金属有机化学第十章

polyethylene kH/kD = 1.04
Since this experiment was inconclusive, Grubbs designed a stereochemical probe to determine if an -H/D agostic interaction affected the insertion process. The Cossee mechanism should give a nearly 1:1 mixture of diastereomers, since there is no primary isotope effect in this mechanism. The Rooney-Green mechanism would be expected to give a unequal product distribution. A 50:50 diastereomeric ratio of products was produced in the reaction. (J. Am. Chem. Soc. 1985, 107, 3377)
Ziegler was originally studying the Aufbau reaction, which uses trialkyl aluminum compounds to make oligomers from ethylene. Ziegler found that adding transition metals salts resulted in significant changes in the reactivity.
These systems would polymerize ethylene, but slowly. Propylene and other higher alkenes could not be polymerized. The major advance was the introduction of the activator methylaluminoxane (MAO) by Kaminsky (Macromol. Chem. Rapid. Comm. 1983, 4, 417)
J. Chem. Soc., Chem. Comm. 1978, 604
The major difference between the two mechanisms, should be in the kinetic isotope effect. The Green-Rooney mechanism would require a large kinetic isotope effect (ca. 3). Grubbs first carried out the polymerization of a mixture of ethylene and ethylene-d4. (J. Am. Chem. Soc. 1982, 104, 4479), No isotope effect was observed, however this result did not definitively rule out an -elimination mechanism.
The major problems with heterogeneous systems are:
• There are a variety of different catalytic sites with different activity producing polymers with different molecular weights and tactcities. The polymers produced are always blends. • Heterogeneous systems are difficult to tune, because there is not much that can be varied and they are generally poorly understood. Although fairly selective systems have been developed, the work is largely empirical.
K. Ziegler (Angew. Chem., 1952, 64, 233):
This was a heterogeneous process. Insoluble metal salts and trialkylaluminum species were mixed in a hydrocarbon solvent under ethylene pressure.
Initiation:
The active species is believed to be a coordinatively unsaturated metal alkyl formed by reaction of the metal halide salt with the alkyl aluminum species.
The major advantage is that they are heterogeneous and can be used under industrially desirable conditions either in the gas phase or as suspensions.
Mechanism of Alkene Polymerization:
Linear Low Density (LLDPE) High Density (HDPE) Polypropylene Ethylene-Propylene rubber
7,959 13,906 15,448 340
Heterogeneous Olefin Polymerization Catalysts: The Ziegler-Natta systems.
Propagation:
The exact nature of the propagation step was controversial for some time. Cossee proposed a mechanism involving simple migratory insertion of an alkene into a metal alkyl Tetrahedron Lett. 1960, 1(38), 12-16
Chain Tain transfer mechanisms have been identified by modeling studies. -H elimination
Hydrogenation
Chain transfer to aluminum
Chain transfer to monomer
-R elimination -R
Homogeneous Ziegler-Natta-type polymerization catalysts
The first homogeneous systems were based on metallocene dichlorides, and were designed to mimic the activation process believed to occur in the heterogeneous systems.
90 % crystalline
X-ray and IR studies showed that the crystalline material had the same configuration at each chiral center. The polymer was isotactic, (via infra)
G. Natta (J. Am. Chem. Soc. 1955, 77, 1708): Soon after, Natta showed that the Ziegler catalyst would polymerize propylene to give polypropylene.
40 % crystalline material
= open coordination site
Green, and Rooney proposed an alternative mechanism based on an -elimination to give a carbene. Since there were few examples of isolated alkene alkyl complexes that undergo migratory insertion, they felt that an alternative mechanism was necessary. Formation of a metalacycle followed by reductive elimination gives a new metal-alkyl.
Ziegler and Natta shared the 1963 Nobel Prize in chemistry for their contributions.
These systems are still widely used in industry, but from the beginning there was interest in developing homogenous, single-site catalysts for olefin polymerization. These were desired both as model compounds for mechanistic studies and to provide more consistent and tunable catalysts. We will discuss this in detail later.