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Vol. 5 BUTADIENE POLYMERS 317

BUTADIENE POLYMERS

Introduction

1,3-Butadiene was first prepared in 1863, and its conjugated structure was pro-posed in 1886 (1,2). However, the ability of butadiene to polymerize was notrecognized until almost 50 years later when, in 1909, a rubbery polymer wasfirst reported as being prepared from butadiene via thermal polymerization (3).Shortly thereafter, the more controlled polymerization of butadiene initiated bysodium metal was reported in 1911 (4). During this time period, a sharp rise innatural rubber prices prompted the Bayer Corp. to develop methyl rubber from2,3-dimethylbutadiene. Though interest in synthetic rubber faded after WorldWar I, in 1926 a rise in price of natural rubber prompted the German companyI. G. Farbenindustrie to resume research on the sodium-initiated polymerizationof butadiene. This work eventually led to the German commercialization of twosynthetic rubbers: Buna 32 and Buna 115 (from butadiene and natrium). Con-currently, in the 1920s research on the emulsion polymerization of butadiene wasbeing carried out in Germany and the United States. The first butadiene–styrenecopolymer prepared from an emulsion polymerization (Buna S) at I. G. Farbenin-dustrie proved to be superior to polybutadiene (5). From this work, Buna-N, acopolymer of butadiene and acrylonitrile, was developed for its solvent and oilresistance. Although the products of this work were inferior to natural rubber,their technology, with considerable modification and improvement, formed thebasis for synthetic rubber production (GR-S and GR-N) in the United States. Un-der the government-established Rubber Reserve in World War II, GR-S and SBRbecame a general-purpose rubber with an annual production of ca 717,700 t in1945.

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

318 BUTADIENE POLYMERS Vol. 5

Butadiene Monomer

1,3-Butadiene, CH2 CH CH CH2, is the simplest conjugated diene, and itsstructure has received much theoretical attention because of its symmetry andsimplicity (6). Heats of hydrogenation and combustion reveal that the stabi-lization 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 bondlengths C C, 0.1337 nm; C C, 0.1483 nm; and C H, 0.1082 nm; the bond anglesare C C C 122.4◦ and C C H 119.8◦ (8). The values for the C C bond lengthspredicted by valence bond approximations are in close agreement with these ob-served figures (9). The C C single bond between the two double bonds is shorterthan the usual 0.154 nm of an isolated single C C bond. Some of the proposedexplanations for this shortened bond, which indicates some double bond character,can be expressed in polar resonance or terminal diradical structures with somelong bond or interaction between the end carbons. However, because the energiesof these structures are high, molecular orbital calculations indicate that there islittle resonance in the ground state and that the bond lengths are determined bythe state of hybridization of carbon (10). The resistance to rotation about the cen-tral bond is attributed to π -conjugation and leads to two conformers, the nonpolars-trans and polar s-cis form as seen in the conformational equation (1).

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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 theextent of only 3% at room temperature. The energy difference between the twoforms 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 theorigin of each of the transitions in the 230–135 nm region has been identified(17). In hexane solution butadiene absorbs at λmax 217 nm, ε = 21,000 (18). Thebathochromic effect of conjugation is evident upon comparison with those valuesfor ethylene, λmax 170 nm, ε = 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 constantshave been calculated from the pure rotational Raman spectrum (23).

The complex proton magnetic resonance spectrum of 1,3-butadiene has beenanalyzed, and the calculated spectrum for this AA′BB′CC′ system has been de-termined (24,25). The temperature-sensitive coupling constant for the protons oncarbons 2 and 3 suggests an equilibrium between the predominant s-trans con-former and a skewed conformer having out-of-plane double bonds (25). The com-paratively simple 13C spectrum shows little effect of conjugation on the chemicalshifts of the carbons (26).

The microwave spectrum has been examined in an effort to detect thes-cis conformer (27). The appearance potentials and relative abundances of the


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

Property Value Reference

Molecular weight 54.09 30Boiling point, ◦C

At 101.3 kPaa −4.413 31At 200 kPa 14.44 31At 245 kPa 21 32

Melting point, ◦C −113 32Freezing point in air at 101.3 kPa, ◦C −108.915 31Density at saturation pressure, g/mL

At 20◦C 0.6211At 25◦C 0.6149 31

Pressure coefficient of boiling point at 101.3 kPa, ◦C/Pa 4.502 31Vapor pressure of liquid at 21◦C, kPaa 24.5Molar volume at 20◦C, liquid, mL/mol 87.08 31Critical temperature, ◦C 152 31Critical pressure, MPab 4.33 31Critical density, g/mL 0.245 31Critical volume, mL/g 4.09 31Critical PV/RT 0.271 31Enthalpy of combustion �Hc

0, at 25◦C, 101.3 kPa, all gases, kJ/molc −137.65 31Enthalpy of formation �Hf

0, at 25◦C, gas, kJ/mol 6.393 31,33Entropy S0, at 25◦C, J/(mol·K)c 15.92 31Free energy of formation �Ff

0, at 25◦C, gas, kJ/mol 8.607 31Heat content function (Ho

0 − Ho0)/T, at 25◦C, J/(mol·K) 2.906 31,33

Free energy function (F0 − Ho0)/T, at 25◦C, J/(mol·K) −13.02 31,33

Entropy S0, at 250, for the ideal gas state, J/(mol·K) 15.92 31,33Enthalpy H0 − Ho

0, at 25◦C, for the ideal gas state, J/mol 866.54 31,33Heat capacity Cp

0, for the ideal gas state, at 25◦C, mJ/(g·K) 77.56 31,33Surface tension at 20◦C, mN/m (=dyn/cm) 13.41 34Solubility parameter, (J/m3)1/2 14.5 × 103 35Flash point, ◦C −76.1 32Lower explosion limit, % 2.0 31Upper explosion limit, % 11.5 31Autoignition temperature, ◦C 420 31Vapor density, g/L 1.87 31aTo convert kPa to mm Hg, multiply by 7.5.bTo convert MPa to psi, multiply by 175.cTo convert J to calories, divide by 4.184.

principal ions from the mass spectrum (28) of butadiene, as well as x-ray data forthe crystalline material (29), have been reported.

Physical Properties. At room temperature 1,3-butadiene is a highly re-active, colorless gas with a mildly aromatic odor (30). Physical properties aregiven in Table 1. Plots of heat of vaporization, vapor pressure, vapor and liq-uid heat capacity, liquid density, surface tension, and vapor and liquid viscos-ity are available as functions of temperature (32), as are plots of vapor andliquid thermal conductivity and heat of formation and free energy of forma-tion of the gaseous butadiene (36). The vapor pressure of butadiene in Pascals


Table 2. Solubility of Butadiene inOrganic Solvents at 20◦Ca

Solvent Butadiene, vol%

Acetone 65Benzene 66.3Dichloromethane 91.4Amyl acetate 115.3aRef. 37.

can be calculated at any temperature in degrees Celsius by using the Antoineequation:

logP = A− BC + 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, ace-tone, benzene, and hexane (Table 2) (38). Solubilities in liquid ammonia, methanol,n-octane, toluene, xylene, ethylene glycol, acetone (39), and water (40,41) havebeen reported.

Reactions. Because of the rich chemistry associated with conjugated dienestructures, the reactivity of 1,3-butadiene has been studied extensively. However,the hundreds of polymers and copolymers described in the literature represent byfar 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 5.33and 120 kPa (40 and 900 mm Hg). Below an oxygen pressure of 5.33 kPa (40 mmHg) the oxidation rate decreases with decreasing oxygen pressure; increased tem-perature and free-radical initiators accelerate the reaction. The polyperoxide iscomposed 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 secondphase in neat butadiene.

Butadiene dimerizes by a Diels–Alder reaction thereby forming mainly4-ethenylcyclohexene (4-vinylcyclohexene) (eq. (2)). In this dimerization reaction,butadiene acts as both diene and dienophile. The dimerized product is alwayspresent in butadiene unless freshly distilled. Although this reaction occurs spon-taneously, selective catalysts based upon nitrosyliron compounds do exist (44).Vinylcyclohexene is a starting material for plasticizers and antioxidants. It shouldbe noted that this reaction also gives smaller amounts of 1,2-divinylcyclobutaneand 1,5-cyclooctadiene (45).

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Fig. 1. Butadiene manufacture in the United States. —�—, Co-produced; — —, On-purpose; —�—, Other.

Manufacture.Ethylene Coproduction. Historically, butadiene was first prepared in pilot

plant quantities via an uneconomical electric arc process. However, the primarysource of butadiene in the world today is as a by-product of thermal pyrolysis ofhydrocarbon feedstocks in ethylene production. In the United States, productionof coproduct butadiene exceeded that of “on-purpose” butadiene for the first timein 1977 and by 1990 high cost on-purpose butadiene production was essentiallyeliminated in the United States (Fig. 1) (46,47). In 1996, the total US productionof butadiene was 1.75 million, 93% of which was co-produced (47). Steam crackingof hydrocarbons yields varying amounts of butadiene, depending on the nature ofthe feedstock, the volume of ethylene produced, and the severity of the crackingoperations (48–50). For example, when feedstocks are switched from atmosphericgas oils and napthas to propane and butane, yields of butadiene drop by as muchas 60% (51).

