Catalytic Hydroxylation of Polyethylenes

Polyolefins account
for 60% of global plastic consumption, but
many potential applications of polyolefins require that their properties,
such as compatibility with polar polymers, adhesion, gas permeability,
and surface wetting, be improved. A strategy to overcome these deficiencies
would involve the introduction of polar functionalities onto the polymer
chain. Here, we describe the Ni-catalyzed hydroxylation of polyethylenes
(LDPE, HDPE, and LLDPE) in the presence of mCPBA as an oxidant. Studies with cycloalkanes and pure, long-chain
alkanes were conducted to assess precisely the selectivity of the
reaction and the degree to which potential C–C bond cleavage
of a radical intermediate occurs. Among the nickel catalysts we tested,
[Ni­(Me4Phen)3]­(BPh4)2 (Me4Phen = 3,4,7,8,-tetramethyl-1,10-phenanthroline) reacted with
the highest turnover number (TON) for hydroxylation of cyclohexane
and the highest selectivity for the formation of cyclohexanol over
cyclohexanone (TON, 5560; cyclohexanol/(cyclohexanone + ε-caprolactone)
ratio, 10.5). The oxidation of n-octadecane occurred
at the secondary C–H bonds with 15.5:1 selectivity for formation
of an alcohol over a ketone and 660 TON. Consistent with these data,
the hydroxylation of various polyethylene materials by the combination
of [Ni­(Me4Phen)3]­(BPh4)2 and mCPBA led to the introduction of
2.0 to 5.5 functional groups (alcohol, ketone, alkyl chloride) per
100 monomer units with up to 88% selectivity for formation of alcohols
over ketones or chloride. In contrast to more classical radical functionalizations
of polyethylene, this catalytic process occurred without significant
modification of the molecular weight of the polymer that would result
from chain cleavage or cross-linking. Thus, the resulting materials
are new compositions in which hydroxyl groups are located along the
main chain of commercial, high molecular weight LDPE, HDPE, and LLDPE
materials. These hydroxylated polyethylenes have improved wetting
properties and serve as macroinitiators to synthesize graft polycaprolactones
that compatibilize polyethylene–polycaprolactone blends.