Coordination−Insertion Copolymerization of Fundamental Polar Monomers

Polyethylene materials with in-chain ketones from nonalternating catalytic copolymerization

[Figure: see text].

Palladium-Catalyzed Living/Controlled Vinyl Addition Polymerization of Cyclopropenes

Despite the various utilities of cyclopropenes (CPEs) in organic synthesis and ring-opening metathesis polymerization (ROMP), their vinyl addition polymerization has been sporadically explored, and the corresponding living/controlled polymerization remains a formidable challenge. The major obstacle is the intrinsic instability of the intermediate and the kinetic barrier for propagation. Herein a living/controlled vinyl addition polymerization of 3-methyl-3-carboxymethyl CPEs, catalyzed by [Pd(π-allyl)Cl]₂ ligated by a sulfinamide bisphosphine ligand, is demonstrated. A plot of the number-average molecular weight (M_n) versus the conversion was found to be linear during the polymerization, with the molecular weight dispersity (M_w/M_n) remaining narrow. The M_n values increased linearly with the increase in the initial feed ratio of monomer to catalyst. Furthermore, controlled block copolymerization via sequential monomer addition was successful. All of these points corroborate the living nature of this polymerization. The synergistic coordination action of the catalyst ligand and the lateral carbonyl group in the cyclopropene moiety plays a key role in achieving the efficient polymerization in a living/controlled manner.

Gold-Catalyzed Post-Polymerization Modification of Commodity Aromatic Polymers

Synthetic aromatic polymers are ubiquitous and indispensable to modern life, industry, and the global economy. The direct functionalization of these materials remains a considerable challenge on account of their unreactive aromatic C-H bonds and robust physical properties. Here, we demonstrate that homogeneous gold catalysis offers a mild, chemoselective, and practical approach to functionalize high-volume commodity aromatic polymers. Utilizing a gold-catalyzed intermolecular hydroarylation between a methyl ester functionalized alkyne, methyl propiolate, and nucleophilic arenes within polystyrene (PS) results in direct functionalization of phenyl rings with 1,2-substituted methyl acrylate functional groups. The reactivity and functionalization depend on the steric and electronic environment of the catalyst, counterion pairing, and method of activation. The reactivity is broad in scope, enabling the functionalization of arenes within commercial polysulfone (PSU) and waste polyethylene terephthalate (PET). These reactions open new opportunities to chemically transform aromatic polymers and modify their physical properties.

Effect of para-Substituents in Ethylene Copolymerizations with 1-Decene, 1-Dodecene, and with 2-Methyl-1-Pentene Using Phenoxide Modified Half-Titanocenes-MAO Catalyst Systems

Effect of para-substituents in the ethylene (E) copolymerization with 1-decene (DC), 1-dodecene (DD), and with 2-methyl-1-pentene (2M1P) using a series of Cp*TiCl2 (O-2,6-i Pr2 -4-R-C6 H2 ) [R=H (1), t Bu (2), Ph (3), CHPh2 (4), CPh3 (5), SiMe3 (6), SiEt3 (7), and newly prepared 4-t BuC6 H4 (8) and 3,5-Me2 C6 H3 (9)]-MAO catalyst systems has been studied. The activities in these copolymerization reactions were affected by the para-substituent, and the SiMe3 (6), SiEt3 (7) and 3,5-Me2 C6 H3 (9) analogues showed the higher activities at 50 °C in the E copolymerization reactions with DC (1.06-1.44×106  kg-polymer/mol-Ti⋅h), DD (1.04-1.88×106  kg-polymer/mol-Ti⋅h) than the others, whereas no significant differences were observed in the comonomer incorporations. Complexes 6 and 7 also showed the higher activities at 50 °C in the E/2M1P copolymerization, and the 2M1P incorporation was affected by the para-substituent and the polymerization temperature; complex 9 showed better 2M1P incorporation at 25 °C.

Distorted Sandwich a-Diimine Pd(II) Catalyst: Linear Polyethylene and Synthesis of Ethylene/Acrylate Elastomers

The introduction of m-xylyl substituents to α-diimine Pd(II) catalyst promotes living ethylene polymerization at room temperature and low pressure to yield high molecular weight polyethylene (PE) with low branching (<17/1000C). m-Xylyl groups provide a highly effective blockage to the axial sites of the catalytic center and form a distorted sandwich geometry. The shielding prevents chain-transfer and easy accessibility of polar monomers, leading to a living polymerization. Conducting a light irradiation as part of the one-step metal-organic insertion light initiated radical (MILRad) process leads to diblock copolymers of ethylene and acrylates. Incorporation of different acrylate block sequences can significantly modify the mechanical and chemical properties of block copolymers which can be modulated to be a hard plastic, elastomer, or semi-amorphous polymer.

