Strategies for Catalytic Polymerization of Polar Monomers - Late Transition Metal Polymerization Catalysis - page 311

Fig. 9.5 Hammett plot for migratory insertion of alkenes into
palladium(II)-methyl bond.

complex leads to a lower insertion barrier). Another surprising observation is that
the observed correlation extends to propene. The slower insertion and polymeriza-
tion rates of propene, when compared to ethene, are usually attributed to the ster-
ic bulk of the methyl group of the former [15, 19]. Our results suggest that, at
least for the late transition metal compounds, it is the donating ability of the
methyl group, rather than its size, which results in slower rate for propene inser-
tion and, hence, polymerization. This argument applies even for the sterically en-
cumbered Brookhart-type system. Likewise, for acrylates, the precoordination of
the ester group appears to have little effect on its migratory insertion rate.
What is the implication of our work with respect to the metal-catalyzed poly-
merization of polar vinyl monomers? First, for the late metal compounds, the po-
lar vinyl monomers can clearly outcompete ethene and simple 1-alkenes with re-
spect to insertion. However, the ground-state destabilization of the alkene complex
that favors the migratory insertion of the polar vinyl monomers is a two-edged
sword because it biases the alkene coordination towards ethene and 1-alkenes. In-
deed, we have observed the near quantitative displacement of vinyl bromide by
propene to form 7 from 3 (Scheme 9.1). Thus, the extent of incorporation of the
polar vinyl monomer in the polymer will depend on the opposing trends in al-
kene coordination and migratory insertion. The above discussion does not take
into account the problem of functional group coordination for acrylates or

-halide

abstraction for vinyl halides.
There are several possible approaches to circumventing functional group coordi-
nation. One obvious route is to decrease the electrophilicity of the metal center,
e.g. by moving from cationic to neutral complexes. In recent theory paper [16],
Ziegler has shown that the interaction of the ester group of acrylate with the me-
tal center is weaker in neutral complexes when compared with the corresponding
cationic species. On the other hand, the bonding of the olefinic functionality is
not significantly affected because the weaker alkene to metal charge transfer in