Co-ordination catalysts

The catalysts used are themselves complexes produced by interaction of alkyls of metals in Groups l-IIl of the Periodic Table with halides and other derivatives of Groups IV-VIII metals. Although soluble co-ordination catalysts are known, those used for the manufacture of stereoregular polymers are usually solid or adsorbed on solid particles. 

As indicated by the title, these processes are largely due to the work of Ziegler and coworkers. The type of polymerisation involved is sometimes referred to as co-ordination polymerisation since the mechanism involves a catalyst-monomer co-ordination complex or some other directing force that controls the way in which the monomer approaches the growing chain. The co-ordination catalysts are generally formed by the interaction of the alkyls of Groups I-III metals with halides and other derivatives of transition metals in Groups IV-VIII of the Periodic Table. In a typical process the catalyst is prepared from titanium tetrachloride and aluminium triethyl or some related material. 

As with polybut-l-ene and many other vinyl monomers that contain an asymmetric carbon, isotactic, syndiotactic and atactic stmctures may be drawn. Using co-ordination catalysts such as mixtures of cobalt chlorides, aluminium alkyls, pyridine and water high-1,2 (high vinyl) polymers may be obtained. One product marketed by the Japan Synthetic Rubber Company (JSR 1,2 PBD) is 91% 1,2, and 51-66% of the 1,2 units are in the syndiotactic state. The molecular mass is said to be several hundred thousand and the ratio MJM is in the range 1.7-2.6. 

Usually the Zigler-Natta co-ordination initiator system is used to graft oc-olefins onto other polymers to give stereo block/graft copolymers, which contain isotactic/atactic sequences. In the Zigler-Natta co-ordination catalyst [69] system, the diethyl aluminium hydride reacts with pendant groups to form macromolecular trialkyl aluminium. The residual initiator is freed by extraction methods. 

It has been noted (Guan et al, 1999) that polyethylene is not just branched (as in linear low-density polyethylene, where branching is controlled by the copolymerization of 1-hexene, or in low-density polyethylene, where it is uncontrolled due to back-biting) but may, under certain circumstances, also be hyperbranched. The mechanism for hyperbranching is to create the branch point by controlled isomerization of the active site by an appropriate choice of both co-ordination catalyst and pressure. 

The measured values of the reactivity ratios for copolymerization reactions involving olefins (particularly those with four or more carbon atoms) where polymerization has been initiated by co-ordination catalysts has been examined by Kissin. In general there is a tendency for block formation to take place and the distribution of monomer units in the copolymers is in general accord with... 

It is also possible to observe alternating co-ordination of monomers at active sites in polymerizations initiated by co-ordination catalysts for other reasons. 

It has been previously noted that co-ordination catalysts were originally employed by Ziegler in 1953 to effect the polymerization of ethylene. Natta extended the use of the catalysts to higher olefins and obtained isotactic polypropylene in 1954. The first commercial production of the polymer was by Montecatini in 1957. Polypropylene is now an important commercial polymer, being used principally in the injection moulding of diverse articles and for fibres and films. 

The general mechanism of polymerization of vinyl compounds by co-ordination catalysts is described in Section 1.4.2.1 and the same principles probably apply to the polymerization of conjugated dienes. 

Details of production processes for the polymerization of isoprene by lithium and alkyllithium compounds do not appear to be available. However, the reaction proceeds under mild conditions (i.e., at room temperature or slightly above and at atmospheric pressure) and a typical process is probably not unlike that described above for co-ordination catalysts. 

The polymerization of butadiene is exactly comparable to that of isoprene (Section 18.3.3). Similarly, butadiene may be polymerized by a variety of methods, the choice of which determines the microstructure of the resulting polymer (Table 18.3). In the case of polybutadiene, of course, 1,2- and 3,4-addition cannot be distinguished. As mentioned in Section 18.4.1, sodium-polymerized butadiene has been produced commercially (and may still be in the U.S.S.R.), but at the present time most large-scale processes are based on either co-ordination catalysts (particularly titanium tetraiodide- and cobalt-containing catalysts) or alkyllithium catalysts. These processes are operated similarly to... 

Another great challenge in polyolefin technology has been the production of polyolefins with polar functionalities. The most advanced method for producing functional polyolefins, in addition to free radical reactions and grafting reactions, is copolymerization with co-ordination catalysts. 

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