Polymerization stereospecific type

A Rohm and Haas group in 1958 (13, 21), reported that methyl methacrylate can be polymerized stereospecifically by an anionic type initiator under homogeneous conditions. Table 6 summarizes their results. 

The coordinate type catalysts are also effective for thiirane polymerizations. The types of systems used are also similar. Thus diethylzinc and in particular diethylzinc/water mixtures have been studied [44]. Other studies made using triethylaluminium and diethylcadmium indicated that these metal alkyls all behave similarly. The reactions seem to be rather complex, and, as also was the case with the epoxides, no well defined kinetic studies have appeared. The polymers produced are of high molecular weight and are often crystalline. Thus stereospecific polysulphides have been reported. Again the bulk of the studies involve PS. Stereoselective polymerization of racemic monomer has been accomplished [45, 46] using a catalyst prepared from diethylzinc and (+) borneol. The marked difference between PO and PS in their polymer-... 

The most efficient enantioface discriminating agents seem to be transition metal complexes covalently bound to the growing chain end, which are also able to achieve a very high regio-selectivity in the attack to the double bond. Unfortunately, the type of monomers which are polymerized stereospecifically with this type of catalysts are mainly unsaturated hydrocarbons. Propylene (14) and butadiene (46) can be polymerized by the above catalysts both to isotactic and syndiotactic polymers. 

The first stereoregular 1,4-polybutadiene was obtained by Morton and his associates by using an Alfin catalyst in 1947 (34) and had a predominantly trans structure. The structure of this polymer was determined by careful X-ray and IR-excimination (61). A few years later the first synthesis of 1,4-cis-polyisoprene was accomplished (62) by using a Li metal dispersion. In the same period, Ziegler-type catalysts were reported to polymerize stereospecifically to 1,4-trans- or 1,4-cis-polymers a number of conjugated dienes. More recently, new types of catalysts for the stereospecific polymerization of dienes were prepared starting with TT-allyl derivatives of transition metals. Some catalysts for the stereospecific 1,4-polymerization of butadiene are indicated in Scheme 12. 

Polymerizations catalyzed with coordination compounds are becoming more important for obtaining polymers with special properties (linear and stereospecific). The first linear polyethylene polymer was prepared from a mixture of triethylaluminum and titanium tetrachloride (Ziegler catalyst) in the early 1950s. Later, Natta synthesized a stereoregular polypropylene with a Ziegler-type catalyst. These catalyst combinations are now called Zieglar-Natta catalysts. 

Most polysilanes are synthesized by the Wurtz-type coupling reaction, which is non-stereospecific, and hence the configuration is usually undefined, although in a few cases, stereochemically defined polysilanes have been synthesized (ROP of all-anti l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclotetrasilane99 101 and masked disilene polymerization).61... 

One of the first reactions to be studied systematically in a channel-type clathrate was the polymerization of olefins or diolefins. The ordering of the monomers within the clathrate lattice leads to stereoregular products that are not available by other techniques (232-234). Such radiation-induced stereospecific polymerization has been reported for a number of clathrate hosts (235). 

He was a Professor of Industrial Chemistry, School of Engineering, Polytechnic Institute of Milan, Milan, Italy since 1937. He became involved with applied research, which led to the production of synthetic rubber in Italy, at the Institute in 1938. He was also interested in the synthesis of petrochemicals such as butadiene and, later, oxo alcohols. At the same time he made important contributions to the understanding of the kinetics of some catalytic processes in both the heterogeneous (methanol synthesis) and homogeneous (oxosynthesis) phase. In 1950, as a result of his interest in petrochemistry, he initiated the research on the use of simple olefins for the synthesis of high polymers. This work led to the discovery, in 1954, of stereospecific polymerization. In this type of polymerization nonsymmetric monomers (e.g., propylene, 1-butene, etc.) produce linear high polymers with a stereoregular structure. 

Solution polymerization is bulk polymerization in which excess monomer serves as the solvent. Solution polymerization, used at approximately 13 plants, is a newer, less conventional process than emulsion polymerization for the commercial production of crumb mbber. Polymerization generally proceeds by ionic mechanisms. This system permits the use of stereospecific catalysts of the Ziegler-Natta or alkyl lithium types which make it possible to polymerize monomers into a cis structure characteristic that is very similar to that of natural rubber. This cis structure yields a rubbery product, as opposed to a trans stmcture which produces a rigid product similar to plastics. 

In the polymer field, reactions of this type are subject to several limitations related to the structure and symmetry of the resultant polymers. In effect, the stereospecific polymerization of propylene is in itself an enantioface-diflferen-tiating reaction, but the polymer lacks chirality. As already seen in Sect. V-A there are few intrinsically chiral stractures (254) and even fewer that can be obtained from achiral monomers. With two exceptions, which will be dealt with at the end of this section, optically active polymers have been obtained only from 1- or 1,4-substituted butadienes, fiom unsaturated cyclic monomers, fiom substituted benzalacetone, or by copolymerization of mono- and disubstituted olefins. The corresponding polymer stmctures are shown as formulas 32 and 33, 53, 77-79 and 82-89. These processes are called asymmetric polymerizations (254, 257) the name enantiogenic polymerization has been recently proposed (301). 

Considerable interest has been shown in the new processes of stereospecific polymerization, not only so far as they concern the production of new classes of polymers, having unusual characteristics and improved properties, but also because they are representative of a peculiar new type of heterogeneous catalysis, of great interest from the practical and the theoretical points of view 1-5). 

Complexes of type III and also traces of soluble halides of strongly electropositive transition metals, being able to form complexes with metallorganic compounds of the type I, II, increase the activity of the stereospecific catalysts formed by the action of metallorganic compounds on crystalline substrates 10,11). They can also polymerize in a stereospecific way in the presence of crystalline substrates of transition metals (for instance, CoCU) which are not by themselves sufficiently electropositive, (when used in the presence of metallorganic compounds) to polymerize the a-olefins 10, 11). 

The results summarized in the previous sections show that, for the considered sample of a-TiCU, there are two types of adsorption of the triethyl-aluminum on a-titanium trichloride, one of them is connected with active centers which are directly active in the stereospecific polymerization of propylene. 

The AlR3-acid amide (1 1) catalyst system thus proved to be an excellent catalyst for the stereospedfic polymerization of acetaldehyde and to be far superior to the AlR3-ketone (1 1) catalyst system. Both linear and cyclic acid amides gave a similar degree of stereospecificity with good reproducibility (35). We considered that the former catalyst system would be more suitable for our purpose, because this type of... 

The strategy we adopted for attacking our problem, i.e., the complete understanding of the stereoregulation mechanism in the stereospecific polymerization reaction, has been successfully applied to the stereospecific polymerization of acetaldehyde and propylene oxide. The same strategy should be applicable also to other types of catalysts and monomers, even if the difficulty encountered in the experimental performance is greater. The fruitful harvest must await future investigation. 

The study of the steric polymerization course when varying the type of initiator and the polymerization conditions, may be of considerable interest for a better knowledge of stereospecific catalysts. 

The development of organometallic initiators, both of the lithium type and of the transition-metal coordination type, occurred rapidly in the decade following the late 1950s. The lithium initiators were developed without the fanfare of coordination-type initiators. This situation developed because of the remarkable ability of the coordination catalysts to induce stereospecific polymerization of a-olefins. 

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