Significance The Phillips catalyst—CrO x /SiO 2 —produces 40–50% of global high-density polyethylene, yet several fundamental mechanistic controversies surround this catalyst. What is the oxidation state and nuclearity of the active Cr sites? How is the first Cr–C bond formed? How does the polymer propagate and regulate its molecular weight? Here we show through combined experimental (infrared, ultraviolet-visible, X-ray near edge absorption spectroscopy, and extended X-ray absorption fine structures) and density functional theory modeling approaches that mononuclear tricoordinate Cr(III) sites immobilized on silica polymerize ethylene by the classical Cossee–Arlman mechanism. Initiation (C–H bond activation) and polymer molecular weight regulation (the microreverse of C–H activation) are controlled by proton transfer steps.

Silica-supported tantalum hydride, (SiO)2Ta-H (1), proves to be the first single-site catalyst for the direct non-oxidative coupling transformation of methane into ethane and hydrogen at moderate temperatures, with a high selectivity (>98%). The reaction likely involves the tantalum-methyl-methylidene species as a key intermediate, where the methyl ligand can migrate onto the tantalum-methylidene affording the tantalum-ethyl.

The insertion of an olefin into a preformed metal-carbon bond is a common mechanism for transition-metal-catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal-carbon bond is not present in the precatalyst. The Phillips catalyst, CrO3 dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first CrC bond is formed remain unknown. We synthesized well-defined dinuclear Cr(II) and Cr(III) sites on silica. Whereas the Cr(II) material was a poor polymerization catalyst, the Cr(III) material was active. Poisoning studies showed that about 65 % of the Cr(III) sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si-(μ-OH)-Cr(III) species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr(III) O bonds.

While Ziegler-Natta (ZN) polymerization is one of the most important catalytic industrial processes, the atomic-scale nature of the catalytically active surface species remains unknown. Coupling high-resolution solid-state NMR spectroscopy with periodic DFT calculations, we demonstrate that the major surface species in the Ziegler-Natta pre-catalyst corresponds to an alkoxy Ti(IV) surface species, which probably results from the ring opening of THF on a cationic Ti(IV) species.

Since the discovery of the Ziegler-Natta catalyst [1,2] for the coordinative polymerization of ethylene, continual chemical and process optimizations have led to a broad range of commodity polyolefins with enhanced properties. [3][4][5] Despite these extensive efforts and numerous breakthroughs, telechelic polyethylenes (PEs), in which both chain ends feature the same functional group (X-PE-X) or chemically distinct groups (X-PE-Z), are yet to be accessed using catalytic ethylene polymerization. Telechelic polymers have important commercial applications as cross-linkers, chain linkers, or building blocks, [6] highlighting the opportunities reliant on the development of telechelic PE production. In this context, catalytic polymerization of ethylene, producing many PE chains per transition-metal center, is the best route to overcome the cost limitation [7] presented by other strategies, while reliably attaining the crystalline, insoluble, thermoplastic properties of high density PE.Previous methods to produce telechelic PE have involved polymerization of butadiene followed by functionalization and hydrogenation, [8] partial hydrogenation of polybutadiene followed by metathesis degradation of the interior olefin groups, [9] ring-opening metathesis polymerization of a cyclic olefin followed by functionalization and hydrogenation, [10,11] and the living coordinative polymerization of olefins. [12,13] These techniques have produced valuable materials for the fundamental understanding of structure-property relationships. They are, however, either multistep processes, noncatalytic (using stoichiometric quantities of high-cost initiators), or employ monomers, such as butadiene or cyclic olefins, that are expensive compared to ethylene and consequently incompatible with the prerequisites of industrial production.In the field of catalytic ethylene polymerization, the scope of end-functional PE production using high-volume methods is limited by both the range of efficient, quantitative, and selective transformations of transition-metal-bound polymer chain ends and by competition from chain-transfer reactions, in particular b-hydrogen transfer, that deactivate the chain end. Approaches to overcoming these limitations have emphasized the exploitation of the reactivity of the polymer-metal bond present in living systems, [14,15] in which chain transfer reactions are absent. The development of complexes that mediate catalyzed chain growth (CCG) [16] of PE chains on a main-group metal has facilitated the introduction of PE end functions under catalytic conditions. In CCG polymerization, reversible PE chain transfer, which is rapid in comparison to propagation, occurs between a catalytic amount of a transition metal (on which the chains propagate) and a main-group metal used as the chain-transfer agent (CTA). [17][18][19] A PE-Mg-PE intermediate can be produced by CCG on magnesium using [(C 5 Me 5 ) 2 NdCl 2 Li(OEt 2 ) 2 ] in combination with a dialkyl magnesium as CTA. [20,21] The nucleophilic MgÀC bonds of PE-Mg-PE can then be e...