An analysis of the typical by-products in the C4 stream derived from crackinga full-range Middle East naphtha is given in Table 3. The range reflects composi-tion at medium and high severity during steam cracking.

Table 3. Typical Composition of a CrudeC4 Streama

Component Wt%

C3 hydrocarbons 0.3Butanes 6.5–3.41-Butene 16.0–13.7cis-2-Butene 5.3–4.8trans-2-Butene 6.6–5.8Isobutene 27.4–22.21,3-Butadiene 37.0–47.5Acetylenes 0.4–1.8C5 hydrocarbons 0.5aRef. 48.


A spike in crude oil prices in late 1999 and 2000 has led many steam-crackingoperations to switch from heavy to light feedstocks, resulting in lower yieldsof butadiene. This factor, combined with historically poor investment economicsfor added butadiene extraction capacity, should lead to tight butadiene suppliesthrough 2004 when additional extraction capacity is expected to become fullyon-line. Further expansion of butadiene extraction capacity in North America willbe limited by crude butadiene supplies unless new domestic ethylene capacity isadded (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 requirestemperatures of about 620◦C and involves passing n-butane over a fixed bed reac-tor containing a chromium–alumina catalyst. Single-pass conversions reach only30–40% requiring multiple recycling passes of unreacted intermediate butylenesand n-butane. The other primary means of dehydrogenation is conducted by pass-ing a preheated stream of n-butene, steam, and air over fixed bed reactors contain-ing a heterogenous autoregenerative catalyst. Bismuth molybdate catalysts havebeen studied in this process (53), but patents suggest that zinc ferrite catalystshave been used commercially (54–57). Nickel phosphate and Li–Sn–P catalystshave been patented by Phillips (58,59).

Other techniques for the preparation of butadiene have also been used com-mercially, including the aldol condensation of acetaldehyde and the reaction ofacetylene and formaldehyde. However, these processes are no longer employedcommercially. Both India and the former USSR have developed ethanol-basedproduction of butadiene (60–64). While these routes have a major advantage interms of low capital and operational costs, raw material costs are prohibitive inmost areas (47). Such plants are only viable in areas where there is little domesticpetroleum but abundant ethanol supply.

Purification. For polymerization, butadiene that is at least 99 mol% pure isrequired. Although alkynes are the most troublesome impurities, separation of thebutadiene from other C4 products is also necessary. Simple fractional distillationis effective for removing the light (C3) and heavy (C5) ends from butadiene, but notfor removing the various C4 species because of the closeness of the boiling points toeach other and to butadiene. Further complicating purification, butadiene formsazeotropes with n-butane and 2-butene. The most widely used recovery systemsare extraction with aqueous cuprous ammonium acetate (CAA) and solvent ex-tractions with furfural, acetonitrile, dimethylformamide, dimethylacetamide, orN-methylpyrrolidinone (65,66).

Cuprous ammonium acetate extraction. Butadiene is purified by aqueousCAA extraction in a liquid–gas countercurrent process developed by Exxon (67–69). The cuprous salt forms a soluble addition complex with butadiene, whichis decomposed by heat; thus the process is adaptable to countercurrent multi-stage equipment. Typically, the C4 hydrocarbon mixture with a butadiene contentof 30–40% contacts the CAA solution in a countercurrent fashion in a series ofmixer–settlers. Cooling to ca −15◦C is required to promote complex formation.The more saturated hydrocarbons, butanes, and butenes are first removed by dis-tillation. Butadiene is released from the complex by further heating to 80◦C. Afterammonia is removed by washing with water, distillation produces butadiene thatis 98–99% pure. Acetylenes and allenes are extracted with the butadiene but must


be removed; acetylenes can lead to foaming and plugging by polymer formation(70–76).

Solvent extraction. This purification method is based on the alterationof the relative volatilities of the C4 hydrocarbons by selective solvents. Thevolatilities 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 impu-rities 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 requireprehydrogenation. They employ countercurrent extraction of the C4 stream wherethe butanes and butenes are removed at the top, and the butadiene-rich solventat the bottom. The butadiene is stripped from the solvent and further purified byfractional distillation. The butadiene is at least 99% pure and contains a few ppmacetylenes.

Specifications and Standards. Commercial polymerization-grade buta-diene is at least 99% pure; higher purity grades up to 99.86% are available fromspecialty gas suppliers (85). A representative specification and analysis is givenin Table 4. A pure grade has been analyzed for hydrocarbon impurities by gaschromatography (see Table 5).

Health and Safety.Toxicity. Short-term exposure to <1000 ppm butadiene has not been asso-

ciated with significant adverse acute effects; however, higher levels may causeeye irritation or transient nervous system depressant effects. The InternationalAgency for Research in Cancer assessed human and laboratory cancer data, andjudged there to be limited evidence in humans for cancer, sufficient evidence inanimals, and an overall qualification as a “2A” chemical (probably carcinogenicto humans) (89). The monograph cites several studies of rubber and monomer

Table 4. Specification and Analysis of 1,3-Butadienea Inhibited with p-tert-butylcatechol(ASTM D1157)

ASTMSpecification Analysis Standard

Appearance Clear and free ofsuspended matter

Conjugated diene, as 1,3-butadiene, min wt% 99.0 99.1Acetylenes, α, as vinylacetylene, max ppm 500 150 D1020Butadiene dimer, max wt% 0.2 0.01 D2426Nonvolatile matter, max wt% 0.1 0.01 D1025C5 hydrocarbons, max wt% 0.05 0.01Carbonyl, as acetaldehyde, max ppm 50 <20 D1089Acetylenes, vinyl, max ppm 10 0–5 D2593Peroxides, as hydrogen peroxide, max ppm 10 nil D1022Sulfur, as H2S, max ppm 10 nil D1072aRef. 86.


Table 5. Impurities in High Purity 1,3-Butadiene

Concentration, Concentration,Impuritya ppmb Impuritya ppmb

Methane 10 Neopentane N/AEthane 7 Butane 210Ethylene 12 1-Butene 5500Acetylene 0.3 Isobutene 2500Cyclopropane 0.4 1,2-Butadiene N/APropane 5 cis-2-Butene 270Propene 46 trans-2-Butene 390Propadiene 70 1-Butyne N/APropyne 13 1-Butene-3-yne N/AIsobutane 61aRef. 87.bRef. 88.

production workers, some of which contain suggestive evidence for leukemogenicactivity for butadiene. However, there is uncertainty of butadiene’s associationwith leukemia because of inconsistent findings in data between studies (lym-phosarcomas vs leukemias, decreased cancer risk association with longer employ-ment, other chemical factors implicated in etiology). Support for a lack of humancarcinogenicity for butadiene despite positive mouse findings (multiple tissues atlow exposures) is that the mouse enzymatically transforms the chemical to mu-tagenic and carcinogenic metabolites (eg, butadiene diepoxide) whereas rats andhumans are less prone to do so (90). The US EPA considers butadiene a “probablehuman carcinogen” (91). The potential for butadiene to cause mutations in ro-dents is known, while effects associated with reproductive and/or developmentalfunctions are currently under investigation.

Butadiene had previously been considered nontoxic (92). It is mildly narcoticwhen inhaled at concentrations below 2%. At higher concentrations it has anes-thetic effects that can cause respiratory paralysis and death (93). Butadiene is anirritant of the eyes and upper respiratory tract; contact of the liquid with the skinmay have frostbite-like effects.

Occupational Guidelines. US OSHA has set an 8-h time-weighted averageair standard for occupational settings of 1 ppm, and 5 ppm for a 15 min short-termexposure limit (94). The ACGIH has established a threshold limit value of 2 ppmfor butadiene, and considers the chemical a “suspected human carcinogen.” Anodor threshold for the chemical is reported as 1.0–1.6 ppm in air (95). Potentialnonoccupational exposures to the public occur as a result of the presence of buta-diene in combustion gases (fossil fuels, tobacco smoke).

Handling and Storage. Butadiene is dangerous because of its explosivenesswhen mixed with air (38). Because of the high vapor pressure of the liquid, it mustbe kept under pressure. Butadiene reacts with air to form explosive peroxides andmust be inhibited. Peroxides or rusty iron surfaces may initiate the formation of“popcorn” polymer which can plug pipelines. Butadiene material fires are foughtlike other hydrocarbon fires, with carbon dioxide or dry chemical extinguishers.Safe handling and storage require careful exclusion of oxygen, thorough ground-ing of all equipment, and avoidance of excessive temperatures in closed vessels.


