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BUTADIENE POLYMERS Introduction 1,3-Butadiene was first prepared in 1863, and its conjugated structure was proposed in 1886 (1,2). However, the ability of butadiene to polymerize was not recognized until almost 50 years later when, in 1909, a rubbery polymer was first reported as being prepared from butadiene via thermal polymerization (3). Shortly thereafter, the more controlled polymerization of butadiene initiated by sodium metal was reported in 1911 (4). During this time period, a sharp rise in natural rubber prices prompted the Bayer Corp. to develop methyl rubber from 2,3-dimethylbutadiene. Though interest in synthetic rubber faded after World War I, in 1926 a rise in price of natural rubber prompted the German company I. G. Farbenindustrie to resume research on the sodium-initiated polymerization of butadiene. This work eventually led to the German commercialization of two synthetic rubbers: Buna 32 and Buna 115 (from butadiene and natrium). Concurrently, in the 1920s research on the emulsion polymerization of butadiene was being carried out in Germany and the United States. The first butadiene–styrene copolymer prepared from an emulsion polymerization (Buna S) at I. G. Farbenindustrie proved to be superior to polybutadiene (5). From this work, Buna-N, a copolymer of butadiene and acrylonitrile, was developed for its solvent and oil resistance. Although the products of this work were inferior to natural rubber, their technology, with considerable modification and improvement, formed the basis for synthetic rubber production (GR-S and GR-N) in the United States. Under the government-established Rubber Reserve in World War II, GR-S and SBR became a general-purpose rubber with an annual production of ca 717,700 t in 1945.

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

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Butadiene Monomer 1,3-Butadiene, CH2 CH CH CH2 , is the simplest conjugated diene, and its structure has received much theoretical attention because of its symmetry and simplicity (6). Heats of hydrogenation and combustion reveal that the stabilization contributed by the conjugation of the double bonds is 12.5–14.6 kJ/mol (3–3.5 kcal/mol) (7). Electron diffraction reveals a planar molecule with bond lengths C C, 0.1337 nm; C C, 0.1483 nm; and C H, 0.1082 nm; the bond angles are C C C 122.4◦ and C C H 119.8◦ (8). The values for the C C bond lengths predicted by valence bond approximations are in close agreement with these observed figures (9). The C C single bond between the two double bonds is shorter than the usual 0.154 nm of an isolated single C C bond. Some of the proposed explanations for this shortened bond, which indicates some double bond character, can be expressed in polar resonance or terminal diradical structures with some long bond or interaction between the end carbons. However, because the energies of these structures are high, molecular orbital calculations indicate that there is little resonance in the ground state and that the bond lengths are determined by the state of hybridization of carbon (10). The resistance to rotation about the central bond is attributed to π-conjugation and leads to two conformers, the nonpolar s-trans and polar s-cis form as seen in the conformational equation (1).

(1) Although at dry ice temperature the s-cis form predominates (11), chemical (12) and spectroscopic (13,14) evidence suggests that s-cis-butadiene is present to the extent of only 3% at room temperature. The energy difference between the two forms has been variously determined as 7.1 ± 2.1 kJ/mol (1.7 ± 0.5 kcal/mol) (15) and 9.6 kJ/mol (2.3 kcal/mol) (16). The ultraviolet spectrum of gaseous butadiene is highly complex, but the origin of each of the transitions in the 230–135 nm region has been identified (17). In hexane solution butadiene absorbs at λmax 217 nm,  = 21,000 (18). The bathochromic effect of conjugation is evident upon comparison with those values for ethylene, λmax 170 nm,  = 17,000 in the vapor (19). The infrared spectrum has been recorded in the gaseous (20), liquid (20,21), and solid states. Raman spectra (14,22) are also available. Rotational constants have been calculated from the pure rotational Raman spectrum (23). The complex proton magnetic resonance spectrum of 1,3-butadiene has been analyzed, and the calculated spectrum for this AA BB CC system has been determined (24,25). The temperature-sensitive coupling constant for the protons on carbons 2 and 3 suggests an equilibrium between the predominant s-trans conformer and a skewed conformer having out-of-plane double bonds (25). The comparatively simple 13 C spectrum shows little effect of conjugation on the chemical shifts of the carbons (26). The microwave spectrum has been examined in an effort to detect the s-cis conformer (27). The appearance potentials and relative abundances of the

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Table 1. Physical and Thermodynamic Properties of 1,3-Butadiene Property

