سلام
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1.
Ziegler-Natta and Metallocene Polymerisation
As a result of the work of Ziegler in Germany, Natta in Italy and Pease and Roedel in the United States, the process of co-ordination polymerisation, a process related to ionic polymerisation, became of significance in the late 1950s.
This process is today used in the commercial manufacture of polypropylene and polyethylene and has also been used in the laboratory for the manufacture of many novel polymers. In principle the catalyst system used governs the way in which a monomer and a growing chain approach each other and because of this it is possible to produce stereoregular polymers.
One way in which such stereospecificity occurs is by the growing polymer molecule forming a complex with a catalyst which is also complexed with a monomer molecule.
In this way growing polymers and monomers are brought
together in a highly specific fashion. The product of reaction of the growing polymer molecule and the monomer molecule is a further growing molecule which will then again complex itself with the catalyst and the cycle may be repeated.
The catalysts used are themselves complexes produced by interaction of alkyls of metals in Groups 1-111 of the Periodic Table with halides and other derivatives of Groups IV-VI11 metals. Although soluble co-ordination catalysts are known,
those used for the manufacture of stereoregular polymers are usually solid or adsorbed on solid particles.
A number of olefins may be polymerised using certain metal oxides supported on the surface of an inert solid particle. The mechanism of these polymerisation reactions is little understood but is believed to be ionic in nature.
Following the considerable commercial success of Ziegler-Natta polymer- isation systems which made possible high density polyethylene, polypropylene, ethylene-propylene rubbers and a number of speciality materials, a considerable 38 The Chemical Nature oj Plastics body of research was devoted to attempt a better understanding of the polymerisation mechanism. Cossee proposed that a metal atom in the catalyst system formed a temporary bond simultaneously with a growing polymer chain and with the double bond of the monomer. This caused the chain end to be electrically attracted to the monomer resulting in fusion of chain end and
monomer generating a new chain end and allowing the process to repeat. The Ziegler-Natta catalysts were, however, complex mixtures of solid and liquid compounds and so attempts were made to produce model systems for study using a catalyst of uniform structure containing a single metal atom. Such systems are referred to as being single-sited and the Ziegler-Natta systems as multi-sited.
Research work eventually concentrated around what became known as metallocene systems. At risk of considerable over-simplification these may be regarded as consisting of a metal atom, usually titanium or zirconium, linked to two rings of 5-carbon atoms and to two other groups, usually single carbon atoms with attached hydrogens. The 5-carbon rings are hinged together by other atoms
in a form reminiscent of a partly opened clamshell and these partly enclose the metal atom. By varying the nature of the hinge atoms, by the use of substituents on the 5-carbon rings, by modifying the symmetry of the ‘clam-shell’ by the positioning of the substituents and by the use of cocatalysts such as methyl aluminoxanes, the accessibility of monomer, and in due course, polymer chain to the metal atom can be carefully controlled.
In turn this can lead to control of the following factors:
(a) what monomer can be polymerised (it may be possible to polymerise just one of a mixture of monomers);
(b) the frequency of termination reactions leading to narrow molecular weight distributions;
(c) the direction of approach of monomer to the chain end leading to closely controlled stereoregular polymers. (The main types
of stereoregular polymer are discussed further in Section 4.3.)
2.other refrence:
PRINCIPLESOF
POLYMERIZATION
FourthEdition
By: GEORGEODIAN
CollegeofStatenIsland
CityUniversityofNewYork
StatenIsland,NewYork
page 640…………………… STEREOCHEMISTRYOF POLYMERIZATION
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