Benzyl esters of optically active malic acid stereocopolymers as obtained by ring‐opening polymerization of (R)‐(+) and (S)‐(‐)‐benzyl malolactonates

Abstract

Abstract (R)‐(+) and (S)‐(‐)‐benzyl malolactonates were copolymerized anionically by using triethylamine and aluminum porphyrin as initiators. Configurational composition of resulting optically active poly(benzyl β‐malate) stereocopolymers depends on the enantiomeric purity in the monomer feed. Stereochemistry during the multistep synthesis starting from aspartic acid enantiomers used as precursors is discussed. It is shown that natural L(+)‐aspartic acid yields (‐)‐poly(benzyl β‐malate) which degrades to L(‐)‐malic acid in alkaline aqueous medium. Configurational structures were investigated by 13 C NMR (75.47 MHz) and splittings of stereosensitive resonances are discussed in terms of stereosequence distributions. Results show that stereocopolymerization proceeds through random addition of chiral repeating units according to Bernoullian statistics. In order to exemplify the effect of enantiomeric composition on physical properties, thermal characteristics have been investigated by DSC for the whole series of stereocopolymers. Samples with more than 70% (S) or (R)‐units are semicrystalline. Melting temperature and crystallinity increase with stereoregularity.