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

The ROMP of 7-tert-butoxynorbornadiene initiated by Mo(=CHCMe2Ph)(=NC6H3-2, 6-i-Pr2)(OR)2 [1, OR = OCMe(CF3)2; 2, OR = OCMe3] and Ru(=CHPh)Cl2(PCy3)2 (3) has been studied at ambient temperature by NMR in CDCl3, CD2Cl2 and C6D6. The reaction with 1 as initiator is extremely fast, but with 2 and 3 the reaction proceeds at measurable speed, being fastest in CDCl3 and slowest in C6D6. The polymer formed in CD2Cl2 using 1 contains 47% syn units and mainly cis double bonds; that formed using 2 contains 34% syn units and about 17% cis double bonds; and that formed using 3 contains less than 7% syn units and 32% cis double bonds. The carbene proton of the propagating carbenes derived from 1 and 2 gives two groups of 1H NMR duoblets, attributed to species having syn and anti units adjacent to the Mo center, but that derived from 3 gives only a single group of signals in which the fine structure is independent of spectrometer frequency and therefore not due to spin coupling. With 3 as initiator, after polymerization is complete, the initiator is partially regenerated at the expense of the of the propagating species indicating the occurence of secondary metathesis reactions at the chains ends, leading to a decrease in the number of chain ends in the polymer. Under conditions where the chain length is low, the shorter chains may be detected by GPC and NMR. Such reactions occur intermolecularly resulting in an increase in the average molecular weight; they may also occur by ring-closing metathesis, though this has not been proved. kp/ki values were derived and in three cases individual values of kp and ki were determined. With 3 as initiator in CD2Cl2 the presence of a five-fold excess of PCy3 caused a marked diminution in the rate of ROMP showing that the reactions proceeds mainly via addition of monomer to monophosphine complexes, Ru(=CHR)Cl2(Pcy3).