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15 N‐NMR spectroscopy. 34. Stereospecificity of the polymerization of D , L ‐leucine N ‐carboxyanhydride and γ‐OMe‐ D , L ‐glutamic acid N ‐carboxyanhydride
Author(s) -
Kricheldorf H. R.,
Hull William E.
Publication year - 1982
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360210813
Subject(s) - chemistry , tacticity , pyridine , polymerization , benzylamine , stereospecificity , monomer , polymer chemistry , nuclear magnetic resonance spectroscopy , triethylamine , polymer , stereochemistry , medicinal chemistry , organic chemistry , catalysis
Abstract D , L ‐Leucine‐ N ‐carboxyanhydride ( D , L ‐Leu‐NCA) and γ‐methyl‐ D , L ‐glutamic acid N ‐carboxyanhydride (γ‐OMe‐ D , L ‐Glu‐NCA) were synthesized with ca. 2.5% 15 N enrichment. Their polymerizations were conducted under a variety of conditions using benzylamine, triethylamine potassium tert‐butanolate, and pyridine as initiators. The 40.55‐MHz 15 N‐nmr spectra of the resulting stereocopolypeptides measured in trifluoroacetic acid display at least four signals, representing the isotatic, syndiotactic, and two heterotactic triads. From the signal intensities it was concluded that these NCAs behave nearly identically. With benzylamine initiation the formation of isotactic blocks is slightly favored, and they are still more predominant when strong bases are used as initiators. Initiation by pyridine favors the formation of syndiotactic sequences. However, in all cases the average lengths of the stereoblocks never exceeded 4 monomer units. The low stereospecificity of most polymerizations of D , L ‐NCAs is confirmed by the high degree of solubility of the resulting poly( D , L ‐amino acids) in aprotic solvents. Penultimate effects are weak or absent, so that most polymerizations follow Bernoullian type statistics. Deviations from these statistics were found for polymerizations in pyridine.