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Polybutadiene
Polybutadiene
Polybutadiene
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About 70% of polybutadiene is used in tire manufacturing

Polybutadiene [butadiene rubber, BR] is a synthetic rubber. It offers high elasticity, high resistance to wear, good strength even without fillers, and excellent abrasion resistance when filled and vulcanized. "Polybutadiene" is a collective name for homopolymers formed from the polymerization of the monomer 1,3-butadiene. The IUPAC refers to polybutadiene as "poly(buta-1,3-diene)". Historically, an early generation of synthetic polybutadiene rubber produced in Germany by Bayer using sodium as a catalyst was known as "Buna rubber". Polybutadiene is typically crosslinked with sulphur, however, it has also been shown that it can be UV cured when bis-benzophenone additives are incorporated into the formulation.[1]

Polybutadiene rubber (BR) accounted for about 28% of total global consumption of synthetic rubbers in 2020, whereas styrene-butadiene rubber (SBR) was by far the most important grade (S-SBR 12%, E-SBR 27% of the entire synthetic rubber market). It is mainly used in the manufacture of tires, which consumes about 70% of the production. Another 25% is used as an additive to improve the toughness (impact resistance) of plastics such as polystyrene and acrylonitrile butadiene styrene (ABS).[2] Polybutadiene is also used to manufacture golf balls, various elastic objects and to coat or encapsulate electronic assemblies, offering high electrical resistivity.[3]

History

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Sergei Vasilyevich Lebedev, a Russian chemist, was the first to polymerize butadiene

The Russian chemist Sergei Vasilyevich Lebedev was the first to polymerize butadiene in 1910.[4][5] In 1926 he invented a process for manufacturing butadiene from ethanol, and in 1928, developed a method for producing polybutadiene using sodium as a catalyst.

The government of the Soviet Union strove to use polybutadiene as an alternative to natural rubber and built the first pilot plant in 1930,[6] using ethanol produced from potatoes. The experiment was a success and in 1936 the Soviet Union built the world's first polybutadiene plant in which the butadiene was obtained from petroleum. By 1940, the Soviet Union was by far the largest producer of polybutadiene with 50,000 tons per year.[7]

Following Lebedev's work, other industrialized countries such as Germany and the United States developed polybutadiene and SBR as an alternative to natural rubber.

In the mid-1950s there were major advances in the field of catalysts that led to the development of an improved versions of polybutadiene. The leading manufacturers of tires and some petrochemical companies began to build polybutadiene plants on all inhabited continents; the boom lasted until the 1973 oil crisis. Since then, the growth rate of the production has been more modest, focused mainly in the Far East.

In Germany, scientists from Bayer (at the time a part of the conglomerate IG Farben) reproduced Lebedev's processes of producing polybutadiene by using sodium as a catalyst. For this, they used the trade name Buna, derived from Bu for butadiene, Na for sodium (natrium in Latin, Natrium in German).[6] They discovered that the addition of styrene to the process resulted in better properties, and thus opted for this route. They had invented styrene-butadiene, which was named Buna-S (S for styrene).[8][9]

Although the Goodrich Corporation had successfully developed a process for producing polybutadiene in 1939,[10] the U.S. federal government opted for the use of Buna-S to develop its synthetic-rubber industry after its entry into the World War II,[6] using patents of IG Farben obtained via Standard Oil. Because of this, there was little industrial production of polybutadiene in America during this time.

After the war, the production of synthetic rubber was in decline due to the decrease in demand when natural rubber was readily available again. However, interest was renewed in the mid-1950s after the discovery of the Ziegler–Natta catalyst.[11] This method proved to be much better for tire manufacturing than the old sodium polybutadiene. The following year, Firestone Tire and Rubber Company was first to produce low cis polybutadiene using butyllithium as a catalyst.

The relatively high production costs were a hindrance to commercial development until 1960 when production on a commercial scale began.[11] Tire manufacturers like Goodyear Tire and Rubber Company[12] and Goodrich were the first to produce plants for high cis polybutadiene, this was followed by oil companies like Shell and chemical manufacturers such as Bayer.

Initially, with plants built in the United States and France, Firestone enjoyed a monopoly on low cis polybutadiene, licensing it to plants in Japan and the United Kingdom. In 1965, the Japanese JSR Corporation developed its own low cis process and began licensing it during the next decade.

The 1973 oil crisis marked a halt to the growth of synthetic rubber production; the expansion of existing plants almost ceased for a few years. Since then, the construction of new plants has mainly taken place in industrializing countries in the Far East (such as South Korea, Taiwan, Thailand, and China), while Western countries have chosen to increase the capacity of existing plants.

