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 whi ch the C=C bond is trans, is formed after a few hours, reaching a maxi mum of more than half the original initiator concentration, and then d eclines 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 t he monomer concentration, tending towards first-order at low [M] and z ero-order at high [M]. This is interpreted in terms of a mechanism inv olving an equilibrium between the syn and anti rotamers of the initato r in which the conversion of the dominant syn rotamer I(s) into the mi nor 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 rat e 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 a tactic, with every carbon except C-9 sensitive to tacticity. Polymer m ade from the single enantiomer is isotactic, allowing m and r assignme nts to be made in the C-13 NMR spectrum of the polymer of racemic mono mer.