Neutral Unsymmetrical Coordinated Cyclophane Polymerization Catalysts

Cyclophane structures can control steric pressure in the otherwise open spaces of square‐planar d⁸‐metal catalysts. This elegant concept was so far limited to symmetrical coordinated metals. We report how a cyclophane motif can be generated in ligands that chelate via two different donors. An ancillary second imine in the versatile κ²‐N,O‐salicylaldiminato catalyst type enables ring closure via olefin metathesis and selective double bond hydrogenation to yield a 30‐membered ring efficiently. Experimental and theoretical analyses show the ancillary imine is directed away from the active site and inert for catalysis. In ethylene polymerization the cyclophane catalyst is more active and temperature stable vs. an open structure reference, notably also in polar solvents. Increased molecular weights and decreased degrees of branching can be traced to an increased energy of sterically demanding transition states by the encircling cyclophane while chain propagation remains highly efficient.

Fluorinated α-Diimine Nickel Mediated Ethylene (Co)Polymerization

Fluorine substituents in transition metal catalysts are of great importance in olefin polymerization catalysis; however, the comprehensive effect of fluorine substituents is elusive in seminal late transition metal α-diimine catalytic system. In this contribution, fluorine substituents at various positions (ortho-, meta-, and para-F) and with different numbers (F_n ; n=0, 1, 2, 3, 5) were installed into the well-defined N-terphenyl amine and thus were studied for the first time in the nickel α-diimine promoted ethylene polymerization and copolymerization with polar monomers. The position of the fluorine substituent was particularly crucial in these polymerization reactions in terms of catalytic activity, polymer molecular weight, branching density, and incorporation of polar monomer, and thus a picture on the fluorine effect was given. As a notable result, the ortho-F substituted α-diimine nickel catalyst produced highly linear polyethylenes with an extremely high molecular weight (M_w =8703 kDa) and a significantly low degree of branching of 1.4/1000 C; however, the meta-F and/or para-F substituted α-diimine nickel catalysts generated highly branched (up to 80.2/1000 C) polyethylenes with significantly low molecular weights (M_w =20-50 kDa).

Influences of Ligand Backbone Substituents on Phosphinecarbonylpalladium and -nickel Catalysts for Ethylene Polymerization and Copolymerization with Polar Monomers

A series of phosphinecarbonylpalladium and -nickel catalysts bearing various substituents on the ligand backbone were prepared, characterized, and used in ethylene polymerization and copolymerization with polar monomers. The Pd and Ni catalysts can achieve high activities as well as high polymer molecular weights in both ethylene polymerization and copolymerization with polar monomers. The electron-donating group from the carbonyl side can effectively increase the polymer molecular weights. Utilization of a cyclic backbone structure can increase the catalytic activities at the expense of the polymer molecular weights. Moreover, installation of a pyridyl moiety in the ligand backbone can enable Lewis acid responsiveness and can enhance the polymerization activities. These results suggest the importance of the ligand backbone for the properties of catalysts in ethylene polymerization and copolymerization reactions.

A disubstituted-norbornene-based comonomer strategy to address polar monomer problem

The transition-metal-catalyzed copolymerization of olefins with polar comonomers is a direct strategy to access polar-functionalized polyolefins in an economical manner. Due to the intrinsic poisoning effect of polar groups towards Lewis acidic metal centers and the drastic reactivity differences of polar comonomers versus non-polar olefins, it is challenging to develop catalysts that provide the desired polymer molecular weight, comonomer incorporation, and activity. In this contribution, we tackle this issue from a comonomer perspective using 5,6-disubstituted norbornenes, which are highly versatile, easily accessible, inexpensive, and capable of introducing two functional groups in a single insertion. More importantly, they are only mildly poisoning due to the presence of long spacers between double bonds and polar groups, and are not prone to β-hydride elimination due to their cyclic structures. As strong π-donors, they can competitively bind to metal centers versus olefins. Indeed, phosphine-sulfonate palladium catalysts can catalyze the copolymerization of ethylene with 5,6-disubstituted norbornenes and simultaneously achieve a high polymerization activity, copolymer molecular weight, and comonomer incorporation. The practicality of this system was demonstrated by studying the properties of the resulting polymers, copolymerization in hydrocarbon solvents or in bulk, recovery/utilization of unreacted comonomer, molecular weight modulation, and large-scale synthesis.