Inhibited butadiene of polymerization purity is available in cylinders (3.8–454 L),tank cars, tank trucks, and pipelines (96,97). The cylinders may be shipped bymotor or rail freight (98). Containers are subject to regulations of the Departmentof Transportation and must be labeled “Flammable Gas” (96). Butadiene is usu-ally stored in bulk under refrigeration rather than pressure in order to suppressformation of the dimer 1-vinylcyclohexene (99). Smaller quantities in cylindersunder pressure should be kept away from sources of ignition and heat.

Polymer Structure

Microstructure refers to the disposition of the double bonds present in the polymerchain and macrostructure refers to long chain features such as extent of branching,molecular weight, and molecular weight distributions.

Polybutadiene Microstructure. Butadiene polymerizes by addition.Having two double bonds, it forms a variety of polymer structures. One of these,known as the vinyl or 1,2-type, results from addition across just one of the doublebonds (eq. 3).

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Actually, three vinyl structures are possible: the isotactic (1), in which all thevinyl groups are attached to a backbone carbon with the same spatial arrange-ment, either the D or L isomer; the syndiotactic (2), in which the vinyl groupsare attached to backbone carbons with alternating D and L configuration; and theatactic with a random mixture of the D and L isomers.

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A second type of structure arises by 1,4 addition when both double bondsparticipate. The resulting double bond in the polymer backbone permits two 1,4


structures, the cis-1,4 (or Z) (3) and the trans-1,4 (or E) (4) isomers. In the absenceof side chains, there is no tacticity that can arise from 1,4-addition.

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(4)

In an actual polymer it is possible to have any or all of the several forms in asingle chain. The structures indicated have all been synthesized individually, aswell as in various mixtures, to give polymers with a preponderance of either 1,2-or 1,4 units.

The microstructure of polybutadienes has been determined by infrared (100–102) and Raman spectroscopy (103). The 965 cm− 1 band is due to trans doublebonds; the 740 cm− 1 band to cis double bonds in polymers of high cis content,shifting to 725 cm− 1 for low cis; and the 911 cm− 1 band is due to the vinyl doublebonds.

Sequence and tacticity information can be obtained from both 1H and 13C nmrspectra; 1H is most useful at high fields where separations of peaks are sufficientto allow determination of cis, trans, and vinyl contents from both the olefinic andaliphatic resonance peaks (104). Spin decoupling of the olefinic peaks gives triaddistributions of the cis and trans isomers (105). At low fields, determination of only1,2- and 1,4 units is possible because of the severe overlapping of the resonances.The 13C nmr spectrum of a polybutadiene contains two groups of resonances aris-ing from aliphatic and olefinic carbons (106,107). Microstructures (108), sequencedistribution (107), and tacticity of the vinyl units can be determined (109,110).

The glass-transition temperature (Tg) and the melting point (Tm) ofpolybutadienes are microstructure-dependent (Table 6).

cis-1,4, trans-1,4, and vinyl polybutadiene microstructures are well un-derstood in terms of conditions influencing their formation during polymeriza-tion and the effect of the population distribution and tacticity of these mi-crostructures on the thermal properties of the polymer. However, literature citesanother microstructure inherent to polybutadiene that is not as well under-stood. Certain polymerization conditions favor formation of a cyclic structurein the growing polymer chain during anionic polymerization. Unlike polysilox-ane polymerizations in which a percentage of macrocyclic polymer is regularlyformed (115), or post-conversion reactions which cyclize polyisoprene (116), thiscyclic form of polybutadiene appears to form during normal anionic polymeriza-tion reactions. This structure was mentioned in the anionic polymerization ofpoly(2-phenylbutadiene) as the authors noted lower levels of unsaturation than


Table 6. Glass-Transition Temperature and MeltingTemperatures of Selected Polybutadienes

Polybutadiene Tg, ◦C Tm, ◦Ca

cis-1,4- −106b 2trans-1,4- −107b

Form I 97Form II 145Syndiotactic 1,2- −28c 156Isotactic 1,2- 126Atactic, 1,2- −4c

11% 1,2- −94d NoneaRef. 111.bRef. 112.cRef. 113.dRef. 114.

expected, and found evidence for five-membered rings using ozonolysis/mass spec-troscopy techniques (117). Most early work citing the cyclic structure involvesmethods producing low molecular weight polybutadiene oils and resins (118,119).These references discuss possible polymerization conditions conducive for cyclicstructure formation, including monomer-starved conditions and elevated tem-peratures. Several subsequent publications developed these ideas further andproposed a backbiting mechanism that involves the two penultimate monomericgroups in the chain (120,121). Noting that the polymerization conditions in thesereferences involve polar modification, it would appear that relatively high vinyllevels are inherent to the formation of cyclic polybutadiene. The most in-depthwork to date involves nuclear magnetic resonance analysis of commercially avail-able polybutadiene resins and develops evidence for vinyl-substituted cyclopen-tane units as part of the polymer backbone (122,123). Experimental evidence in-cluded in these references cites a range of incorporation of cyclic structure between0 and 45% by weight. In-depth polymerization studies to elucidate the thermody-namic and kinetic conditions favoring this structure formed by anionic polymeriza-tion techniques have not been published, and the contribution to glass-transitiontemperature and other thermal properties of cyclic polybutadiene elastomers hasnot been established.

Many references exist for cyclic diene structures formed during certaincationic polymerizations (124–127). The exact mechanism involved in the forma-tion of the ladder-like fused ring structure has yet to be determined.

The sequence distribution of two copolymerizing monomers depends on thecatalyst or initiator used, the method of polymerization, and the concentration andreactivities of the monomers. Reactivity ratios for many monomer pairs have beenmeasured for free-radical, anionic, and coordination polymerization of butadiene(128).

Polybutadiene Macrostructure. Macrostructure refers to the molecu-lar weight, molecular weight distribution, and extent of branching in a polymer.Branch sites along a polymer backbone can be either comb-like short-chain ran-dom branching (in the case of polybutadiene, most often derived from graftingreactions), random long-chain branching, or highly branched dendritic structures


(129). Often the nature and extent of branching and the breadth of the molecularweight distribution is dependent on the catalyst/initiator system and polymeriza-tion conditions.

Free-Radical Polymerization. The highly reactive nature of radical speciesresults in polymerization products with high levels of branching. The relativelevels of recombination and disproportionation termination reactions determinesthe breadth of the molecular weight distribution. Chain-transfer agents also affectthe macrostructure of the polymer.

Ziegler–Natta Polymerization. The macrostructure of Ziegler–Nattapolybutadiene varies considerably and is most dependent on the metal catalyst’sability to undergo chain-transfer events. Typically, early transition metals areless likely to participate in metal-hydride elimination chain-transfer reactionsthan the more electron-rich late transition metals (130). This phenomenon al-lows early metal systems, such as titanium, to catalyze the formation of poly-mers with narrow molecular weight distributions and low branching. Conversely,nickel systems are dominated by chain-transfer reactions and therefore generatepolybutadienes with very large molecular weight distributions. Cobalt, also a latetransition metal, catalyzes the formation of branched polybutadiene presumablyby reinsertion of a terminated conjugated diene end group. Another factor control-ling the macrostructure of Ziegler–Natta polybutadiene is the propensity of thecatalyst to chain transfer to the aluminum co-catalyst. This type of chain-transferreaction is dominant in neodymium systems leading to linear polymers but withbroad molecular weight distributions.

Anionic Polymerization. Complementing the diversity in microstructureinherent to anionic chemistry, living anionic polymerization based on alkali metalalkyl initiating systems can also afford a wide range of macrostructural possibil-ities for polybutadiene products.

Batch polymerization methods can produce narrower molecular weight dis-tributions than continuous processes (131), resulting in materials with lower hys-teresic properties. Low vinyl polybutadiene can be produced with either linear orbranched macrostructure. Polymerizations of polybutadiene performed at mod-erate temperatures produce linear chains in comparison to high temperatureprocesses which give rise to random thermal branching. Divinylbenzene is a di-functional monomer that is commonly added to promote long-chain branching.This common cold-flow-reducing agent effectively incorporates into the polymerchain to generate a macromonomer that can further polymerize to give long-chainbranching (132). Coupling agents of various functionality can be added at full con-version to produce star polymers with enhanced rheological properties (133,134).These branched materials not only have greatly reduced cold flow, but can alsooffer improved high shear mixing and extruding properties over more linear coun-terparts (135,136).

In contrast to the inherent linearity exhibited by unmodified low vinylpolybutadiene polymerized at moderate temperature, medium to high vinylpolybutadiene produced in continuous systems have the tendency to be highlybranched. The common use of alkali metal alkoxides with or without additionalpolar modifiers to increase the vinyl content results in a superbasic system witha high tendency to metallate the polymer chain (137–141). The metallated sitecan reinitiate in the presence of continuously fed monomer and produce random


long-chain branching (142,143). Proper design of the continuous polymeriza-tion process or choosing initiator/alkali metal alkoxide/polar modifier systemswith decreased superbasic activity can reduce the overall levels of branching(144,145).