Value

Molecular weight 54.09 Boiling point, ◦ C At 101.3 kPaa −4.413 At 200 kPa 14.44 At 245 kPa 21 Melting point, ◦ C −113 Freezing point in air at 101.3 kPa, ◦ C −108.915 Density at saturation pressure, g/mL At 20◦ C 0.6211 At 25◦ C 0.6149 Pressure coefficient of boiling point at 101.3 kPa, ◦ C/Pa 4.502 Vapor pressure of liquid at 21◦ C, kPaa 24.5 Molar volume at 20◦ C, liquid, mL/mol 87.08 Critical temperature, ◦ C 152 Critical pressure, MPab 4.33 Critical density, g/mL 0.245 Critical volume, mL/g 4.09 Critical PV/RT 0.271 Enthalpy of combustion
Hc 0 , at 25◦ C, 101.3 kPa, all gases, kJ/molc −137.65 Enthalpy of formation
H f 0 , at 25◦ C, gas, kJ/mol 6.393 Entropy S0 , at 25◦ C, J/(mol·K)c 15.92 Free energy of formation
F f 0 , at 25◦ C, gas, kJ/mol 8.607 Heat content function (H o 0 − H o 0 )/T, at 25◦ C, J/(mol·K) 2.906 Free energy function (F 0 − H o 0 )/T, at 25◦ C, J/(mol·K) −13.02 Entropy S0 , at 250 , for the ideal gas state, J/(mol·K) 15.92 Enthalpy H 0 − H o 0 , at 25◦ C, for the ideal gas state, J/mol 866.54 Heat capacity Cp 0 , for the ideal gas state, at 25◦ C, mJ/(g·K) 77.56 Surface tension at 20◦ C, mN/m (=dyn/cm) 13.41 Solubility parameter, (J/m3 )1/2 14.5 × 103 Flash point, ◦ C −76.1 Lower explosion limit, % 2.0 Upper explosion limit, % 11.5 Autoignition temperature, ◦ C 420 Vapor density, g/L 1.87

Reference 30 31 31 32 32 31

31 31 31 31 31 31 31 31 31 31,33 31 31 31,33 31,33 31,33 31,33 31,33 34 35 32 31 31 31 31

a To

convert kPa to mm Hg, multiply by 7.5. convert MPa to psi, multiply by 175. c To convert J to calories, divide by 4.184. b To

principal ions from the mass spectrum (28) of butadiene, as well as x-ray data for the crystalline material (29), have been reported. Physical Properties. At room temperature 1,3-butadiene is a highly reactive, colorless gas with a mildly aromatic odor (30). Physical properties are given in Table 1. Plots of heat of vaporization, vapor pressure, vapor and liquid heat capacity, liquid density, surface tension, and vapor and liquid viscosity are available as functions of temperature (32), as are plots of vapor and liquid thermal conductivity and heat of formation and free energy of formation of the gaseous butadiene (36). The vapor pressure of butadiene in Pascals

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Table 2. Solubility of Butadiene in Organic Solvents at 20◦ Ca Solvent

Butadiene, vol%

Acetone Benzene Dichloromethane Amyl acetate a Ref.

65 66.3 91.4 115.3

37.

can be calculated at any temperature in degrees Celsius by using the Antoine equation: logP = A−

B C+t

with the constants A = 9.74022 (6.85941 for mm Hg), B = 935.531, and C = 239.554 (31). Butadiene is soluble in organic solvents such as ethanol, diethyl ether, acetone, benzene, and hexane (Table 2) (38). Solubilities in liquid ammonia, methanol, n-octane, toluene, xylene, ethylene glycol, acetone (39), and water (40,41) have been reported. Reactions. Because of the rich chemistry associated with conjugated diene structures, the reactivity of 1,3-butadiene has been studied extensively. However, the hundreds of polymers and copolymers described in the literature represent by far the most important commercial uses for butadiene. Butadiene and atmospheric oxygen form an explosive polyperoxide (42,43). At 50◦ C the liquid-phase reaction is independent of oxygen pressure between 5.33 and 120 kPa (40 and 900 mm Hg). Below an oxygen pressure of 5.33 kPa (40 mm Hg) the oxidation rate decreases with decreasing oxygen pressure; increased temperature and free-radical initiators accelerate the reaction. The polyperoxide is composed of equal amounts of 1,4- and 1,2-butadiene units separated by peroxide, O2 , units. It is only slightly soluble in butadiene and accumulates as a second phase in neat butadiene. Butadiene dimerizes by a Diels–Alder reaction thereby forming mainly 4-ethenylcyclohexene (4-vinylcyclohexene) (eq. (2)). In this dimerization reaction, butadiene acts as both diene and dienophile. The dimerized product is always present in butadiene unless freshly distilled. Although this reaction occurs spontaneously, selective catalysts based upon nitrosyliron compounds do exist (44). Vinylcyclohexene is a starting material for plasticizers and antioxidants. It should be noted that this reaction also gives smaller amounts of 1,2-divinylcyclobutane and 1,5-cyclooctadiene (45).

(2)

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Fig. 1. Butadiene manufacture in the United States. —䉬—, Co-produced; — —, Onpurpose; —䉱—, Other.

Manufacture. Ethylene Coproduction. Historically, butadiene was first prepared in pilot plant quantities via an uneconomical electric arc process. However, the primary source of butadiene in the world today is as a by-product of thermal pyrolysis of hydrocarbon feedstocks in ethylene production. In the United States, production of coproduct butadiene exceeded that of “on-purpose” butadiene for the first time in 1977 and by 1990 high cost on-purpose butadiene production was essentially eliminated in the United States (Fig. 1) (46,47). In 1996, the total US production of butadiene was 1.75 million, 93% of which was co-produced (47). Steam cracking of hydrocarbons yields varying amounts of butadiene, depending on the nature of the feedstock, the volume of ethylene produced, and the severity of the cracking operations (48–50). For example, when feedstocks are switched from atmospheric gas oils and napthas to propane and butane, yields of butadiene drop by as much as 60% (51). An analysis of the typical by-products in the C4 stream derived from cracking a full-range Middle East naphtha is given in Table 3. The range reflects composition at medium and high severity during steam cracking.