Ferrocenylmethylphosphanes and the Alpha Process for Methoxycarbonylation: The Original Story

The Lucite Alpha process is the predominant technology for the preparation of acrylics. This two-stage process involves the palladium-catalysed formation of methyl propanoate from ethene, CO, and methanol, followed by the oxidative formylation of methyl propanoate into methyl methacrylate. A range of bis-1,2-disubstituted aminomethylferrocenes has been prepared and characterised. These complexes serve as precursors to a variety of bulky ferrocenylmethyldiphosphanes that, in turn, function as ligands in the palladium-catalysed process. We describe the crystal structures of five ligand precursors and provide a rationale for their design. In situ catalyst testing on palladium complexes derived from ferrocenylphosphanes demonstrates that these are highly selective (>99.5%) catalysts for the formation of methyl propanoate from ethene, CO, and methanol and have turnover numbers exceeding 50,000. This article credits those researchers who worked on this project in the early days, who received little or no credit for their achievements and endeavours.

Synthesis of Nonalternating Polyketones Using Cationic Diphosphazane Monoxide-Palladium Complexes

Copolymerization of olefin with carbon monoxide has received considerable interest from both academia and industry, and the introduction of polar carbonyl group renders the resultant polyketones with excellent mechanical strength, crystallinity, photodegradability, hydrophilicity, surface, and barrier properties. However, most of the reported polyketones are difficult to be processed because of limited solubility in common solvents and high melting temperature (T_m ∼ 260 °C) resulting from the strictly alternative structure. Nonalternating copolymerization of ethylene with CO is a very promising method to circumvent the problem of processability of traditional perfectly alternating polyketone. In the contribution, the palladium coordinated diphosphazane monoxide substituted by strong electron-donating groups is discovered to be highly reactive for producing nonalternating polyketones, and up to 24.2% extra ethylene incorporation has lowered T_m values to 147 and 165 °C and further improved thermal stability (T_d ∼ 339 °C) of the resultant materials. Our data demonstrates that cationic palladium complexes can also exhibit excellent reactivity and an unprecedented nonalternating degree in this copolymerization.

Hydrogen-Bonding-Induced Heterogenization of Nickel and Palladium Catalysts for Copolymerization of Ethylene with Polar Monomers

The practical synthesis of polar-functionalized polyolefins using transition-metal-catalyzed copolymerization of olefins with polar monomers is a challenge; the use of heterogeneous catalysts is little explored. Herein, we report the synthesis of heterogeneous naphthoquinone-based nickel (Ni/SiO₂ ) and palladium (Pd/SiO₂ ) catalysts through hydrogen bonding interactions of the ligands with the silica surface. Ni/SiO₂ exhibits high activities (up to 2.65×10⁶  g mol^-1  h^-1 ) during the copolymerization of ethylene with 5-hexene-1-yl-acetate, affording high-molecular-weight (M_n up to 630 000) polar-functionalized semicrystalline polyethylene (comonomer incorporation up to 2.8 mol %), along with great morphology control. The resulting copolymers possess improved surface properties and great mechanical properties. Pd/SiO₂ can mediate ethylene copolymerization with polar monomers with moderate activity to produce high-molecular-weight copolymers with tunable comonomer incorporation.

Organo-Cobalt Complexes in Reversible-Deactivation Radical Polymerization

Cobalt complexes have played an essential role in different chemical reactions. One of them that has attracted substantial attention in polymer science is cobalt mediated radical polymerization (CMRP), which is famous for its remarkable efficiency in controlling the radical polymerization of vinyl acetate (VAc) and other less active monomers (LAMs). Two pathways, reversible termination (RT) and degenerative transfer (DT), were recognized to control the polymerization in CMRP and could be further used to rationalize the mechanism of other RDRP methods. These control mechanisms were then found to be correlated to the redox potential of cobalt complexes and thus could be judged more quantitatively. The control of polymer composition and tacticity could also be achieved by using CMRP. The hybridization of CMRP and atom transfer radical polymerization (ATRP) could directly synthesize the vinyl acetate/methyl methacrylate and vinyl acetate/styrene block copolymers in one pot. The copolymer of acrylates and 1-octene could be obtained by visible-light-induced CMRP. With the addition of bulky Lewis acid, CMRP of N,N-dimethylacrylamide (DMA) showed high isotacticities with the contents of meso dyads (m) and meso triads (mm) up to 94 % and 87 %, respectively, and generated the crystalline PDMA with T_m as high as 276 °C. This personal account reviewed the development of CMRP with the mechanistic understanding, the control of composition and stereoselectivity of the polymeric products, and its perspective.

