How Chain Transfer Leads to a Uniform Polymer Particle Morphology and Prevents Reactor Fouling

The effect of adding diethyl zinc as a chain transfer agent during the polymerization of propylene in heptane performed at 80 degrees C was studied. Although it was expected that the chain transfer would stop after precipitation of the polymer, the polymer molecular weight continued to increase throughout the whole of the polymerization. The presence of diethyl zinc had an additional effect that the polymerizations were devoid of reactor fouling. To unravel this phenomenon, the polymer particle morphology was studied. Under the conditions applied, surprisingly, uniform platelet-shaped polymer particles were formed. At high polymer content, these particles aggregate into microfibrillar structures consisting of nematic columnar strands of the same uniform platelets. The polymer particle morphology, as a result of controlled crystallization, is believed to play a crucial role in preventing reactor fouling

In-Reactor Polypropylene Functionalization-The Influence of Catalyst Structures and Reaction Conditions on the Catalytic Performance

To unravel the relationship between silylene-bridged metallocene catalyst structures and polymerization conditions and their effect on the performance in in-reactor functionalization of polypropylene, the behaviors of rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2/MMAO, rac-Me2Si(Ind)2ZrCl2, rac-Me2Si(2-Me-4-Ph-Ind)2HfCl2, and rac-Me2Si(Ind)2HfCl2 in propylene/aluminum alkyl-passivated 10-undecen-1-ol copolymerization were compared. Kinetic analysis revealed higher catalytic activities for zirconocenes compared to analogous hafnocenes. Both the zirconocene and hafnocene with substituted indenyl ligands afforded a higher molecular weight capability, improved stereo-selectivity, and enhanced ability to incorporate functionalized comonomers compared to their non-substituted congeners. An in-depth study of polypropylene functionalization using the best performing catalyst system, rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2/MMAO, at temperatures ranging from 40 to 100 °C, revealed a linear inversely proportional correlation of polymerization temperature with functionalized comonomer reactivity (↑Tp → ↓ r1), copolymer molecular weight (↑Tp → ↓Mn), and melting temperature (↑Tp → ↓Tm). While performing well under standard laboratory polymerization conditions, rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2/MMAO showed limited molecular weight and stereo-selectivity capabilities under high-temperature (130-150 °C) solution process conditions. Although immobilization of rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 onto silica, allowing it to be used under industrially relevant slurry and gas-phase conditions, led to an active catalyst, it failed to incorporate any functionalized comonomer

Preparation of Well-Compatibilized PP/PC Blends and Foams Thereof

The performance of polypropylene-poly(ethylene brassylate) block and graft copolymers and a polypropylene-polycaprolactone graft copolymer as compatibilizers for polypropylene-rich polypropylene/bisphenol A polycarbonate (PP/PC, 80/20 wt/wt) blends was elucidated. The copolymers were synthesized either by metal-catalyzed ring-opening polymerization or transesterification of a presynthesized polyester, initiated by hydroxyl-functionalized PPs, which themselves were obtained by catalytic routes or reactive extrusion, respectively. Spectroscopic fingerprints of the copolymers from liquid-state nuclear magnetic resonance (NMR) in combination with scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), dynamic mechanical thermal analysis (DMTA), and rheology analyses of the blends indicated that the compatibilizers spontaneously organize at the interface of the two immiscible polymers leading to the formation of uniform, stable, nanophase morphologies. The effect of the compatibilizers on the performance of the PP/PC blends was evaluated, and well-compatibilized PP/PC blends showed improved melt strength and strain hardening when compared to pure PP. This was verified by the successful foam extrusion using isobutane as a blowing agent of well-compatibilized PP/PC blends to low-density PP-based foams, for which normally long-chain branched PP is required

Aluminum Complexes of Bidentate Fluorinated Alkoxy-Imino Ligands: Syntheses, Structures, and Use in Ring-Opening Polymerization of Cyclic Esters.