Table 3. Typical Composition of a Crude C4 Streama Component C3 hydrocarbons Butanes 1-Butene cis-2-Butene trans-2-Butene Isobutene 1,3-Butadiene Acetylenes C5 hydrocarbons a Ref.

48.

Wt% 0.3 6.5–3.4 16.0–13.7 5.3–4.8 6.6–5.8 27.4–22.2 37.0–47.5 0.4–1.8 0.5

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A spike in crude oil prices in late 1999 and 2000 has led many steam-cracking operations to switch from heavy to light feedstocks, resulting in lower yields of butadiene. This factor, combined with historically poor investment economics for added butadiene extraction capacity, should lead to tight butadiene supplies through 2004 when additional extraction capacity is expected to become fully on-line. Further expansion of butadiene extraction capacity in North America will be limited by crude butadiene supplies unless new domestic ethylene capacity is added (52). In addition, dehydrogenation of n-butane or n-butene also affords butadiene. For example, the Houdry process for conversion of n-butane to butadiene requires temperatures of about 620◦ C and involves passing n-butane over a fixed bed reactor containing a chromium–alumina catalyst. Single-pass conversions reach only 30–40% requiring multiple recycling passes of unreacted intermediate butylenes and n-butane. The other primary means of dehydrogenation is conducted by passing a preheated stream of n-butene, steam, and air over fixed bed reactors containing a heterogenous autoregenerative catalyst. Bismuth molybdate catalysts have been studied in this process (53), but patents suggest that zinc ferrite catalysts have been used commercially (54–57). Nickel phosphate and Li–Sn–P catalysts have been patented by Phillips (58,59). Other techniques for the preparation of butadiene have also been used commercially, including the aldol condensation of acetaldehyde and the reaction of acetylene and formaldehyde. However, these processes are no longer employed commercially. Both India and the former USSR have developed ethanol-based production of butadiene (60–64). While these routes have a major advantage in terms of low capital and operational costs, raw material costs are prohibitive in most areas (47). Such plants are only viable in areas where there is little domestic petroleum but abundant ethanol supply. Purification. For polymerization, butadiene that is at least 99 mol% pure is required. Although alkynes are the most troublesome impurities, separation of the butadiene from other C4 products is also necessary. Simple fractional distillation is effective for removing the light (C3 ) and heavy (C5 ) ends from butadiene, but not for removing the various C4 species because of the closeness of the boiling points to each other and to butadiene. Further complicating purification, butadiene forms azeotropes with n-butane and 2-butene. The most widely used recovery systems are extraction with aqueous cuprous ammonium acetate (CAA) and solvent extractions with furfural, acetonitrile, dimethylformamide, dimethylacetamide, or N-methylpyrrolidinone (65,66). Cuprous ammonium acetate extraction. Butadiene is purified by aqueous CAA extraction in a liquid–gas countercurrent process developed by Exxon (67– 69). The cuprous salt forms a soluble addition complex with butadiene, which is decomposed by heat; thus the process is adaptable to countercurrent multistage equipment. Typically, the C4 hydrocarbon mixture with a butadiene content of 30–40% contacts the CAA solution in a countercurrent fashion in a series of mixer–settlers. Cooling to ca −15◦ C is required to promote complex formation. The more saturated hydrocarbons, butanes, and butenes are first removed by distillation. Butadiene is released from the complex by further heating to 80◦ C. After ammonia is removed by washing with water, distillation produces butadiene that is 98–99% pure. Acetylenes and allenes are extracted with the butadiene but must

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be removed; acetylenes can lead to foaming and plugging by polymer formation (70–76). Solvent extraction. This purification method is based on the alteration of the relative volatilities of the C4 hydrocarbons by selective solvents. The volatilities follow the order of the boiling points. 1-Butene (bp = −6.3◦ C), for example, is more volatile than butadiene (bp = −4.4◦ C); n-butane (bp = −0.5◦ C) is less volatile. Selective solvents change this order, making the impurities more volatile than butadiene. Hence, separations become easier. Furfural (77), acetonitrile (78), N,N-dimethylformamide (79), N,N-dimethylacetamide (80), and N-methylpyrolidinone (81) appear to be most widely used (82). β-Methoxyproprionitrile (78), dimethylsulfoxide (83), and N-acetylmorpholine (84) have also been studied. The commercial processes generally do not require prehydrogenation. They employ countercurrent extraction of the C4 stream where the butanes and butenes are removed at the top, and the butadiene-rich solvent at the bottom. The butadiene is stripped from the solvent and further purified by fractional distillation. The butadiene is at least 99% pure and contains a few ppm acetylenes. Specifications and Standards. Commercial polymerization-grade butadiene is at least 99% pure; higher purity grades up to 99.86% are available from specialty gas suppliers (85). A representative specification and analysis is given in Table 4. A pure grade has been analyzed for hydrocarbon impurities by gas chromatography (see Table 5).

