Ring-Opening Metathesis Polymerization of 7-tert-Butoxybicyclo[2.2.1]hepta-2,5-diene Initiated by Well-Defined Molybdenum and Ruthenium Carbene Complexes

Abstract

The ROMP of 7-tert-butoxynorbonadiene initiated by Mo(=CHCMe 2 Ph)(=NC 6 H 3 -2,6-i-Pr 2 )(OR) 2 [1, OR = OCMe(CF 3 ) 2 ; 2, OR = OCMe 3 ] and Ru(=CHPh)Cl 2 (PCy 3 ) 2 (3) has been studied at ambient temperature by NMR in CDCl 3 , CD 2 Cl 2 and C 6 D 6 . The reaction with 1 as initiator is extremely fast, but with 2 and 3 reaction proceeds at measurable speed, being fastest in CDCl 3 and slowest in C 6 D 6 . The polymer formed in CD 2 Cl 2 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 give two groups of 1 H NMR doublets, 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 intiator is partiallu regenerated at the expense 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 intermolecurlarly resulting in an increase in the average molecular weight; they may also occur by ring-closing metathesis, though this has not been proved. k p /k i values were derived and in three cases individual values of k p and k i were determined. With 3 as initiator in CD 2 Cl 2 the presence of a five-fold excess of PCy 3 caused a marked diminution in the rate of ROMP showing that the reaction proceeds mainly via addition of monomer to monophosphine complexes, Ru(=CHR)Cl 2 (PCy 3 ).