Stereo- and regioselective cyclopolymerization of chiral 1,7-octadiynes

Abstract

A series of diastereomeric 4,5-diethoxycarbonyl-1,7-octadiynes were synthesized from which the pure diastereomers were obtained by selective Soxhlet extraction in CHCl₃. The concept of “small alkoxides” was used with varying size of the alkoxide ligands and all the monomers were cyclopolymerized with two different Schrock-type initiators. As a result, poly(1,7-octadiyne)s consisting virtually solely of six membered repeat units were obtained via regioselective α-insertion. The structure of the polymers and the regioselectivity of insertion were further confirmed by comparing the ¹³C NMR shifts of model compounds with those of the corresponding polymers. The living character of the polymerization was demonstrated by kinetic studies and end group analysis via MALDI-ToF MS; diblock copolymers were successfully prepared from 1,7-octadiynes and 1,6-heptadiynes. Finally, the tacticity of the cyclopolymers was elucidated from NMR of poly(1,7-octadiyne) synthesized from 1,7-octadiyne-4,4-dicarboxylic esters containing enantiomerically pure menthyl ester groups. These investigations revealed the formation of highly tactic polymers (>86%) with at least predominantly trans-configured exocyclic double bonds.