Stereosequencing in poly(leucine) obtained from stereoselective polymerization of racemic leucine N‐carboxy anhydride

Abstract

Polymerizations of mixtures of D-and L-isomers of a-amino acid N-carboxy anhydrides follow normal copolymerization behavior, and sometimes even ideal copolymerization, which is conventionally interpreted as leading to a random distribution of monomer units along the polymer chains. Equations are derived which relate the fraction of isotactic L-diads to experimental data from the previously reported methods involving sequential and stereospecific cleavage with enzymes. The application of these equations to poly(o,~-leucines), which were prepared by polymerization of racemic leucine Ncarboxy anhydride with chiral and achiral primary amines as initiators, indicated that the polymers contain between 94 and 98% isotactic diads. The poly(leucines) thus consist of long stereoblocks despite the observed ideal copolymerization behavior.

15N‐nmr spectroscopy. 33. Assignments of isotactic and syndiotactic sequences in poly(D,L‐amino acids)

“…Using Eqs. (5)- (8), this means k l > h3 and k z > kq. Again we find that the stereospecificity of a heterostereocopolymerization agrees with that of the two corresponding sterecopolymerizations, namely, D,L-Leu-NCA and D , L -A~~-N C A~~,~~:…”

“…VL.L~"-D.A~~ = k4[-L-Leu] -[D-Ala-NCA] (8) where V is reaction rate and leu] and [-D-Ala] are molar concentrations of active chain ends.…”

“…For the last 20 years the stereoselectivity of copolymerizations of D-and L-amino acid N-carboxyanhydrides (NCAs) has been the object of numerous inve~tigations.l-~ The methods used to tackle this problem were rather indirect, namely, comparison of optical rotations, CD spectra, kinetic investigations of polymerization rates, and, in one case, enzymatic degradations combined with a chromatographic analysis of the degradation products. 8 Only recently could we demonstrate9J0 that high-resolution 13C-and 15N-nmr spectra allow one to study the tacticity of poly(D,L-amino acids) directly. lH-nmr at 400 and 500 MHz also shows tacticity effects; however, the signal patterns are complicated and not well-resolved, so that their routine application for analytical purposes is not straightforward.…”

Stereochemistry and macromolecules: Principles and applications

“…In actual practice, the task of delineating the mechanism of a polymerization reaction involving stereoisomers requires not only a determination of which stereoisomer is incorporated into the polymer (stereoselectivity) but also the determination of the sequence distribution of monomer units in the polymer and the tacticity of substituents. Elegant examples of the elucidation of the stereochemistry of polymerization reactions can he found in the work of Price (47) on the polymerization of propylene oxide and of Elias (48) for the polymerization of rr-aminoacid -carboxy anhydride systems. Examination of the stereochemistry of polymerization reactions has proven to he an exciting and rewarding research endeavor which can he equally enjoyable for students to understand.…”