In 1987, Bayer started to use neodymium-based catalysts to catalyze polybutadiene. Soon thereafter other manufacturers deployed related technologies such as EniChem (1993) and Petroflex (2002).

In the early 2000s, the synthetic rubber industry was once again hit by one of its periodic crises. The world's largest producer of polybutadiene, Bayer, went through major restructuring as it was troubled by financial losses; between 2002 and 2005 it closed its cobalt-polybutadiene plants in Sarnia (Canada) and Marl (Germany),[13] transferring their production to neodymium plants in Port Jérôme (France) and Orange (USA).[14] During the same time, the synthetic rubber business was transferred from Bayer to Lanxess, a company founded in 2004 when Bayer spun off its chemicals operations and parts of its polymer activities.[15]

Polymerization of butadiene

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1,3-Butadiene is an organic compound that is a simple conjugated diene hydrocarbon (dienes have two carbon-carbon double bonds). Polybutadiene forms by linking many 1,3-butadiene monomers to make a much longer polymer chain molecule. In terms of the connectivity of the polymer chain, butadiene can polymerize in three different ways, called cis, trans and vinyl. The cis and trans forms arise by connecting the butadiene molecules end-to-end, so-called 1,4-polymerisation. The properties of the resulting isomeric forms of polybutadiene differ. For example, "high cis"-polybutadiene has a high elasticity and is very popular, whereas the so-called "high trans" is a plastic crystal with few useful applications. The vinyl content of polybutadiene is typically no more than a few percent. In addition to these three kinds of connectivity, polybutadienes differ in terms of their branching and molecular weights.

The trans double bonds formed during polymerization allow the polymer chain to stay rather straight, allowing sections of polymer chains to align to form microcrystalline regions in the material. The cis double bonds cause a bend in the polymer chain, preventing polymer chains from aligning to form crystalline regions, which results in larger regions of amorphous polymer. It has been found that a substantial percentage of cis double bond configurations in the polymer will result in a material with flexible elastomer (rubber-like) qualities. In free radical polymerization, both cis and trans double bonds will form in percentages that depend on temperature. The catalysts influence the cis vs trans ratio.

Types

[edit]

The catalyst used in the production significantly affects the type of polybutadiene product.

Typical polybutadiene composition by catalyst[16]
Catalyst Molar proportion (%)
cis trans vinyl
Neodymium 98 1 1
Cobalt 96 2 2
Nickel 96 3 1
Titanium 93 3 4
Lithium 10–30 20–60 10–70

High cis polybutadiene

[edit]

This type is characterized by a high proportion of cis (typically over 92%)[17] and a small proportion of vinyl (less than 4%). It is manufactured using Ziegler–Natta catalysts based on transition metals.[18] Depending on the metal used, the properties vary slightly.[16]

Using cobalt gives branched molecules, resulting in a low viscosity material that is easy to use, but its mechanical strength is relatively low. Neodymium gives the most linear structure (and therefore higher mechanical strength) and a higher percentage of 98% cis.[19] Other less-used catalysts include nickel and titanium.[16]

Low cis polybutadiene

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Using an alkyllithium (e.g. butyllithium) as the catalyst produces a polybutadiene called "low cis" which typically contains 36% cis, 54% trans and 10% vinyl.[18]

Despite its high liquid-glass transition, low cis polybutadiene is used in tire manufacturing and is blended with other tire polymers, also it can be advantageously used as an additive in plastics due to its low contents of gels.[20]

High vinyl polybutadiene

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In 1980, researchers from the Japanese company, Zeon, discovered that high-vinyl polybutadiene (over 70%), despite having a high liquid-glass transition, could be advantageously used in combination with high cis in tires.[21] This material is produced with an alkyllithium catalyst.

High trans polybutadiene

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Polybutadiene can be produced with more than 90% trans using catalysts similar to those of high cis: neodymium, lanthanum, nickel. This material is a plastic crystal (i.e. not an elastomer) which melts at about 80 °C. It was formerly used for the outer layer of golf balls. Today it is only used industrially, but companies like Ube are investigating other possible applications.[22]

Other

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Metallocene polybutadiene
[edit]

The use of metallocene catalysts to polymerize butadiene is being explored by Japanese researchers.[23] The benefits seem to be a higher degree of control both in the distribution of molecular mass and the proportion of cis/trans/vinyl. As of 2006, no manufacturer produces "metallocene polybutadiene" on a commercial basis.