Ethylene Polymerization Activity of (R3P)Ni(codH)+ (cod = 1,5-cylcooctadiene) Sites Supported on Sulfated Zirconium Oxide

PAr₃ containing o-OMe, o-Me, or o-Et substituents reacts with Brønsted sites on sulfated zirconium oxide (SZO) to form [HPAr₃][SZO]. The phosphonium sites on this material react with bis(cyclooctadiene)nickel [Ni(cod)₂] to form [Ni(PAr₃)(codH)][SZO] that are active in ethylene polymerization reactions. Selective poisoning studies with pyridine show that ∼90% of the Ni(PAr₃)(codH)⁺ sites in this material are active in polymerization reactions.

Modulation of N^N'-bidentate chelating pyridyl-pyridylidene amide ligands offers mechanistic insights into Pd-catalysed ethylene/methyl acrylate copolymerisation

The efficient copolymerisation of functionalised olefins with alkenes continues to offer considerable challenges to catalyst design. Based on recent work using palladium complexes containing a dissymmetric N^N'-bidentate pyridyl-PYA ligand (PYA = pyridylidene amide), which showed a high propensity to insert methyl acrylate, we have here modified this catalyst structure by inserting shielding groups either into the pyridyl fragment, or the PYA unit, or both to avoid fast β-hydrogen elimination. While a phenyl substituent at the pyridyl side impedes catalytic activity completely and leads to an off-cycle cyclometallation, the introduction of an ortho-methyl group on the PYA side of the N^N'-ligand was more prolific and doubled the catalytic productivity. Mechanistic investigations with this ligand system indicated the stabilisation of a 4-membered metallacycle intermediate at room temperature, which has previously been postulated and detected only at 173 K, but never observed at ambient temperature so far. This intermediate was characterised by solution NMR spectroscopy and rationalises, in part, the formation of α,β-unsaturated esters under catalytic conditions, thus providing useful principles for optimised catalyst design.

Controlled di-lithiation enabled synthesis of phosphine-sulfonamide ligands and implications in ethylene oligomerization

Catalyst design for ethylene oligomerization has attracted significant interest. Herein, we report the synthesis of phosphine-sulfonamide-derived palladium complexes and examine their performance in ethylene oligomerization. Arresting a dilithiation intermediate of N-(2-bromophenyl)-4-methylbenzenesulfonamide (1) at -84 °C selectively produced N-(2-(bis(2-methoxyphenyl)phosphanyl)phenyl)-4-methylbenzenesulfonamide (L1A). However, the same reaction at -41 °C delivered a different ligand; 2-(bis(2-methoxyphenyl)phosphanyl)-4-methyl-N-phenylbenzenesulfonamide (L2A). The generality of our strategy has been demonstrated by preparing N-(2-(diphenylphosphanyl)phenyl)-4-methylbenzenesulfonamide (L1B) and 2-(diphenylphosphanyl)-4-methyl-N-phenylbenzenesulfonamide (L2B). Subsequently, L1A and L1B were treated with a palladium precursor to yield 5-membered complexes C1 and C2, respectively. In contrast, L2A upon treatment with palladium produced a 6-membered metal complex C3. Thus, a small library of 7 palladium complexes (C1-C7) were synthesized by varying the donor moiety (pyridine, DMSO, and acetonitrile). The identity of palladium complexes was unambiguously ascertained using a combination of spectroscopic and analytical methods, including single-crystal X-ray diffraction. The performance of the complexes C1-C7 was investigated in ethylene oligomerization and almost all of them were found to be active. The resultant ethylene oligomers were characterized using 1H and 13C NMR, MALDI-ToF-MS, and GPC. Detailed screening of reaction parameters revealed 100 °C and 40 bars ethylene to be optimal conditions. Complex C5 outperformed other complexes and produced ethylene oligomers with a molecular weight of 1000-1900 g mol-1.

