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Mechanistic Aspects of the Ziegler-Natta Polymerization

Citation

Soto, Jorge Gonzalez (1984) Mechanistic Aspects of the Ziegler-Natta Polymerization. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/AXPG-AG37. https://resolver.caltech.edu/CaltechTHESIS:07072014-083212737

Abstract

The isotope effect on propagation rate was determined for four homogeneous ethylene polymerization systems. The catalytic system Cp2Ti(Et)Cl + EtAlCl2 has a kHp/kDp = 1.035 ± 0.03. This result strongly supports an insertion mechanism which does not involve a hydrogen migration during the rate determining step of propagation (Cossee mechanism). Three metal-alkyl free systems were also studied. The catalyst I2(PMe3)3Ta(neopentylidene)(H) has a kHp/kDp = 1.709. It is interpreted as a primary isotope effect involving a non-linear α-hydrogen migration during the rate determining step of propagation (Green mechanism). The lanthanide complexes Cp*2LuMe•Et2O and Cp*2YbMe•Et2O have a kHp/kDp = 1.46 and 1.25, respectively. They are interpreted as primary isotope effects due to a partial hydrogen migration during the rate determining step of propagation.

The presence of a precoordination or other intermediate species during the polymerization of ethylene by the mentioned metal-alkyl free catalysts was sought by low temperature NMR spectroscopy. However, no evidence for such species was found. If they exist, their concentrations are very small or their lifetimes are shorter than the NMR time scale.

Two titanocene (alkenyl)chlorides (hexenyl 1 and heptenyl 2 were prepared from titanocene dichloride and a THF solution of the corresponding alkenylmagnesium chloride. They do not cyclize in solution when alone, but cyclization to their respective titanocene(methyl(cycloalkyl) chlorides occurs readily in the presence of a Lewis acid. It is demonstrated that such cyclization occurs with the alkenyl ligand within the coordination sphere of the titanium atom. Cyclization of 1 with EtAlCl2 at 0°C occurs in less than 95 msec (ethylene insertion time), as shown by the presence of 97% cyclopentyl-capped oligomers when polymerizing ethylene with this system. Some alkyl exchange occurs (3%). Cyclization of 2 is slower under the same reaction conditions and is not complete in 95 msec as shown by the presence of both cyclohexyl-capped oligomers (35%) and odd number α-olefin oligomers (50%). Alkyl exchange is more extensive as evidenced by the even number n-alkanes (15%).

Cyclization of 2-d1 (titanocene(hept-6-en-1-y1-1-d1)chloride) with EtAlCl2 demonstrated that for this system there is no α-hydrogen participation during said process. The cyclization is believed to occur by a Cossee-type mechanism. There was no evidence for precoordination of the alkenyl double bond during the cyclization process.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Chemistry
Degree Grantor:California Institute of Technology
Division:Chemistry and Chemical Engineering
Major Option:Chemistry
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Grubbs, Robert H.
Thesis Committee:
  • Grubbs, Robert H.
Defense Date:29 July 1983
Non-Caltech Author Email:jorgesoto2020 (AT) gmail.com
Funders:
Funding AgencyGrant Number
Consejo Nacional de Ciencia y Tecnología (CONACYT)UNSPECIFIED
Record Number:CaltechTHESIS:07072014-083212737
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:07072014-083212737
DOI:10.7907/AXPG-AG37
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:8531
Collection:CaltechTHESIS
Deposited By: John Wade
Deposited On:07 Jul 2014 16:16
Last Modified:21 Dec 2019 02:55

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