37.439
