Alternating ring-opening metathesis copolymerization by Grubbs-type initiators with unsymmetrical N-heterocyclic carbenes.

A series of Ru(IV)-alkylidenes based on unsymmetrical imidazolin-2-ylidenes, that is, [RuCl(2){1-(2,4,6-trimethylphenyl)-3-R-4,5-dihydro-(3H)-imidazol-1-ylidene}(CHPh)(pyridin)] (R = CH(2)Ph (5), Ph (6), ethyl (7), methyl (8)), have been synthesized. These and the parent initiators [RuCl(2)(PCy(3)){1-(2,4,6-trimethylphenyl)-3-R-4,5-dihydro-(3H)-imidazol-1-ylidene}(CHC(6)H(5))] (R = CH(2)C(6)H(5) (1), C(6)H(5) (2), ethyl (3)) were used for the alternating copolymerization of norborn-2-ene (NBE) with cis-cyclooctene (COE) and cyclopentene (CPE), respectively. Alternating copolymers, that is, poly(NBE-alt-COE)(n) and poly(NBE-alt-CPE)(n) containing up to 97 and 91% alternating diads, respectively, were obtained. The copolymerization parameters of the alternating copolymerization of NBE with CPE under the action of initiators 1-3 and 5-8 were determined by using both a zero- and first-order Markov model. Finally, kinetic investigations using initiators 1-3, 6, and 7 were carried out. These revealed that in contrast to the 2nd-generation Grubbs-type initiators 1-3 the corresponding pyridine derivatives 6 and 7 represent fast and quantitative initiating systems. Hydrogenation of poly(NBE-alt-COE)(n) yielded a fully saturated, hydrocarbon-based polymer. Its backbone can formally be derived by 1-olefin polymerization of CPE (1,3-insertion) followed by five ethylene units and thus serves as an excellent model compound for 1-olefin polymerization-derived copolymers.