International audienceThe coordination chem. of bidentate fluorinated alkoxy-imino ligands onto Al(III) centers was studied. The proligands (CF3)2C(OH)CH2C(R1):NR2 (1, \ONR1,R2\H; R1 = Ph, R2 = Bn (a, Bn = CH2Ph), Ph (c), Cy (d, Cy = cyclohexyl); R1 = Me, R2 = Bn (b)) react selectively with AlMe3 (0.5 or 1.0 equiv) and AlMe2(OiPr) or Al(OiPr)3 (0.5 equiv) to give the corresponding monoligand compds. \ONR1,R2\AlMe2 (2a-d) and the bis-ligand compds. \ONR1,R2\2AlMe (3a-d) and \ONR1,R2\2Al(OiPr) (4a-c). X-ray diffraction studies revealed that \ONPh,Bn\AlMe2 (2a), \ONMe,Bn\AlMe2 (2b), \ONMe,Bn\2AlMe (3b), \ONPh,Ph\2AlMe (3c), \ONMe,Bn\2Al(OiPr) (4b), and \ONPh,Ph\2Al(OiPr) (4c) all adopt a mononuclear structure in the solid state; four-coordinate \ONR1,R2\AlMe2 and five-coordinate \ONR1,R2\2AlMe and \ONR1,R2\2Al(OiPr) feature resp. distorted-tetrahedral and trigonal-bipyramidal geometries. The 1H, 13C\1H\, and 19F\1H\ NMR data indicate that the structures obsd. in the solid state are retained in CD2Cl2 or C6D6 soln. at room temp. The binary systems \ONR1,R2\AlMe2 (2)/BnOH and discrete \ONR1,R2\2Al(OiPr) (4) are effective catalysts for the controlled ROP of ε-caprolactone and rac-lactide, both in bulk molten monomer and in toluene soln./slurry. In contrast to the case of aluminum complexes having a bridged tetradentate fluorinated dialkoxy-diimino ligand that provides isotactic-enriched polylactides, the unbridged compds. \ONR1,R2\2Al(OiPr) (4) produce atactic PLAs. The key element which appears to be at the origin of the absence of stereocontrol is the lack of bridge between the two imino-alkoxy moieties, possibly via a decrease in the rigidity of the compds. and/or a different positioning of N,O vs. N,N heteroatoms in axial sites. [on SciFinder(R)

Metal-Based Catalysts for Controlled Ring-Opening Polymerization of Macrolactones: High Molecular Weight and Well-Defined Copolymer Architectures

This contribution describes our recent results regarding the metal-catalyzed ring-opening polymerization of pentadecalactone and its copolymerization with ε-caprolactone involving single-site metal complexes based on aluminum, zinc, and calcium. Under the right conditions (i.e., monomer concentration, catalyst type, catalyst/initiator ratio, reaction time, etc.), high molecular weight polypentadecalactone with <i>M</i>_n up to 130 000 g mol^–1 could be obtained. The copolymerization of a mixture of ε-caprolactone and pentadecalactone yielded random copolymers. Zinc and calcium-catalyzed copolymerization using a sequential feed of pentadecalactone followed by ε-caprolactone afforded perfect block copolymers. The blocky structure was retained even for prolonged times at 100 °C after full conversion of the monomers, indicating that transesterification is negligible. On the other hand, in the presence of the aluminum catalyst, the initially formed block copolymers gradually randomized as a result of intra- and intermolecular transesterification reactions. The formation of homopolymers and copolymers with different architectures has been evidenced by HT-SEC chromatography, NMR, DSC and MALDI-ToF-MS

Potential of Functionalized Polyolefins in a Sustainable Polymer Economy: Synthetic Strategies and Applications

ConspectusPolymers play a crucial role in our modern life as no other material exists that is so versatile, moldable, and lightweight. Consequently, the demand for polymers will continue to grow with the human population, modernization, and technological developments. However, depleted fossil resources, increasing plastic waste production, ocean pollution, and related growing emission of greenhouse gases has led to a change in the way we think about the use of polymers. Although polymers were never designed to be recycled, it is clear that a linear polymers economy is no longer sustainable. The design for recycling and reuse and life-cycle analyses will become increasingly important factors when deciding on which polymer to choose for a certain application. Of all polymers, polyolefins have the lowest life-cycle environmental impact and even outperform renewable polymers. However, polyolefins are chemically inert and reveal a low surface energy. Combining their excellent mechanical properties with the ability to adhere to other materials or create self-assembled or nanostructured materials would widen the application window of polyolefins even more.This Account covers part of our personal account in the field of functionalized polyolefin synthesis and their application development. We start with addressing the challenge of finding suitable catalysts that tolerate nucleophilic functionalities, which tends to poison most electrophilic catalysts even when passivated with, for example, an aluminum alkyl. We argued that lowering of the oxidation state of a titanium-based catalyst might lower the electrophilicity of the metal center. Indeed, this simple approach resulted in an unprecedentedly high tolerance toward aluminum alkyl-passivated alkenols during their copolymerization with ethylene. Interestingly, catalyst deactivation was much less pronounced during the copolymerization of propylene and aluminum-passivated alkenols, clearly demonstrating the protective effect of the methyl branch in the growing polymer. Because the use of randomly functionalized polypropylenes is rather underdeveloped, as compared to the corresponding randomly functionalized polyethylenes, we focused on potential applications of the former material. Atactic or low-crystalline hydroxyl- and carboxylic acid-functionalized propylene-based co- and terpolymers form elastomers with interesting properties that can be influenced by enhancing the hydrogen bonding within the system or by creating ionomers. The polar functionalities cluster together in domains that can host small polar molecules such as, for example, a pH indicator, thus affording useful sensors. The functionalized polyolefins can also be used as precursors for amphiphilic graft copolymers, undergoing self-assembly and therefore being suitable for nanoporous membrane preparation. The graft copolymers also proved to be effective compatibilizers in various polymer blends