Health and Safety. Toxicity. Short-term exposure to 200◦ C) material and a low melting material (90◦ C), both produced from cobalt catalysts, have been commercialized by Ube

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Industries (286) and Japan Synthetic Rubber Co. (JSR) (287) respectively. The high melting polymer is made from a preformed cobalt octoate/ triisobutylaluminum/butadiene/carbon disulfide catalyst formed in a ratio of 1:3:20:1 and charged at a level between 0.1 and 0.2 mmphm. High crystalline material having greater than 98% 1,2-microstructure is obtained with a melt temperature of 195–205◦ C. Use of other modifiers such as diethyl fumarate results in a lower degree of crystallinity and a melt temperature of 150◦ C (288). The very low melting syndiotactic 1,2-polybutadiene from JSR utilizes a cobalt–phosphine catalyst system. The catalyst is generated by treating bis(triphenylphosphine)cobalt dibromide with triisobutlyaluminum in the presence of water in a ratio of 1:20:10. Typically, the polymer has 90% 1,2-microstructure and a melt temperature of 90◦ C. Mechanistic studies aimed at uncovering the differences in these systems have been described (289,290). A very unique system for the polymerization of syndiotactic 1,2-polybutadiene with varying degrees of crystallinity and melts ranging from 120 to 190◦ C has been disclosed by The Goodyear Tire & Rubber Co (291). In this example, cobalt octoate treated with butadiene, triisobutylaluminum, and carbon disulfide has been shown to polymerize butadiene in an emulsion polymerization. This example represents a rare case of a Ziegler–Natta polymerization process tolerant of water. The lack of reactivity towards water could be due to the formation of an impenetrable capsule of crystalline polymer around the cobalt catalyst. In this fashion, only the lipophilic butadiene is able to diffuse into the region of active catalyst. Frontiers of Ziegler–Natta Catalysts. There are a number of areas of Ziegler–Natta polybutadiene research that have attracted recent attention. These include the use of new co-catalysts, metallocene, or single-site catalysts, and development of supported Ziegler–Natta catalysts for gas-phase polymerization. Much of this work can be tied to the discovery by Kaminsky and Sinn (292–294) that methylaluminoxane (MAO) acts as an extremely efficient co-catalyst in metallocene-catalyzed ethylene polymerization. Examination of MAO in Ziegler–Natta type conjugated diene polymerization includes work on catalysts such as vanadium (295,296), titanium (297–299), cobalt (300–302), nickel (298,303,304), and neodymium (305–307). In certain cases, the use of MAO in place of traditional trialkylaluminum co-catalysts has resulted in the formation of highly active cis catalysts even in the absence of a halogen source. The use of MAO with homogeneous metallocene catalysts for butadiene polymerization has focused mainly on cyclopentadienyltitanium trichloride systems (298,299,309–311). Although this system has comparable activity to traditional Ziegler–Natta catalysts, the overall cis content is only 80%. Unlike the plastics industry where metallocenes and well-defined single-site catalysts have rapidly gained commercial success, the use of these systems for polybutadiene production has not, as of yet, matured. However, one area that is certain to be dominated by this class of catalyst is the area of gas-phase polymerization. In-roads to such a process for high cis-polybutadiene have been made by Bayer through the use of an inorganic supported neodymium allyl/MAO complex (312). Similarly, supported cyclopentadienyltitanium complexes, also activated with MAO, have been used for the gas-phase polymerization of high trans-polybutadiene (313). Nickel (314) and cobalt (315) systems have also been disclosed.

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Anionic Polybutadiene. Anionic polymerization (qv) is perhaps the most versatile technology for producing polybutadiene in terms of microstructural and macrostructural control. The relative stability of the anionic propagating center allows for excellent control and manipulation of the conformation of enchained monomeric units and the overall structure of the polymer backbone. Coordination catalyst technologies are best employed if highly stereospecific and/or highly tactic microstructures are desired. Although high trans-1,4- and high cis-1,4-polybutadiene elastomers can be produced by a variety of Ziegler–Natta technologies (316), a wider range of thermal and rheological properties can be realized with anionic polymerization systems resulting in a broader range of applications for this technology. As mentioned previously, during World War I, alkali metals were investigated early in the search for a suitable elastomeric substitute for natural rubber. German and Russian researchers used sodium metal to polymerize dienes (317,318), but these materials proved ill-suited for tire applications. An alkali metal catalyst was later developed by Firestone researchers (319), which was used to produce cis-1,4-polybutadiene on a commercial scale. Alkyl lithium initiators have since been utilized to produce very high molecular weight polybutadienes with approximately 70% cis-1,4 structures. Lithium-based initiators produce the lowest vinyl content and highest 1,4-enchainment when compared to the other alkali metals. These properties were desired when efforts to emulate natural rubber dominated synthetic rubber research. Presently, research efforts have shifted focus from low vinyl products to higher vinyl, higher glass-transition temperature materials. This shift follows a trend toward higher performance tires with improved traction and handling characteristics, while not sacrificing wear and fuel economy properties. Anionically prepared polybutadienes with elevated vinyl contents and controlled long-chain branching characteristics can be used in many applications when a balance between traction and fuel economy is desired. Anionic polymerization of 1,3-butadiene allows for a wide range of heterotactic vinyl enchainment levels. The structures obtained in hydrocarbon solvents with alkali metal initiators are shown in Table 9. Lithium polybutadiene maintains the lowest vinyl, highest 1,4 content possible via anionic polymerization. The microstructure produced via a free-radical emulsion polymerization is provided as a reference. Polar modifiers are most often employed to control the level of vinyl structures formed during living lithium-based anionic polymerization processes in nonpolar solvents. Relatively small amounts of added polar modifier, based on the Table 9. Microstructure of Alkali Metal Initiated Polybutadienesa Catalyst or condition Lithium Sodium Potassium Rubidium Cesium Emulsion a Refs.