Synthesis of Polybutadiene

Free-Radical Polybutadiene. Free-radical emulsion polymerizationof butadiene has yet to gain commercial importance. Unlike emulsionstyrene–butadiene copolymers, which exhibit excellent abrasion resistance,crack-growth resistance, and easy processing (146), emulsion polybutadiene doesnot perform as well in typical rubber applications as its solution counterpart.The main difference between these two types of rubber stems from the lackof Tg control in the free-radical polymerization process. The indiscriminate na-ture of free-radical chain propagation results in a polybutadiene containing amixture of 1,4- and 1,2-microstructure (147). Although the cis/trans ratio of the1,4-enchainment can be controlled with temperature, the total vinyl content re-mains constant between 15 and 20% (Table 7). In most cases, the resulting polybu-tadiene will have a Tg near −80◦C, which is a full 20◦ higher than that of highcis-polybutadiene produced with Ziegler–Natta catalysts (qv).

Another drawback of emulsion polybutadiene is the rapid formation of gelduring the polymerization. In order to avoid significant quantities of gel it is nec-essary to terminate the polymerization below 60% conversion. In this fashion, asmonomer concentration is depleted, the reaction of a growing chain with the back-bone of another chain can be avoided. One material that actually takes advantageof the rapid recombination of growing chains in a free-radical polymerization ishydroxytelechelic polybutadiene (HTPB). HTPB is typically prepared in a butanolsolvent with the use of hydrogen peroxide as an initiatior (149–156). Terminationof growing chains via primary radical recombination results in the formation ofdihydroxy functional polybutadienes. Molecular weights are limited, but neverthe-less a market exists for oligomers of both carboxytelechelic and hydroxytelechelicpolybutadienes. Advances in controlled radical polymerization techniques (157–161) have led to a number of recent studies covering hydroxy radical initiatedbutadiene polymerizations (162,163).

Ziegler–Natta Polybutadiene. Ziegler–Natta polybutadiene is thegeneric name given to materials produced from polymerization catalysts basedon transition metals (see ZIEGLER–NATTA CATALYSTS). These catalysts follow a

Table 7. Effect of Temperature on the Microstructure of EmulsionPolybutadienesa

Polymerization temp, ◦C cis-1,4, % trans-1,4, % 1,2, %

−33 5.4 78.9 15.65 13.0 69.9 16.550 19.0 62.7 18.870 20.8 59.4 19.8aRef. 148.


coordination polymerization mechanism and typically employ insoluble metalhalides or soluble metal carboxylates in conjunction with aluminum alkylco-catalysts (164,165). The commercial success of Ziegler–Natta type polymeriza-tions is related to the ability of this broad class of catalyst to afford stereoregularpolybutadienes. All the three types of possible microstructure for polybutadiene—1,4-cis, 1,4-trans, and 1,2-vinyl—can be achieved in near 100% regularity withthe appropriate metal catalyst. The most significant of these materials being highcis-polybutadiene. This polymer, with a Tg below −100◦C, has very good abrasionresistance, high resilience, and low heat buildup and is used in many tire-relatedapplications. Such properties have made cis-polybutadiene the second largest vol-ume synthetic rubber, behind only emulsion SBR (146).

cis-1,4-Polybutadiene. At least four different metal catalysts have beencommercialized for the production of cis-polybutadiene, namely, titanium, cobalt,nickel, and most recently neodymium. Each of these catalysts generate polymersthat vary in microstructure, such as cis content, as well as in macrostructure,which includes molecular weight, molecular weight distribution, and degree ofbranching. The end user must consider both the microstructure and macrostruc-ture obtained with each catalyst system when optimizing an elastomer for a givenapplication (166).

Titanium Catalysts. The titanium catalyst was the first system tobe commercialized in the 1950s by Phillips Petroleum (167). Generally,titanium-catalyzed polybutadiene is characterized by its low branch content andnarrow molecular weight distribution leading to good tensile properties and fa-tigue resistance but poor processibility (168,169). The catalyst is comprised of atitanium halide precursor and an aluminum alkyl co-catalyst. The most studiedsystem consists of treating titanium tetraiodide with three to eight equivalents oftriisobutyl aluminum (TIBA) in an aromatic solvent at 30◦C. This results in poly-merization of butadiene to cis-polybutadiene having a cis content greater than 90%(170,171). Polymer molecular weight is inversely dependent on catalyst concentra-tion with high molar mass polymers being obtained at low catalyst levels. To pre-pare materials that are easily processed, approximately 1–2 millimoles of titaniumtetraiodide per hundred grams of monomer (mmphm) is required (172). The cata-lyst is heterogeneous, and although catalyst activity is sensitive to the Al/Ti ratio,the overall cis content is only slightly affected. The cis content is, however, verysensitive to the type of halogen employed in the polymerization. As Table 8 shows,the cis content drops significantly as iodide is replaced with more electronegativehalides. The dependence of microstructure on halide is apparent for all of thecis-specific transition metal catalysts except neodymium. A number of alternative

Table 8. Cis Content %, as a Function of Metal Halide andTransition Metal in Polybutadiene Polymerizationa

Transition Metal I Br Cl F

Titanium 93 87 75 35Cobalt 50 91 98 93Nickel 10 80 85 98Neodymium 97 97 96 96aRefs. 173 and 174.


catalysts to the classic binary titanium system exist, including combinations ofTIBA/I2/TiCl4 (175–177) (C2H5)2 AlI/TiCl4 (178), (C2H5)3Al/TiCl4/TiI4 (179,180),and DIBAlH/AlI3/TiCl4 (181). The TIBA/I2/TiCl4 systems used in a molar ratio of8:1.5:1 has been optimized for conversion, molecular weight, and cis-1,4-contentand has been used in the commercial production of cis-polybutadiene (182,183).

Cobalt Catalysts. The preparation of high cis-polybutadiene with cobalt cat-alysts was first disclosed by B. F. Goodrich and is characterized by a high degreeof branching and good processibility (184). By analogy to the titanium halidecatalysts, early cobalt work focused on CoCl2 as the catalyst precursor. Unliketitanium, the use of trialkyl aluminum species does not lead to an active catalyst.Instead, an alkyl aluminum halide must be employed as co-catalyst in these sys-tems (185–191). Moreover, maximum activity appears to be linked to the Lewisacidity of the co-catalyst. For example, the use of aluminum ethyl sesquichloride[Al2(C2H5)3Cl3] in place of diethyl aluminum chloride results in a more activecatalyst. Increasing the strength of the Lewis acid even further, by using ethylaluminum dichloride or aluminum trichloride, results in competitive gel forma-tion. The balance of activity and gel content has been achieved through the use ofwater as a third catalyst component. Maximum activity is achieved when wateris added at approximately 10 mol% to the diethyl aluminum chloride (192). Abovethis concentration, water begins to act as a catalyst poison retarding the rate ofpolymerization. Presumably the water is reacting with (C2H5)2AlCl to generatethe Lewis acidic ethylchloroaluminoxane [O(AlC2H5Cl)2] (193). Degree of branch-ing, as a measure of solution viscosity at equal molecular weight, decreases withincreasing water content.

Catalyst systems based on CoCl2 are heterogeneous in nature and requirecatalyst levels as high as 10 mmphm in order to achieve good conversion. Muchlower levels are used when soluble cobalt salts are employed. These salts includepyridine adducts of CoCl2, cobalt octanoates, cobalt acetylacetonates, and cobaltversatates in the plus two or three oxidation state. When soluble salts are uti-lized, catalyst levels can drop to as low as 0.01 mmphm of cobalt with Al/Co ratiosof 100–1000/1. Typical cis contents range from 95 to 98%. Catalyst activity andmolecular weight are also dependent on the solvent used for polymerization. Ben-zene is the solvent of choice resulting in the most active polymerizations (194);however, mixtures of aromatic and aliphatic solvents are used in production plantsin an effort to reduce the amount of benzene emissions (195). Molecular weightmust also be regulated in these systems through the use of a chain-transfer agent.Suitable transfer agents are those that regulate molecular weight without ad-versely affecting the rate of chain propagation and polymer microstructure. For thecobalt systems hydrogen, ethylene, propylene, and allenes such as 1,2-butadieneare all effective regulators (196).

