Photooxidation of polyolefins and their light stability

It is shown that all peculiarities of polypropylene photooxidation and alteration of its mechanical properties in this process are a process of second-order termination reaction. The influence of natural conditions (varied temperature and light intensity) on this process are considered.     

Metallocene carbene chemistry

The Group IV bent metallocenes Cp₂M (M = Ti, Zr, Hf) were involved in carbene-related chemistry in various ways. Examples from four different areas are used to illustrate this. In situ generated Cp₂M: species exhibit some carbene-like character. They add to olefins, and their addition products can incorporate additional unsaturated organic reagents, e.g., alkyne, to form five-membered metallacycles. The high oxophilicity of the Group IV metals helps the addition of alkene-, aryne-, ²-ketone-, ²-aldehyde-, or butadiene-containing reagents to a great variety of metal carbonyl compounds to form the Fischer-type carbene complexes. The resulting zirconoxycarbene complexes have found some application in organometallic chemistry and in organic synthesis. Reactive [Cp₂M^IVR]⁺ cations can be stabilized by the addition of the Arduengo carbenes that serve as bulky two-electron donor ligands. First examples were structurally characterized. Dialkylimidazol-2-ylidenes also add to the electrophilic MX₄ compounds yielding stable trans-bis(imidazol-2-ylidene)MX₄ systems. Several examples are presented where ruthenium carbene complexes are used for carrying out catalytic olefin metathesis reactions at pendant olefinic substituents at the bent metallocenes. These reactions have led to the formation of novel bimetallic metallocene systems, as well as to new ansa-metallocenes. These catalytic reactions have helped in the current efforts to develop a functional group chemistry at the sensitive Group IV bent metallocene frameworks.     

Metallocene and Organo-main Group Trifluoromethanesulfonates

The substituted metallocene compounds Cp₂M(OSO₂CF₃)₂ (Cp=η⁵-C₅H₅; M = Ti, Zr, Hf, Nb, Mo), CpTi(OSO₂CF₃)₃ · 0.75(1,2-dimethoxyethane), and the organo-main group compounds (C₆H₅)₃M′(OSO₂CF₃) (M′=Si, Ge), (C₆H₅)₂Sn(OSO₂CF₃)₂ and (C₆H₅)₃Sb(OSO₂CF₃)₂, were synthesized from the corresponding chloride or bromide compounds and silver trifluoromethanesulfonate (triflate) and characterized spectroscopically, including a detailed analysis of their IR spectra. Triflate coordination is typically monodentate, but CpTi(OSO₂CF₃)₃ · 0.75(1,2-dimethoxyethane) and the organo-germanium triflate show evidence of bidentate CF₃SO ₃⁻ ligands and are likely to have polymeric structures. Conductance measurements in nitromethane, acetone and acetonitrile have demonstrated the ease of triflate substitution, with a limited kinetic study of acetone solvation of the Ti and Hf compounds supporting an associative pathway. Three new catalysts for the polymerization of tetrahydrofuran, Cp₂HfCl₂/Ag(OSO₂CF₃), CpTiCl₃/Ag(OSO₂CF₃) and (C₆H₅)₂SiCl₂/Ag(OSO₂CF₃), are also reported.     

Modern Metallocene Chemistry and Catalysis

Anfang Juli wurde Hans‐Herbert Brintzinger, Konstanz, die Ehrendoktorwürde der Universität Heidelberg verliehen. Anlässlich dieser Ehrung lud der Sonderforschungsbereich „Molekulare Katalysatoren: Struktur und Funktionsdesign”︁ (SFB 623) zum Symposium „Modern Metallocene Chemistry and Catalysis”︁ ein.     

Hard versus Soft Materials as Supports for Metallocene and Post‐Metallocene Catalysts

The influence of organic supports on the polymerization behavior of post‐metallocene catalysts is studied and compared with similarly supported titanium and zirconium metallocenes. The effects of the immobilization, activation, and polymerization process were studied by video microscopy, laser confocal fluorescence microscopy, SEM, and TEM. A model for the polymerization process for a catalyst supported on latex particles was developed from the results obtained. Organic supports based on latex particles are easily adjustable for different catalysts due to the versatile functionalization of the surfaces and can be applied to different types of olefin polymerization catalysts. They can be considered as an alternative to SiO₂ or MgCl₂ supports.

