In situ dissolved polypropylene prediction by Raman and ATR-IR spectroscopy for its recycling.

Monitoring the dissolution of polyolefins using online spectroscopy analysis is addressed in this work, with the aim of optimizing plastic recycling processes. Two in situ spectroscopic methods are used to predict the dissolved polymer content: Raman spectroscopy and attenuated total reflectance infrared spectroscopy. Commercially available polypropylenes are considered. Different solvents are selected based on their affinity with polypropylene. Partial least squares regression is employed to identify models predicting the polymer concentration for each solvent from the online spectra. Raman spectroscopy was found to give a better prediction. It was therefore used to study different parameters influencing the dissolution process, such as solvent type, temperature and polymer form.

Spectroscopic Signature and Structure of the Active Sites in Ziegler-Natta Polymerization Catalysts Revealed by Electron Paramagnetic Resonance.

Despite decades of extensive studies, the atomic-scale structure of the active sites in heterogeneous Ziegler-Natta (ZN) catalysts, one of the most important processes of the chemical industry, remains elusive and a matter of debate. In the present work, the structure of active sites of ZN catalysts in the absence of ethylene, referred to as dormant active sites, is elucidated from magnetic resonance experiments carried out on samples reacted with increasing amounts of BCl3 so as to enhance the concentration of active sites and observe clear spectroscopic signatures. Using electron paramagnetic resonance (EPR) and NMR spectroscopies, in particular 2D HYSCORE experiments complemented by density functional theory (DFT) calculations, we show that the activated ZN catalysts contain bimetallic alkyl-Ti(III),Al species whose amount is directly linked to the polymerization activity of MgCl2-supported Ziegler-Natta catalysts. This connects those spectroscopic signatures to the active species formed in the presence of ethylene and enables us to propose an ethylene polymerization mechanism on the observed bimetallic alkyl-Ti(III),Al species based on DFT computations.

Monofunctional and Telechelic Polyethylenes Carrying Phosphonic Acid End Groups.

Monofunctional or telechelic polyethylenes (PEs) carrying phosphonic acid end groups are obtained from functional PE produced by catalyzed chain growth (CCG) on magnesium. CCG is first used to produce iodo-end-functionalized PE (PE-I) that is efficiently turned into phosphonate end-functionalized PE (PE-P(O)(OEt)2 ) in the presence of triethylphosphite through the Michaelis-Arbuzov reaction. A simple treatment of the resulting PE-P(O)(OEt)2 with bromotrimethylsilane leads to the targeted phosphonic acid end-functionalized PE (PE-P(O)(OH)2 ) for the first time. Vinyl-end-functionalized analogs (Vin-PE-P(O)(OEt)2 ) are produced using vinyl-end-functionalized PE-I (Vin-PE-I) recently obtained through CCG. A cross-metathesis reaction is then employed to couple Vin-PE-P(O)(OEt)2 and produce after treatment with bromotrimethylsilane the corresponding unprecedented α-ω-(diphosphonic acid) telechelic PE ((OH)2 (O)P-PE-P(O)(OH)2 ).

Toward Anisotropic Hybrid Materials: Directional Crystallization of Amphiphilic Polyoxazoline-Based Triblock Terpolymers.

We present the design and synthesis of a linear ABC triblock terpolymer for the bottom-up synthesis of anisotropic organic/inorganic hybrid materials: polyethylene-block-poly(2-(4-(tert-butoxycarbonyl)amino)butyl-2-oxazoline)-block-poly(2-iso-propyl-2-oxazoline) (PE-b-PBocAmOx-b-PiPrOx). The synthesis was realized via the covalent linkage of azide-functionalized polyethylene and alkyne functionalized poly(2-alkyl-2-oxazoline) (POx)-based diblock copolymers exploiting copper-catalyzed azide-alkyne cycloaddition (CuAAC) chemistry. After purification of the resulting triblock terpolymer, the middle block was deprotected, resulting in a primary amine in the side chain. In the next step, solution self-assembly into core-shell-corona micelles in aqueous solution was investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Subsequent directional crystallization of the corona-forming block, poly(2-iso-propyl-2-oxazoline), led to the formation of anisotropic superstructures as demonstrated by electron microscopy (SEM and TEM). We present hypotheses concerning the aggregation mechanism as well as first promising results regarding the selective loading of individual domains within such anisotropic nanostructures with metal nanoparticles (Au, Fe3O4).

Divinyl-end-functionalized polyethylenes: ready access to a range of telechelic polyethylenes through thiol-ene reactions.

Telechelic α,ω-iodo-vinyl-polyethylenes (Vin-PE-I) were obtained by catalytic ethylene polymerization in the presence of [(C5 Me5 )2 NdCl2 Li(OEt2 )2 ] in combination with a functionalized chain-transfer agent, namely, di(10-undecenyl)magnesium, followed by treatment of the resulting di(vinylpolyethylenyl)magnesium compounds ((vinyl-PE)2 Mg) with I2 . The iodo-functionalized vinylpolyethylenes (Vin-PE-I) were transformed into unique divinyl-functionalized polyethylenes (Vin-PE-Vin) by simple treatment with tBuOK in toluene at 95 °C. Thiol-ene reactions were then successfully performed on Vin-PE-Vin with functionalized thiols in the presence of AIBN. A range of homobifunctional telechelic polyethylenes were obtained on which a hydroxy, diol, carboxylic acid, amine, ammonium chloride, trimethoxysilyl, chloro, or fluoroalkyl group was installed quantitatively at each chain end.

Proton transfers are key elementary steps in ethylene polymerization on isolated chromium(III) silicates.

Mononuclear Cr(III) surface sites were synthesized from grafting [Cr(OSi(O(t)Bu)3)3(tetrahydrofurano)2] on silica partially dehydroxylated at 700 °C, followed by a thermal treatment under vacuum, and characterized by infrared, ultraviolet-visible, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopy (XAS). These sites are highly active in ethylene polymerization to yield polyethylene with a broad molecular weight distribution, similar to that typically obtained from the Phillips catalyst. CO binding, EPR spectroscopy, and poisoning studies indicate that two different types of Cr(III) sites are present on the surface, one of which is active in polymerization. Density functional theory (DFT) calculations using cluster models show that active sites are tricoordinated Cr(III) centers and that the presence of an additional siloxane bridge coordinated to Cr leads to inactive species. From IR spectroscopy and DFT calculations, these tricoordinated Cr(III) sites initiate and regulate the polymer chain length via unique proton transfer steps in polymerization catalysis.

Polymerization of ethylene by silica-supported dinuclear Cr(III) sites through an initiation step involving C-H bond activation.

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.

A well-defined silica-supported aluminium alkyl through an unprecedented, consecutive two-step protonolysis-alkyl transfer mechanism.

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)].

Non-oxidative coupling reaction of methane to ethane and hydrogen catalyzed by the silica-supported tantalum hydride: ([triple bond]SiO)2Ta-H.

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.