1,721,132 research outputs found
STEREOCHEMISTRY OF POLYMERIZATION OF SOME alpha-OLEFINS IN THE PRESENCE OF ZIEGLER-TYPE CATALYSTS
No full textPolymerization mechanism of conjugated dienes in the presence of Ziegler-Natta type catalysts: Theoretical study of butadiene and isoprene polymerization with CpTiCl3-MAO initiator
No full textThe mechanism of butadiene and isoprene polymerization in the presence of CpTiCl3-MAO has been investigated by means of ab initio computations. According to experimental evidence, the catalytically active species are assumed to be [CpTi-P](+) organotitanium cations (P = growing polymer chain). The results show that (i) in the active species, P is coordinated to Ti both with the pi allyl group of its ending unit and with the pi bond of the penultimate unit; (ii) the cis eta(4) coordination of an incoming monomer to the active species requires the breakage of the latter interaction and the change of the allyl coordination mode from eta(3) to eta(1) That significant rearrangement of P is predicted to be the rate-determining step of the whole propagation reaction and is predicted to be much easier when the ending unit of P is butenyl rather than 2-methylbutenyl. That finding accounts both for the large difference observed in the homopolymerization rates of butadiene and isoprene and for the fact that in copolymerizations the two monomers exhibit almost the same reactivity
Isotactic Polymerization of Propene With Heterogeneous Catalytic-systems - A Partial Tentative Rationalization of the Behavior of Some 3rd Components
No full textThe stereoregulating effects of Me benzoate (I) [93-58-3] in TiCl4-MgCl2-Et3Al Ziegler catalysts for polymn. of C3H6 [115-07-1] could be attributed to slow ligand exchangers for the nonstereospecific sites and to the deactivation of the nonstereospecific sites in the presence of I. The carbon atoms of I (as a fourth component in the catalyst) were incorporated into the polymer via alkylation of I by the Ti-C bond of the active catalytic sites. The polymn. kinetics was discussed
Mechanism of isoprene and butadiene polymerization in the presence of CpTiCl3-MAO initiator: A theoretical study
No full textThe mechanism of butadiene and isoprene polymerization in the presence of CpTiCl3-MAO initiator has been investigated by means of EHT and ab-initio computations, with the purpose of understanding the factors responsible for the very different homopolymerization rates of the two monomers. The catalytically active species is assumed to be the highly unsaturated [CpTi-R](+) organotitanium cation (R = growing polymer chain). It is shown that coordination of an incoming monomer to the active species forces the ending unit of R to rearrange from the initial eta(3) allylic coordination to the a one. This step is energetically much easier when the ending unit is butenyl rather than 2-methylbutenyl. No significant differences between butadiene and isoprene have been found for the insertion step; therefore, the large difference in the homopolymerization rates of butadiene and isoprene appears to be due to the highest reactivity of the growing chain ending with a butenyl unit
Isotactic Polymerization of Propene - Homogeneous Catalysts Based On Group-4 Metallocenes Without Methylalumoxane
No full textC-13-enriched end groups of poly(3-methyl-1-pentene) prepared in the presence of metallocene catalysts
No full textPolymerization of both (S)-3-methyl-1-pentene and (R,S)-3-methyl-1-pentene in the presence of two metallocene catalysts prepared from metallocene precursors of different symmetry (C-2 and C-s) and C-13-enriched MAO shows that the insertion of the monomer in the initiation step is diastereoselective to an extent similar to that previously observed in the presence of the heterogeneous isotactic specific catalyst delta-TiCl3-Al((CH3)-C-13)(3). Surprisingly, the catalyst prepared with the metallocene precursor of C-s symmetry promotes coisotactic polymerization of the two enantiomers of 3-methyl-1-pentene but only dimerization of (S)-3-methyl-1-pentene
Reductive decomposition of cationic half-titanocene(IV) complexes, precursors of the active species in syndiospecific styrene polymerization
No full textMixtures of Cp*TiR(3) (with Cp* = C-5(CH3)(5) and R = CH3, (CH3)-C-13) and equimolar amounts of B(C6F5)(3) or C(C6H5)(3)B(C6F5)(4), active in the syndiospecific polymerization of styrene, have been investigated by means of ESR spectroscopy. At room temperature the formation of Cp*TiR(+) complexes has been observed. Rapid insertion of styrene occurs in Ti-R bonds of these cationic Ti(lll) complexes, strongly suggesting their role in the stereospecific polyinsertion
International Symposium on Future Technology for Polyolefin and Olefin Polymerization Catalysis
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