     

茂金属催化剂的负载化

茂金属催化剂是继齐格勒-纳塔催化剂之后的新一代烯烃聚合催化剂,与多活性中心催化剂相比,这种单一活性中心类型的催化剂具有聚合活性较高、聚合物分子量分布较窄等特点。其中,负载化是茂金属催化剂实用化的重要课题,它克服了均相催化体系中聚合物形貌不可控的缺点,降低了助催化剂的用量。近些年来茂金属负载化的研究仍很活跃。本文总结了近十年来茂金属负载化的研究进展,主要对载体的形貌、载体的改性处理、茂金属催化剂的负载方法和负载化茂金属催化剂的应用进行了评述并展望了茂金属催化剂负载化的发展趋势。     

Epitaxy of helical polyolefins

The structural rules which govern the epitaxial crystallisation of polymers - and especially polyolefins - on organic substrates are established. Illustrative examples involve isotactic and syndiotactic polypropylenes and poly( l-butene). Investigation of the film structure by electron microscopy, electron diffraction and atomic force microscopy reveals some unprecedented features, including in particular the selection of the contact plane according to the chirality of its constituent helices, and direct observation of both right and left hands of polyolefin helices.     

Plasma fluorination of polyolefins

ESCA and contact angle measurements were used to characterize the surfaces of Polyethylene and polypropylene films exposed to SF₆, CF₄, and C₂F₆ plasmas. None of these gases polymerized in the plasma. However, all plasma treatments grafted fluorinated functionalities directly to the polymer surfaces. SF₆ plasmas graft fluorine atoms to a polyolefin surface. CF₄ plasmas also react by a mechanism dominated by fluorine atoms, but with some contribution from CF_x-radical reactions. Although C₂F₆ does not polymerize, the mechanism of grafting is still dominated by the reactions of CF_x radicals. For all gases studied, the lack of polymerization is attributed to competitive ablation and polymerization reactions occurring under conditions of ion bombardment.     

Catalytical and thermal pyrolysis of polyolefins

Polyolefins have a high potential for alternative oil production since they contain only carbon and hydrogen atoms. By pyrolysis of these materials up to 95% can be obtained as oil and gas. Upgrading the products by catalytic cracking of polyolefins is a subject of growing interest in the last years as less energy is needed for the pyrolysis and more valuable products are formed. Numerous studies have been reported in which a variety of catalysts such as zeolites, silica-alumina, mesoporous MCM-41, solid acids and reaction conditions have been investigated. In our studies we used Lewis acids and mixtures of Ziegler–Natta catalyst such as TiCl₄, AlCl₃ to pyrolyse polypropylene. Experiments were carried out in a batch reactor as well as in a fluidized bed process. The pyrolysis temperature can be decreased by 100 °C compared to runs without catalysts. A drastic increase in the amount of low boiling compounds (C4 hydrocarbons) can be observed by the use of the catalysts instead of longer chained hydrocarbons.     

Post-polymerization functionalization of polyolefins

Polyolefins are macromolecular alkanes and include the most familiar and most commercially produced plastic, polyethylene. The low cost of these materials combined with their diverse and desirable property profiles drive such large-scale production. One property that renders polyolefins so attractive is their resistance to harsh chemical environments. However, this attribute becomes a severe limitation when attempting to chemically convert these plastics into value-added materials. Functionalization of polymers is a useful methodology for the generation of new materials with wide ranging applications, and this tutorial review describes both new and established methods for the post-polymerization modification of polyolefins.     

