Monofunctional Hyperbranched Ethylene Oligomers

The
neutral κ2N,O-salicylaldiminato
Ni­(II) complexes [κ2N,O-{(2,6-(3′,5′-R2C6H3)2C6H3-NC­(H)-(3,5-I2-2-O-C6H2)}]­NiCH3(pyridine)]
(1a-pyr, R = Me; 1b-pyr, R = Et; 1c-pyr, R = iPr) convert ethylene to hyperbranched low-molecular-weight
oligomers (Mn ca. 1000 g mol–1) with high productivities. While all three catalysts are capable
of generating hyperbranched structures, branching densities decrease
significantly with the nature of the remote substituent along Me >
Et > iPr and oligomer molecular weights increase.
Consequently, only 1a-pyr forms hyperbranched structures
over a wide range of reaction conditions (ethylene pressure 5–30
atm and 20–70 °C). An in situ catalyst system achieves
similar activities and identical highly branched oligomer microstructures,
eliminating the bottleneck given by the preparation and isolation
of Ni–Me catalyst precursor species. Selective introduction
of one primary carboxylic acid ester functional group per highly branched
oligoethylene molecule was achieved by isomerizing ethoxycarbonylation
and alternatively cross metathesis with ethyl acrylate followed by
hydrogenation. The latter approach results in complete functionalization
and no essential loss of branched oligomer material and molecular
weight, as the reacting double bonds are close to a chain end. Reduction
yielded a monoalcohol-functionalized oligomer. Introduction of one
reactive epoxide group per branched oligomer occurs completely and
selectively under mild conditions. All reaction steps involved in
oligomerization and monofunctionalization are efficient and readily
scalable.