ABSTRACT
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-abundant nickel catalysts for the reaction, which would be especially desired for industrial applications: a Ni/phosphine phenolate (PO) precatalyst yielded regioregular polypropylene with the incorporation of some allyl monomers into the main polymer chain. The installation of a chiral menthyl substituent on the phosphine allowed for moderate stereoselectivity to be achieved, though the applicable polar monomers currently remain limited. The Account concludes with a discussion of the factors that affect the insertion mode of propylene and polar comonomers in copolymerization reactions, beginning with our recent computational study, and finishing with work from ourselves and others covering both comonomer and precatalyst steric and electronic profiles with reference to the observed regioselectivity.
ABSTRACT
UV-induced photochemical transformations of the paramagnetic [Cr(CO)4(Ph2PCH2CH2CH2PPh2)]+ complex (abbreviated [Cr(CO)4(dppp)]+) in dichloromethane was investigated by CW EPR spectroscopy. Room-temperature UV irradiation results in the rapid transformation of [Cr(CO)4(dppp)]+ into trans-[Cr(CO)2(dppp)2]+. However, low-temperature (77-120 K) UV irradiation reveals the presence of an intermediate mer-[Cr(CO)3(κ1-dppp)(κ2-dppp)]+ complex which photochemically transforms into trans-[Cr(CO)2(dppp)2]+. The derived spin Hamiltonian parameters for these complexes were confirmed by DFT calculations. The photoinduced reaction is shown to be concentration-dependent, leading to a distribution of the three complexes ([Cr(CO)4(dppp)]+, mer-[Cr(CO)3(κ1-dppp)(κ 2-dppp)]+, and trans-[Cr(CO)2(dppp)2]+). A bimolecular photoinduced mechanism is proposed to account for the formation of mer-[Cr(CO)3(κ1-dppp)(κ2-dppp)]+ and trans-[Cr(CO)2(dppp)2]+.
