Polybutadienes stereoregular

Ziegler-Natta catalysts currently produce linear polyethylene (non-branched), stereoregular polypropylene, cis-polybutadiene, and other stereoregular polymers. 

The properties of elastomers are much improved by strain-induced crystallization, which occurs only in polymers with high stereoregularity. The polymerization of butadiene using completely soluble catalysts composed of a) rare earth carboxylates, b) Lewis acids and c) aluminum alkyls leads to polymers with up to 99 % cis-1,4 configuration. These polymers show more strain-induced crystallization than the commercial polybutadienes and consequently their processability is much improved. 

Polymerization Temperature. The stereoregularity of polybutadienes prepared with the BuLi-barium t-butoxide-hydroxide catalyst in toluene is exceedingly temperature dependent. Figure 6 compares the trans-1,4 dependence for polybutadiene prepared with BuLi, alone, and with the BuLi-barium t-butoxide-hydroxide complex in toluene (the molar ratio of the initial butadiene to BuLi concentration was 500). The upper curve demonstrates that the percent trans content increased rapidly from 627. to 807. trans-1,4 as the temperature decreased from 75°C to 22°C. From 22°C to 5°C, the microstructure does not change. The increase in trans-1,4 content occurred with a decrease in cis-1,4 content, the amount of vinyl unsaturation remaining at 5-87.. For the polybutadienes prepared using BuLi alone, there is only a very slight increase in the trans-1,4 content as the polymerization temperature is decreased. 

The polybutadienes prepared with these barium t-butoxide-hydroxide/BuLi catalysts are sufficiently stereoregular to undergo crystallization, as measured by DTA ( 8). Since these polymers have a low vinyl content (7%), they also have a low gl ass transition temperature. At a trans-1,4 content of 79%, the Tg is -91°C and multiple endothermic transitions occur at 4°, 20°, and 35°C. However, in copolymers of butadiene (equivalent trans content) and styrene (9 wt.7. styrene), the endothermic transitions are decreased to -4° and 25°C. Relative to the polybutadiene, the glass transition temperature for the copolymer is increased to -82°C. The strain induced crystallization behavior for a SBR of similar structure will be discussed after the introduction of the following new and advanced synthetic rubber. 

The stereoregularity of butadiene based polymers prepared in cyclohexane with Ba-Mg-Al catalysts depends on polymerization temperature and catalyst concentration. Trans-1,4 content increases nonlinearly with a decrease in polymerization temperature over the range of 80° to 30°C (Figure 11) and/or a decrease in the initial molar ratio of butadiene to dialkyl-magnesium from 3400 to 400 (Figure 12). For polybutadienes prepared with relatively large amounts of catalyst at 30°C, the trans-1,4 content approaches a limiting value of about 907.. 

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). 

Stereoregular Polymerization of Dienes. Coordination polymerization exhibiting high degree of chemo- and stereoselectivity is the method of choice to synthe-size c -1,4-polybutadiene the commercially most important product (see... 

Polybutadiene. Most polybutadiene is made by an emulsion process with a free radical initiator. If stereoregular cis-1,4-polybutadiene is desired, a titanium-based Ziegler-Natta catalyst is used. The catalyst is similar to those used for polyethylene and polypropylene in type and mechanism. 

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... 

Some of the polybutadienes obtained with transition metal-based coordination catalysts have practical significance the most important is cA-1,4-polybutadiene, which exhibits excellent elastomeric properties. As regards isoprene polymers, two highly stereoregular polyisoprenes, a cA-1,4 polymer (very similar to natural rubber) and a trans- 1,4-polymer (of equal structure to that of gutta percha or balata) have been obtained with coordination catalysts. Various polymers of mixed 3,4 structure, amorphous by X-ray, were also obtained [7]. 

Explain why polybutadienes obtained with rare-earth metal-based catalysts exhibit a higher degree of stereoregularity (higher contents of cis-1,4 monomeric units) than those derived from polymerisations in the presence of transition metal-based catalysts. 

High -cis polybutadiene has relatively high heat resistance, which is advantageous in the processing of HIPS. On the other hand, this type of polybutadiene crystallizes at about 0 °C, owing to its stereoregular structure, with the consequence that the low-temperature toughness of polystyrene, produced in this way, is reduced. 

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. 

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]. 

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]. 

Scheme 1. The structurally different stereoregular polybutadienes and polyisoprenes obtainable from the monomers by 1,4-, 1,2- or 3,4-C-C bond linking, respectively.

However, the most interesting products could be obtained upon the radiolysis of butadiene derivatives included in a host matrix ( - ) For a number of monomers with a non-centrosymmetric molecular structure it could be demonstrated that y-irradiation leads to stereoregular, optically active polymers in a direct asymmetric induction. Especially these studies indicate that apart from polymerization in solution using optically active catalyst systems (A), the solid state polymerization represents a suitable method to obtain stereoregular polybutadienes. 

Polymer concrete based on two kinds of liquid rubbers was investigated type A, low molecular polybutadiene (Butarez , Liten [1,4-cis 25%-39% 1,4-trans 35%-40%, 1,2-vinyl 28%-35%]) and type B, stereoregular low molecular rubber (Polyoil 110/130 , Ricon (1,4-cis 70%-80% 1,4-trans 20%-30%, 1,2-vinyl l%-2%). The main physical-mechanical properties are shown in Table 2.5. 

Rhodium trichloride has been used to catalyze a number of organic reactions. Thus in aqueous emulsions it induces the stereoregular polymerization of butadiene to trans-polybutadiene and it catalyzes the isomerization of various alkenes in ethanolic solutions. 

Recently, several lanthanide complexes, based on Nd, Ln, Sm, or Yb, have been smdied for controlling stereoregularity and activity of diene polymerizations and copolymerizations. Specifically, Nd-based catalysts have shown very good stereoregularity control over the production of high CM-polybutadiene rubber [36]. 

Polymerizations in thiourea canal complexes yield high melting crystalline rra/w-1,4-polybutadiene, 2,3-dimethylbutadiene, 2,3-dichlorobutadiene, and 1,3-cyclohexadiene. Cyclo-hexadiene monoxide, vinyl chloride, and acrylonitrile also form stereoregular polymers. On the other hand, polymerizations of isobutylene and of vinylidine chloride fail to yield stereospecific polymers. 

Stereoregular polybutadiene rubber (formation of homogeneons macromolecnle growth centres). 

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. 

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