Nickel Catalysts. Nickel-catalyzed high cis-polybutadiene, first developedby Bridgestone (197–199), is typically less branched than commercial cobaltpolybutadiene but exhibits an equally high cis content of 98%. The polymeris also characterized by a large molecular weight distribution of 4–5 giv-ing the material good processibility. The commercial process utilizes a solublenickel(II)carboxylate, aluminum trialkyl, and a halogen source (200). Of thesethree components, the process is most dependent on the choice of halogen. As wasseen in Table 5, use of nickel-iodide yields mostly trans-polybutadiene while Br, Cl,


and F all favor cis polymerization (201–203). Initially boron trifluoride etheratewas used as a halogen source but later hydrogen fluoride was also implemented inlarge-scale production (204). Catalyst activity is similar to the cobalt system, withreasonable rates and molecular weights being achieved with 0.01 mmphm nickel.Typical Al/Ni ratios are 10–50/1 with an F/Al ratio of 2–3/1. Unlike the cobaltsystem, chain transfer readily competes with propagation in the nickel system re-sulting in broad molecular weight distributions. Maximum molecular weights areachieved during the first 50% of conversion and then level off for the remainderof the polymerization (205). The final molecular weight is inversely dependent oncatalyst levels. Alternatively, to obtain low molecular weights an olefin containingmolecular weight modifier such as 1-butene or 1,5-hexadiene may be used whenlow catalyst levels are employed. Studies with nickel octanoate have shown thatmicrostructure is a function of molecular weight (206). As the molecular weightincreases the cis content increases at the expense of both the trans and vinyl;the amount of trans is always about twice the amount of vinyl. At 10,000 Mnthe cis content increases to over 90% and at 100,000 Mn or more, cis content is>96%. Recent advances have also been made in the control of nickel polybutadienemacrostructure, resulting in the commercial availability of a branched, improvedprocessing material (207).

Because of the unique ability of nickel to readily form stable metal-allylcomplexes, many mechanistic studies of this system have been undertaken. Fromthese, it is clear that nickel allyl species are indeed involved in the coordinationpolymerization of polybutadiene (208–212). Whether generated in situ or addeddirectly as catalyst, the type of polymer produced from π allyl NiX complexes isdetermined by the nature of X, electron-donating or accepting additives, and thetransition metal. The chloride complex gives cis polymer, the iodide trans, andthe bromide a mixed microstructure (213). Catalytic activity is low and yields arepoor with the latter catalysts. Complexes of more electronegative anions, suchas trichloroacetate and trifluoroacetate, exhibit high catalytic activity and affordpolymers of very high cis contents (214). Electron-donating additives, such asoxygen, ethers, alcohols, and phosphites, tend to inhibit π -allyl polymerizationand increase the trans content. On the other hand, electron-accepting additives,such as tin and titanium halides, metal salts, and iodine, increase the activity ofπ -allyl nickel halides. π -Allyl nickel chloride retains the high cis microstructurein the polymer with most added metal salts. Iodide complexes of these electronacceptors produce high cis polymers, rather than the expected trans polymer.

It is also clear that in many instances the stereoregular outcome of monomerinsertion is dependent on the initial mode of monomer coordination and rateof anti-syn allyl isomerization. For example, in a now classic experiment (215–217), the polymerization of butadiene with π -allyl nickel(II)trifluoroacetate inthe absence of an external ligand yields high cis-polybutadiene. Upon additionof a trialkylphosphite, however, the stereo control is completely reversed andtrans-polybutadiene is formed. One explanation put forth to explain this behavioris that in the absence of a ligand the incoming monomer can bind to the metalin an η4-cis configuration (218,219). This bonding mode will result in the forma-tion of a syn-allyl upon migration of the growing polymer chain and subsequentlya cis configuration in the polymer. Conversely, if a ligand is present, the incom-ing monomer will favor η2-monodentate complexation. Migration of the growing


Fig. 2. Coordination and insertion mechanisms leading to cis- and trans-1,4-polybutadiene.

polymer into this bond will result in the generation of an anti-allyl leading totrans polymer (Fig. 2). The mechanism is, however, dependent on the assump-tion that no isomerization from η3-syn-allyl to η3-anti-allyl occurs or that η4-transcoordination of butadiene is not favored (220,221). A comparison of the mecha-nism of isolated nickel-allyl catalyzed diene polymerization with the traditionalZiegler–Natta nickel system has been reviewed (222).

Neodymium Catalysts. The most recent catalyst system developed for thecommercial production of high cis-polybutadiene is based on the rare earth metalneodymium. Although it was recognized in the 1960s that lanthanide catalysts, ingeneral (223–225), and neodymium, in particular, were viable cis-polybutadienecatalysts, it was not until the 1980s that the process was commercialized (226–229). Polymer produced from this class of catalyst is characterized by a linearmacrostructure and a very high cis content of greater than 98% giving the rubberexcellent abrasion and fatigue performance. The linearity of the material, coupledwith high molecular weight, makes this rubber more difficult to process than theother high cis polymers. This problem has been addressed and there are now ver-sions of neodymium polybutadiene available, which contain long chain branchesreducing cold flow and improving processability while maintaining a high cis mi-crostructure (230,231).

The chemistry of the neodymium catalyst has very recently been re-viewed (233,234), and although the resulting polymer is similar to other highcis-polybutadienes the change from a transition metal based system (Ti, Co, Ni)to a lanthanide metal with valance f -electrons creates striking differences in thepolymerization chemistry. There are two main types of catalyst systems (235).The first is a ternary system based on soluble neodymium carboxylates in con-junction with a trialkylaluminum cocatalyst and a halogen source (236,237). Thesecond system is a binary catalyst comprising of an insoluble neodymium halide


complexed with three equivalents of a Lewis base such as alcohols, amines, orphosphonates and a trialkylaluminum (238–240). In general, the two systems be-have similarly; however, the homogeneous ternary system appears to have gainedacceptance commercially (241). As with the transition metal based systems, ahalogen must be present for high cis-polymerization; however, any halogen willsuffice (172). Chloride and bromide derivatives make more active catalysts butthe cis content is equally high for fluoride or iodide complexes. Typical cata-lyst concentrations are between 0.10 and 0.30 mmphm Nd, placing the activityof these systems between titanium on the low side and cobalt or nickel on thehigh side. Branched trialkylaluminum and dialkylaluminum hydrides used in anAl/Nd ratio of 10–40/1 in conjunction with two to three equivalents to Nd of ahalide source such as diethylaluminum chloride or tert-butylhalides treated witha branched long-chain neodymium carboxylate give the highest activity (242).Polymer molecular weight is inversely proportional to overall catalyst levels aswell as aluminum alkyl concentration. As the aluminum to Nd ratio increases, themolecular weight of the polymer decreases (243). This effect is most pronouncedfor diisobutylaluminum hydride, and in light of the fact that typical chain-transferagents such as hydrogen, olefins, and 1,2-butadiene are ineffective in this system(244), aluminum hydrides are often used as molecular weight regulators. Anotherdifference between the Nd catalysts and Ti, Co, or Ni, is that Nd is more efficientin hydrocarbon solvents than in aromatics (245). Presumably, benzene can com-pete with monomer for open coordination sites on the large f -block metal throughη6-coordination. Enhanced reactivity in hydrocarbon solvents makes this class ofcatalysts very amenable to commercial solution polymerization.

trans-1,4-Polybutadiene. Polybutadiene with a high trans microstructureis a crystalline resin-like material with a melt temperature of 95◦C for the alphaform and 145◦ for the beta form. A number of metal catalysts have been used toprepare high trans-polybutadiene including vanadium (246–249), titanium (250–252), cobalt (253), nickel (254,255), neodymium (256,257), and rhodium (258). Asis the case for the cis specific catalyst, these trans systems are typically comprisedof an insoluble metal halide or soluble metal carboxylate in conjunction with analuminum alkyl co-catalyst. In general, trans catalysts are less active than ciscatalysts.

Vanadium Catalysts. The vanadium system is heterogeneous in nature andcombines VCl3 with triethylaluminum in an Al/V ratio of 2–10/1 to achieve a99% trans-polybutadiene (259,260). Formation of high molecular weight polymercoupled with the highly crystalline nature of the material causes the polymerto precipitate during the polymerization. Encapsulation of active catalyst in theinsoluble polymer particles can be a conversion-limiting occurrence. From a pro-cess standpoint, dispersion of the heterogeneous catalyst particles becomes a veryimportant issue. Along these lines, mixed metal systems of VCl3 with TiCl3 ina 1:4 ratio have been found to create a highly dispersed active trans catalyst(261).

Cobalt Catalysts. Cobalt catalysts have the advantage of being readily avail-able as soluble metal salts. Combining cobalt carboxylates with diethyl aluminumchloride in the presence of a Lewis base such as triethylamine at an Al/Co/NR3 ra-tio of 10:1:10 results in reasonable conversion of butadiene to polybutadiene witha 95% trans content (253). Control of microstructure is possible through changes


in catalyst concentration and make-up allowing for a range of trans contents from72 to 95% with the rest of the polymer being made up of mostly 1,2-vinyl.

