سال:1393 | دوره: | شماره: |تعداد مقالات:7

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Some innovative solutions are proposed to the problem of the unavoidable physical migration of antioxidants from plastic films for packaging, in order to minimize the consequent undesirable effect of food contamination. In previous exploratory tests, phenolic antioxidant co-units were achieved and incorporated into polyethylene chain and now the work is extended to create new families of polymeric additives properly designed for specific materials. An effective route was designed to synthesize the functionalized comonomer, analogues of commercial 2, 6- t -butyl-4-methoxy-phenol (BHA), containing eight methylene units as spacer between the aromatic ring and the polymerizable olefinic double bond (C8). Ethylene/1-hexene/C8 terpolymers, with 1-hexene concentration in the typical range found in commercial polyethylene grades, and propylene/C8 copolymers with microstructure similar to those of commercial packaging polypropylenes were produced. A careful 13C NMR study was conducted for the precise determination of the functionalized comonomer content on all terpolymer and copolymer samples. The samples melt blended with additive-free commercial LDPE and PP matrices, individually, were analyzed in terms of thermal and thermo-oxidative stability and compared with LDPE and PP films containing the traditional BHA additive analogue. The results demonstrate that, in either way, the polymeric additives exert a very positive effect on the degradation temperature of the polymeric matrices, retarding the thermo-oxidative sequence of reaction.

Afourth-generation Ziegler-Natta catalyst was prepared to synthesize polypropylene (PP), which was stabilized by in situ polymerization employing lignin as antioxidant. The antioxidant properties of lignin were compared with those of the commercial antioxidant Irganox 1010. The presence of lignin in the reaction medium slightly decreased the catalytic activity of the reaction. The isotacticity index (I.I.) of PP synthesized with lignin (PP-lig) was not affected by the presence of the additive in the reaction medium. The thermal properties, characterized by differential scanning calorimetry, showed slightly decreased degree of crystallinity (Xc), but the melting temperature (Tm) and crystallization temperature (Tc) were not affected when compared with the neat polymer. Lignin showed good activity as a stabilizer by thermogravimetry. The initial temperature of degradation (Tonset) increased when compared to the pure PP and PP stabilized with the commercial antioxidant. The lower carbonyl index of the PP, evaluated by infrared spectroscopy (FTIR) after thermo-oxidative treatment, also revealed the stabilizing action of lignin.

AFI Zr-based catalyst of bis [N- (3, 5-dicumylsalicylidene) -2′, 6′-diisopropylanilinato] zirconium (IV) dichloride was prepared and used for polymerization of ethylene. The effects of reaction conditions on the polymerization were examined in detail. The increase in ethylene pressure and rise in polymerization temperature up to 35°C were favorable for catalyst/MAO to raise the catalytic activity as well as the viscosity-average molecular weight (Mv) of polyethylene. The activity of the catalyst was linearly increased with increasing MAO concentration and no optimum activity was observed in the range studied. Although introduction of the bulky cumyl and 2′, 6′-diisopropyl alkyl substitution groups on ortho positions to the phenoxy-oxygen and on phenyl ring on the N, respectively enhanced the viscosity average molecular weight (Mv) of the obtained polymer strongly, diminished the activity of the catalyst. Neither the activity of the catalyst nor the (Mv) of the obtained polymer were sensitive to hydrogen concentration. However, higher amount of hydrogen could slightly increase the activity of the catalyst. The (Mv) of polyethylene ranged from 2.14×106 to 2.77×106 at the monomer pressure of 3 and 5 bar, respectively which are much higher than those of the reported FI Zr-based catalysts.

Primary MgCl2.3.3EtOH adduct (PCT1) was prepared by melt quenching method and then submitted into a programmed thermal dealcoholation process using a fluidized bed reactor. During thermal dealcoholation program, different MgCl2.nEtOH support samples with n=3.0, 2.7, 2.4, and 2.1 were selected and named as PCT2 to PCT5, respectively. Structural analysis of the support samples, by moving from PCT1 to PCT5, showed a significant increase in the surface area from 7.4 m2/g to 12.8 m2/g, together with reduced peak heights at 2 q»8.9 and 9.7o. After characterization of support samples, the final catalysts were prepared by reaction of the samples with TiCl4 and tested in slurry phase propylene polymerization. The prepared catalysts showed similar stereospecificities but different activities in polymerization tests, so that, with progression of dealcoholation from PCT1 to PCT2 the catalyst activity was reached its maximum value of 2.9 kgPP/g Cat.h, and then by further dealcoholation, from PCT2 to PCT5, the activity decreased gradually. In the last stage of the project, the effect of time interval between thermal dealcoholation and catalyst preparation, termed as "storage time" was studied on crystal and morphological characteristics of the two best adduct samples, namely MgCl2.2.4EtOH and MgCl2.3.0EtOH. The storage time greatly affected the adducts and the resulting catalysts, and in this respect the best catalyst activity was achieved for those prepared immediately after adduct formation.

The stereospecific polymerization of conjugated dienes began in 1954 with the first catalysts obtained by combining TiCl4 or TiCl3 with aluminum-alkyls, i.e. the catalytic systems previously employed for ethylene and propylene polymerizations. Subsequently, many other catalytic systems were obtained and examined by a combination of transition metal or lanthanide compounds with appropriate alkylating agents. With the advent of MAO as alkylating agent, at the beginning of the 1980s, new catalytic systems were introduced, in some cases much more active and stereospecific than those based on common aluminum-alkyls. Starting from the 2000s, in the wake of what happened in the case of mono-olefins, a new generation of catalysts based on complexes of transition metals and lanthanides with various ligands containing donor atoms such as P, N, O (e.g., phosphines, imines, imino-pyridines, cheto-imines) has been introduced. These systems have proved particularly active and able to provide polymers with controlled microstructure (i.e., cis -1, 4; 1, 2; mixed cis -1, 4/1, 2 with a variable 1, 2 content) from several types of 1, 3-dienes, permitting indeed to establish new correlations between the catalyst structure, the monomer structure and the polymer microstructure, and to improve our knowledge on the polymerization mechanism of 1, 3-dienes. This paper provides an exhaustive overview of the latest developments in the field of stereospecific polymerization of 1, 3-butadiene.

Highly active metallocenes and other single site catalysts have opened up the possibility of polymerizing cycloolefins such as norbornene (N) or of their copolymerization with ethene (E) or propene (P). The polymers obtained show exciting structures and properties. E-N copolymers are industrially produced materials, with variable and high glass transitions depending on the wide range of their microstructures. By realizing the possibility in great variety of stereoregularity of propene and norbornene units and the difference in comonomer distribution, P-N copolymers were expected to have fine tuned microstructures and properties. Moreover, P-N copolymers should be characterized by higher Tg-values than E-N copolymers with the same norbornene content and molar mass. A review of the state of the art of P-N copolymerization by ansa -metallocenes of C2 symmetry, namely rac -Et (Ind) 2ZrCl2 (I-I) and rac -Me2Si (Ind) 2ZrCl2 (I-II), and rac -Me2Si (2-Me-Ind) 2ZrCl2 (I-III), and of catalysts of Cs symmetry, namely (tBuNSiMe2Flu) TiMe2 (IV-I) and derivatives, is given here. Special emphasis is given to microstructural studies of P-N copolymers, including stereo- and regioregularity of propene units as well as of comonomer distribution, stereoregularity of norbornene units, and the structure of chain end-groups. This information allows us to find a rationale for the catalytic activities and the copolymer properties.