Syndiotactic and isotactic 1,2-polybutadienes

Theoretically, the simplest conjugated diolefin can be transformed into four stereoregular pol3raiers. From 1,4 addition, the possible structures are ds- and tra s-l,4-polybutadienes. The remaining two arise from 1,2 addition and are syndiotactic and isotactic 1,2-polybutadienes. All four have become experimental realities using Ziegler catalysts. 

Sodium-catalyzed polybutadienes contain a preponderance of 1,2-structures but since there are also significant quantities of other microstructures the products are not stereoregular. Since the discovery of the Ziegler-Natta catalyst systems both syndiotactic and isotactic 1,2-polybutadienes have been prepared. The syndiotactic polymers are obtained by the use of aluminium triethyl and halogen-free compounds of vanadium, molybdenum and cobalt, particularly the acetyl acetonates. 

It has been postulated that the syn TT-ahyl stmcture yields the trans-1 4 polymer, and the anti TT-ahyl stmcture yields the cis-1 4 polymer. Both the syn and anti TT-ahyl stmctures yield 1,2 units. In the formation of 1,2-polybutadiene, it is beheved that the syn TT-ahyl form yields the syndiotactic stmcture, while the anti TT-ahyl form yields the isotactic stmcture. The equihbtium mixture of syn and anti TT-ahyl stmctures yields heterotactic polybutadiene. It has been shown (20—26) that the syndiotactic stereoisomers of 1,2-polybutadiene units can be made with transition-metal catalysts, and the pure 99.99% 1,2-polybutadiene (heterotactic polybutadiene) [26160-98-5] can be made by using organolithium compounds modified with bis-pipetidinoethane (27). At present, the two stereoisomers of 1,2-polybutadiene that are most used commercially are the syndiotactic and the heterotactic stmctures. 

The polymerization of butadiene to 1.2 polymers with anionic Ziegler type catalysts has been studied by Natta and co-workers (46). They have shown that isotactic 1.2-polybutadiene can be produced by the use of catalysts which are made up of components which have basic oxygen and nitrogen structures such as triethylaluminum with cobalt acetylacetonate or with chromium acetylacetonate. Natta and co-workers have shown that either syndiotactic or isotactic structures are produced depending on the ratio of aluminum to chromium. Syndiotactic structures are obtained at low aluminum to chromium ratios while isotactic polybutadiene is obtained at high ratios. The basic catalyst component is characteristic of syndiotactic catalysts. Natta, Porri, Zanini and Fiore (47) have also produced 1.2 polybutadiene using... 

As regards high Irons- 1,4-poly butadiene, it has a few applications, especially as a blend with natural rubber. Syndiotactic 1,2-polybutadiene is a unique material that combines the properties of plastic and rubber. These properties lead to applications both as a thermoplastic resin and as a rubber. As regards isotactic 1,2-polybutadiene, one may note that its properties have not excited sufficient interest for commercial development. 

Polybutadiene. Unlike cis- and trans-1,4-polybutadiene, high vinyl 1,2-polybutadiene has a chiral center which can exist in one of three different stere-ochemically related forms. The material can either be atactic, leading to an amorphous elastomer, or it can be isotactic or syndiotactic, both of which are crystalline. The elastomeric amorphous form has found utility in tire tread applications (271) and although certain molybdenum (272) coordination catalysts can produce this material, commercialization has focused on anionic alkali metal initiators modified with Lewis bases. Of the two crystalline forms, isotactic 1,2-polybutadiene with a melting temperature of 126° C is the most elusive isomer. A few chromium systems based on soluble salts and aluminum alkyls have been reported to give 45% of the isotactic polymer in a mixture of the atactic isomer (273,274). 

CM-1,4-Polybutadiene is one of the most important rubbers used for technical purposes and is produced with a high degree of stereoregularity using conventional Ziegler-Natta catalysts. Moreover, the other possible stereoregular microstructures are also known for PBD (tra s-l,4-polybutadiene, isotactic 1,2-polybutadiene, and syndiotactic 1,2-polybutadiene). 

Isotactic 1,2-polybutadiene has so far only been obtained using catalyst systems composed of aluminum alkyls and soluble chromium compounds such as Cr(acac)3, Cr(C=NPh)6, Cr(CO)6, and Cr(CO)3Py3. In all of these cases, high Al Cr ratios and ageing of the catalytic system are crucial to obtain prevalently isotactic polymer otherwise, syndiotactic polymer is produced. This behavior indicates that the catalytic species actually responsible for the isotactic polymerization are formed by reduction of the initial Cr complex by the alkylaluminum reagent. 

Three kinds of polymer segments are formed in the polymerization of dienes 1-4 cis-, 1-4 trans-, and 1-2 segments (or 3-4 in polymerization of isoprene or other monosubstituted dienes). The latter may form isotactic or syndiotactic diads when the proportion of the 1-2 form is sufficiently high, e.g. a syndiotactic, highly 1-2 polybutadiene was described recently by Ashitaka et al. 123), although the so far examined 1-2 polybutadienes produced by homogeneous anionic polymerization were found to be atactic (unpubl. results of Bywater, Worsfold). 