Ethylene polymerizations were performed in toluene using the neodymocene complex (C5Me5)2NdCl2Li­(OEt2)2 or {(Me2Si­(C13H8)2)­Nd­(μ-BH4)­[(μ-BH4)­Li­(THF)]}2 in combination with n-butyl-n-octylmagnesium used as both alkylating and chain transfer agent. The kinetics were followed for various [Mg]/[Nd] ratios, at different polymerization temperatures, with or without ether as a cosolvent. These systems allowed us to (i) efficiently obtain narrowly distributed and targeted molar masses, (ii) characterize three phases during the course of polymerization, (iii) estimate the propagation activation energy (17 kcal mol–1), (iv) identify the parameters that control chain transfer, and (v) demonstrate enhanced polymerization rates and molar mass distribution control in the presence of ether as cosolvent. This experimental set of data is supported by a computational investigation at the DFT level that rationalizes the chain transfer mechanism and the specific microsolvation effects in the presence of cosolvents at the molecular scale. This joint experimental/computational investigation offers the basis for further catalyst developments in the field of coordinative chain transfer polymerization (CCTP).

Impregnation of [(AliBu(3))(Et(2)O)] on partially dehydroxylated SBA-15 affords a mesoporous material bearing the well-defined single site surface aluminium species [(≡SiO)(2)Al(iBu)(Et(2)O)].

Low-temperature skeletal cleavage and the formation of CÀC bonds are of prime importance in the petrochemical industry because the transformation of crude oil into hydrocarbons having different numbers of carbon atoms is often necessary. In this regard, the skeletal transformation of olefins into valuable products remains an important challenge in chemistry. Any new reaction related to this challenge is important. In 1991, we discovered that the highly electrophilic earlytransition-metal hydride [(SiO) 3 Zr-H] supported on silica [1][2][3][4] could activate the C À H and C À C bonds of alkanes or polyolefins and could also catalyze the hydrogenolysis of these hydrocarbons into a range of gasolines.[5] Later, in 1997, we found that the highly electrophilic silica-supported tantalum hydride [(SiO) 2 TaH] [6,7] could transform any light alkane into its lower and higher homologues by both cleavage and formation of C À H and C À C bonds. We called this new catalytic reaction "alkane metathesis" by analogy to "olefin metathesis" [Eq.(1)].[6-8] Herein we disclose that tantalumhydride (TaH) supported on fibrous silica nanospheres (KCC-1) can catalyze, in the presence of hydrogen, the direct conversion of olefins into alkanes having higher and lower numbers of carbon atoms; therefore we refer to the reaction as "hydro-metathesis" [Eq. (2)]. This novel reaction hasexcellent catalytic performance and unexpectedly high turnover numbers as compared to the now classical alkane metathesis. For the first time, this silica-supported tantalum hydride shows remarkable catalytic stability, with an excellent potential of regeneration.In the case of propane metathesis, kinetic studies carried out at very low contact time, in a continuous flow reactor, revealed that the primary products of this reaction were olefins and H 2 .[9] This observation, among many others, as well as elementary steps known in tantalum organometallic chemistry, led us to propose a mechanism based on the following key steps: 1) paraffin C À H bond activation leading to a metal/alkyl species with subsequent formation of an olefin and a metal hydride by b-hydride elimination; 2) a-hydrogen abstraction from the same metal/alkyl species leading to the formation of a metallocarbene; 3) olefin metathesis on this metallocarbene; and 4) hydrogenation of the new olefins on the metal hydride (see Scheme S1 in the Supporting Information).[9] Thus, the tantalum hydride in this metathesis reaction acts as a trifunctional single-site system (dehydrogenation/metathesis/hydrogenation).In this work, we observed that TaH/KCC-1 not only transforms any olefin in the presence of hydrogen at moderate temperatures into the expected corresponding alkane, but also transforms the same olefin into alkanes having a higher and lower number of carbon atoms. Importantly, in our quest of nanocatalysts, [10] we used our recently discovered high-surface-area silica nanospheres having a unique fibrous morphology (KCC-1) as the catalyst support.[11]The KCC-1-supported tantalum hydride (TaH/KCC-1) ...