Microphase Separation and Crystallization in H‑Bonding End-Functionalized Polyethylenes

Well-defined, crystalline, low molar mass polyethylene PE_<i>x</i> (where <i>x</i> is the molar mass 1300 and 2200 g mol^–1) bearing thymine (Thy) or 2,6-diaminotriazine (DAT) end groups have been synthesized from amino-terminated PE. Either double-layer or monolayer solid-state morphologies were attained depending on the nature of the end-group(s). PE₁₃₀₀-NH₂, PE₁₃₀₀-DAT, and the equimolar blend PE₁₃₀₀-Thy/DAT-PE₁₃₀₀ all organized into double-layer structures composed of extended PE chains sandwiched between H-bonding chain-ends. The double-layered morphology arose from the microphase separation of the polar end-groups and the nonpolar PE chains and was frozen by the crystallization of the PE domains. The regularity of the PE lamellar stacking was higher for the stronger and more directional associated pair Thy/DAT compared with samples of either PE-NH₂ or PE-DAT. For PE₁₃₀₀-Thy, the mesoscopic organization was driven by the crystallization of Thy domains prior to crystallization of the PE chains, forcing the small proportion of nonfunctionalized PE chains to segregate and crystallize separately to the PE-Thy chains. The confinement of PE chains between Thy domains lead to a conventional monolayer form in which extended PE chains were interdigitated. The volume fraction of Thy or DAT end-groups was a key parameter in the organization in all these systems: the PE crystallinity was higher with longer PE chains (i.e., a low volume fraction of Thy or DAT units), but the mesoscopic organization of the supramolecular PE was less regular

Ethylene–Butadiene Copolymerization by Neodymocene Complexes: A Ligand Structure/Activity/Polymer Microstructure Relationship Based on DFT Calculations

Ethylene/butadiene copolymerization can be performed by neodymocene catalysts in the presence of an alkylating/chain transfer agent. A variety of polymerization activities and copolymer microstructures can be obtained depending on the neodymocene ligands. For a set of four catalysts, namely (C₅Me₅)₂NdR, [Me₂Si­(3-Me₃SiC₅H₃)₂]­NdR, [Me₂Si­(C₅H₄)­(C₁₃H₈)]­NdR and [Me₂Si­(C₁₃H₈)₂]­NdR, we report a DFT mechanistic study of this copolymerization reaction performed in the presence of dialkylmagnesium. Based on the modeling strategy developed for the ethylene homopolymerization catalyzed by (C₅Me₅)₂NdR in the presence of MgR₂, our model is able to account for the following: (i) the formation of Nd/Mg heterobimetallic complexes as intermediates, (ii) the overall differential activity of the catalysts, (iii) the copolymerization reactivity indexes, and (iv) the specific microstructure of the resulting copolymers, including branching and cyclization. The analysis of the reaction mechanisms and the energy profiles thus relates ligand structure, catalyst activity, and polymer microstructure and sets the basis for further catalyst developments

Deciphering the Mechanism of Coordinative Chain Transfer Polymerization of Ethylene Using Neodymocene Catalysts and Dialkylmagnesium

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 <i>n</i>-butyl-<i>n</i>-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)

Borate and MAO Free Activating Supports for Metallocene Complexes

Fluorinated activating supports (AS) for metallocene complexes were prepared via treatment of silica with AlEt₃ or AlEt₂F followed by pyrolysis and combustion steps, and a subsequent fluorination step when AlEt₃ was used. This new family of activators appears to be universal for metallocene complexes leading to catalysts displaying high activities in ethylene polymerization without the addition of MAO. A productivity of 3200 g g_AS^–1 was obtained in 1 h with the catalyst <i>rac</i>-Et­(Ind)₂ZrCl₂/AS₈/Al­(<i>i</i>Bu)₃ at 80 °C under 10 bar of ethylene. An isotactic polypropylene with a melting transition at 145 °C was prepared using <i>rac</i>-Me₂Si­(2-Me-benz­(e)­Ind)₂ZrCl₂ activated by AS9 and Al­(<i>i</i>Bu)₃. The spherical particle morphology of polyolefins was particularly adapted to slurry processes employed in industry

Structural and Mechanical Properties of Supramolecular Polyethylenes

Thymine (Thy) or 2,6-diamino-1,3,5-triazine (DAT) end-groups were efficiently installed on well-defined polyethylenes (PEs) synthesized by catalyzed chain growth (CCG) polymerization. Mono- and bifunctional low-molar mass PEs (1200–1500 g·mol^–1) formed lamellar morphologies with long-range order upon cooling from the melt due to microphase segregation of polar supramolecular units and apolar PE chains. Crystallization of Thy functions into rigid planes at 180 °C induced very long-range lamellar order in Thy-functionalized PEs and dramatically suppressed PE crystallization (from 67% to 19%). DAT-functionalized PEs, whose end-groups do not crystallize, showed slightly reduced PE crystallinity (62%) and less long-range order, since assembly was instead driven by PE crystallization. Mechanical analysis of the bifunctional PEs demonstrated high moduli roughly proportional to PE crystallinity but low strains at break due to the absence of chain entanglements and/or tie chains between crystalline lamellae. This work offers important insights for designing supramolecular systems with tunable thermal and mechanical properties

