Quasi‐living copolymerization of ethylene with 1‐hexene by heteroligated (salicylaldiminato β‐enaminoketonato) titanium complexes

A series of heteroligated (salicylaldiminato)(β‐enaminoketonato)titanium complexes [3‐Bu^t‐2‐OC₆H₃CH = N(C₆F₅)] [PhN = C(R₁)CHC(R₂)O]TiCl₂ [ 3a : R₁ = CF₃, R₂ = ^tBu; 3b : R₁ = Me, R₂ = CF₃; 3c : R₁ = CF₃, R₂ = Ph; 3d : R₁ = CF₃, R₂ = C₆H₄Ph(p ); 3e : R₁ = CF₃, R₂ = C₆H₄Ph(o ); 3f : R = CF₃, R₂ = C₆H₄Cl(p ); 3g : R₁ = CF₃; R₂ = C₆H₃Cl₂(2,5); 3h : R₁ = CF₃, R₂ = C₆H₄Me(p )] were investigated as catalysts for ethylene (co)polymerization. In the presence of modified methylaluminoxane as a cocatalyst, these complexes showed activities about 50%–1000% and 10%–100% higher than their corresponding bis(β‐enaminoketonato) titanium complexes for ethylene homo‐ and ethylene/1‐hexene copolymerization, respectively. They produced high or moderate molecular weight copolymers with 1‐hexene incorporations about 10%–200% higher than their homoligated counterpart pentafluorinated FI‐Ti complex. Among them, complex 3b displayed the highest activity [2.06 × 10⁶ g/mol_Ti?h], affording copolymers with the highest 1‐hexene incorporations of 34.8 mol% under mild conditions. Moreover, catalyst 3h with electron‐donating group not only exhibited much higher 1‐hexene incorporations (9.0 mol% vs. 3.2 mol%) than pentafluorinated FI‐Ti complex but also generated copolymers with similar narrow molecular weight distributions (M _w/M _n = 1.20–1.26). When the 1‐hexene concentration in the feed was about 2.0 mol/L and the hexene incorporation of resultant polymer was about 9.0 mol%, a quasi‐living copolymerization behavior could be achieved. ¹H and ¹³C NMR spectroscopic analysis of their resulting copolymers demonstrated the possible copolymerization mechanism, which was related with the chain initiation, monomer insertion style, chain transfer and termination during the polymerization process. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 2787–2797     

“Experimentation and modeling for the apparent elongation viscosity of polymer melts with the White–Metzner model”

The measurement of the apparent elongation viscosity (η_e) of several polyolefin melts was conducted in this study by using the isothermal fiber‐spinning method. The White–Metzner (W–M) model was used to analyze the spinning flow of the polymer melts and, thus, the elongation viscosity was predicted at elongation strain rates ranging from 0 to approximately 5 s^?1. The values of the model parameters required in the W–M model were obtained by curve fitting the experimental data obtained from the shear measurements. The elongation viscosity predicted using the W–M model was in good agreement with the experimental results of fiber spinning. In addition, η_e could also be estimated directly from the measured shear viscosity (η_S) with a formulation using the W–M model; the subsequently obtained elongation viscosity and Trouton ratio (T_R) were reasonable within a wide range of strain rates. Based on the experimental and theoretical results, the polyolefin with a high molecular weight was observed to have high elongation viscosity, and the polymer with a broad molecular weight distribution also possessed high η_e. The T_R value of the commercial polypropylene (PP‐1040) began to increase from 3 at a deformation rate of 0.1 s^?1 and grew up asymptotically to 10, whereas the T_R of high‐density polyethylene (HDPE‐606) remained nearly at 3 within the entire range of strain rates. Copyright © 2014 John Wiley & Sons, Ltd.     

1H NMR assignment of oligomeric grafts of maleic anhydride‐grafted polyolefin

The ¹ H NMR assignment of oligomeric grafts of maleic anhydride (MA)‐grafted polyolefin (PO), MA‐g‐PO hereafter, was experimentally demonstrated for the first time using NMR spectroscopy. ¹³ C DEPT, ¹ H‐¹ H DQF‐COSY, and ¹ H T₂‐edited spectroscopy of MA‐g‐PO proved that peaks of the intermediate methine protons of succinic anhydride oligomeric grafts, which are nearly tetrameric, are observed at 2.5–3.5 ppm and show broadening. Copyright © 2012 John Wiley & Sons, Ltd.     

X-ray microtomography studies of nascent polyolefin particles polymerized over magnesium chloride-supported catalysts

Drastic changes occur during the initial stages of the α-olefin polymerization over heterogeneous catalysts. Fragmentation of the support takes place as polymer is formed at the active sites within the voids of the support/catalyst. Magnesium chloride-supported titanium catalyst/polymer particles have been analyzed employing high-resolution computed microtomography (CMT) using synchrotron radiation at Brookhaven National Laboratory. The changes in morphology, the spatial distribution of the support/catalyst fragments, porosity, and polymer distribution in single growing polypropylene and polyethylene particles have been studied. These studies documented considerable macroporosity ( > 2 μm in size) within the growing catalyst/support/polymer particles. The largest pores may be due to agglomeration of smaller subparticles. Our results confirm that the initial fragmentation of the support proceeds readily and uniformly to yield a multi-grain growth of subparticle agglomerates. The support/catalyst fragments appear to be distributed relatively uniformly within the growing polymer particle. The surface of the subparticle agglomerates is accessible through the void-space between growing catalyst/particle grains. This may facilitate monomer transport to the activate sites through the polymer/catalyst particles. © 1993 John Wiley & Sons, Inc.     