320 and 321.

cis-1,4, %

trans-1,4, %

1,2, %

35 10 15 7 6 18

52 25 40 31 35 64

13 65 45 62 59 18

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Fig. 3. Glass-transition temperature as a function of vinyl content.

stoichiometry of anionic propagating centers, can have profound vinyl-directing effects (322,323). The vinyl content correlates directly to the glass-transition temperature, as seen in Figure 3 (271) and in other references (324,325). Higher modifier to chain-end ratios produce higher vinyl levels. The vinyl-directing strength of most polar modifiers can also be a function of polymerization temperature (326). Increased polymerization temperatures decrease the vinyl-directing strength and result in lower glass-transition temperatures for the product. In commercially produced polybutadiene, the vinyl content can be controlled by the choice of polar modifier, its relative concentration to the propagating centers, and the polymerization temperature. Polybutadiene products with elevated vinyl contents can be made at normal polymerization temperatures (50–100◦ C) with many conventional polar modifiers, including tetrahydrofuran (327), chelating diamines (328), bi-1,3-dioxanes (329), bisdipiperidinoethane (330), and alkyl ethers derived from tetrahydrofurfuryl alchohols (331). Recently, with varying success, attempts have been made to model the vinyl-directing strengths of polar modifiers based on a number of parameters (332,333). Polymerization propagation rate constants for unmodified lithium systems have a fractional order dependence on chain-end concentration and are relatively slow when compared to modified systems. The dienyl-lithium chain end has a tendency to aggregate in hydrocarbon solutions (334), and give lower polymerization rates. The order of aggregation is reported to be dependent on factors such as chain-end concentration (335,336), monomer concentration, temperature, and solvent (337). The use of polar modification not only increases vinyl contents, but generally interferes with aggregation and drives the equilibrium to monomeric chain-end species with subsequent increases in the polymerization rates. In addition to the use of specific Lewis bases to influence the vinyl content of products formed via anionic polymerizations, the use of alkali metal alkoxide salts in lithium polymerizations can also affect the ratio of vinyl enchainment (338). Salts of lithium have little to no effect on microstructure at typical molecular weights and commercial polymerization temperatures, while those of sodium, potassium, and rubidium increase the vinyl content significantly. The addition of

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polar solvents into these anionic systems also increases the vinyl content of the product. Commercial lithium-based anionic polybutadienes at typical molecular weights and polymerization temperatures, regardless of polar modification, have cis-/trans-1,4 ratios of approximately 0.5–0.7. Higher cis contents can be attained for unmodified polybutadiene in solution at very low initiator concentrations (very high molecular weights) or in neat monomer (339). High trans-1,4 structure can also be promoted in lithium-based systems. The use of barium salts with lithium can produce elevated trans-1,4 content relative to cis-1,4 at lower temperatures (340,341). Other systems that produce trans-1,4 products include complexes of dibutylmagnesium or alkyl lithium and potassium salts (342,343). The products of these systems, polymerized in hydrocarbon solvents, displayed both a soluble high vinyl fraction and an insoluble high trans-1,4 fraction. Low Vinyl Polybutadiene. Typical glass-transition temperatures range between −95 and −80◦ C for commercial unmodified to slightly modified lithium polybutadiene (10–30% vinyl). These products are made by a variety of producers via living anionic initiators resulting in amorphous polybutadiene of 10% vinyl, 35% cis-1,4, and 55% trans-1,4 structure. Several review articles and books describing the anionic polymerization to make low vinyl polybutadiene have been published (323,327,344,345). These materials generally show good wear properties because of the low glass-transition temperatures when compounded in tire formulations. Hysteresis is generally lower than higher vinyl products but remains a function of the long-chain branching content. Low vinyl polybutadienes do have lower green strengths than the high cis-1,4 Ziegler–Natta catalyzed products, because of a lack of strain crystallization. However, where amorphous characteristics are desired, optimally compounded carbon black stock prepared with lithium polybutadiene shows very little difference in performance compared to the Ziegler–Natta catalyzed high cis-1,4 material. It is believed that the essential requirement is a high overall 1,4 content regardless of whether it is primarily cis or more equally distributed between cis and trans. High cis-1,4-polybutadienes (Ziegler–Natta) do show improved tack and process more easily, primarily because of macrostructural differences including broader molecular weight distributions and higher branching levels. Medium and High Vinyl Polybutadiene. Many tire performance properties are closely related to the glass-transition temperature of the polymer system used (324,325,346). As the glass-transition temperature is increased, wet and dry traction is increased, but abrasion resistance is compromised (347). Again, hysteretic properties are more a function of macrostructure than glass-transition temperature. Figure 4 depicts the trends in tire performance properties as a function of polybutadiene vinyl content (271). An interest in the medium to high vinyl polybutadienes has been encouraged by rising styrene monomer prices. Medium vinyl polybutadienes of similar T g offer an alternative to poly(styrene–butadiene) copolymers in the tire industry and can also be used in other applications, including shoe soles, conveyer belts, and other engineered rubber products (348). Commercial medium vinyl materials typically display glass-transition temperatures in the −75 to −55◦ C range (35–55% vinyl). The vinyl content is increased relative to low vinyl polybutadiene by polar modification of the living anionic chain end.

