Regio- and stereospecific cyclopolymerization of 1,6-heptadiynes

Abstract

The synthesis of poly(ene)s exclusively based on one single repetitive unit, i.e. 1,2-cyclopent-2-enylenvinylenes is described. Polymers containing >96% 1,2-cyclopent-2-enylenvinylene units were obtained by low-temperature-initiated cyclopolymerization of diethyl dipropargylmalonate (DEDPM) by Mo(N-2,6-i-Pr2-C6H3)(CHCMe2Ph)(OCH(CH3)2)2. In the presence of quinuclidine, 100% five-membered ring structures were realized at room temperature, e.g. using Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(OC(CH3)3)2. 4-(Ethoxycarbonyl)-4-(1S,2R,5S)-(+)-menthyl-1,6-heptadiyne was cyclopolymerized to determine the configuration of the double bond (i.e. the cis/trans ratio) and the tacticity of the poly(ene) backbone. Poly(4-(ethoxycarbonyl)-4-(1S,2R,5S)-(+)-menthyl-1,6-heptadiyne) again consisted of >96% five-membered rings and possessed an all-trans tactic, presumably isotactic structure. A linear plot of number of monomers added (N) versus molecular weights suggested that the cyclopolymerization of DEDPM proceeded in a living manner. At least a class V living system was confirmed by the stepwise synthesis of poly(DEDPM). Molecular weight distributions (PDIs) of 1.16–1.37 result from ratios of the rate constants for polymerization (kp) to initiation (ki), kp/ki>1. Influences of temperature, the base and steric and electronic effects of the arylimido and alkoxy ligand are presented. Complementary to the use of well-defined Schrock initiators, poly(DEDPM) containing exclusively five-membered rings was accessible by the use of the quaternary systems MoCl5–n-Bu4Sn–EtOH–quinuclidine and MoOCl4–n-Bu4Sn–EtOH–quinuclidine. Though virtually identical polymers were obtained, the polymerization systems themselves were controlled, yet not living as is the case with Schrock initiators. Selected polymer properties such as stability, degradation behavior, conductivity, and thermoresponsive behavior will be discussed.