سال:1394 | دوره: | شماره: |تعداد مقالات:6

Beside Ziegler-Natta and Phillips catalysts the development of methylaluminoxane (MAO) as cocatalyst in combination with metallocenes or other transition metal complexes for the polymerization of olefins has widely increased the possibilities in controlling the polymer composition, polymer structure, tacticity and special properties with high precision. These catalysts allow the synthesis of isotactic, isoblock, syndiotactic, stereoblock or atactic polymers, as well as polyolefin composite materials with superior properties such as film clarity, tensile strength and lower content of extractables. Metallocene and other single site catalysts are able to copolymerize ethene and propene with short and long chained a-olefins, cyclic olefins, or polar vinyl monomers such as ethers, alcohols or esters, especially, if the polar monomers are protected by aluminum alkyls. Different vinyl ethers such as vinyl-ethyl ether, vinyl-propyl ether, vinyl-hexyl ether, and 2, 7-octadienyl methyl ether (MODE) were copolymerized with olefins using triisobutyl aluminum as protecting agents. Polar monomers could be incorporated into the polymer chain by up to 16 mol%. Such copolymers show better gas barrier and surface properties, as well as solvent resistance and they are suitable for blends of polyolefins with polyethers and other polar polymers because of an excellent adhesion of the two polymers.

Ethylene polymerization catalysts became available in an enormous variety. The challenge in this research is to find catalysts that are able to connect ethylene molecules in such a way that not only linear chains are produced but variations like branched materials that possess very interesting mechanical properties like linear low density polyethylene (LLDPE). In this contribution, three different types of catalysts are presented that are able to do not only one job at a time but three. These are “intelligent catalysts”. Catalysts of type 1 are homogeneous metallocene complexes that can be activated with methylaluminoxane (MAO). With ethylene they produce their own support and they become heterogeneous catalysts (self-immobilization) and they prevent fouling in polymerization reactors. The produced resin has evenly distributed ethyl branches (without a comonomer) with unique properties and the MAO that is necessary in the activation step can be recycled. Catalysts of type 2 are dinuclear complexes with two different active sites. One centre can oligomerize ethylene and the other one can copolymerize the in statu nascendi produced oligomers with ethylene to give branched LLDPE (a molecule as the smallest reactor for LLDPE) and/or bimodal resins. Catalysts of type 3 are MAO activated iron di (imino) pyridine complexes that are able to oligomerize ethylene to give not only oligomers with even numbered carbon atoms but also odd numbered ones. In this reaction, one catalyst does three jobs at a time: oligomerization, isomerization and metathesis of ethylene.

The data on the effect of ethylene concentration on polymerization rate for several modifications of modern highly active titanium–magnesium catalysts TiCl4/MgCl2 are presented. These catalysts differ in titanium content and conditions of support preparation, activities, and the shape of kinetic curves. It is found that the observed order of polymerization rate with respect to ethylene in the range of ethylene pressures of 0.5-6 bar is 1.8-2.1 for all catalysts used (polymerization at 80oC, AlEt3 used as a cocatalyst). When AlEt3 was replaced with Al (i-Bu) 3, the reaction order decreased to 1.3-1.4. In order to elucidate the possible reasons for the observed high order with respect to ethylene, we analyzed the data on the effect of monomer concentration on the molecular weight of polyethylene. The results gave grounds for suggesting that the observed order with respect to monomer is attributable to the effect of ethylene concentration on the number of active sites. The possible reaction scheme explaining the nonlinear dependence of the polymerization rate on monomer concentration was proposed based on these data.

Pyrolysis of low density polyethylene (LDPE) by equilibrium fluid catalytic cracking (FCC) was studied in a stirred reactor under different process parameters. In this work, the effect of process parameters such as degradation temperature (420-510oC), catalyst/polymer ratio (0-60%), carrier gas type (H2, N2, ethylene, propylene, Ar and He), residence time and agitator speed (0-300 rpm) on the condensate yield (liquid, gas and coke) and product composition were considered. Reaction products were determined by GC analysis and shown to contain naphthenes (cycloalkanes), paraffins (alkanes), olefins (alkenes) and aromatics. Higher temperature and more catalyst amount enhanced LDPE cracking. The maximum “fuel like” condensed product yield was attained at 450oC and 10% catalyst, respectively and gaseous products increased with increases in temperature. Hydrogen as a reactive carrier gas increased the condensed and paraffinic product yield. Appropriate heat transfer (by stirring) increased the catalyst efficiency in a stirred reactor.

Monolithic aerogels of high molecular weight polyethylene (Mw=3×106- 6×106 g/mol) have been prepared by solvent extraction with supercritical carbon dioxide from thermoreversible gels prepared in decalin. These low density and highly porous aerogels present an apparent porosity up to 90%. The aerogel morphology observed by scanning electron microscopy (SEM) is characterized by spherulitic structures being interconnected by fibers. The X-ray diffraction experiments show that PE aerogels are highly crystalline with a degree of crystallinity of c.a.80% and PE chains being packed into the typical orthorombic unit cell. The combined SEM and N2 sorption investigations show that PE aerogels are essentially macroporous with a small amount of mesopores. The oil-sorption performance of polyethylene aerogels has been also evaluated in this study in order to assess a possible use of these materials for oil spillage recovery and results show that aerogel macropores allow a very fast sorption kinetics with a 100% oil weight uptake obtained in less than 1 min.

Heterogeneous Ziegler-Natta and homogeneous metallocene catalysts exhibit greatly different active site nature in olefin polymerization. In our previous study [1], it was reported that MgCl2-supported titanocene catalysts can generate both Ziegler-Natta-type and metallocene-type active sites according to the type of activators. The dual active site nature of the supported titanocene catalysts was further explored in the present study: The influence of the ligand structure of titanocene precursors was studied on the nature of active sites when supported on MgCl2 in ethylene and propylene homopolymerization, and ethylene/1-hexene copolymerization. It was found that the reducibility of titanocene precursors by alkylaluminum is closely related to the appearance of the dual active site nature, while the kind of olefin did not affect the type of active sites formed during polymerization. The Ziegler-Natta-type active sites produced poorly isotactic polypropylene and less branched polyethylene, while the metallocene-type active sites produced atactic polypropylene and exhibited much higher incorporation efficiency for 1-hexene.