CONFIGURATIONAL STATISTICS OF RANDOM POLY(LACTIC ACID) CHAINS .2. THEORY

The conformational energy of the L-lactyl residue of poly(L-lactic acid), P-L-LA, is calculated as a function of rotation angles ϕ and ψ about the O–Cα and Cα–C bonds, respectively, the ester bond being planar trans. Methods of calculation correspond to those applied previously to various polypeptides. Despite the close structural similarity of P-L-LA to poly-L-alanine, differences in the conformational energy result from the difference of –10° between the COCα angle compared with the CNCα angle of the latter chain. Dipolar interactions, though much smaller than in the polypeptides, are significantly important. The conformational energy contour map over ϕp and ψ is dominated by four well-defined minima. The two of lowest energy, situated approximately at the g+t and g+g+ conformations, suffice for interpretation of the configurational characteristics of the chain. The calculated characteristic ratio agrees very well with that observed. The temperature coefficient d In (r2)0/dT is predicted to be negative, but much smaller in magnitude than experiments indicate. © 1969, American Chemical Society. All rights reserved.