Nickel Catalysts. The use of a Lewis base to convert an otherwise cis cat-alyst into a trans catalyst has already been demonstrated for nickel (212–214).Similarly, the cis-specific nickel catalyst can also be converted into a trans catalystby replacing Ni–F complexes with Ni–I. Polymers obtained from these systems canhave up to 97% trans content (262).

Neodymium Catalysts. Neodymium catalysts are known to polymerize bu-tadiene to trans-polybutadiene in the absence of a halogen source. For example,when Nd(versatate)3 is treated with magnesium dialkyl in a critical ratio of 1:10,a polymer with 90–95% trans content is obtained (256). Molecular weights aretypically below 100,000 and the catalyst must be used at levels above 0.25 mm-phm to obtain reasonable conversion. More recently, a trans-specific system hasbeen described in which a neodymium phosphate is treated with four equivalentsof butyl lithium, yielding a high trans polymer in good conversion (257). Inter-estingly, a neodymium halide species treated with tributylphosphate and dibutylmagnesium also yielded high trans-polybutadiene (263).

Rhodium Catalysts. An extremely interesting system for the produc-tion of high trans-1,4-polybutadiene is based on a rhodium catalyst usedin an emulsion-type aqueous polymerization (258,264). In a typical recipe,rhodium(III)nitrate or rhodium(III)chloride is shaken in the presence of wa-ter, sodium dodecylbenzenesulfonate, and butadiene achieving 99% trans-1,4-polybutadiene, albeit in low conversion (265). The polymerization is very depen-dent on the emulsifier present, with the most active catalysts being generatedfrom long-chain alkyl sulfates or aryl sulfonates (266). In the absence of an alu-minum alkyl co-catalyst, the dependence on the sulfonate group may arise fromthe need for rhodium alkyls to form from the migration of an Rh-sulfonate bond(267). The kinetics of this system has been explored (268,269) and a very thoroughreview exists (270).

1,2-Polybutadiene. Unlike cis- and trans-1,4-polybutadiene, high vinyl1,2-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 amor-phous 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 thismaterial, commercialization has focused on anionic alkali metal initiators mod-ified with Lewis bases. Of the two crystalline forms, isotactic 1,2-polybutadienewith a melting temperature of 126◦C is the most elusive isomer. A few chromiumsystems based on soluble salts and aluminum alkyls have been reported to give45% of the isotactic polymer in a mixture of the atactic isomer (273,274).

Most Ziegler–Natta catalysts for high vinyl 1,2-polybutadiene yields syndio-tactic polymer with a melting temperature that ranges between 90 and 220◦Cdepending on the degree of crystallinity. The microstructure of this material wasfirst recognized by Natta (272) in 1955 and can be prepared with cobalt (275–280),vanadium (281), molybdenum (282,283), chromium (274,284), and titanium (285)salts treated with aluminum alkyl co-catalysts.

A high melting (>200◦C) material and a low melting material (90◦C),both produced from cobalt catalysts, have been commercialized by Ube


Industries (286) and Japan Synthetic Rubber Co. (JSR) (287) respec-tively. The high melting polymer is made from a preformed cobalt octoate/triisobutylaluminum/butadiene/carbon disulfide catalyst formed in a ratio of1:3:20:1 and charged at a level between 0.1 and 0.2 mmphm. High crystallinematerial having greater than 98% 1,2-microstructure is obtained with a melt tem-perature of 195–205◦C. Use of other modifiers such as diethyl fumarate results ina lower degree of crystallinity and a melt temperature of 150◦C (288). The very lowmelting syndiotactic 1,2-polybutadiene from JSR utilizes a cobalt–phosphine cat-alyst system. The catalyst is generated by treating bis(triphenylphosphine)cobaltdibromide 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 of90◦C. Mechanistic studies aimed at uncovering the differences in these systemshave been described (289,290).

A very unique system for the polymerization of syndiotactic 1,2-poly-butadiene with varying degrees of crystallinity and melts ranging from 120 to190◦C has been disclosed by The Goodyear Tire & Rubber Co (291). In this ex-ample, cobalt octoate treated with butadiene, triisobutylaluminum, and carbondisulfide has been shown to polymerize butadiene in an emulsion polymerization.This example represents a rare case of a Ziegler–Natta polymerization processtolerant of water. The lack of reactivity towards water could be due to the forma-tion 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 ofactive catalyst.

Frontiers of Ziegler–Natta Catalysts. There are a number of areas ofZiegler–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 polymer-ization. Much of this work can be tied to the discovery by Kaminsky andSinn (292–294) that methylaluminoxane (MAO) acts as an extremely efficientco-catalyst in metallocene-catalyzed ethylene polymerization. Examination ofMAO in Ziegler–Natta type conjugated diene polymerization includes work on cat-alysts 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 inplace of traditional trialkylaluminum co-catalysts has resulted in the formation ofhighly active cis catalysts even in the absence of a halogen source. The use of MAOwith homogeneous metallocene catalysts for butadiene polymerization has fo-cused mainly on cyclopentadienyltitanium trichloride systems (298,299,309–311).Although this system has comparable activity to traditional Ziegler–Natta cata-lysts, the overall cis content is only 80%. Unlike the plastics industry where met-allocenes and well-defined single-site catalysts have rapidly gained commercialsuccess, the use of these systems for polybutadiene production has not, as ofyet, matured. However, one area that is certain to be dominated by this classof catalyst is the area of gas-phase polymerization. In-roads to such a processfor high cis-polybutadiene have been made by Bayer through the use of an in-organic supported neodymium allyl/MAO complex (312). Similarly, supported cy-clopentadienyltitanium complexes, also activated with MAO, have been used forthe gas-phase polymerization of high trans-polybutadiene (313). Nickel (314) andcobalt (315) systems have also been disclosed.


Anionic Polybutadiene. Anionic polymerization (qv) is perhaps the mostversatile technology for producing polybutadiene in terms of microstructural andmacrostructural control. The relative stability of the anionic propagating centerallows for excellent control and manipulation of the conformation of enchainedmonomeric units and the overall structure of the polymer backbone. Coordi-nation catalyst technologies are best employed if highly stereospecific and/orhighly tactic microstructures are desired. Although high trans-1,4- and highcis-1,4-polybutadiene elastomers can be produced by a variety of Ziegler–Nattatechnologies (316), a wider range of thermal and rheological properties can berealized with anionic polymerization systems resulting in a broader range of ap-plications for this technology.

As mentioned previously, during World War I, alkali metals were investigatedearly in the search for a suitable elastomeric substitute for natural rubber. Ger-man and Russian researchers used sodium metal to polymerize dienes (317,318),but these materials proved ill-suited for tire applications. An alkali metal cata-lyst was later developed by Firestone researchers (319), which was used to producecis-1,4-polybutadiene on a commercial scale. Alkyl lithium initiators have sincebeen utilized to produce very high molecular weight polybutadienes with approx-imately 70% cis-1,4 structures. Lithium-based initiators produce the lowest vinylcontent and highest 1,4-enchainment when compared to the other alkali metals.These properties were desired when efforts to emulate natural rubber dominatedsynthetic rubber research. Presently, research efforts have shifted focus from lowvinyl products to higher vinyl, higher glass-transition temperature materials. Thisshift follows a trend toward higher performance tires with improved traction andhandling characteristics, while not sacrificing wear and fuel economy properties.Anionically prepared polybutadienes with elevated vinyl contents and controlledlong-chain branching characteristics can be used in many applications when abalance between traction and fuel economy is desired.

Anionic polymerization of 1,3-butadiene allows for a wide range of hetero-tactic vinyl enchainment levels. The structures obtained in hydrocarbon solventswith alkali metal initiators are shown in Table 9. Lithium polybutadiene main-tains the lowest vinyl, highest 1,4 content possible via anionic polymerization. Themicrostructure produced via a free-radical emulsion polymerization is providedas a reference.

Polar modifiers are most often employed to control the level of vinyl struc-tures formed during living lithium-based anionic polymerization processes in non-polar solvents. Relatively small amounts of added polar modifier, based on the

Table 9. Microstructure of Alkali Metal Initiated Polybutadienesa

Catalyst or condition cis-1,4, % trans-1,4, % 1,2, %

Lithium 35 52 13Sodium 10 25 65Potassium 15 40 45Rubidium 7 31 62Cesium 6 35 59Emulsion 18 64 18aRefs. 320 and 321.


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

stoichiometry of anionic propagating centers, can have profound vinyl-directingeffects (322,323). The vinyl content correlates directly to the glass-transition tem-perature, as seen in Figure 3 (271) and in other references (324,325). Highermodifier to chain-end ratios produce higher vinyl levels. The vinyl-directingstrength of most polar modifiers can also be a function of polymerization tem-perature (326).