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Fig. 4. Tire performance properties as a function of vinyl content.

Amorphous high vinyl polybutadienes (>70% vinyl) have glass-transition temperatures higher than −40◦ C (349). These materials are typically made with lithium initiators combined with a mixed alkali metal alkoxide and/or polar modifier system. As a result, commercial high vinyl polybutadienes are typically quite branched but display excellent traction properties. These materials are most often used in silica-filled tire applications. Functionalization. Anionically prepared polybutadiene offers the unique advantage of near quantitative chain-end functionalization. Through a variety of functional initiating and/or terminating agents, a variety of reactive moieties can be attached to the polymer termini (350). For example, there are several methods available that introduce hydroxyl groups at both the α- and ω-chain ends (351,352). In addition, functional initiators can be utilized to provide hydroxyl, amine, and tin functionality (353). Cyclic amine functionalization at either termini can provide hysteresis improvements in carbon black filled compounds (354,355). The polymer macrostructure can also be modified by adding functional linking agents such as tin (or silicon) tetrachloride. Tin-coupled polymers give benefits in processing and have been reported to also decrease compound hysteresis (356–358). Using a combination of functional initiators and terminators, polybutadienes containing two types of end groups can be synthesized (359), enabling greater control of post-conversion polymer chemistries. Cationic Polymerization. Like anionic polymerization, cationic polymerization is an addition reaction involving a relatively stable ionic propagating species. Cationic polymerization proceeds by an attack on the monomer by an electrophilic species, resulting in heterolytic cleavage of a double bond producing a carbocation. In the case of olefins, the carbocation is of the carbenium form. The most complete reference to date acknowledging the importance of cationic polymerization chemistry is a 1975 book (360). Early research demonstrated that 1,3-butadiene can be polymerized by strong organic acids (361) or Lewis acids at low temperatures (361–363). Typical

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microstructures were primarily trans-1,4 linkages with the remainder being mostly vinyl. For the synthesis of low molecular weight oils and resins, aliphatic ethers were used in conjunction with metal halide/hydrogen halide mixtures (364). Later studies reported polybutadiene produced from combinations of diethyl aluminum chloride and various salt hydrates in benzene at 20◦ C (125). These same studies highlighted the disagreement between theoretical unsaturation amounts and that which were actually found. An intramolecular backbiting mechanism producing pendent cyclohexene groups was postulated. An overview of many papers proposing that cationically polymerized diene monomers by a mixed initiating system of magnesium bromide/titanium tetrachloride in benzene produced cyclized structures was published in 1969 (365). Structures inferred by infrared spectroscopy included cyclic ladder units alternating with linear trans-1,4 segments that generate a heat-stable form of cyclopolybutadiene (126,366). Similar studies using highly acidic alkyl aluminum chloride/titanium tetrachloride mixtures in n-heptanes produced insoluble polymer (367). The insolubility was attributed to either cross-linking or a ladder-like structure comprised of joined rings. These cyclic structures were found to be inherent to most cationically polymerized dienes in nonpolar solvents (368). It has been reported that these cyclic structures could also be produced by typical cationic polymerization in the presence of ubiquitous proton-donating impurities, chain-transfer reactions (to monomer and aromatic solvents), and more conventional elimination reactions (369). In contrast, high molecular weight soluble cationically polymerized polybutadiene products can be produced with relatively high rates with the ion pair (C2 H5 )2 Al+ C2 H5 AlCl3 − in conjunction with cobalt. By mixing either (C2 H5 )2 AlCl with C2 H5 AlCl or (C2 H5 )3 Al with AlCl3 , this ion pair can produce high cis-1,4 microstructures (>90%) (370). It should be noted that the only cationically polymerized butadiene product commercialized is DuPont’s Budium ® , which is an oligomeric resin used for tin-can linings. This product is produced using BF3 ·(C2 H5 )2 O/H2 O in hexane at low temperature (371).

