Nature of the Propagating Species in Ring-Opening Metathesis Polymerizations of Oxygen-Containing Monomers Using Well-Defined Ruthenium Initiators

Abstract

The propagating alkylidene species that arise during the ring-opening metathesis polymerization (ROMP) of a range of strained bicyclic monomers initiated by RuCl2(PCy3)(2)(=CHPh) [1]. and RuCl2(PCy3)(=CH-o-O-i-PrC6H4) [2] have been identified using H-1 NMR spectroscopy. For polymerizations initiated by 1 in which the monomers do not contain oxygen, a single set of two NMR signals are observed for the resting state of the propagating species. These correspond to bisphosphine-ruthenium complexes with the proximal backbone double bond either cis or trans. When the monomers contain oxygen, an additional set of two NMR signals are observed for the resting state of the propagating species. These correspond to monophosphine species which also have oxygen from the polymer backbone complexed to the ruthenium, again with the proximal backbone double bond either cis or trans. Resonances attributed to bisphosphine and monophosphine species have been conclusively assigned by the addition of CuCl or PCy3. Assignments were further confirmed by the study of comparable ROMP reactions initiated by 2. Bisphosphine propagating species are not normally observed during ROMP reactions initiated by 2. If the monomer contains oxygen, then, in addition to the monophosphine species with complexation of the -O-Pr-i group to the ruthenium, additional monophosphine propagating species may be observable due to oxygen in the polymer backbone chelating to the Ru center. Bisphosphine propagating species and RuCl2(PCy3)(2)(=CH-o-O-i-PrC6H4), 3, are observable using H-1 NMR spectroscopy if PCy3 is added to 2 prior to the ROMP of 5-exo,6-endo-dicarbomethoxynorbornene, 4. The amount of added PCy3 is found to affect the relative intensities of the propagating bisphosphine species and 3 as well as the rate of monomer consumption and the polydispersity of the resulting polymer.