Copolymerization of Butadiene and Styrene

It is interesting to note, that there are publications on the Nd-catalyzed copolymerization of BD and styrene (St) as well as on the selective polymerization of BD which is performed in the presence of St as the solvent. The copolymerization of BD and St cannot be achieved with standard binary or ternary catalyst systems which yield BR with a high cis- 1,4-content. This 

One catalyst system which allows for the copolymerization of BD and St is based on NdV and is activated by MAO. It is important to note that this catalyst system does not contain a halide donor. By the addition of cy-clopentadienyl derivatives (e.g. cyclopentadiene, indene, anthracene etc.) St incorporation is increased. It may be speculated that by the influence of MAO a proton is abstracted from the cyclopentadienyl derivatives added and that the resulting cyclopentadienyl-type anions coordinate to the active Nd sites [498,499]. In this way the activity pattern of Nd is changed and copolymerization of BD and St is made possible. As shown in the attached Table 28 increases of St incorporation occur simultaneously with decreases of cis-1,4-contents. 

Also Nd( 3- C3Hs)3-based catalyst systems have been applied successfully for the copolymerization of BD and St. The cis- 1,4-content of incorporated BD moieties is around 10% [500,501]. 

Another catalyst system which allows for the copolymerization of BD and St comprises the catalyst components Nd(OCOCCl3)3/TIBA/DEAC [177]. The catalyst yields low cis- 1,4-contents and high contents of trans-1,4- and 1,2-structures in the BD units. The content of incorporated St increases with increasing polymerization temperature. 

For the copolymerization of BD and St Zhang et al. use various Nd precursors in combination with TIBA and DIBAC [502]. These authors obtain copolymers with a content of incorporated styrene of ca. 10-20 mol %. At this level of incorporated styrene the BD moieties exhibit cis- 1,4-contents between 90 to 97%. The amount of incorporated St increases with increasing polymerization temperature and with increasing St content in the monomer feed. It is demonstrated that the cis- 1,4-content of the incorporated BD moieties decreases with increasing content of incorporated St. In this study styrene incorporation depends on the composition of the catalyst system. The highest amount of styrene is incorporated at the molar ratios of TiBA/ Nd = 20 and Cl/ Nd = 3. 

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Fig. 21. Copolymerization of butadiene and styrene initiated by BuLi in cyclohexane at 50 °C (A. A. Korotkov and al., Ref.143 )...

Butadiene-Styrene Copolymers from Ba-Mg-Al Catalyst Systems. Figure 13 shows the relationship between copolymer composition and extent of conversion for copolymers of butadiene and styrene (25 wt.7. styrene) prepared in cyclohexane with Ba-Mg-Al and with n-BuLi alone. Copolymerization of butadiene and styrene with barium salts and Mg alkyl-Al alkyl exhibited a larger initial incorporation of styrene than the n-BuLi catalyzed copolymerization. A major portion of styrene placements in these experimental SBR s are more random however, a certain fraction of the styrene sequences are present in small block runs. 

Copolymerization of butadiene and styrene in hexane with a number of initiators, such as lithium morpholinide, lithium dialkylamide, lithium piperidinide, etc., has also been examined. In general, the microstructure and styrene content of the polymers are dependent on the type of initiator and the polymerization conditions. Detailed results including a postulated mechanism for these polymerizations are discussed. 

Lithium diethylamide has been shown to be an effective initiator for the homopolymerization of dienes and styrene llr2). It is also known that such a polymerization process is markedly affected by the presence of polar compounds, such as ethers and amines (2,3). However, there has been no report of the use of a lithium amide containing a built-in polar modifier as a diene polymerization initiator. This paper describes the preparation and use of such an initiator, lithium morpholinide. A comparison between polymerization with this initiator and lithium diethyl amide, with and without polar modifiers, is included. Furthermore, we have examined the effects of lithium-nitrogen initiators on the copolymerization of butadiene and styrene. 

