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

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 alpha-insertion. The structure of the polymers and the regioselectivity of insertion were further confirmed by comparing the C-13 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.