P. M. Nedorezova

A New Catalytic System (2-PhInd)2ZrMe2/Al(iso-Bu)3 for the Synthesis of Stereoblock Polypropylene

A new efficient two-component homogeneous system consisting of (2-PhInd)2ZrMe2 and Al(iso-Bu)3 cocatalyst was developed for the synthesis of elastomeric stereoblock polypropylene. The catalytic behavior of this system was studied in toluene and liquid propylene. It was shown that a change in the Al/Zr molar ratio enables one to vary the system activity, the kinetic characteristics of the polymerization process, and the properties of polypropylene over wide ranges. For example, an increase in the Al/Zr ratio from 50 to 300 mol/mol is accompanied by an increase in the activity by a factor of 4, an increase in the molecular weight from 80 000 to 220 000, and a decrease in isotactic pentad content from 39 to 16%. It was found that the polymerization rate is proportional to the concentration of Al(iso-Bu)3 and to the square of the monomer concentration. Some assumptions about the nature of active centers and about reactions leading to the formation of these centers were made. A model for the formation of sequences of isotactic and atactic blocks was proposed. This model takes into account the multiple inversion of the active center (fast dynamic equilibrium) from the iso-specific to the nonspecific state during chain growth. This inversion is due to the coordination of an additional Al(iso-Bu)3 molecule. This model satisfactorily describes the pentad composition of polypropylene samples as a function of the Al/Zr ratio.

Synthesis of propylene-ethylene copolymers in liquid propylene using ansa-metallocenes of the C 1 symmetry

The copolymerization of ethylene with propylene in the liquid propylene initiated by ansa-metallocenes of the C 1 symmetry, rac-[1-(9-η5-fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-η5-indenyl)ethane]zirconium dichloride and rac-[1-(9-η5-fluorenyl)-2-(5,6-cyclopenta-2-methyl-1-η5-indenyl)ethane]hafnium dichloride, activated by methylaluminoxane has been studied. Triisobutylaluminum has been used as a cocatalyst. The propylene-ethylene copolymers thus prepared contain 5–60 mol % ethylene units. The reactivity ratios have been measured. In the case of the zirconocene-based catalyst, the molecular mass of the copolymers decreases with an increase in the content of ethylene units. The reverse situation is observed in the case of the hafnocene-based catalytic system. The copolymers are characterized by the low T g values (down to −45°C). Incorporation of a small amount of ethylene units (5 mol %) results in a rise in the elastomeric behavior of the polymers.

Thermooxidative destruction of polypropylene-graphite composites☆

Abstract Thermooxidation of composites prepared by polymerization filling of polypropylene (PP) with natural and synthetic graphite was studied. Graphite retards PP oxidation; the dependence of the oxidation rate on the form and dispersity of graphite and on the composition of the composites was determined. Chain termination at the graphite surface appears as the chief retarding mechanism.

Homo-and copolymerization of vinylcyclohexane with α-olefins in the presence of heterogeneous and homogeneous catalytic systems

The polymerization and copolymerization of vinylcyclohexane with α-olefins in the presence of several heterogeneous and homogeneous catalytic systems were studied. It was shown that, with respect to activity in the polymerization of vinylcyclohexane, the tested catalysts can be arranged in the following order: α-TiCl3 < titanium-magnesium catalyst < metallocene catalyst. Poly(vinylcyclohexane) prepared with heterogeneous catalytic systems is a solid semicrystalline polymer. The properties of polymers synthesized with homogeneous systems differ substantially depending on the type of the metallocene used. In the presence of metallocenes with a C2 symmetry, crystalline powderlike products arise, while in the case of metallocenes with C1 and C s symmetries, polymerization yields amorphous viscous products. Molecular-mass distributions of poly(vinylcyclohexane) samples prepared using both heterogeneous titanium-magnesium catalysts and homogeneous metallocene complexes show a bimodal pattern, indicating the heterogeneity of active centers of these catalysts. Upon introduction of a comonomer (ethylene, propylene, and 1-hexene) into the reaction mixture, the activity of all studied catalytic systems increases. When Me2C(3-Me-Cp)(Flu)ZrCl2 and rac-Me2SiInd2ZrCl2 are used as catalysts, the degree of crystallinity of the copolymers grows owing to the presence of ethylene or propylene units in poly(vinylcyclohexane) chains.

Bulk propylene polymerization in the presence of ansa-metallocenes of C2 and C1 symmetries: From a rigid thermoplastic to elastomeric stereoblock polypropylene

Bulk propylene polymerization in the presence of ansa-metallocenes with C2 and C1 symmetries has been studied. The catalytic activity, polymerization kinetics, and the molecular weight of polypropylene (PP) depend strongly on catalyst formation conditions. Mixtures of rac and meso isomers of metallocenes make it possible to rapidly produce a high-molecular-weight isotactic PP with high stereoregularity and mechanical characteristics and thus skip the stage of the isolation of pure rac isomer in the catalyst synthesis. The ability of triisobutylaluminum to serve as a cocatalyst is studied for ansa-metallocenes of C1 symmetry. In this case, the molecular weight of PP is higher, indicating that organoaluminum compounds participate in chain termination reactions. An increase in the reaction temperature results in an increase in the stereoregularity and crystallinity of PP. Polypropylene synthesized using ansa-metallocenes of C1 symmetry has good elastomeric properties.