The copolymerizations of butadiene and styrene with these four amides were carried out in hexane with the insoluble initiators at several different temperatures. In general, the styrene content in the lithium diethylamide or lithium di-n-butylamide initiated butadiene-styrene polymerizations is higher than in the lithium dimethylamide or lithium di-i-propylamide system (Table VII). A high styrene content (23.4% to 25.3%) in the lithium diethylamide and the lithium di-n-butylamide systems was obtained at 50°C polymerization temperature. From this result, it appears that the best copolymerization temperature for these systems is 50 C. 

The copolymerization of butadiene and styrene with these two initiators was performed at several different temperatures and initiator concentrations. In general, a low styrene content (10-12%) was observed regardless of the polymerization temperature and initiator concentrations (Tables IX and X). 

COPOLYMERIZATION OF BUTADIENE AND STYRENE WITH LITHIUM PIPERIDINIDE IN HEXANE 3 ... 

The best evidence for the photolytic decomposition of mercaptans and disulfides into free radicals involves photoinitiation of polymerization of olefins. Thus, photolysis of disulfides initiates the copolymerization of butadiene and styrene,154 as well as the polymerization of styrene207 and of acrylonitrile.19 Thiophenol and other thiols promote polymerization upon ultraviolet irradiation.19 Furthermore, the exchange of RS-groups between disulfides and thiols is greatly accelerated by light. Representative examples are benzothiazolyl disulfide and 2-mercapto-thiazole,90 tolyl disulfide and p-thiocresol, and benzyl disulfide and benzylmercaptan.91 The reaction probably has a free radical mechanism. Similar exchange reactions have been observed of RS-groups of pairs of disulfides have been observed.19... 

As in the case of olefin or diene homopolymerization by RLi, copolymerization is particularly sensitive to solvent effects. Initial-charge (all monomers added together) copolymerization of butadiene and styrene tends to result in a tapered block copolymer (a block of butadiene with increasing levels of styrene, followed by a block of styrene) in hydrocarbon solvents and a random copolymer (a uniform distribution of butadiene and styrene) in polar media. 

An alternative rationale for the unusual RLi (hydrocarbon) copolymerization of butadiene and styrene has been presented by O Driscoll and Kuntz (71). Rather than invoking selective solvation, these workers stated that classical copolymerization kinetics is sufficient to explain this copolymerization. They adapted the copolymer-composition equation, originally derived from steady-state assumptions for free-radical copolymerizations, to the anionic copolymerization of butadiene and styrene. Equation (20) describes the relationship between the instantaneous copolymer composition c/[M,]/rf[M2] with the concentrations of the two monomers in the feed, M, and M2, and the reactivity ratios, rt, r2, of the monomers. The rx and r2 values are measures of the preference of the growing chain ends for like or unlike monomers. 

Initial-charge RLi copolymerization of butadiene and styrene in hydrocarbons tends toward a tapered-block placement of monomer units. However, it is possible to generate a random copolymer in such solvents if the B S monomer ratio is kept constant by programmed monomer addition or continuous copolymerization (77-78a). 

Polar modifiers tend to increase the reactivity and uptake of styrene. Figure 10 shows a diglyme-modified copolymerization of butadiene and styrene and should be compared with the unmodified reaction shown in Fig. 9. It can also be seen that the modified copolymerization is sensitive to temperature. The higher uptake of styrene occurs at the lower copolymerization temperature (28). 

Korotkov and Rakova (53) found that butadiene was more active in copolymerization with isoprene with lithium catalyst, although in homopolymerization isoprene is three times faster. Korotkov and Chesnokova (33) studied the copolymerization of butadiene and styrene with n-butyl lithium in benzene. Butadiene polymerized before much of the styrene was consumed. They claimed the formation of block polymers consisting initially of polybutadiene and the polystyrene chain attached. 

