Introducing polar functional groups into widely used polyolefins can enhance polymer surface, rheological, mixing, and other properties, potentially upgrading polyolefins for advanced, value‐added applications. The metal catalyst‐mediated copolymerization of non‐polar olefins with polar comonomers represents the seemingly most straightforward, atom‐ and energy‐efficient approach for synthesizing polar functionalized polyolefins. However, electrophilic early transition metal (groups 3 and 4)‐catalyzed processes which have achieved remarkable success in conventional olefin polymerizations, encounter severe limitations here, largely associated with the Lewis basicity of the polar co‐monomers. In recent years, however, new catalytic systems have been developed and successful strategies have emerged. In this Minireview, we summarize the recent progress in early transition metal polymerization catalyst development, categorized by the catalytic metal complex and polar comonomer identity. Furthermore, we discuss advances in the mechanistic understanding of these polymerizations, focusing on critical challenges and strategies that mitigate them.

Polyethylene terephthalate (PET) is selectively depolymerized by a carbon-supported single-site molybdenum-dioxo catalyst to terephthalic acid (PTA) and ethylene. The solventless reactions are most efficient under 1 atmosphere of H 2. The catalyst exhibits high stability and can be recycled multiple times without loss of activity. Waste beverage bottle PET or a PET + polypropylene (PP) mixture (simulating the bottle + cap) proceeds at 260 8C with complete PET deconstruction and quantitative PTA isolation. Mechanistic studies with a model diester, 1,2-ethanediol dibenzoate, suggest the reaction proceeds by initial retro-hydroalkoxylation/b-CÀO scission and subsequent hydrogenolysis of the vinyl benzoate intermediate. Polymer-based plastics are among the most widely used synthetic materials worldwide and are essential for modern life and the global economy. By 2050, their annual production should reach % 1.12 billion tons. [1] Since almost all plastics are produced from fossil feedstocks their impact on finite natural resources is a concern, as is waste plastic accumulation and the worldwide environmental consequences. [2] Underlying this accumulation is a linear economic model whereby most products are discarded after use. In contrast, a circular economy in which waste plastics are recycled and repurposed is far more rational. [3] Due to the great popularity of polyesters, there is a rising need for their recycling. Current technologies are: 1) Thermomechanical recycling in which sorted polyethylene terephthalate (PET) waste is remelted and reprocessed. The high temperatures lead to significant thermal degradation, and lower grade plastics with inferior optical, thermal, and mechanical properties. 2) Chemical recycling in which one or more component monomers is recovered. [4] The attraction here is that valuable monomers can be repurposed for known or new materials having identical or higher performance. The most common chemical process for polyesters such as PET is glycolytic, methanolytic, or hydrolytic trans-esterification. These are catalyzed by metal acetates, [5] titanium complexes, [6] metal chlorides, [7] metallic [8] and metal oxide nanoparticles, [9] ionic liquids, [10] and bases. [11] Hydrosilylation [12] and microbial agents [13] were recently shown to also affect PET deconstruction. The principal limitation of such processes is formation of difficultly separated side products and the requirement of large solvent and degradation agent excesses (Figure 1 A). In principal, catalytic hydrogenolysis is attractive for deconstructing polyesters, however, there has been limited research. Recently, homogeneous Ru pincer complexes, such as Milsteins catalyst, [14] and Ru complexes with tridentate phosphine ligands, LRu(tmm) (L = triphos, triphos-xyl, tmm = trimethylenemethane), [15a] were shown to catalyze PET hydrogenolysis to the corresponding alcohols. [15] Although H 2 is a cost-effective reductant, these processes require high H 2 pressures, long reaction times (16-48 h), use of solvents, and expensive, ai...

Direct coordinative copolymerization of ethylene with functionalized co-monomers is a long-sought-after approach to introducing polyolefin functionality. However, functional-group Lewis basicity typically depresses catalytic activity and co-monomer incorporation. Finding alternatives to intensively studied group 4 d and late-transition-metal catalysts is crucial to addressing this long-standing challenge. Shown herein is that mono- and binuclear organoscandium complexes with a borate cocatalyst are active for ethylene + amino olefin [AO; H C=CH(CH ) NR ] copolymerizations in the absence of a Lewis-acidic masking reagent. Both activity (up to 4.2×10 kg mol ⋅h atm ) and AO incorporation (up to 12 % at 0.2 m [AO]) are appreciable. Linker-length-dependent (n) AO incorporation and mechanistic probes support an unusual functional-group-assisted enchainment mechanism. Furthermore, the binuclear catalysts exhibit enhanced AO tolerance and enhanced long chain AO incorporation.