Catalytic degradation of polyolefins

Silica, alumina, silica-aluminas and zeolites were screened as catalysts for polyolefins degradation. The degradations were carried out at temperatures from 200°C till to 600°C under vacuum. The degradation products were analyzed using gas chromatography-mass spectrometry (GC-MS). The comparison between these products and those obtained by thermal degradation in the absence of catalyst gives us many informations about the different mechanisms of the two degradation reactions. Kinetic measurements and thermogravimetry provided further useful data for screening the catalysts, finding the best operating conditions.     

Nucleating and clarifying agents for polyolefins

Nucleating agents are used in semi‐crystalline polymer systems (mainly polyolefins) to increase the rate of crystallization. These agents increase degree of crystallinity and lamellae thickness and, hence, improve stiffness and strength. In addition to altered mechanical characteristics, clarifying agents do also improve optical properties. This paper provides a brief overview on nucleating/clarifying agents. The efficiency of such products (e.g. Na‐benzoate, sorbitol acetals) for polypropylene (PP homopolymer, PP copolymer) is evaluated. The clarifiers Irgaclear D ((1,3:2,4)‐dibenzylidene sorbitol) and Irgaclear DM (1,3:2,4‐Bis‐(p‐methylbenzylidene) sorbitol) provide a 3‐dimensional fibrillar structure upon cooling from the polymer melt. This magnified nucleation density yields spherulites with reduced size (< 1micron) ‐ in comparison to non‐nucleated products. Addition of Irgaclear to polypropylene leads to increased clarity, reduced haze and enhanced stiffness of molded parts. Higher recrystallization temperatures results in shorter cycle times and increased throughput during the molding process.     

Interfacial interactions and structure of polyolefins

The elucidation of structural aspects of polyolefins (isotactic and syndiotactic polypropene and isotactic poly(1-butene)) oriented by epitaxial crystallization is briefly described. Investigations are performed by a combination of electron and Atomic Force Microscopy. AFM helps assess submolecular details in well defined crystallographic planes selected via the epitaxial crystallization.     

茂金属催化剂的负载化

负载化是茂金属催化剂发展的一个新方向。本文系统地描述了茂金属负载化催化剂的主要制备方法;分析了活性中心的本质、结构以及生成机理;并且讨论了负载化对茂金属催化剂烯烃聚合性能的影响。     

Cross metathesis functionalization of polyolefins

A cross metathesis strategy is reported for the post-polymerization functionalization of the pendant vinyl groups present in a range of polyolefin architectures. This represents a general strategy for the synthesis of tailored random and block copolymers as well as homopolymers.     

Adhesion mechanisms of grafted polyolefins

By comparing surface properties assessed by wettability measurements with polymer-metal adhesion strength determined by peeling for different grafted polyolefins (acrylic acid grafted polyethylene, acrylic acid and maleic anhydride grafted poly(ethylene-vinyl acetate), maleic anhydride grafted polypropylene), a general adhesion mechanism can be proposed. The surface polarity is not the main determining parameter for adhesion. Polyolefin-aluminium adhesion is obtained through chemical bonding of acid groups grafted in the polymer bulk and basic groups present on the aluminium.     

Biofouling and biodegradation of polyolefins in ocean waters

High density polyethylene (HDPE), low density polyethylene (LDPE) and polypropylene (PP) coupons were immersed for a period of 6 months in Bay of Bengal near Chennai Port (Port) and Fisheries Survey of India (FSI). Samples were retrieved every month and the extent of biofouling and biodegradation were monitored by measuring biological and physicochemical parameters. Dissolved oxygen and oxidation reduction potential were higher at Port than at FSI. Total suspended solids and organic matter were more on PP, followed by HDPE and LDPE indicating hydrophobic surfaces favour more biofouling. Pseudomonas sp., anaerobic, heterotrophic and iron-reducing bacteria were observed on polymer surface. Biofouling was found to depend on the season, loading being highest in the month of August. Chlorophyll was higher at FSI than at Port due to higher pollution levels and also being closer to the shores. Maximum weight loss was seen in LDPE (1.5-2.5%), followed by that in HDPE (0.5-0.8%) and finally in PP (0.5-0.6%) samples deployed at Port in the six month time period.