Kuntz (33) reported on the copolymerization of butadiene and styrene in n-heptane at 30° using n-butyl lithium. Although styrene homopolymerized six times faster than butadiene, the copolymerization rate was initially the same as that of butadiene homopolymerization and then increased markedly. It was found that about 80% of the styrene remained when 90% of the butadiene was consumed and that the increase in rate coincided with the almost complete consumption of butadiene. With added tetrahydrofuran, the rate of polymerization was faster and about 30% styrene was found in the initial copolymer. 

Butadiene-Styrene Rubber occurs as a synthetic liquid latex or solid rubber produced by the emulsion polymerization of butadiene and styrene, using fatty acid soaps as emulsifiers, and a suitable catalyst, molecular weight regulator (if required), and shortstop. It also occurs as a solid rubber produced by the solution copolymerization of butadiene and styrene in a hexane solution, using butyl lithium as a catalyst. Solvents and volatiles are removed by processing with hot water or by drum drying. 

Redox systems are used for polymerizations at lower temperatures. Many of these redox initiator couples were developed for the emulsion copolymerization of butadiene and styrene, since the 5-10"C cold recipe yields a better rubber than the hot SO C emulsion polymerization. 

Rubbers, or elastomers, stretch readily to elongate by a factor of 10 before breaking. Natural rubber, obtained from the sap of certain trees, can be hardened and toughened by addition of sulfur in the vulcanization process. Synthetic rubber is produced by addition copolymerization of butadiene and styrene. 

Hou and Wakatsuki [197] reported a cationic ternary system composed of samarocene aluminate Cp 2Sm( j,-Me)2AlMe2 (95) and TIBA and [PhsC] [B(C6Fs)4], showing living mode for the copolymerization of butadiene and styrene... 

Free-radicals generated in many oxidation-reduction (or redox) reactions can be used to initiate chain poymerization. An advantage of this type of initiation is that, depending on the redox system used, radical production can occur at high rates at moderate (0-50°C) and even lower temperatures. Redox systems are generally used in polymerizations only at relatively low temperatures, a significant commercial example being the production of styrene-butadiene rubber by emulsion copolymerization of butadiene and styrene at 5-10°C ( cold recipe ). 

Next in importance is the polymerization of butadiene, if the use of sodium is ignored in the production of such inorganic compound as sodium cyanide, sodium peroxide, and titanium. Buna rubber, prepared by the sodium-catalyzed copolymerization of butadiene and styrene, was of considerable importance during World War II, especially in Germany. More recently, Morton s alfin catalyst has caught the attention of the rubber industry because of the exceptional quality of polybutadiene prepared by his techniques. 

Commercial SBR is produced by either emulsion or solution copolymerization of butadiene and styrene. Emulsion copolymerization is either a cold (41°F) or hot (122°F) process. The copolymers from the hot and cold processes have principal differences in molecular weight, molecular weight distribution, and microstructure, as shown in Table 5.1. The solution copolymerization process for the production of... 

Harkins conclusions relate to the polymerization of hydrocarbon monomers which are miscible with their polymers and which have very low s(4ubilities in water, but which can be solubilized in larger quantities in the interior of the micelles of ionic surfactants. Most of the wartime work relates to the etmilsion copolymerization of butadiene and styrene, monomers which fulfil these critma. Model experiments concentrate on styrene homopolymerization because it can be handled more conveniently and is less toxic than some other common monomers. However, conclusions based on experiments with styrene may need modification before they can be extended to more polar monomers (e.g. methyl methacrylate, vinyl acetate) which have significantly higher solubilities in water or which have only limited miscibility with their polymer (e.g. acrylonitrile, vinyl chloride) or which produce polymras with a significant degree of crystallinity (e.g. vinylidene chloride, tetrafluoroethylene). 