Potential of Functionalized Polyolefins in a Sustainable Polymer Economy:Synthetic Strategies and Applications

ConspectusPolymers play a crucial role in our modern life as no other material exists that is so versatile, moldable, and lightweight. Consequently, the demand for polymers will continue to grow with the human population, modernization, and technological developments. However, depleted fossil resources, increasing plastic waste production, ocean pollution, and related growing emission of greenhouse gases has led to a change in the way we think about the use of polymers. Although polymers were never designed to be recycled, it is clear that a linear polymers economy is no longer sustainable. The design for recycling and reuse and life-cycle analyses will become increasingly important factors when deciding on which polymer to choose for a certain application. Of all polymers, polyolefins have the lowest life-cycle environmental impact and even outperform renewable polymers. However, polyolefins are chemically inert and reveal a low surface energy. Combining their excellent mechanical properties with the ability to adhere to other materials or create self-assembled or nanostructured materials would widen the application window of polyolefins even more.This Account covers part of our personal account in the field of functionalized polyolefin synthesis and their application development. We start with addressing the challenge of finding suitable catalysts that tolerate nucleophilic functionalities, which tends to poison most electrophilic catalysts even when passivated with, for example, an aluminum alkyl. We argued that lowering of the oxidation state of a titanium-based catalyst might lower the electrophilicity of the metal center. Indeed, this simple approach resulted in an unprecedentedly high tolerance toward aluminum alkyl-passivated alkenols during their copolymerization with ethylene. Interestingly, catalyst deactivation was much less pronounced during the copolymerization of propylene and aluminum-passivated alkenols, clearly demonstrating the protective effect of the methyl branch in the growing polymer. Because the use of randomly functionalized polypropylenes is rather underdeveloped, as compared to the corresponding randomly functionalized polyethylenes, we focused on potential applications of the former material. Atactic or low-crystalline hydroxyl- and carboxylic acid-functionalized propylene-based co- and terpolymers form elastomers with interesting properties that can be influenced by enhancing the hydrogen bonding within the system or by creating ionomers. The polar functionalities cluster together in domains that can host small polar molecules such as, for example, a pH indicator, thus affording useful sensors. The functionalized polyolefins can also be used as precursors for amphiphilic graft copolymers, undergoing self-assembly and therefore being suitable for nanoporous membrane preparation. The graft copolymers also proved to be effective compatibilizers in various polymer blends

Aluminum Complexes of Fluorinated β-Diketonate Ligands: Syntheses, Structures, Intramolecular Reduction, and Use in Ring-Opening Polymerization of Lactide

International audienceRoutes toward heteroleptic Al complexes supported by fluorinated β-diketonate ligands have been studied. Reactions of pro-ligands (acacR1,R2)H (R1 = R2 = CF3, "(hfacac)H"; R1 = CF3, R2 = tBu) with 0.5 equiv of AlMe3 or AlMe2Cl systematically yielded the corresponding homoleptic complexes Al(hfacac)3 (1) and Al(acacCF3,tBu)3 (2). Compound 2 exists in CD2Cl2 solution as a mixture of fac- and mer-isomers. Heteroleptic complexes [Al(acacR1,R2)2(OiPr)]n (R1 = R2 = CF3, 3; R1 = CF3, R2 = tBu, 4) were cleanly prepared from the reaction of the corresponding (acacR1,R2)H pro-ligands and 0.5 equiv of AlMe2(OiPr). Reaction of (hfacac)H and 0.5 equiv of Al(OiPr)3 at room temperature afforded 3 contaminated by other products, of which [κ2:μ2-(hfacac)5(OiPr)4Al3] (5) was isolated. When the same reaction was carried out at 55 °C, (S,R)-[κ,μ:κ2-(4H-hfacac)(hfacac)Al(THF)]2 (6), which contains a μ-bridging dianionic ligand (4H-hfacac)2− that arises from the reduction of one carbonyl group in (hfacac)−, was isolated in moderate yield (22%). Single-crystal X-ray diffraction studies revealed that complexes 3, 6, and 7 ([Al(hfacac)2(OH)]2, the hydrolysis product of 3) are dinuclear in the solid state with μ-bridging isopropoxide or hydroxide groups, while 5 features a symmetric trinuclear structure with the two terminal Al atoms supported by two (hfacac)− ligands and bridged via μ-isopropoxide groups to a central Al atom supported by a single (hfacac)− ligand. The Al-OiPr complexes 3 and 4 are effective initiators for the ring-opening polymerization of racemic lactide in THF or toluene solutions, giving atactic PLAs, end-capped by OiPr and OH groups, with controlled molecular weights (Mn up to 30 300 g*mol−1) and relatively narrow polydispersities (Mw/Mn = 1.10−1.32)