Polymerization Processes Polybutadiene can be prepared by a number of processes, including bulk, solution, suspension, emulsion, and gas-phase polymerizations. Of the commercially significant polymerization processes, production of polybutadiene by solution technologies are the most predominant. The relative popularity of these processes follow the limitations dictated by the chemistries most practiced when producing polybutadiene commercially, including Ziegler–Natta, anionic, free-radical, and single-site technologies. Several of the more commercially relevant processes are outlined in detail below. Solution Polymerization Process. Solution polymerization processes are often used when polymerization thermodynamics are largely exothermic, as in the case of polybutadiene. The solvent not only acts as a diluent, but also allows for efficient transfer of the heat of polymerization to a heat sink. Given the proper

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choice of solvent, lower solution viscosities can also be maintained. Polybutadiene polymerizations can be carried out in aliphatic, cycloaliphatic, or aromatic solvents. Chain transfer to solvent may be a concern, as are impurities inherent to the solvent feed stream. Solution polymerization systems may be either batch or continuous in operation, with the broadness of the molecular weight distribution dependent on the configuration used. Ziegler–Natta and anionic polymerization technologies are the most common types used in solution polybutadiene production. Commercial high cis-polybutadiene can be made via either technology in solution systems, though Ziegler–Natta systems predominate by a wide margin. High trans-polybutadiene can also be produced by Ziegler–Natta catalysts. High vinyl elastomeric materials are typically produced via anionic solution polymerization. Ziegler–Natta solution polymerization processes are very sensitive to impurities. Both the monomer and solvent streams must be well purified. Carbonyl and acetylenic impurities, common in crude monomer streams, must be removed along with the common polymerization inhibitor tert-butylcatechol. Molecular weight control is often accomplished by the addition of chain-transfer agents in certain Ziegler–Natta systems. In anionic solution systems the feed stocks are typically dried over various types of dessicants because the systems are sensitive to water contamination. When using continuous anionic solution polymerization systems, it is necessary to employ low (ppm) concentrations of a chain-transfer agent in order to discourage gelation and fouling; 1,2-butadiene is often used for this purpose in commercial applications. Alkyl-lithium-initiated polybutadiene is less prone to contain gel and does not contain the heavy metal catalyst residues associated with Ziegler–Natta catalyzed products. The polybutadiene produced by solution processes must be adequately desolvated in order to maintain low residual monomer and solvent in the product. Steam stripping is often employed in commercial processes in order to recover the solvent for recycling, followed by mechanical drying (extruder) and/or hot air drying of the wet polymer crumb. Emulsion Polymerization. The free-radical polymerization of butadiene in a homogeneous system results in chain lengths that are too short for high quality elastomers. If the free-radical concentration in homogeneous media is high enough to give a useful reaction rate, it is also high enough to favor termination reactions or monomer depletion before high molecular weights are achieved. In contrast to homogeneous free-radical systems, emulsion free-radical polymerization affords high propagation rates and high molecular weight products of elastomeric quality (372,373). The propagating radical chains are somewhat physically isolated and thus prevented from recombining as rapidly as they would in solution or bulk media. The emulsion process allows convenient temperature control, which is advantageous in the highly exothermic polymerization of butadiene (1.4 kJ/g). Heat removal and temperature control are considerably more difficult at high reaction rates in viscous solution or bulk processes. A typical emulsion system contains water, monomer(s), initiator, emulsifier (soap), and a molecular weight modifier. Upon vigorous stirring, an emulsion consisting of monomer droplets dispersed in an aqueous phase is formed. The aqueous

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phase contains the initiators, dispersed monomer droplets, and micelles formed from the emulsifier. Primary initiation centers, formed in the aqueous phase, diffuse into the micelles where polymerization proceeds (374,375). The polymer particle formed within the micelle then tends to absorb monomer from the surrounding aqueous phase because the monomer in the vicinity of the polymer particle has been consumed by polymerization. Continued uptake of monomer results in growth of the polymer particle at the expense of the monomer droplets. As the polymer particles grow, the emulsifier adsorbs on the increasing surface area and eventually leads to the disappearance of micellar soap as well as the monomer droplet phase. Relatively little polymer is believed to form in the aqueous phase or in the monomer droplet phase. In a continuous process, butadiene, soap, initiator, and activator (an auxiliary initiating agent) are pumped continuously from storage tanks through a series of agitated reactors at such a rate that the desired degree of conversion is reached at the last reactor. A terminator is added, the latex warmed with steam, and the unreacted butadiene flashed off. After addition of an antioxidant, the latex is coagulated by the addition of brine, followed by dilute sulfuric acid or aluminum sulfate. The coagulated crumb is washed, dried, and baled for shipment. Gas-Phase Polymerization Processes. Gas-phase polymerization is the newest process in development for the commercial polymerization of conjugated dienes. Although primarily utilized for the polymerization of ethylene and propylene monomers, commercial gas-phase processes are being extended to include the manufacture of polybutadiene. Many polymer manufacturers have reported researching gas-phase processes for diene monomers, and several have established significant patent portfolios including Amoco, Bayer, Exxon, Mitsui, and Union Carbide. To date, Bayer appears to be closest to commercializing a gas-phase process for producing polybutadiene rubber (376,377). The purported benefits of gas-phase technology include the reduction in overall waste streams, including no waste water, lower solid waste, and reduced overall emissions. Solvent recycling is no longer necessary, and costly polymer product isolation and drying steps are not required. Products are typically isolated from the reactor as powder or crumb. Production costs have the potential to be significantly lower than either solution or emulsion technologies. The chemistries utilized in gas-phase technologies employ the same Ziegler–Natta (314,315) and single-site (metallocene) catalysts (313) described in the processes included below. In gas-phase systems, however, the catalysts are generally solid-supported, but produce the same range of polybutadiene microstructures inherent to the nonsupported catalyst. Several patents also include anionic polymerization systems as useful in gas-phase processes (378). Kinetic modeling work has also been done to better predict the gas-phase polymerization behavior of 1,3-butadiene (379).