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In principal, the direct copolymerization of ethylene with polar comonomers should be the most efficient means to introduce functional groups into conventional polyolefins but remains af ormidable challenge.D espite the tremendous advances in group 4-centered catalysis for olefin polymerization, successful examples of ethylene + polar monomer copolymerization are rare,e specially without Lewis acidic masking reagents.Here we report that certain group 4catalysts are very effective for ethylene + CH 2 =CH(CH 2 ) n NR 2 copolymerizations with activities up to 3400 Kg copolymer mol À1 -Zr h -1 atm -1 ,and with comonomer enchainment up to 5.5 mol % in the absence of masking reagents.G roup 4c atalyst-aminoolefin structure-activity-selectivity relationships reflect the preference of olefin activation over free amine coordination, which is supported by mechanistic experiments and DFT analysis.These results illuminate poorly understood facets of d 0 metal-catalyzed polar olefin monomer copolymerization processes.Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: https://doi.

Here we report the combined application of high-resolution solid-state 13C–CPMAS-NMR and FT-IR spectroscopy, elemental analysis, kinetic poisoning/active site counting, variable dielectric constant medium, and DFT computation to characterize the surface chemistry of a pyridylamido hafnium complex (Cat1, L1-HfMe2, L1 = 2,6-diisopropyl-N-{(2-isopropylphenyl)­[6-(naphthalen-1-yl)­pyridin-2-yl]­methyl}­aniline) adsorbed on Brønsted acidic sulfated zirconia (ZrS). The spectroscopic and DFT results indicate protonolytic formation of organohafnium cations having a largely electrostatic pyridylamido-Hf-CH3 +···ZrS– interaction with elongated Hf···OZrS distances of ∼2.14 Å. High-molecular-weight polyethylenes and ethylene/1-octene copolymers are obtained with this supported catalyst without an activator/cocatalyst. The DFT calculations reveal that the first ethylene insertion into the Hf-methyl bond has a lower barrier than the corresponding insertion into the Hf-aryl bond of this single-site heterogeneous catalyst.

Homobimetallic Hf(IV) complexes, L 2 -Hf 2 Me 5 (3) and L 2 -Hf 2 Me 4 (4) (L 2 = N,N′-{[naphthalene-1,4-diylbis(pyridine-6,2-diyl)]bis[(2-isopropylphenyl)-methylene)]bis(2,6-diisopropylaniline}), were synthesized by reaction of the free ligand L 2 with the appropriate Hf precursor and were characterized in solution (NMR) and in the solid state (X-ray diffraction). In 3, L 2 acts as a dianionic tridentate ligand for one Hf metal center and as a monoanionic bidentate ligand for the other, whereas in 4, both Hf units are tricoordinated to opposite sides of L 2 . In the solid state, the Hf···Hf distance is significantly different in 3 vs 4 (6.16 vs 8.06 Å, respectively), but in solution, the structural dynamics of the two linked metallic units in bis-activated complex 3 accesses conformers with far closer Hf···Hf distances (∼3.2 Å). Once activated with Ph 3 C + B(C 6 F 5 ) 4 − (B 1 ) or PhNMe 2 H + B(C 6 F 5 ) 4 − (NB), 3 exhibits pronounced bimetallic cooperative effects in ethylene homopolymerization and ethylene +1-octene copolymerization vs the monometallic analogue L 1 -HfMe 2 (1, L 1 = 2,6-diisopropyl-N-{(2-isopropylphenyl)[6-(naphthalen-1-yl)pyridin-2-yl]methyl}aniline) and bimetallic 4, producing polyethylene with 5.7 times higher M w and poly(ethylene-co-1-octene) with 2.4 times higher M w and 1.9 times greater 1-octene enchainment densities than 1. The activation chemistry of 3 and 4 with 1 or 2 equiv of B 1 and NB is characterized in detail by NMR spectroscopy. In sharp contrast to 1, which undergoes Hf− C naph protonolysis followed by naphthyl remetalation with NB as the cocatalyst, activation of 3 with B 1 or NB proceeds by consecutive −CH 3 protonolysis/abstractions at each Hf center, explaining the higher polymerization activity of 3/NB versus 1/ NB. All product polymers have narrow (2−3) PDIs, and this is explained by NMR evidence for very fast exchange of alkyl moieties between the two active Hf metal centers. Key experimental findings are supported by DFT analysis.

Ethylene/polar monomer coordination copolymerization offers an attractive way of making functionalized polyolefins. However, ethylene copolymerization with industrially relevant short chain length alkenoic acid remain a big challenge. Here we report the efficient direct copolymerization of ethylene with vinyl acetic acid by tetranuclear nickel complexes. The protic monomer can be extended to acrylic acid, allylacetic acid, ω-alkenoic acid, allyl alcohol, and homoallyl alcohol. Based on X-ray analysis of precatalysts, control experiments, solvent-assisted electrospray ionization-mass spectrometry detection of key catalytic intermediates, and density functional theory studies, we propose a possible mechanistic scenario that involves a distinctive vinyl acetic acid enchainment enabled by Ni···Ni synergistic effects. Inspired by the mechanistic insights, binuclear nickel catalysts are designed and proved much more efficient for the copolymerization of ethylene with vinyl acetic acid or acrylic acid, achieving the highest turnover frequencies so far for both ethylene and polar monomers simultaneously.