Dialkenylmagnesium Compounds in Coordinative Chain Transfer Polymerization of Ethylene. Reversible Chain Transfer Agents and Tools To Probe Catalyst Selectivities toward Ring Formation

A range of dialkenylmagnesium compounds ([CH₂CH­(CH₂)_<i>n</i>]₂Mg; <i>n</i> = 1–6) were synthesized and used as chain transfer agents (CTA) with either (C₅Me₅)₂NdCl₂Li­(OEt₂)₂ (<b>1</b>) or [Me₂Si­(C₁₃H₈)₂Nd­(BH₄)₂Li­(thf)]₂ (<b>2</b>) neodymium precursors for the polymerization of ethylene. In all cases, the systems followed a controlled coordinative chain transfer polymerization mechanism. The intramolecular insertion of the vinyl group on the CTA in growing chains is possible and led to the formation of cyclopentyl, cyclohexyl, and possibly cycloheptyl chain ends. While the production of cyclopentyl- or cyclohexyl-capped polyethylene chains can be quantitative (<i>n</i> = 2–5), the integrity of this double bond can also be kept if <i>n</i> is higher than 6. In comparison to <b>1</b>/CTA catalytic systems, <b>2</b>/CTA catalytic systems showed a higher propensity to produce cycloalkyl chain ends. This was ascribed to the lower steric demand around the active site, as shown by DFT calculations. In addition, the formation of bis­(cyclopentylmethyl)­magnesium from dipentenylmagnesium using a catalytic amount of <b>2</b> was shown

Dialkenylmagnesium Compounds in Coordinative Chain Transfer Polymerization of Ethylene. Reversible Chain Transfer Agents and Tools To Probe Catalyst Selectivities toward Ring Formation

A range of dialkenylmagnesium compounds ([CH₂CH­(CH₂)_<i>n</i>]₂Mg; <i>n</i> = 1–6) were synthesized and used as chain transfer agents (CTA) with either (C₅Me₅)₂NdCl₂Li­(OEt₂)₂ (<b>1</b>) or [Me₂Si­(C₁₃H₈)₂Nd­(BH₄)₂Li­(thf)]₂ (<b>2</b>) neodymium precursors for the polymerization of ethylene. In all cases, the systems followed a controlled coordinative chain transfer polymerization mechanism. The intramolecular insertion of the vinyl group on the CTA in growing chains is possible and led to the formation of cyclopentyl, cyclohexyl, and possibly cycloheptyl chain ends. While the production of cyclopentyl- or cyclohexyl-capped polyethylene chains can be quantitative (<i>n</i> = 2–5), the integrity of this double bond can also be kept if <i>n</i> is higher than 6. In comparison to <b>1</b>/CTA catalytic systems, <b>2</b>/CTA catalytic systems showed a higher propensity to produce cycloalkyl chain ends. This was ascribed to the lower steric demand around the active site, as shown by DFT calculations. In addition, the formation of bis­(cyclopentylmethyl)­magnesium from dipentenylmagnesium using a catalytic amount of <b>2</b> was shown

Enhanced Spin Capturing Polymerization of Ethylene

Enhanced spin capturing polymerization (ESCP)a recent and versatile technique in the field of controlled radical polymerizationachieves control over molecular weights and the synthesis of complex copolymer structures for a wide range of monomers. In the present work, the use of ESCP was extended to the radical polymerization of ethylene under mild conditions (low temperature and medium ethylene pressure) using a nitrone as spin trapping agent. It was demonstrated that the evolution of polyethylene (PE) molecular weight can be accurately described by classical ESCP kinetic equations. A PE bearing a midchain alkoxyamine function was thus obtained with high selectivity (90%). A more complex structure was produced from the radical polymerization of ethylene in the presence of a midchain alkoxyamine-functionalized polystyrene (PS) synthesized by ESCP in the form of ABA triblock copolymer (where A is polystyrene and B polyethylene)

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-<i>block</i>-poly(2-(4-(<i>tert</i>-butoxycarbonyl)amino)butyl-2-oxazoline)-<i>block</i>-poly(2-<i>iso</i>-propyl-2-oxazoline) (PE-<i>b</i>-PBocAmOx-<i>b</i>-P<i>i</i>PrOx). The synthesis was realized <i>via</i> 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-<i>iso</i>-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)

Completely Miscible Polyethylene Nanocomposites

A route to fully miscible polyethylene (PE) nanocomposites has been established based on polymer-brush-coated nanoparticles. These nanoparticles can be mixed with PE at any ratio, with homogeneous dispersion, and without aggregation. This allowed a first systematic study of the thermomechanical properties of PE nanocomposites without interference from aggregation effects. We observe that the storage modulus in the semicrystalline state and the softening temperature increase significantly with increasing nanoparticle content, whereas the melt viscosity is unaltered by the presence of nanoparticles. We show that the complete miscibility with the semicrystalline polymer matrix and the improvement of thermomechanical properties in the solid state is caused by the PE-coated nanoparticles being nucleating agents for the crystallization of PE. This provides a general route to fully miscibility nanocomposites with semicrystalline polymers