POLY(ETHYLENE OXIDE-CO-BETA-BENZYL L-ASPARTATE) BLOCK-COPOLYMERS - INFLUENCE OF THE POLY(ETHYLENE OXIDE) BLOCK ON THE CONFORMATION OF THE POLY(BETA-BENZYL L-ASPARTATE) SEGMENT IN ORGANIC-SOLVENTS

The conformation of the poly(beta-benzyl L-aspartate) segment in the p oly(ethylene oxide-co-beta-benzyl L-aspartate) (PEO/PBLA) block copoly mers (M(w) of PEO = 5000 and 20 units of beta-benzyl L-aspartate) was investigated by H-1 NMR, by specific optical rotation measurements, an d by 2D H-1,H-1 NOESY NMR in chloroform, dimethyl sulfoxide (DMSO), an d mixtures of chloroform/DMSO. The comparison between the H-1 NMR spec tra of the block copolymer in CDCl3 and the one in DMSO-d(6) showed th at the PBLA blocks adopt a different conformation depending upon the s olvents. The specific rotation of the block copolymer at 546 nm demons trated that the PBLA segments adopt a left-handed alpha-helix conforma tion in chloroform. PBLA homopolymer with the same molecular weight as the PBLA segment in the block copolymer showed no evidence of alpha-h elix formation in the same conditions, indicating that the PEO segment in the block copolymer is essential to allow PBLA to take the alpha-h elix structure. On the other hand, the specific rotation of the PEO/PB LA block copolymer in DMSO at the same wavelength showed that the PBLA segments have a random-coil conformation in this solvent. Moreover. t he measurement of the specific rotation of the block copolymer in mixt ures of x% CHCl3/(100 - x)% DMSO (0 less than or equal to x less than or equal to 100) demonstrated that the left-handed alpha-helix conform ation adopted by the PBLA segments in chloroform is stable. Conversely , the PBLA homopolymer cannot be solubilized in mixtures of CHCl3/DMSO , although this homopolymer is soluble in pure CHCl3 and in pure DMSO. The 2D H-1,H-1 NOESY NMR spectrum in CDCl3 gave evidence of interacti ons between the methylene protons (CO2CH2C6H5) of the PBLA segments an d the methylene protons of the PEO blocks on one hand and between the benzyl protons of the PBLA segments and the methylene protons of the P EO blocks on the other. Such interactions are not observed in DMSO-d(6 ). Thus, the stability of the left-handed alpha-helix conformation of the PBLA blocks (having low molecular weights) in chloroform can be ex plained by the interactions existing between the PBLA and the PEO bloc ks, Moreover, these interactions also allow the solubilization of the block copolymer in mixtures of CHCl3/DMSO.