Zirconocene-catalyzed stereoselective cyclocopolymerization of 2-methyl-1,5-hexadiene with propylene

International audienceThe copolymerization of 2-methyl-1,5-hexadiene (MHD) with propylene has been studied with different single-site group 4 metal catalysts. Systems based on ansa-zirconocene precursors such as rac-{Me2Si(2-Me-4-Ph-Ind}ZrCl2 (1) and C1- or CS-symmetric ansa-{CpCR2Flu}ZrCl2 (2 and 3, respectively), once activated by MAO, are highly active (20–600 kgpol gcat−1 h−1 at 60–70 °C) and yield copolymers in which MHD is cyclopolymerized as methylene-(1-methyl)-1,3-cyclopentane (MMCP) units. 13C NMR studies revealed, depending on the symmetry of the precatalyst used, either highly isotactic (1, 2) or syndiotactic (3) polypropylene (PP) backbones, with isolated MMCP units. Fully trans-diastereoselective cyclopolymerization of MHD was observed with 1/MAO, while a mixture of trans and cis MMCP rings was observed with 2 and 3/MAO. The amount of MMCP units in PP (0.2–1.6 mol%) can be controlled by the amount of MHD in the feed. In contrast, the constrained geometry catalyst system based on {C5Me4SiMe2NtBu}TiCl2 (4) and MAO showed a much lower productivity (ca. 3 kgpol gcat−1 h−1 at 60 °C) and yielded a regioirregular, atactic copolymer in which MHD is simply vinyl-inserted in quite moderate amounts (0.2 mol%)

Engineering Advanced Morphologies for Structurally Reinforced Polyolefins

The primary objective of this research is to develop new methods to enhance the mechanical properties of isotactic polypropylene (iPP). Two complementary methods were developed to produce reinforced iPP-nanographite nanocomposites. In the first method, nanocomposites were prepared through an in-situ metallocene-catalyzed polymerization technique. In the second method, a new compounding strategy was used to prepare iPP-nanographite nanocomposites with improved spatial size distribution of nanoparticle agglomerates. Finally, a new process referred to as Melt-Mastication (MM) was developed as a means to improve the mechanical properties of pure iPP through generating unique and beneficial crystal morphologies. Reinforced iPP-nanographite nanocomposites were prepared through an in-situ polymerization technique and compared to analogous composites prepared by conventional melt processing. In-situ preparation of iPP-nanogrpahite nanocomposites was accomplished via single site metallocene catalyzed polymerization of propylene within a toluene dispersion of xGnP nanoparticles. Mechanical analysis showed iPP-nanographite nanocomposites demonstrated improved stiffness and strength relative to neat iPP. The results are discussed with regard to the thermal and morphological properties. A new polymer processing method referred to as “Melt-Mastication” (MM) was developed as a means to augment the crystal morphology of iPP and thereby enhance the thermal and physical properties. Melt-Mastication is a low temperature mixing technique that subjects an iPP melt to flow induced crystallization within a chaotic flow field. Thermal calorimetry and SAXS showed that MM substantially increases the lamellar crystal thickness and crystallinity of iPP, resulting in a 50% improvement to yield strength, 55% improvement to elastic modulus, and improved temperature stability. The property improvements were attributed to a unique hierarchical organization of lamellar crystals produced by MM, distinct from conventionally prepared iPP materials. Finally, Melt-Mastication was repurposed as a compounding method for preparation of iPP-nanographite nanocomposites with enhanced nanographite dispersion. Due to flow induced crystallization, the process viscosity increases significantly during Melt-Mastication, which produces higher mixing torque and therefore shear resulting in the fragmentation of nanoparticle agglomerates. The spatial size distribution of nanographite agglomerates was evaluated via a quantitative stereological technique, and a model for agglomeration in shear flow is proposed

Propylene Polymerization Using 4th Generation Ziegler-Natta Catalysts: Polymerization Kinetics and Polymer Microstructural Investigation