The most successful method developed for the production of a general-purpose synthetic rubber was the emulsion copolymerization of butadiene and styrene (SBR), which still represents the main process in use today (Blackley, 1975 Hofmann, 1989 Blow, 1971 Brydson, 1981 Bauer, 1979 Sun and Wusters, 2004 Demirors, 2003). The general principles of copolymerization will be discussed in a later section, but it is instructive at this point to examine the other main features of this system. The types of recipes used are seen in Table 2.5 (Bauer, 1979). The recipes shown are to be considered only as typical, as they are subject to many variations. It should be noted that the initiator in the 50°C recipe (hot rubber) is the persulfate, whereas in the 5°C recipe (cold mbber) the initiator consists of a redox system comprising the hydroperoxide-iron(II)-sulfoxylate-EDTA. In the latter case, the initiating radicals are formed by the reaction of the hydroperoxide with the ferrous iron, whose concentration is... 

The possibilities inherent in the anionic copolymerization of butadiene and styrene by means of organolithium initiators, as might have been expected, have led to many new developments. The first of these would naturally be the synthesis of a butadiene-styrene copolymer to match (or improve upon) emulsion-prepared SBR, in view of the superior molecular weight control possible in anionic polymerization. The copolymerization behavior of butadiene (or isoprene) and styrene is shown in Table 2.15 (Ohlinger and Bandermann, 1980 Morton and Huang, 1979 Ells, 1963 Hill et al., 1983 Spirin et al., 1962). As indicated earlier, unlike the free radical type of polymerization, these anionic systems show a marked sensitivity of the reactivity ratios to solvent type (a similar effect is noted for different alkali metal counterions). Thus, in nonpolar solvents, butadiene (or isoprene) is preferentially polymerized initially, to the virtual exclusion of the styrene, while the reverse is true in polar solvents. This has been ascribed (Morton, 1983) to the profound effect of solvation on the structure of the carbon-lithium bond, which becomes much more ionic in such media, as discussed previously. The resulting polymer formed by copolymerization in hydrocarbon media is described as a tapered block copolymer it consists of a block of polybutadiene with little incorporated styrene comonomer followed by a segment with both butadiene and styrene and then a block of polystyrene. The structure is schematically represented below ... 

Wofford, C.F., Hsieh, H.L., 1969. Copolymerization of butadiene and styrene by initiation with alkylUthium and alkali metal tert-butoxides. J. Polym Sci. Part A Polym. Chem. 7 (2), 461-469. 

The copolymerization of butadiene and styrene gives a random styrene butadiene copolymer. 

Buna-S or SS (GR-S) n. A synthetic elastomer produced by the copolymerization of butadiene and styrene. Manufactured by Hills, Germany. 

Fig. 4. Copolymerization of butadiene and styrene in different solvents at 50°C. Parts of butadiene/styrene/solvent/n-butyllithium = 75/25/1000/0.13 (2.0 mmol). From Ref. 50 reprinted by permission of the Rubber Division of the American Chemical Society.

Random Styrene-Diene Copolymers. Random copolymers of butadiene (SBR) or isoprene (SIR) with styrene can be prepared by addition of small amounts of ethers, amines, or alkali metal alkoxides with alkylhthium initiators. Random copolymers are characterized as having only small amounts of block styrene content. The amoimt of block styrene can be determined by ozonoly-sis or, more simply, by integration of the nmr region corresponding to block polystyrene segments (S = 6.5-6.94 ppm) (180). Monomers reactivity ratios of tb = 0.86 and rs = 0.91 have been reported for copolymerization of butadiene and styrene in the presence of 1 equiv of TMEDA ([TMEDAMRLi] = 1) (181). However, the random SBR produced in the presence of TMEDA will incorporate the butadiene predominantly as 1,2 imits. At 66°C, 50% 1,2-butadiene microstructure will be obtained for copolymerization in the presence of lequiv of TMEDA (134). In the presence of Lewis bases, the amounts of 1,2-polybutadiene enchainment decreases with increasing temperature. 

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