posted on 2016-12-09, 18:49authored byMitchell
L. Smith, Amanda K. Leone, Paul M. Zimmerman, Anne J. McNeil
Polymerizing
electron-deficient arenes in a controlled, chain-growth
fashion remains a significant challenge despite a decade of research
on catalyst-transfer polycondensation. The prevailing hypothesis is
that the chain-growth mechanism stalls at a strongly associated metal–polymer
π-complex, preventing catalyst turnover. To evaluate this hypothesis,
we performed mechanistic studies using thiazole derivatives and identified
approaches to improve their chain-growth polymerization. These studies
revealed a surprisingly high barrier for chain-walking toward the
reactive C–X bond. In addition, a competitive pathway involving
chain-transfer to monomer was identified. This pathway is facilitated
by ancillary ligand dissociation and N-coordination to the incoming
monomer. We found that this chain-transfer pathway can be attenuated
by using a rigid ancillary ligand, leading to an improved polymerization.
Combined, these studies provide mechanistic insight into the challenges
associated with electron-deficient monomers as well as ways to improve
their living, chain-growth polymerization. Our mechanistic studies
also revealed an unexpected radical anion-mediated oligomerization
in the absence of catalyst, as well as a surprising oxidative addition
into the thiazole C–S bond in a model system.