Ring-opening metathesis polymerization of 7-methylbicyclo[2.2.1]hepta-2,5-diene initiated by well-defined molybdenum and ruthenium carbene complexes

Polymers of 7-methylnorbornadiene (7-MNBD) have been prepared by ring-opening metathesis polymerization (ROMP) using Mo(CHCMe2Ph)(NC6H3-2,6-i-Pr2)(OR)2 [1, OR=OCMe(CF3)2; 2, OR=OCMe3], Mo(CHCMe3)(NC6H3-2,6-i-Pr2)(OR)2 (2′) and Ru(CHPh)Cl2(PCy3)2 (3) as initiators in CD2Cl2. The structures of the polymers were investigated by 1H- and 13C-NMR spectroscopy. In all cases the ROMP was very fast and the initial product was essentially a polymer of anti units, linked by 88% cis double bonds for initiation by 1, 68% for 2 or 2′, and 17% for 3. The tacticities of the polymers varied from atactic using 1 as initiator to fully tactic (t/m and c/r) using 2 or 2′ as initiator. Values of kp/ki were derived from the proportion of initiator consumed and the initial ratio of monomer to initiator: kp/ki=550 for 1, 1100 for 2, 64 for 2′, and 61 for 3. Two kinds of secondary reaction were observed: the first gave rise to a second set of 13C-NMR peaks (10–20% of the intensity of the main peaks), and the second, observed only with 1, was characterised by sudden gelation of the reaction mixture some time after the monomer had been consumed, and was followed by a marked fall in the intensity of the main 13C-NMR peaks. These effects are attributed to secondary intra- and inter-molecular metathesis reactions, respectively. The high-trans polymer could be fully hydrogenated, but the high-cis polymer could only be partially hydrogenated.