Increased polymerization temperatures decrease the vinyl-directingstrength and result in lower glass-transition temperatures for the product. Incommercially produced polybutadiene, the vinyl content can be controlled by thechoice of polar modifier, its relative concentration to the propagating centers, andthe polymerization temperature. Polybutadiene products with elevated vinyl con-tents can be made at normal polymerization temperatures (50–100◦C) with manyconventional polar modifiers, including tetrahydrofuran (327), chelating diamines(328), bi-1,3-dioxanes (329), bisdipiperidinoethane (330), and alkyl ethers derivedfrom tetrahydrofurfuryl alchohols (331). Recently, with varying success, attemptshave been made to model the vinyl-directing strengths of polar modifiers basedon a number of parameters (332,333).

Polymerization propagation rate constants for unmodified lithium systemshave a fractional order dependence on chain-end concentration and are relativelyslow when compared to modified systems. The dienyl-lithium chain end has atendency to aggregate in hydrocarbon solutions (334), and give lower polymeriza-tion rates. The order of aggregation is reported to be dependent on factors suchas chain-end concentration (335,336), monomer concentration, temperature, andsolvent (337). The use of polar modification not only increases vinyl contents, butgenerally interferes with aggregation and drives the equilibrium to monomericchain-end species with subsequent increases in the polymerization rates.

In addition to the use of specific Lewis bases to influence the vinyl contentof products formed via anionic polymerizations, the use of alkali metal alkoxidesalts in lithium polymerizations can also affect the ratio of vinyl enchainment(338). Salts of lithium have little to no effect on microstructure at typical molecu-lar weights and commercial polymerization temperatures, while those of sodium,potassium, and rubidium increase the vinyl content significantly. The addition of


polar solvents into these anionic systems also increases the vinyl content of theproduct.

Commercial lithium-based anionic polybutadienes at typical molecularweights and polymerization temperatures, regardless of polar modification, havecis-/trans-1,4 ratios of approximately 0.5–0.7. Higher cis contents can be attainedfor unmodified polybutadiene in solution at very low initiator concentrations (veryhigh molecular weights) or in neat monomer (339). High trans-1,4 structure canalso be promoted in lithium-based systems. The use of barium salts with lithiumcan 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 ofdibutylmagnesium or alkyl lithium and potassium salts (342,343). The productsof these systems, polymerized in hydrocarbon solvents, displayed both a solublehigh vinyl fraction and an insoluble high trans-1,4 fraction.

Low Vinyl Polybutadiene. Typical glass-transition temperatures range be-tween −95 and −80◦C for commercial unmodified to slightly modified lithiumpolybutadiene (10–30% vinyl). These products are made by a variety of producersvia 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 de-scribing the anionic polymerization to make low vinyl polybutadiene have beenpublished (323,327,344,345). These materials generally show good wear proper-ties because of the low glass-transition temperatures when compounded in tireformulations. Hysteresis is generally lower than higher vinyl products but re-mains a function of the long-chain branching content. Low vinyl polybutadienesdo have lower green strengths than the high cis-1,4 Ziegler–Natta catalyzed prod-ucts, because of a lack of strain crystallization. However, where amorphous char-acteristics are desired, optimally compounded carbon black stock prepared withlithium polybutadiene shows very little difference in performance compared tothe Ziegler–Natta catalyzed high cis-1,4 material. It is believed that the essen-tial requirement is a high overall 1,4 content regardless of whether it is primarilycis or more equally distributed between cis and trans. High cis-1,4-polybutadienes(Ziegler–Natta) do show improved tack and process more easily, primarily becauseof macrostructural differences including broader molecular weight distributionsand higher branching levels.

Medium and High Vinyl Polybutadiene. Many tire performance propertiesare 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 trac-tion is increased, but abrasion resistance is compromised (347). Again, hystereticproperties are more a function of macrostructure than glass-transition tempera-ture. Figure 4 depicts the trends in tire performance properties as a function ofpolybutadiene vinyl content (271).

An interest in the medium to high vinyl polybutadienes has been encour-aged by rising styrene monomer prices. Medium vinyl polybutadienes of similarTg offer an alternative to poly(styrene–butadiene) copolymers in the tire industryand can also be used in other applications, including shoe soles, conveyer belts,and other engineered rubber products (348). Commercial medium vinyl materi-als typically display glass-transition temperatures in the −75 to −55◦C range(35–55% vinyl). The vinyl content is increased relative to low vinyl polybutadieneby polar modification of the living anionic chain end.


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

Amorphous high vinyl polybutadienes (>70% vinyl) have glass-transitiontemperatures higher than −40◦C (349). These materials are typically made withlithium initiators combined with a mixed alkali metal alkoxide and/or polar mod-ifier system. As a result, commercial high vinyl polybutadienes are typically quitebranched but display excellent traction properties. These materials are most oftenused in silica-filled tire applications.

Functionalization. Anionically prepared polybutadiene offers the uniqueadvantage of near quantitative chain-end functionalization. Through a variety offunctional initiating and/or terminating agents, a variety of reactive moieties canbe attached to the polymer termini (350). For example, there are several methodsavailable 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 tinfunctionality (353). Cyclic amine functionalization at either termini can providehysteresis improvements in carbon black filled compounds (354,355). The polymermacrostructure can also be modified by adding functional linking agents suchas tin (or silicon) tetrachloride. Tin-coupled polymers give benefits in processingand have been reported to also decrease compound hysteresis (356–358). Using acombination of functional initiators and terminators, polybutadienes containingtwo types of end groups can be synthesized (359), enabling greater control ofpost-conversion polymer chemistries.

Cationic Polymerization. Like anionic polymerization, cationic polymer-ization is an addition reaction involving a relatively stable ionic propagatingspecies. Cationic polymerization proceeds by an attack on the monomer by anelectrophilic species, resulting in heterolytic cleavage of a double bond producinga carbocation. In the case of olefins, the carbocation is of the carbenium form.The most complete reference to date acknowledging the importance of cationicpolymerization chemistry is a 1975 book (360).

Early research demonstrated that 1,3-butadiene can be polymerized bystrong organic acids (361) or Lewis acids at low temperatures (361–363). Typical


microstructures were primarily trans-1,4 linkages with the remainder beingmostly vinyl. For the synthesis of low molecular weight oils and resins, aliphaticethers were used in conjunction with metal halide/hydrogen halide mixtures (364).

Later studies reported polybutadiene produced from combinations of diethylaluminum chloride and various salt hydrates in benzene at 20◦C (125). These samestudies highlighted the disagreement between theoretical unsaturation amountsand that which were actually found. An intramolecular backbiting mechanismproducing pendent cyclohexene groups was postulated. An overview of many pa-pers proposing that cationically polymerized diene monomers by a mixed initi-ating system of magnesium bromide/titanium tetrachloride in benzene producedcyclized structures was published in 1969 (365). Structures inferred by infraredspectroscopy included cyclic ladder units alternating with linear trans-1,4 seg-ments that generate a heat-stable form of cyclopolybutadiene (126,366). Simi-lar studies using highly acidic alkyl aluminum chloride/titanium tetrachloridemixtures in n-heptanes produced insoluble polymer (367). The insolubility wasattributed to either cross-linking or a ladder-like structure comprised of joinedrings.

These cyclic structures were found to be inherent to most cationicallypolymerized dienes in nonpolar solvents (368). It has been reported that thesecyclic structures could also be produced by typical cationic polymerization in thepresence of ubiquitous proton-donating impurities, chain-transfer reactions (tomonomer and aromatic solvents), and more conventional elimination reactions(369).

In contrast, high molecular weight soluble cationically polymerized polybu-tadiene products can be produced with relatively high rates with the ion pair(C2H5)2Al+ C2H5AlCl3

− in conjunction with cobalt. By mixing either (C2H5)2AlClwith C2H5AlCl or (C2H5)3Al with AlCl3, this ion pair can produce high cis-1,4microstructures (>90%) (370).

It should be noted that the only cationically polymerized butadiene prod-uct commercialized is DuPont’s Budium®, which is an oligomeric resin used fortin-can linings. This product is produced using BF3·(C2H5)2O/H2O in hexane atlow temperature (371).

Polymerization Processes

Polybutadiene can be prepared by a number of processes, including bulk, solu-tion, suspension, emulsion, and gas-phase polymerizations. Of the commerciallysignificant polymerization processes, production of polybutadiene by solution tech-nologies are the most predominant. The relative popularity of these processes fol-low the limitations dictated by the chemistries most practiced when producingpolybutadiene commercially, including Ziegler–Natta, anionic, free-radical, andsingle-site technologies. Several of the more commercially relevant processes areoutlined in detail below.