Copolymers
[edit]

1,3-butadiene is normally copolymerized with other types of monomers such as styrene and acrylonitrile to form rubbers or plastics with various qualities. The most common form is styrene-butadiene copolymer, which is a commodity material for car tires. It is also used in block copolymers and tough thermoplastics such as ABS plastic. This way a copolymer material can be made with good stiffness, hardness, and toughness. Because the chains have a double bond in each and every repeat unit, the material is sensitive to ozone cracking.

Production

[edit]

The annual production of polybutadiene was 2.0 million tons in 2003.[18] This makes it the second most produced synthetic rubber by volume, behind the styrene-butadiene rubber (SBR).[16][24]

The production processes of high cis polybutadiene and low cis used to be quite different and were carried out in separate plants. Lately, the trend has changed to use a single plant to produce as many different types of rubber as possible, including, low cis polybutadiene, high cis (with neodymium used as a catalyst) and SBR.

Processing

[edit]

Polybutadiene rubber is seldom used alone, but is instead mixed with other rubbers. Polybutadiene is difficult to band in a two roll mixing mill. Instead, a thin sheet of polybutadiene may be prepared and kept separate. Then, after proper mastication of natural rubber, the polybutadiene rubber may be added to the two roll mixing mill. A similar practice may be adopted, for example, if polybutadiene is to be mixed with Styrene Butadiene Rubber (SBR). *Polybutadiene rubber may be added with Styrene as an impact modifier. High dosages may affect clarity of Styrene.

In an internal mixer, natural rubber and/or styrene-butadiene rubber may be placed first, followed by polybutadiene.

The plasticity of polybutadiene is not reduced by excessive mastication.

Uses

[edit]

The annual production of polybutadiene is 2.1 million tons (2000). This makes it the second most produced synthetic rubber by volume, behind styrene-butadiene rubber (SBR).[25]

Tires

[edit]
Racing tires

Polybutadiene is largely used in various parts of automobile tires; the manufacture of tires consumes about 70% of the world production of polybutadiene,[19][20] with a majority of it being high cis. The polybutadiene is used primarily in the sidewall of truck tires, this helps to improve fatigue to failure life due to the continuous flexing during run. As a result, tires will not blow out in extreme service conditions. It is also used in the tread portion of giant truck tires to improve the abrasion, i.e. less wearing, and to run the tire comparatively cool, since a very small proportion of the energy from flexing is converted into heat. Both parts are formed by extrusion.[26]

Its main competitors in this application are styrene-butadiene rubber (SBR) and natural rubber. Polybutadiene has the advantage compared to SBR in its lower liquid-glass transition temperature, which gives it a high resistance to wear and a low rolling resistance.[19][27] This gives the tires a long life and low fuel consumption. However, the lower transition temperature also lowers the friction on wet surfaces, which is why polybutadiene almost always is used in combination with any of the other two elastomers.[16][28] About 1 kg of polybutadiene is used per tire in automobiles, and 3.3 kg in utility vehicles.[29]

Plastics

[edit]

About 25% of the produced polybutadiene is used to improve the mechanical properties of plastics, in particular of high-impact polystyrene (HIPS) and to a lesser extent acrylonitrile butadiene styrene (ABS).[20][30] The addition of between 4 and 12% polybutadiene to polystyrene transforms it from a fragile and delicate material to a ductile and resistant one.

The quality of the process is more important in the use in plastics than in tires, especially when it comes to color and content of gels which have to be as low as possible. In addition, the products need to meet a list of health requirements due to its use in the food industry.

Golf balls

[edit]
A cross section of a golf ball; its core consists of polybutadiene

Most golf balls are made of an elastic core of polybutadiene surrounded by a layer of a harder material. Polybutadiene is preferred to other elastomers due to its high resilience.[31]

The core of the balls are formed by compression molding with chemical reactions. First, polybutadiene is mixed with additives, then extruded, pressed using a calender and cut into pieces which are placed in a mold. The mold is subjected to high pressure and high temperature for about 30 minutes, enough time to vulcanize the material.

The golf ball production consumes about 20,000 tonnes of polybutadiene per year (1999).[20]

Other uses

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  • Polybutadiene rubber may be used in the inner tube of hoses for sandblasting, along with natural rubber, to increase resilience. This rubber can also be used in the cover of hoses, mainly pneumatic and water hoses.
  • Polybutadiene rubber can also be used in railway pads, bridge blocks, etc.
  • Polybutadiene rubber can be blended with nitrile rubber for easy processing. However large use may affect the oil resistance of nitrile rubber.
  • Polybutadiene is used in the manufacturing of the high-restitution toy Super Ball.[32] Due to the high resilience property, 100% polybutadiene rubber based vulcanizate is used as crazy balls — i.e. a ball if dropped from 6th floor of a house will rebound up to 5½ to 6th floor (assuming no air resistance).
  • Polybutadiene is also used as binder in combination with an oxidizer and a fuel in various Solid Rocket Boosters such as Japan's H-IIB launch vehicle, and ESA's Ariane 5; commonly is employed as hydroxyl-terminated polybutadiene (HTPB) or carboxyl-terminated polybutadiene (CTPB).

