Strategies for Catalytic Polymerization of Polar Monomers - Late Transition Metal Polymerization Catalysis - page 307

Fig. 9.2 Modes of bonding for functionalized norbornene de-
rivatives.
This has two detrimental effects on polymerization. First, chelation strengthens
the metal-olefin interaction, thereby raising the barrier for the insertion step. Sec-
ond, it forces insertion through the endo face, in sharp contrast to the known pro-
pensity for norbornene to insert into metal-carbon bonds through the less hin-
dered exo face [3a, 5]. Consistent with this hypothesis has been our observation of
the preferential uptake of the exo isomer in the polymerization of functional nor-
bornene derivatives by [Pd(PR 3 )(Me)] + . For example, Fig. 9.3 shows the uptake pro-
file versus time for the polymerization of 5-norbornene-2-carboxylic acid ethyl es-
ter starting with a monomer isomer ratio of 22% exo to 78% endo . Indeed, under
certain conditions a polymer can be obtained from the exo isomer but not the
endo isomer [10].
Given the published studies that indicate that the insertion of nonfunction-
alized norbornenes into metal-carbon bonds occurs with exo , exo - stereochemistry
(e.g., Fig. 9.1) we sought to determine whether the presence of an endo functional-
ity would change the insertion stereochemistry. Because of the relative instability
of the catalytically active palladium species, detailed studies encompassing the co-
ordination and insertion of norbornene derivatives were carried using the model
Fig. 9.3 Plot of unreacted monomer versus time for the polymerization of 5-norbornene-2-car-
boxylic acid ethyl ester.