Sequential Insertion of Alkynes, Alkenes, and CO into the Pd–C Bond of ortho-Palladated Primary Phenethylamines: from η3-Allyl Complexes and Enlarged Palladacycles to Functionalized Arylalkylamines

The eight-membered metallacycles arising from the insertion of 1 equiv of alkyne into the Pd–C bond of ortho-metalated homoveratrylamine and phentermine can further react with alkenes to give two different types of mononuclear complexes depending on the nature of the olefin. When terminal alkenes (styrene and ethyl acrylate) are used, a mixture of the anti/syn η³-allyl Pd(II) complexes are isolated, which evolve slowly to the syn isomers by heating the mixtures appropriately. These η³-allyl Pd(II) complexes do not react with CO or weak bases, but when they are treated with a strong base, such as KO^tBu, they afford Pd(0) and the functionalized starting phenethylamines containing a 1,3-butadienyl substituent in an ortho position. When 2-norbornene was used instead of terminal alkenes, the strained olefin inserts into the alkenyl Pd(II) complex to afford a 10-membered norbornyl palladium(II) complex, in which the new C,N-chelate ligand is coordinated to the metal through an additional double bond, occupying three coordination positions. The reactivity of these norbornyl complexes depends on the substituents on the inserted alkenyl fragment, and thus they can further react with (1) KO^tBu, to give Pd(0) and a tetrahydroisoquinoline nucleus containing a tricyclo[3.2.1]octyl ring, or (2) CO and TlOTf, to afford Pd(0) and amino acid derivatives or the corresponding lactones arising from an intramolecular Michael addition of the CO₂H group to the α,β-unsaturated ester moiety. Crystal structures of every type of compound have been determined by X-ray diffraction studies.

Efficient Suppression of Chain Transfer and Branching via Cs -Type Shielding in a Neutral Nickel(II) Catalyst

An effective shielding of both apical positions of a neutral NiII active site is achieved by dibenzosuberyl groups, both attached via the same donors' N-aryl group in a Cs -type arrangement. The key aniline building block is accessible in a single step from commercially available dibenzosuberol. This shielding approach suppresses chain transfer and branch formation to such an extent that ultrahigh molecular weight polyethylenes (5×106  g mol-1 ) are accessible, with a strictly linear microstructure (<0.1 branches/1000C). Key features of this highly active (4.3×105  turnovers h-1 ) catalyst are an exceptionally facile preparation, thermal robustness (up to 90 °C polymerization temperature), ability for living polymerization and compatibility with THF as a polar reaction medium.

Toward the Copolymerization of Propylene with Polar Comonomers

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.

2,4,6-Triphenylpyridinium: A Bulky, Highly Electron-Withdrawing Substituent That Enhances Properties of Nickel(II) Ethylene Polymerization Catalysts

The reactivity of Ni^II and Pd^II olefin polymerization catalysts can be enhanced by introduction of electron-withdrawing substituents on the supporting ligands rendering the metal centers more electrophilic. Reported here is a comparison of ethylene polymerization activity of a classical salicyliminato nickel catalyst substituted with the powerful electron-withdrawing 2,4,6-triphenylpyridinium (trippy) group to the -CF₃ analogue. The trippy substituent is substantially more electron-withdrawing (σ_meta =0.63) than the trifluoromethyl group (σ_meta =0.43) which results in a ca. 8-fold increase in catalytic turnover frequency. An additional advantage of trippy is the high steric bulk relative to the trifluoromethyl group. This feature results in a four-fold increase in polymer molecular weight owing to enhanced retardation of chain transfer. A significant increase in catalyst lifetime is observed as well.

Progress in propylene homo- and copolymers using advanced transition metal catalyst systems

Recent progress on advanced transition metal catalysts for propylene polymerization and copolymerization are reviewed.

Copolymerisation of 1-alkenes with bulky oxygen-containing olefins for dual-stage functionalisation of polyolefins

Pd–Diimine complexes catalyse the copolymerisation of 1-alkenes with olefins having a bulky oxygen-containing substituents, such as cyclic acetal or bicycloorthoester groups, affording the copolymers containing the comonomer unit up to 34%.

Influence of intramolecular π–π and H-bonding interactions on pyrazolylimine nickel-catalyzed ethylene polymerization and co-polymerization

Pyrazolylimine-based nickel catalysts bearing intramolecular π–π and H-bonding interactions show high activity, thermal stability, and Mn of polyethylene.

Computational study of the copolymerization mechanism of ethylene with methyl 2-acetamidoacrylate catalyzed by phosphine-sulfonate palladium complexes

In this research, a computational study is carried out to describe the insertion of a vital monomer, methyl 2-acetamidoacrylate (MAAA), into catalyst A (A = [(POOMe,OMe)PdMe]) (POOMe,OMe = 2[2-MeOC6H4](2-SO3-5-MeC6H4)P).

Custom-made polar monomers utilized in nickel and palladium promoted olefin copolymerization

In this review, the functions of custom-made polar monomers are insightfully emphasized in the preparation of functional polyolefins.