A systematic study of propylene polymerization using a 4th generation Ziegler-Natta catalyst is presented in this thesis. The apparent kinetic rate constants for propylene polymerization were estimated in the presence and absence of hydrogen and/or donor. The estimated activation energies for activation, propagation, and deactivation were found to be close to values previously reported in the literature for similar catalysts. The polypropylene samples were characterized using high-temperature gel permeation chromatography (GPC), carbon-13 nuclear magnetic resonance (13C NMR), and crystallization elution fractionation (CEF). The effect of hydrogen and external electron donor on polypropylene microstructure was investigated at two polymerization temperatures. In addition to the expected electron donor positive effect on tacticity, hydrogen was also found to increase polypropylene tacticity. The effect of changing these polymerization conditions on molecular weight and polydispersity was also investigated. Finally, CEF profiles show how the distribution of polypropylene crystallizability changes by adding hydrogen and electron donor to the reactor. The concentrations of hydrogen and external donor were also varied to study their effect of polymerization kinetics and polymer microstructure. The estimated activation energies were close to those found in the first part of this investigation in the presence and/or absence of donor and hydrogen. A polypropylene microstructural study showed a positive effect of hydrogen concentration on mmmm pentad at low donor concentration, likely due to an increase in stereoselectivity of the aspecific sites by hydrogen. However, increasing donor concentration over a given threshold seems to transform the aspecific sitess into stereospecific sites that are no longer significantly affected by hydrogen. These experimental results were compared to a previously developed Monte Carlo model and found to agree with the trends predicted by simulation. Finally, the effect of diisopropyldimethoxysilane (P), n-propyltrimethoxysilane (N), paraethoxyethylbenzoate (PEEB), and dicyclopentyldimethoxysilane (D) external donors on catalyst activity and stereoselectivity was investigated. P and D donors were more stereoselective than N and PEEB donors; however, D donor had the best activity among all donors investigated. Therefore, D donor was mixed with PEEB to combine its high activity with the self-extinguishing properties of PEEB. The D/PEEB 90/10 (mol/mol) mixture generated a catalyst with good stereoselectivity but poor activity. When the ratio was increased to 95/5 and 98/2, the resulting catalyst had high activity and good stereoselectivity. Interestingly, the D/PEEB combination with just a small fraction of PEEB has a positive effect on the catalysts activation term which may decrease polymerization costs with this system

Ethylene or propylene based copolymers with higher 1-olefins by metallocene catalysts: correlation between microstrusture and properties.

missin

Ethylene or propylene based copolymers with higher 1-olefins by metallocene catalysts: correlation between microstrusture and properties.

missin

Polymerization of sterically hindered a-olefins with single-site group 4 metal catalyst precursors

A variety of group 4 metal catalytic systems (C2-symmetric {EBTHI}-, {SBI}-type zirconocene complexes (C2-1–4); C1-symmetric (C1-5–8) and Cs-symmetric (Cs-9) {Cp/Flu}-type zirconocene complexes; Cp*2ZrCl2 (Cp* 2-10)), half-metallocene complexes (CpTiCl3, HM-11), constrained-geometry (CGC-12) titanium catalysts) and post-metallocene catalysts (Dow’s ortho-metallated amido-pyridino hafnium complex (PM-13)) have been screened in the polymerization of the sterically demanding 3-methylbut-1-ene (3MB1) and vinylcyclohexane (VCH). All systems proved to be sluggishly active under regular conditions (toluene, 20°C; MAO as cocatalyst) towards 3MB1, with productivities in the range 0–15 kg.mol–1.h–1. Higher productivities (up to 75 kg.mol–1.h–1) were obtained in the polymerization of VCH with C1-symmetric metallocene catalysts under the same conditions, while Cs-symmetric systems were found to be completely inactive. For both 3MB1 and VCH, under all conditions tested, the most productive catalyst appeared to be Dow’s post-metallocene system PM-13/MAO. Optimization of the polymerization conditions led to a significant enhancement of the productivities of this catalyst system towards both 3MB1 and VCH up to 390 and 760 kg.mol–1.h–1, respectively (Tpolym = 70°C). 13C NMR spectroscopy studies revealed that all isolated P(3MB1) and P(VCH) polymers were isotactic, regardless the nature/symmetry of the (pre)catalyst used. The nature of the chain-end groups in P(3MB1) is consistent with two different chaintermination mechanisms, namely b-H elimination/transfer-to-monomer for C2-1/MAO and chain-transfer to Me3Al for PM-13/MAO systems, respectively. For polymerization of VCH with PM-13/MAO at 70°C, b-H elimination / transfer-to-monomer appeared to be the main chain termination reaction