Coordination polymerization trans-1,4-polybutadiene

The trans/cis ratio of the product must, therefore, be determined at an earlier reaction stage and most probably by the ratio of species 27a and 27b. Steric or electronic factors affecting this ratio will influence the trans/cis ratio of the resulting 1,4-hexadiene. The phosphine and the cocatalyst effect on the stereoselectivity can thus be interpreted in terms of their influence on the mode of butadiene coordination. Some earlier work on the stereospecific synthesis of polybutadiene by Ni catalyst can be adopted to explain the effect observed here, because the intermediates that control the stereospecificity of the polymerization should be essen-... 

Since butadiene can also undergo coordinated anionic polymerizations, some of the differences in polymer microstructure are attributable to changes in mechanism. Based on the catalysts reported to date, the isotactic and syndiotactic 1,2-polybutadienes appear to arise from coordinated anionic mechanisms. Qs and trans 1,4-polybutadienes can probably be made by all mechanisms, with cis arising from soluble catalysts which are capable of multi-coordination at one metal site. Trans structure is favored by cationic mechanism and by anionic mechanism involving coordination at two metal centers. 

Polybutadiene, CAS 9003-17-2, is a common synthetic polymer with the formula (-CH2CH=CHCH2-)n- The cis form (CAS 40022-03-5) of the polymer can be obtained by coordination or anionic polymerization. It is used mainly in tires blended with natural rubber and synthetic copolymers. The trans form is less common. 1,4-Polyisoprene in cis form, CAS 9003-31-0, is commonly found in large quantities as natural rubber, but also can be obtained synthetically, for example, using the coordination or anionic polymerization of 2-methyl-1,3-butadiene. Stereoregular synthetic cis-polyisoprene has properties practically identical to natural rubber, but this material is not highly competitive in price with natural rubber, and its industrial production is lower than that of other unsaturated polyhydrocarbons. Synthetic frans-polyisoprene, CAS 104389-31-3, also is known. Pyrolysis and the thermal decomposition of these polymers has been studied frequently [1-18]. Some reports on thermal decomposition products of polybutadiene and polyisoprene reported in literature are summarized in Table 7.1.1 [19]. 

In hydrocarbons, a polybutadiene is obtained containing about 35 % 1,4-cis, 54 % 1,4-trans and 11 % 1,2 units, while isoprene is polymerized under the same conditions with a cis selectivity of more than 90 %. By addition of polar ligands, such as tetrahydrofuran, dimethylglycol ether, tetramethylethylendiamine, or dipiperidylethane in the butadiene polymerization, the 1,2-selectivity can be enhanced by up to 100%. The effect increases with the coordination power and probably also with the space-filling ability of the ligand, and decreases with a rise in temperature. 

Medium-c/5 lithium-polybutadiene was first developed by Firestone Tire and Rubber Company in 1955 [86]. Solution polymerization using anionic catalysts is usually based on butyllithium. Alkyllithium initiation does not have the high stereospecificity of the coordination catalysts based on titanium, cobalt, nickel, or neodymium compounds. Polymerization in aliphatic hydrocarbon solvents such as hexane or cyclohexane yields a polymer of about 40 % cis, 50 % trans structure with 10 % 1,2-addition. However, there is no need for higher cis content because a completely amorphous structure is desired for mbber applications the glass transition temperature is determined by the vinyl content. The vinyl content of the polybutadiene can be increased up to 90 % by addition of small amounts of polar substances such as ethers. 

Insertion of the monomer, bonded to the metal in rf-cis fashion, into the metal-polymer bond forms a new 7t-allyl polymer end with the substituent at the anti-position. Successive insertion of the new monomer with if-cis coordination would produce cis- 1,4-polybutadiene. Insertion of if-trans-co-ordinated monomer into the metal-polymer bond leads to trans-1,4-polybu-tadiene via syn zr-allyl intermediates. The above anti Tt-allyl polymer end is often equilibrated with the thermodynamically more favorable syn zr-allyl structure via n-a-n rearrangement. Thus, the ratio of cis-1,4 and trans-1,4 repeating units of the polymer produced depends on the relative rates of the two reactions C-C bond formation between the monomer and the polymer end, and anti to syn isomerization of the zr-allyl end of the growing polymer. If the anti-syn isomerization of the anti zr-allyl polymer end occurs more rapidly than the insertion of a new monomer, the polymer with trans-1,4 units is formed even from 7j4-ds-coordinated monomer. The polymerization catalyzed by Ti, Co, or Ni complexes shows high cis-1,4 selectivity, while that with low monomer concentration results in increase of the trans content of... 

Polybutadiene containing a high content of trans-, 4 structure has been prepared with [(i7 -allyl)NiX]2 (X = Cl, Br, I). Since [(Tj -allyl)NiX]2 exists in the dimer state, it breaks into the monomer state only after coordination with butadiene. Butadiene can be polymerized with (17 -allyl) Ni compounds without base metal alkyls being present . 

The lanthanide coordination catalysts are known to be highly stereospecific for producing high-cis polybutadiene and high-cis polyisoprene as well as high-cis copoljnnerization of the two monomers. In addition, the polymerizations of other conjugated dienes such as trans-piperlene, 2,4-hexadiene,... 

ADMET polymerization has also been applied to 1,5-hexadiene, and polybutadiene (PBD) exclusively in the 1,4 mode was obtained [49]. Unlike PBD produced by ROMP, this ADMET polymer has a trans content of 75% [52]. With [W]l and [Mo]2,theAfn of these polymers was approximately 8.0 X lO gmol , witha poly-dispersity near 2.0. Attempts to polymerize 1,5-hexadiene with [Ru]l, however, resulted in oligomers of approximately 1.0 x lO gmoD, in addition to cyclics and unreacted monomer, even after extended reaction times [53]. This decrease in activity was attributed to stable intramolecular ii-complexation of the distal olefin of the 4-penten-l-ylidene complex to the metal center. Such coordination could obstruct bimolecular coordination of another diene to the metal, and thereby prevent further polymerization. The absence of this effect with Schrock s catalysts was explained in part by the steric congestion around the metal center of those catalysts and the lack of a labile ligand. A number of hydrocarbon dienes have been utilized in ADMET polymerizations [54, 55]. 

Polybutadiene belongs to the most important rubbers for technical purposes. In 1999 more that 2 million tons were produced worldwide, that is about 20% of all synthetic rubbers [11,12]. The cis type made by 1,4-addition is economically the most important polybutadiene [13,14]. Trans- as well as isotactic, syndiotactic, or atactic 1,2-polybutadiene can also be synthesized in good purity with suitable catalysts. For anionic polymerization with butyllithium or the coordinative process with Ziegler catalysts, 1,3-butadiene must be carefully purified from reactive contaminants such as acetylene, aldehydes, or hydrogen sulfide. 

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