Metal Halide-Based Catalysts

The general formula of metal halide-based living polymerization catalysts is expressed as MO,jCl ,-co-catalyst-ROH (M = Mo or W, = 0 or 1, m = 5 or 4). The most important feature of these catalysts is ease of preparation, but their initiation efficiency is low. A typical living polymerization by metal halide-based catalysts has been achieved with 1-chloro-l-octyne as monomer and using MoOCl -w-Bu Sn-EtOH (1 in Table 15.2) as catalyst [41]. Specifically, poly(l-chloro-l-octyne) with a narrow MWD 1.2) is obtained, and the 

The ternary MoOCl -w-Bu Sn-EtOH catalyst induces living polymerization of not only 1-choro-l-alkynes but also PhA with bulky ortho substituents [42,43]. The presence of bulky ortho substituents (e.g., CF, SiMeg) is essential to achieve excellent living polymerization, which is probably because such substituents are able to sterically preclude chain transfer and termination. Stereospecific living polymerization of er -butylacetylene is possible with MoOCl -w-Bu Sn-EtOH, which gives a polymer with a narrow MWD [44]. The NMR spectrum of the formed polymer has shown that the cis content of main-chain double bond reaches 97% for poly(fert-butylacetylene) prepared at -30 °C. 

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Living polymerization by metal halide-based metathesis catalysts... 

Mo and W hexacarbonyls, Mo(CO)6 and W(CO)6, alone do not induce polymerization of acetylenic compounds. However, UV irradiation toward these catalysts in the presence of halogenated compounds can form active species for polymerization of various substituted acetylenes. Carbon tetrachloride, CCI4, when used as the solvent for the polymerization, plays a very important role for the formation of active species, and thus cannot be replaced by toluene that is often used for metal chloride-based catalysts. Although these metal carbonyl-type catalysts are less active compared to the metal halide-based counterparts, they can provide high MW polymers. It is a great advantage that the metal carbonyl catalysts are very stable under air and thus handling is much easier. 

The gas phase reaction of acetylene with hydrogen chloride uses mercuric chloride (8, 9) or other heavy metal halides as catalyst. It is important that the gas streams be dry and free from arsine, phosphine, or sulfur. Because ethylene is priced substantially lower than acetylene, most recent processes substitute ethylene for acetylene, and acetylene-based vinyl chloride plants have been disappearing. 

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

In contrast to the situation with copper-based catalysis, most studies on ruthenium-based catalysts have made use of preformed metal complexes. The first reports of ruthenium-mediated polymerization by Sawamoto and coworkers appeared in I995.26 In the early work, the square pyramidal ruthenium (II) halide 146 was used in combination with a cocatalyst (usually aluminum isopropoxide). 

Ziegler-Natta catalyst. Gi ulio Natta developed a catalyst based on his work with Karl Ziegler for polymerizing vinyl monomers to give stereoregular, tailored, three-dimensional chains. The catalyst is based on aluminum alkyls and TiCU or other transition metal halides. 

From the experimental point of view, a particularly attractive procedure has been developed to produce cyclopentylmethylzinc species from primary or secondary halides, based on the use of diethylzinc and transition metal catalysts (see Section . .1)34. 

Otsuka et al. (110, 112) studied the polymerization of butadiene in the presence of an aged Co2(CO)8/2 MoC15 catalyst. The product obtained was predominantly an atactic poly(l,2-butadiene), the 1,2-structure being favored by low reaction temperature (e.g., at 40° C, 97% 1,2 at 30° C, > 99% 1,2). Similar experiments with a Ni(CO)4/MoCl5 catalyst yielded a polymer with 85% cis- 1,4-structure. The results of Otsuka et al. have been confirmed by Babitski and co-workers (8), who studied the polymerization of butadiene by a large number of binary catalysts, based on transition metal halide, transition metal carbonyl combinations. These systems are of interest as further examples of alkyl-free coordination polymerization catalysts for dienes (9, 15a, 109). Little is known of the origins of stereospecificity of these reactions. 

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