Characterization of mechanically sheared ethylene–propylene copolymers by high resolution NMR

We report high-resolution solution-state NMR experiments on chain ends generated in ethylene–propylene copolymers by mechanical shearing in an extruder. The use of the higher resolution of the ¹³C-NMR spectrum, in a two-dimensional ¹H-¹³C chemical shift correlation experiment, has allowed the complete resolution and assignment of the olefinic chain-end region of the ¹H-NMR spectrum. Simultaneously, the assignments of the ¹³C olefinic resonances, previously identified [A. C. Kolbert, J. G. Didier, and L. Xu, Macromolecules, 29 , 8591 (1996)] are confirmed. An iterative method for calculating the average molecular weight, based on quantitative measurements of the olefinic ¹H-NMR peak intensities is introduced and these results are compared with measurements from ¹³C-NMR and size exclusion chromatography and correlated to reduced viscosities. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1955–1961, 1997     

Thermal expansion of crosslinked closed-cell polyethylene foams

An experimental study on the thermal expansion of a collection of crosslinked low-density polyethylene (LDPE) foams with closed-cell structure is presented. The thermal characterization of these materials, the relationships between the linear thermal expansion coefficient and the structure of the foams, and the determination of the variables that can modify the thermal properties of these products are the goals of this work. The experimental results show that the linear thermal expansion coefficient decreases when the density of the foamed material increases. The gas expansion inside the cells is a mechanism that should be taken into account. Moreover, the thermal expansion also depends on the cellular structure. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2587–2596, 1998     

Effect of deuterium substitution on the surface interactions in binary polymer mixtures

We have examined the effect of deuterium labeling on surface interactions in mixtures of random olefinic copolymers [C₄H₈]_1−x[C₂H₃(C₂H₅)]_x. Based on surface segregation data we have determined a surface energy difference χ_s between pure blend constituents. In each binary mixture components have different fractions x₁, x₂ of the group C₂H₃(C₂H₅), and one component is labeled by deuterium (dx) while the other is hydrogenous (hx). The mixtures are grouped in four pairs of structurally identical blends with swapped labeled constituent (dx₁/hx₂, hx₁/dx₂). For each pair the surface energy parameter χ_s increases when the component with higher fraction x is deuterated, i.e., χ_s(dx₁/hx₂) > χ_s(hx₁/dx₂) for x₁ > x₂. A similar pattern has been found previously for the bulk interaction parameter χ. This is explained by the solubility parameter formalism aided by the lattice theory relating the surface excess to missing-neighbor effect. χ_s has also an additional contribution, insensitive to deuterium swapping effect, and related to entropically driven surface enrichment in a more stiff blend component with a lower fraction x. Both enthalpic and entropic contributions to χ_s seem to depend on the extent of chemical mismatch between blend components. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2691–2702, 1998     

Functionalization of polymers: Epoxidation of polyolefins

In order to produce a polymer with an epoxy group, the epoxidation of the polyolefins poly(ethylene-co-1,9-decadiene) (PED) and polybutadiene (PB) was investigated. For these reactions, in situ generated peroxy acid was most effective and convenient. Among them an H₂O₂–formic acid system gave the best result. The reactivity of PB showed the dependency on the microstructure. The reactivity of the double bond is trans>cis> vinyl(pendant) and the selectivity for the epoxidation is cis∼vinyl(pendant)≫trans.     

Polarized FT-IR photoacoustic spectroscopy on polymer fibers

The orientation of macromolecular chains in two melt-spun polymer fibers (segmented polyurethane elastomer and melt-modified polyolefine fibers) was studied by photoacoustic spectroscopy (PAS) with polarized light in the mid-infrared range. The PAS orientation functions calculated from the photoacoustic signal intensities of orientation sensitive bands describe the orientation of the different chain segments with respect to the fiber axis. Thus, the orientation of hard and soft segments in the unstressed polyurethane fiber is different and both are similiar to that of the corresponding injection molded bars. With increased spinning velocity an improved orientation of the polyolefine chains along the fiber axis, but only in the amorphous regions of the polymer, were detected. Only a slight reduction of orientation was measured as a result of the subsequent chemical crosslinking in the polyolefines.     

Synthesis of maleic anhydride grafted polyethylene and polypropylene,with controlled molecular structures

This article discusses a new chemical route to prepare maleic anhydride (MA) grafted polyethylene and polypropylene polymers with controlled molecular structure, that is, MA grafted content and polymer molecular weight and composition distributions. The chemistry involves a free radical graft reaction of maleic anhydride with poly(ethylene‐co‐p‐methylstyrene) and poly(propylene‐co‐p‐methylstyrene) copolymers. Under a suspension reaction condition, the grafting reaction takes place selectively on the p‐methylstyrene units in the copolymer, due to high reactivity of p‐methyl group and favorable mixing between p‐methylstyrene units and chemical reagents in the swollen amorphous phases. The resulting polymer shows no detectable molecular weight change during the reaction, and the MA grafted content increases with the increase of initiator and p‐methylstyrene concentrations. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 1337–1343, 2000