Isospecific Bulk Polymerization of Propylene with the Use of ansa-Metallocenes as a Mixture of rac and meso Isomers

The effects of various pathways of the formation of catalytic systems based on the rac-Me2SiInd2ZrCl2 metallocene on the activity and properties of polypropylene prepared by the bulk polymerization of propylene were studied in detail. It was found that the conditions of formation of the catalytic system affect not only its activity and the character of kinetic curves but also the molecular weight of the synthesized polymer. Propylene polymerization was studied with the use of a number of bisindenyl derivatives of zirconium, which were mixtures of rac and meso forms that differ in the nature of substituents in the Si bridge or the indenyl ligand. The conclusion was drawn that, with the use of metallocenes as a mixture of rac and meso forms, high-molecular-weight isotactic polypropylene can be prepared with high stereoregularity at a very high rate; thereby, the stage of separation of a pure rac isomer can be excluded in the synthesis of the catalyst.

Using Raman spectroscopy to determine the structure of copolymers and polymer blends

We present Raman structural study of two grades of random propylene/1-octene copolymers with low and high molecular weights and the 1-octene content up to 4.5 mole %. The copolymer spectra are compared with the spectra of the α, γ, and smectic modifications of isotactic polypropylene. Raman investigation has showed that the degree of crystallinity and conformational order of the copolymer macromolecules slightly decrease with the growth of the 1-octene content. The degree of crystallinity is slightly higher for the samples of the high-molecular-weight grade compared to the low-molecular-weight one. Furthermore, we present Raman spectra of polyethylene/polypropylene (90/10) blends, mixed in the polyethylene melt at three different temperatures, corresponding to three different states of polypropylene macromolecules. It was concluded that the degree of crystallinity and conformational order of the polyethylene macromolecules in the blends are the highest for the temperature, at which polypropylene macromolecules have lost their packing and conformational order.

Biodegradation of Poly-ε-caprolactones and Poly-l-lactides by Fungi

This work assessed biodegradation, by Aspergillus, Fusarium, Penicillium and Parengyodontium fungi, of four samples of poly-ε-caprolactone (PCL), three samples of poly-l-lactide (PLA) and one sample of poly-d,l-lactide (DL-PLA) produced by ring-opening polymerization initiated by aluminium complexes of corresponding lactones. Mesophilic fungal strains actively biodegrading PCL (F. solani) and PLA (Parengyodontium album and A. calidoustus) were selected. The rate of degradation by the selected fungi was found to depend on the physicochemical and mechanical properties of the polymers (molecular weight, polydispersity, crystallinity). The most degradable poly-ε-caprolactone sample was shown to have the lowest molecular weight; the most biodegradable polylactide DL-PLA had the lowest crystallinity. Mass spectral analysis of biodegraded polymer residues showed PCL to be degraded more intensively than PLA. It is established that in the case of Parengyodontium album the colonization of the films of polypropylene composites with DL-PLA is observed, which will undoubtedly contribute to their further destruction under the influence of abiotic factors in the environment.

New insights into the structure of polypropylene polymorphs and propylene copolymers probed by low-frequency Raman spectroscopy

We present a low-frequency (or low-wavenumber) Raman spectroscopy study of isotactic polypropylene, namely the different modifications - α, γ, and smectic, with a degree of crystallinity ranging from 0 to 70 %, as well as of random propylene/1-butene and propylene/1-octene copolymers with the incorporated monomer contents of 5.3 and 19.5 mol % for 1-butene and 4.5 and 24.0 mol % for 1-octene. In the Raman spectra of the propylene/1-octene copolymers and the neat isotactic polypropylene, we have found that the band at 105 cm−1, which is connected to the torsion vibrations of CH2-CHCH3 groups in the polymer backbone, can be assigned to the vibrations of helical macromolecules in the α and γ crystallites and the smectic phase. If the smectic phase is absent in the corresponding polymer system, then the intensity of the 105 cm−1 band is related to the molecular vibrations in the crystallites. We have also observed a broadening of the band at 398 cm−1 accompanied by the appearance of a high-wavenumber shoulder in the Raman spectra of the copolymers with the increase in the incorporated monomer content. This experimental finding can be explained by the reduction of the degree of crystallinity, conformational disordering or by the increase in the dimensions of the crystal unit cell.

Nanocarbon filler particles in polymer matrix - Nanosized dielectric probe

Composite materials of polypropylene, graphene nanoplatelets (GNP) or fullerene C60 were synthesized by in situ polymerization. GNP particles consist of 3 - 5 graphene layers and have aspect ratio 40. In composites with pristine GNP particles their aspect ratio is 110, whereas ultrasonic processing reduces it to 40 - 50. This change of aspect ratio of filler particles and their aggregates results in different properties of composites with pristine and sonicated GNP. Percolation threshold for composites with pristine GNP is 0.25% vol. In composites with sonicated GNP it is 2-3% vol. This is due to reduction in the size of filler particles aggregates and more uniform distribution of particles in polymer matrix after ultrasonic treatment. The presence of nanocarbon filler (GNP or fullerene) makes α-transition, associated with the glass transition of the amorphous phase of polypropylene, clearly resolved. Its intensity increases with the concentration of nanofiller, which acts as a dielectric probe.