Conspectus
Polyolefins are produced in vast amounts and
are found in so many
consumer products that the two most commonly produced forms, polyethylene
(PE) and polypropylene (PP), fall into the rather sparse category
of molecules that are likely to be known by people worldwide, regardless
of their occupation. Although widespread, the further upgrading of
their properties (mechanical, physical, aesthetic, etc.) through the
formation of composites with other materials, such as polar polymers,
fibers, or talc, is of huge interest to manufacturers. To improve
the affinity of polyolefins toward these materials, the inclusion
of polar functionalities into the polymer chain is essential. The
incorporation of a functional group to trigger controlled polymer
degradation is also an emerging area of interest. Currently practiced
methods for the incorporation of polar functionalities, such as post-polymerization
functionalization, are limited by the number of compatible polar monomers:
for example, grafting maleic anhydride is currently the sole method
for practical functionalization of PP. In contrast, the incorporation
of fundamental polar comonomers into PE and PP chains via coordination
insertion polymerization offers good control, making it a highly sought-after
process. Early transition metal catalysts (which are commonly used
for the production of PE and PP) display poor tolerance toward the
functional groups within polar comonomers, limiting their use to less-practical
derivatives. As late transition metal catalysts are less-oxophilic
and thus more tolerant to polar functionalities, they are ideal candidates
for these reactions. This Account focuses on the copolymerization
of propylene with polar comonomers, which remains underdeveloped as
compared to the corresponding reaction using ethylene. We begin with
the challenges associated with the regio- and stereoselective insertion
of propylene, which is a particular problem for late transition metal
systems because of their propensity to undergo chain walking processes.
To overcome this issue, we have investigated a range of metal/ligand
combinations. We first discuss attempts with group 4 and 8 metal catalysts
and their limitations as background, and then focus on the copolymerization
of propylene with methyl acrylate (MA) using Pd/imidazolidine–quinolinolate
(IzQO) and Pd/phosphine–sulfonate (PS) precatalysts. Each generated
regioregular polymer, but while the system featuring an IzQO ligand
did not display any stereocontrol, that using the chiral PS ligand
did. A further difference was found in the insertion mode of MA: the
Pd/IzQO system inserted in a 1,2 fashion, while in the Pd/PS system
a 2,1 insertion was observed. We then move onto recent results from
our lab using Pd/PS and Pd/bisphosphine monoxide (BPMO) precatalysts
for the copolymerization of propylene with allyl comonomers. These
P-stereogeneic precatalysts generated the highest isotacticity values
reported to date using late transition metal catalysts. This section
closes with our work using Earth-abundan...