Solution Polymerization Process. Solution polymerization processesare often used when polymerization thermodynamics are largely exothermic, asin the case of polybutadiene. The solvent not only acts as a diluent, but also allowsfor efficient transfer of the heat of polymerization to a heat sink. Given the proper


choice of solvent, lower solution viscosities can also be maintained. Polybutadi-ene polymerizations can be carried out in aliphatic, cycloaliphatic, or aromaticsolvents. Chain transfer to solvent may be a concern, as are impurities inherentto the solvent feed stream. Solution polymerization systems may be either batchor continuous in operation, with the broadness of the molecular weight distri-bution dependent on the configuration used. Ziegler–Natta and anionic polymer-ization technologies are the most common types used in solution polybutadieneproduction. Commercial high cis-polybutadiene can be made via either technol-ogy in solution systems, though Ziegler–Natta systems predominate by a widemargin. High trans-polybutadiene can also be produced by Ziegler–Natta cata-lysts. High vinyl elastomeric materials are typically produced via anionic solutionpolymerization.

Ziegler–Natta solution polymerization processes are very sensitive to impu-rities. Both the monomer and solvent streams must be well purified. Carbonyl andacetylenic impurities, common in crude monomer streams, must be removed alongwith the common polymerization inhibitor tert-butylcatechol. Molecular weightcontrol is often accomplished by the addition of chain-transfer agents in certainZiegler–Natta systems.

In anionic solution systems the feed stocks are typically dried over varioustypes of dessicants because the systems are sensitive to water contamination.When using continuous anionic solution polymerization systems, it is necessaryto employ low (ppm) concentrations of a chain-transfer agent in order to discouragegelation and fouling; 1,2-butadiene is often used for this purpose in commercialapplications. Alkyl-lithium-initiated polybutadiene is less prone to contain gel anddoes not contain the heavy metal catalyst residues associated with Ziegler–Nattacatalyzed products.

The polybutadiene produced by solution processes must be adequately des-olvated in order to maintain low residual monomer and solvent in the product.Steam stripping is often employed in commercial processes in order to recoverthe solvent for recycling, followed by mechanical drying (extruder) and/or hot airdrying of the wet polymer crumb.

Emulsion Polymerization. The free-radical polymerization of butadi-ene in a homogeneous system results in chain lengths that are too short forhigh quality elastomers. If the free-radical concentration in homogeneous me-dia is high enough to give a useful reaction rate, it is also high enough to favortermination reactions or monomer depletion before high molecular weights areachieved.

In contrast to homogeneous free-radical systems, emulsion free-radical poly-merization affords high propagation rates and high molecular weight products ofelastomeric quality (372,373). The propagating radical chains are somewhat phys-ically isolated and thus prevented from recombining as rapidly as they would insolution or bulk media. The emulsion process allows convenient temperature con-trol, which is advantageous in the highly exothermic polymerization of butadiene(1.4 kJ/g). Heat removal and temperature control are considerably more difficultat 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 con-sisting of monomer droplets dispersed in an aqueous phase is formed. The aqueous


phase contains the initiators, dispersed monomer droplets, and micelles formedfrom the emulsifier. Primary initiation centers, formed in the aqueous phase, dif-fuse into the micelles where polymerization proceeds (374,375). The polymer par-ticle formed within the micelle then tends to absorb monomer from the surround-ing aqueous phase because the monomer in the vicinity of the polymer particlehas been consumed by polymerization. Continued uptake of monomer results ingrowth of the polymer particle at the expense of the monomer droplets. As thepolymer particles grow, the emulsifier adsorbs on the increasing surface area andeventually leads to the disappearance of micellar soap as well as the monomerdroplet phase. Relatively little polymer is believed to form in the aqueous phaseor in the monomer droplet phase.

In a continuous process, butadiene, soap, initiator, and activator (an auxiliaryinitiating agent) are pumped continuously from storage tanks through a series ofagitated reactors at such a rate that the desired degree of conversion is reachedat the last reactor. A terminator is added, the latex warmed with steam, andthe unreacted butadiene flashed off. After addition of an antioxidant, the latex iscoagulated by the addition of brine, followed by dilute sulfuric acid or aluminumsulfate. The coagulated crumb is washed, dried, and baled for shipment.

Gas-Phase Polymerization Processes. Gas-phase polymerization isthe newest process in development for the commercial polymerization of conju-gated dienes. Although primarily utilized for the polymerization of ethylene andpropylene monomers, commercial gas-phase processes are being extended to in-clude the manufacture of polybutadiene. Many polymer manufacturers have re-ported researching gas-phase processes for diene monomers, and several haveestablished significant patent portfolios including Amoco, Bayer, Exxon, Mitsui,and Union Carbide. To date, Bayer appears to be closest to commercializing agas-phase process for producing polybutadiene rubber (376,377).

The purported benefits of gas-phase technology include the reduction in over-all waste streams, including no waste water, lower solid waste, and reduced overallemissions. Solvent recycling is no longer necessary, and costly polymer product iso-lation and drying steps are not required. Products are typically isolated from thereactor as powder or crumb. Production costs have the potential to be significantlylower than either solution or emulsion technologies.

The chemistries utilized in gas-phase technologies employ the sameZiegler–Natta (314,315) and single-site (metallocene) catalysts (313) describedin the processes included below. In gas-phase systems, however, the catalystsare generally solid-supported, but produce the same range of polybutadiene mi-crostructures inherent to the nonsupported catalyst. Several patents also includeanionic polymerization systems as useful in gas-phase processes (378). Kineticmodeling work has also been done to better predict the gas-phase polymerizationbehavior of 1,3-butadiene (379).

Economic Aspects

The economics of polybutadiene production are characterized by overcapacity ona 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


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

Capacity Production Net trade Consumption

United States 675 600 51 544Western Europe 498 332 −53 385Japan 344 290 116 191Other 1463 880 −118 837Total 3019 2102 0 1957aRef. 380.

Table 11. Major Producers of Polybutadiene Elastomersa

Annual capacityas of January World

Company and plant location 2000, 103 t capacity, %

Russian companies 395 13Bayer AG (Canada, France, Germany, and United States) 448 15Chinese producers 341 12The Goodyear Tire & Rubber Co. (United States) 230 8Michelin (France) 220 7Enichem (Italy and United Kingdom) 145 5Bridgestone/Firestone (Japan and United States) 134 5Korea Kumho Petrochemical Co., Ltd. (Republic of Korea) 167 6Other 882 29Total 2962 100aRef. 380.

resulting in a drop in global capacity utilization from 82 to 70% (380). Globalproduction figures are tabulated in Table 10. Major polybutadiene producers arelisted in Table 11.

The average annual growth rate in consumption between 1990 and 1999was 2.0% per year on a global basis. However, because of an unusual increaseof natural rubber prices in 1994, polybutadiene use in tires in the United Statesincreased by 6.1% per year from 1994 to 1996 (380). This was, however, followed bythe return of natural rubber prices to their historic norm and a resulting decreasein polybutadiene use in tires. During the period from 1996 to 1999, tire shipmentsincreased by 4.3% whereas polybutadiene use in tires lagged behind—increasingat only 2.5% per year (380). While the combination of predominately small cars,radial tires, and low car sales drastically reduced butadiene production in theearly 1980s, in the 1990s this was followed by a significant market shift to largervehicles, sport-utility vehicles (SUV), and high performance tires. Of the trends inconsumer demand, only high mileage tires demand high levels of polybutadienein the tread compound. High performance tires require higher Tg materials andSUV tires place traction and durability demands on compounds that cannot bemet by polybutadiene.

The portion of polybutadiene going into nontire applications continues toincrease. Between 1980 and 1999, the portion of polybutadiene consumed in tire


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

Clear rubber Clear plastics Oil extended Oil-blackgrade grade 37.5 phr oilc masterbatchd

Jan. 1982 1.49–1.65 1.53–1.67 1.54 1.10Oct. 1989 1.49–1.76 1.53–1.80 1.69 1.23Jan. 1993 1.49–1.76 1.53–1.80 1.67 1.21Jan. 1996 1.71–1.94 1.75–1.94 1.90 1.39–1.50Feb. 2000 1.69–1.96 1.72–1.96 1.85–1.94 1.39–1.50aRef. 380.bPrices are FOB Gulf Coast manufacturing plant, freight collect, for truckload and car quantities.cParts of highly aromatic oil.dContains 77 parts per hundred of resin (phr) of N-302 or N-303 carbon black and 53 phr of highlyaromatic oil.

applications decreased by 11–67% of world production. Use of polybutadiene inhigh impact polystryene modification and ABS resin manufacture account for15–20% of global demand (380,381). Polybutadiene used in these applicationsmust be essentially gel free and colorless. The remaining markets for polybutadi-ene are divided among industries including golf balls and footwear and assortedindustrial products including, but not limited to, conveyor belts, v-belts, seals, gas-kets, and wire insulation. In the mid 1990s the number of rounds of golf playedin the United States increased at roughly a 10% annual rate (380). However, thisrate of growth has not been sustained in the latter part of the 1990s and into the2000s with total rounds of golf being flat in recent years. Despite a leveling off inrounds of golf played in the latter part of the 1990s and into 2000, changes in ballconstruction, 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 polymersold. Typical United States prices are given in Table 12.

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