Butadiene, stereospecific polymerization

Many recent publications have described the stereospecific polymerization of dienes by ir-allyl compounds derived from Cr, Nb, Ni, etc. Of particular interest is the work of Durand, Dawans, Teyssie who have shown that ir-allyl nickel catalysts (XXI) in the presence of certain additives polymerize butadiene stereospecifically (87, 38). The active center results from reaction of acidic additives with the transition metal. 

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. 

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). 

Stereospecific polymerization has particular significance for the preparation of stereoregular polymeric dienes. In the radical polymerization of butadiene or isoprene the molecular chains always consist of varying proportions of adjacent cis- and trans-1,4-units as well as 1,2- and 3,4- linked units, depending on the polymerization conditions but it is now possible, using particular ionic initiation systems to make a synthetic natural rubber that contains more than 90% cfs-l,4-isoprene repeating units (see Example 3-21). 

Stereospecific Polymerization of Butadiene with Ziegler-Natta-Catalysts Preparation of c/s-1,4-Polybutadiene... 

Another important use of BC13 is as a Friedel-Crafts catalyst in various polymerization, alkylation, and acylation reactions, and in other organic syntheses (see Friedel-Crafts reaction). Examples include conversion of cydophosphazenes to polymers (81,82) polymerization of olefins such as ethylene (75,83—88) graft polymerization of vinyl chloride and isobutylene (89) stereospecific polymerization of propylene (90) copolymerization of isobutylene and styrene (91,92), and other unsaturated aromatics with maleic anhydride (93) polymerization of norbomene (94), butadiene (95) preparation of electrically conducting epoxy resins (96), and polymers containing B and N (97) and selective demethylation of methoxy groups ortho to OH groups (98). 

Shen et al.120,121) found that the compounds of lanthanoid metals (from La to Lu) were active for the stereospecific polymerization of butadiene in the presence of alkylaluminum. Recently, Ouyangetal.122) reported that a NdCl3/C2H5OH/Al(C2Hs)3 catalyst exhibited a living character for the polymerization of diene and ethylene at temperatures below —30 °C. Diblock or triblock copolymers of diene and ethylene were obtained upon further addition of a diene monomer to a living polydiene or polyethylene. 

Annex 2 Stereospecific Polymerization of Conjugated Diolefins Butadiene and Isoprene... 

The titanium(II) derivatives Ti(02CCFg)X (where X = Cl or Br) have been prepared by the reaction between Ti(02CCF3)2 and the corresponding thionyl halide in ether. All of these titanium(II) trifluoro-acetates are effective as catalysts for the stereospecific polymerization of butadienes (44). 

Metal trifluoroacetates are used as catalysts in several industrial processes. The use of [(7i-allyl)Ni(02CCF3)2]2 in the stereospecific polymerization of butadiene has been studied in some detail 59, 60, 166,... 

The stereospecific polymerization of butadiene catalyzed by transition metal salts is also-controlled by the nature of the ligand (3, 4, 29, 41, 60) and may involve intermediates similar to those discussed. 

Alcock and coworkers studied the polymerization of butadiene (as well as of monoolefins, acetylene and aromatic olefins) trapped within the tunnel clathrate system of tris((9-phenylenedioxy)cyclotriphosphazene, induced by Co-y-radiation. The host was used in order to find if the concatenation and orientation of the monomer molecules under the steric forces generated within the host crystal lattice will lead to stereospecific polymerization. The clathrate was prepared by addition of liquid butadiene to the pure host at low temperature. The irradiation was conducted at low temperatures. Irradiation of pure butadiene (unclathrated bulk monomer) leads to formation of a mixture of three addition products f,2-adduct, cis- and trons-f,4-adducts. In contrast, the radiation-induced polymerization within the tunnel system of the host yielded almost pure trans-1,4-polybutadiene. A small percentage of f, 2-addition product was observed, but no evidence for the formation of c/s-f,4-adduct was found, confirming the earlier observation by Fin ter and Wegner. The average molecular weight was about 5000,... 

The catalysis of the stereospecific polymerization of conjugated dienes is of considerable interest from both the scientific and the industrial points of view [1,2]. From butadiene and isoprene, as the industrially most important 1,3-dienes, in comparison with the polymerization of olefins many more structurally different stereoregular polymers can be derived cf the structures of the stereoregular polybutadienes and polyisoprenes given in Scheme 1 [106]. 

The synthesis and characterization of anionic allylneodymium(m) complexes and their use as catalysts for the stereospecific polymerization of butadiene have been reported. For the already known tetrakis(allyl) complex Li[Nd(7]3-C3Hs)4]-1.5dioxane a significantly improved method of preparation from anhydrous NdCl3 and LiC3HS dioxane in DME was found (Scheme 29).182... 

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