Isotactic 1,2-polybutadiene

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. 

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. 

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. 

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. 

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). 

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. 

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). 

Hesse and Heusinger130 studied the ESR signal due to Am = 2 transition of radical pairs in a number of /-irradiated polymers including 1,2-polybutadiene (both atactic and isotactic) and 3,4-polyisoprene. It was found that the distance between the radicals in the pair is 0.53 0.04 pm 1.0 0.5% of the radicals in 1,2-polybutadiene and 3,4-isoprene are arranged in pairs. 

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). 

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. 

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 isomerically pure polymers were not available, three different kinds of BR, each relatively high in one of the three kinds of base units were used as standards [35]. The band near 1308 cm 1 was identified [38,39] with the cis isomer and used for analyses [43]. The 1308 cm 1 band is weak and relatively broad, with the appearance of an unresolved doublet (1306,1311 cm 1). The cis band at 730 cm 1 is more frequently used in spite of some difficulties. Relatively pure, crystalline stereoregular polymers have been prepared and structures were determined by X-ray diffraction for cis [44], trans [45] and syndiotactic vinyl [46] and isotactic vinyl [47]. Infrared spectra [48-50] have been published for the four stereoregular polybutadienes, with detailed analyses of the spectra and band assignments for cis [51], trans [51] and syndiotactic vinyl [51] polymers. For the spectrum of isotactic vinyl BR, bands at 1232, 1225, 1109, 943, 876, 807 and 695 cm"1... 

The determination of percentage of styrene and butadiene isomer distribution in copolymers is an extension of the methods for the analysis of polybutadiene. The styrene band at 700 cm 1 is largely independent of the sequence distribution and therefore useful in styrene content determination [76]. A series of bands in the IR spectrum of crystalline isotactic polystyrene at 758, 783, 898, 920, 1053, 1084, 1194, 1261, 1297, 1312 cm"1 have been attributed to the helical structure [77]. The absorption bands for butadiene in SBR are similar to BR structures (Table 3.2a). 

Morton and Taylor (226) found that Alfin catalysts which produce 1,4-polybutadiene are poor catalysts for making isotactic polystyrene and vice versa. They concluded that the monomer is adsorbed or com-plexed and oriented at the surface of the insoluble catalyst. 

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. 

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. 

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

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... 

High density polyethylene (HOPE) Linear low density polyethylene (LLDPE) Isotactic polypropylene (iPP) Syndiotactic polypropylene (sPP) tram-1,4-Polyisoprene Syndiotactic polystyrene (sPS) Cyclooleflns Ethylene-propylene copolymers Styrene-ethylene copolymers cw -1,4-polybutadiene rrarw -1,4-Poly isoprene Random ethylene-a-olefin copolymers Ethylene-propylene rubber (EPR) Ethylene-propylene-diene copolymers (EPDM)... 

It should be noted that the steric effects of the pendant groups considered above are simply additional contributions to the main chain effects. Similarly cis-trans isomerism in polydienes and tacticity variations in certain a-methyl substituted polymers alter chain flexibility and hence affect Tg. Well-known examples of cis-trans variations are polybutadiene cis Tg= — 108°C) and trans(T = — 18°C) or polyisoprene cis Tg = —73°C) and trans T = —53°C). An example of tacticity variation is polyfmethyl methacrylate) for which the isotactic, atactic, and syndiotactic stereostructures are associated with Tg values of 45, 105, and 115°C, respectively. 

The characteristics of the samples used are summarized in Table 1 where SBR, PB and PI designate, respectively, a random copolymer of poly(styrene-random-butadiene), polybutadiene, and polyisoprene. Binary mixtures of PP/EPR (14), X-7G/PET (15), both having (50/50 wt/wt composition), and PS/SB (16) (35/65 w wt) were occasionally us, where PP, EPR, X-7G, PET, PS and SB, designate, respectively, isotactic poly(propylene), a random copolymer of... 

Tranv-1,4- and 1,2-polybutadiene can be hydrohalogenated under mild conditions with gaseous HCl. The same is true of copolymers of butadiene with piperylene and also of isotactic transAA-piperylene. The addition of HCl to the asymmetric double bond is trans for polypiperylene and occurs in a stereoselective way, judging from the NMR spectra. 

Other symbols occasionally used in the literature include (br) for branched materials and (iso), (syndio), and (a) for isotactic, syndiotactic, and atactic structures respectively. No symbol appears to exist for mechanical blends, although these materials are obviously important. Where necessary the symbol -m- will denote a mechanical blend, for example, poly(styrene-m-butadiene) for a mechanical blend of polystyrene with polybutadiene. 

This group of Ziegler-Natta catalysts is stereospecific for the polymerisation of a-olefins and 1,3-dienes the products are mainly isotactic polyolefins (polypropylene, polystyrene, and so on) and 3 5-l,4-polydienes (polybutadiene, polyisoprene, and so on). 

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