Direct Synthesis of Polar-Functionalized Linear Low-Density Polyethylene (LLDPE) and Low-Density Polyethylene (LDPE)

Late-transition-metal
catalysts have great potentials to incorporate
polar comonomers during olefin polymerization. The preparation of
polar-functionalized polyolefins with different microstructures and
topologies is a highly fascinating concept. In this contribution,
we demonstrate this possibility through palladium-catalyzed ethylene
copolymerizations with polar-functionalized α-olefins and their
terpolymerizations with α-olefins. A phosphine–sulfonate–palladium
catalyst (PO-Pd) afforded highly linear polyethylene
during ethylene homopolymerization. Linear low-density polyethylene
(LLDPE) was obtained from the PO-Pd-catalyzed copolymerizations
of ethylene with α-olefins; in these copolymerizations, the
partial or complete replacement of α-olefins with polar-functionalized
α-olefins led to the formation of polar-functionalized LLDPE
(P-LLDPE). A specially designed α-diimine palladium catalyst
(NN-Pd) afforded polyethylenes with tunable branching
densities (16–37 per 1000 carbon atoms), melting points (101–113
°C), and densities (0.89–0.92 g/cm3), which
closely resemble those of low-density polyethylene (LDPE). The NN-Pd-catalyzed copolymerizations of ethylene with polar α-olefins
generated analogues of polar-functionalized low-density polyethylene
(P-LDPE). The mechanical and surface properties of these polar polyolefin
materials were studied in detail, and their properties were further
improved/modified through cross-linking reactions.