METATHESIS POLYMERIZATION OF 1,7,7-TRIMETHYLBICYCLO[2.2.1]HEPT-2-ENE USING A WELL-DEFINED MOLYBDENUM INITIATOR

The polymerization of the title racemic and enantiomeric ( - ) monomer in CD2Cl2 at 20-degrees-C, initiated by Mo(CHCMe2Ph) (N-2,6-i-Pr2C6H3) [OCMe(CF3)2]2, has been studied by 400 MHz H-1 and 100 MHz C-13 NMR spectroscopy. The head-tail alkylidene adduct (P1H), [GRAPHICS] in which the C=C bond is trans, is formed after a few hours, reaching a maximum of more than half the original initiator concentration, and then declines very slowly as more monomer adds to P1H to give a trans, head-tail polymer. The first-order rate constant for the decay of initiator I in the presence of excess monomer M shows a complex dependence on the monomer concentration, tending towards first-order at low [M] and zero-order at high [M]. This is interpreted in terms of a mechanism involving an equilibrium between the syn and anti rotamers of the initator in which the conversion of the dominant syn rotamer I(s) into the minor anti rotamer I(a) is rate determining at high [M] with k(s/a) = 6.1 X 10(-5) s-1, while at low [M] the addition of M to I(a) becomes rate determining. The value of k(s/a) agrees well with the value directly determined in toluene by Schrock and coworkers. The C-13 NMR spectrum of the polymer made from racemic monomer shows it to be essentially atactic, with every carbon except C-9 sensitive to tacticity. Polymer made from the single enantiomer is isotactic, allowing m and r assignments to be made in the C-13 NMR spectrum of the polymer of racemic monomer.