As is well known, the most simple head-to-tail stereoregular vinyl polymers were called isotactic (22-24) and syndiotactic (25) by Natta. The first compounds to be recognized as such were polypropylene and 1,2-polybutadiene, respectively (26). Ideal isotactic vinyl polymers (4, 5, Scheme 1) have all the substituents on the same side of the chain while in syndiotactic polymers (6, 7) the substituents regularly alternate between the two sides of the chain (27). 

Lithium and alkyllithiums in aliphatic hydrocarbon solvents are also used to initiate anionic polymerization of 1,3-butadiene and isoprene.120,183-187 As 1,3-butadiene has conjugated double bonds, homopolymerization of this compound can lead to several polymer structures. 1,4 Addition can produce cis-1,4- or tram-1,4-polybutadiene (19, 20). 1,2 Addition results in a polymer backbone with vinyl groups attached to chiral carbon atoms (21). All three spatial arrangements (isotactic, syndiotactic, atactic) discussed for polypropylene (see Section 13.2.4) are possible when polymerization to 1,2-polybutadiene takes place. Besides producing these structures, isoprene can react via 3,4 addition (22) to yield polymers with the three possible tacticites ... 

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. 

From the 1,2-polybutadiene, analogous to polypropylene, there are three structurally different polymers the isotactic, syndiotactic, and atactic form. All have been isolated -. ... 

Tacticity describes the symmetry of the monomer orientation or stereochemistry of the monomer. In polybutadiene, atactic polymers have no steric order along the polymer. Isotactic polymers have the monomer pendent groups at the same relative position. Syndiotactic 1,2- polybutadiene has pendent groups alternating along the polymer chain. For illustrative purposes atactic, isotactic, and syndiotactic polypropylene are shown in Figure 4.2 [2, 4]. 

Considering only a single repeat unit the 1,2 and 3,4 structures are identical and both polymers may be considered as 1,2-polybutadienes. However, as with polypropylene, polyvinyl chloride, polystyrene and many other thermoplastics, 1,2-polybutadiene possesses an asymmetric carbon atom so that both isotactic and syndiotactic stereoregular forms as well as the stereo-irregular atactic form are possible. 

The ds-1,4 and trans-1,4 are isomers arising from the rigidity of the double bond in the backbone. 1,2-polybutadiene maybe isotactic, syndiotactic, or atactic. If R is other than H,... 

The commercial polydienes are elastomers. Q s-1,4 polybutadiene has a Tg of -100 °C and has a crystalline melting point of less than 0 °C. Q s-1,4 polyisoprene has a Tg of -70 °C and has a crystalline melting point of 35 C. Both polymers crystallize rather slowly. Trans-1,4 polybutadiene and polyisoprenes are crystalline thermoplastics at room temperature. They are not, however, used commercially because of their poor aging characteristics relative to polyolefins. This is associated with the double bonds in their backbones. Polybutadienes with high atactic 1,2-contents have been widely used in the tire industry. Their Tg is about -15 °C. Isotactic and syndiotactic 1,2-polybutadienes are high melting crystalline thermoplastics, but age poorly compared to polyolefins. The 1,2-polybutadienes have been used as packaging for additives in the rubber industry. 

Homopolymerization of butadiene can proceed via 1,2- or 1,4-additions. The 1,4-addition produces the geometrically distinguishable trans or cis stmctures with internal double bonds on the polymer chains, 1,2-Addition, on the other hand, yields either atactic, isotactic, or syndiotactic polymer stmctures with pendent vinyl groups (Eig. 2). Commercial production of these polymers started in 1960 in the United States. Eirestone and Goodyear account for more than 60% of the current production capacity (see Elastomers, synthetic-polybutadiene). 

The versatility of Ziegler-Natta catalysis is shown in the polymerization of butadiene. Polybutadiene may have either a 1,2 or 1,4 configuration. The 1,4 polymer has a double bond as part of the main chain and this can be atactic, isotactic, or syndiotactic. Thus many different polybutadienes can be made and all of them have been made with the aid of Ziegler-Natta catalysts. 

The various regular polymers that can be produced by polymerization of butadiene and isoprene are summarized in reactions (4-3) and (4-4). In addition to the structures shown in these reactions, it should be remembered that 1, 4 polymerization can incorporate the monomer with cis or trans geometry at the double bond and that the carbon atom that carries the vinyl substituent is chiral in 1,2 and 3,4 polymers. It is therefore possible to have isotactic or syndiotactic polybutadiene or polyisoprene in the latter cases. Further, these various monomer residues can alt appear in the same polymer molecule in regular or random sequence. It is remarkable that all these conceivable polymers can be synthesized with the use of suitable catalysts comprising transition metal compounds and appropriate ligands. 

Geometric isomerism. When there are unsaturated sites along a polymer chain, several different isomeric forms are possible. As illustrated in Fig. 14.14, conjugated dienes such as isoprene and chloroprene can be polymerized to give either 1,2-, 3,4, or 1,4-polymer. In the case of 1,4-polymers, both cis and trans configurations are possible. Also, stereoregular (i.e., isotactic and syndiotactic) polybutadienes can be produced in case of 1,2- and 3,4-polymerization. 

Fluorescence depolarisation studies on mixtures of isotactic and syndiotactic polymethylmethacrylate have shown the formation of a 1 2 complex in toluene while fluorescence anisotropy studies on polybutadienes have shown viscous rotational diffusion with slipping boundary conditions OS. Fluorescence studies on quasi-rigid-rod like probes in anisotropic polyethylene have shown... 

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