Economic Aspects The economics of polybutadiene production are characterized by overcapacity on a global basis. Total world capacity increased by 649,000 t from 1995 to 1999; however, actual production increased by only 167,000 t during this same period

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Table 10. Global Capacity and Production for Polybutadiene by Region—1999a Capacity

Production

Net trade

Consumption

675 498 344 1463 3019

600 332 290 880 2102

51 −53 116 −118 0

544 385 191 837 1957

United States Western Europe Japan Other Total a Ref.

380.

Table 11. Major Producers of Polybutadiene Elastomersa

Company and plant location Russian companies Bayer AG (Canada, France, Germany, and United States) Chinese producers The Goodyear Tire & Rubber Co. (United States) Michelin (France) Enichem (Italy and United Kingdom) Bridgestone/Firestone (Japan and United States) Korea Kumho Petrochemical Co., Ltd. (Republic of Korea) Other Total a Ref.

Annual capacity as of January 2000, 103 t

World capacity, %

395 448 341 230 220 145 134 167 882 2962

13 15 12 8 7 5 5 6 29 100

380.

resulting in a drop in global capacity utilization from 82 to 70% (380). Global production figures are tabulated in Table 10. Major polybutadiene producers are listed in Table 11. The average annual growth rate in consumption between 1990 and 1999 was 2.0% per year on a global basis. However, because of an unusual increase of natural rubber prices in 1994, polybutadiene use in tires in the United States increased by 6.1% per year from 1994 to 1996 (380). This was, however, followed by the return of natural rubber prices to their historic norm and a resulting decrease in polybutadiene use in tires. During the period from 1996 to 1999, tire shipments increased by 4.3% whereas polybutadiene use in tires lagged behind—increasing at only 2.5% per year (380). While the combination of predominately small cars, radial tires, and low car sales drastically reduced butadiene production in the early 1980s, in the 1990s this was followed by a significant market shift to larger vehicles, sport-utility vehicles (SUV), and high performance tires. Of the trends in consumer demand, only high mileage tires demand high levels of polybutadiene in the tread compound. High performance tires require higher T g materials and SUV tires place traction and durability demands on compounds that cannot be met by polybutadiene. The portion of polybutadiene going into nontire applications continues to increase. Between 1980 and 1999, the portion of polybutadiene consumed in tire

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Table 12. Typical U.S. List Prices, $/kg, for Selected Grades of Polybutadienea,b

Jan. 1982 Oct. 1989 Jan. 1993 Jan. 1996 Feb. 2000

Clear rubber grade

Clear plastics grade

Oil extended 37.5 phr oilc

Oil-black masterbatchd

1.49–1.65 1.49–1.76 1.49–1.76 1.71–1.94 1.69–1.96

1.53–1.67 1.53–1.80 1.53–1.80 1.75–1.94 1.72–1.96

1.54 1.69 1.67 1.90 1.85–1.94

1.10 1.23 1.21 1.39–1.50 1.39–1.50

a Ref.

380. are FOB Gulf Coast manufacturing plant, freight collect, for truckload and car quantities. c Parts of highly aromatic oil. d Contains 77 parts per hundred of resin (phr) of N-302 or N-303 carbon black and 53 phr of highly aromatic oil. b Prices

applications decreased by 11–67% of world production. Use of polybutadiene in high impact polystryene modification and ABS resin manufacture account for 15–20% of global demand (380,381). Polybutadiene used in these applications must be essentially gel free and colorless. The remaining markets for polybutadiene are divided among industries including golf balls and footwear and assorted industrial products including, but not limited to, conveyor belts, v-belts, seals, gaskets, and wire insulation. In the mid 1990s the number of rounds of golf played in the United States increased at roughly a 10% annual rate (380). However, this rate of growth has not been sustained in the latter part of the 1990s and into the 2000s with total rounds of golf being flat in recent years. Despite a leveling off in rounds of golf played in the latter part of the 1990s and into 2000, changes in ball construction, including the predominance of solid core over wound core golf balls, has maintained stability in this market segment. Prices typically vary depending on the market segment and grade of polymer sold. Typical United States prices are given in Table 12.

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