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Polybutadiene

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Polybutadiene is a homopolymer derived from the of 1,3-butadiene, characterized by its high elasticity, excellent abrasion resistance, and low temperature, making it suitable for demanding mechanical applications. It exists in various microstructures, including high-cis (over 95% cis-1,4 units), medium-cis (around 40% cis-1,4, 50% trans-1,4, and 10% 1,2-vinyl), and high-trans forms, each influencing its properties such as flexibility, strength, and thermal stability. Produced primarily through using catalysts like , , or alkyllithium in solvents such as , polybutadiene ranks as the second most produced globally, with a production of approximately 4.5 million metric tons in 2024. Its key physical properties include a of about 0.90 g/cm³, a of 1.5178, and a temperature below -90°C, enabling low-temperature performance. Approximately 70% of polybutadiene is used in , where it is blended with or rubber to enhance wear resistance and reduce , while other applications include impact modification for and ABS plastics, conveyor belts, and golf ball cores.

History

Early Research and Discovery

The hydrocarbon 1,3-butadiene was first identified in 1886 by the English chemist Henry Edward Armstrong, who isolated it from the products of . This discovery established butadiene as a key with potential for , though its synthetic utility remained unexplored for decades. Early interest in butadiene stemmed from efforts to replicate , particularly during when natural rubber supplies were disrupted. The first attempts at polymerizing occurred in the 1910s, pioneered by Russian chemist Sergei Vasilyevich Lebedev, who achieved thermal polymerization in 1910, producing a dark, viscous material with limited rubber-like properties. These experiments, often involving heat or light exposure, typically yielded low-molecular-weight oils or brittle resins rather than elastomeric polymers suitable for practical use. Despite patents filed around this time, such as those by researchers in 1912 for sodium-catalyzed processes, the resulting materials lacked the elasticity and processability needed for commercial viability. In the 1930s, research advanced with the development of techniques in and the , driven by impending shortages. IG Farbenindustrie in conducted key experiments using radical-initiated of , producing homopolymers with mixed microstructures but inferior mechanical properties, including low elasticity and poor tensile strength. To address these limitations, researchers copolymerized with styrene, yielding Buna-S rubber—a copolymer that served as a wartime substitute for during . However, early polybutadiene homopolymers remained challenging due to uncontrolled 1,2-addition and syndiotactic structures, resulting in brittle or sticky materials that crystallized easily and exhibited inadequate elastomeric behavior.

Commercial Development and Milestones

The commercial development of polybutadiene accelerated in the post-World War II era, building on earlier free-radical methods from the war that produced low-cis variants unsuitable for high-performance applications. A key milestone occurred in 1955 when developed high-cis polybutadiene through lithium-based anionic polymerization, yielding a rubbery with superior elasticity comparable to . This breakthrough, detailed in a filed that year, enabled the production of stereoregular polymers with over 90% cis-1,4 content, addressing previous limitations in mechanical properties and paving the way for and industrial uses. In the late 1950s, cobalt-catalyzed processes were developed for cis-1,4-polybutadiene, achieving around 96% cis content with reduced branching. These methods were rapidly scaled for due to enhanced processability and wear resistance, facilitating commercial viability and integration into blends. Ziegler-Natta catalysts, particularly titanium-based systems, emerged in the late 1950s to produce trans-1,4-polybutadiene variants with crystalline structures suitable for rigid applications. Combinations such as TiCl₄ with aluminum alkyls yielded predominantly trans-1,4 configurations (up to 90%), as described in foundational work by , expanding polybutadiene's utility beyond elastomers. Global production of polybutadiene surged from less than 10,000 metric tons in 1960 to over 1 million metric tons annually by 1970, fueled by rising demand for synthetic rubbers in automotive and consumer goods sectors. This growth reflected the shift to stereospecific processes, with major expansions in the United States and . In the 1960s, developments in facilitated the production of high 1,2-vinyl polybutadiene variants for specialty resins and impact-modified plastics, enhancing global supply chains through technology transfers, particularly in .

Chemical Structure

Butadiene Monomer

1,3-Butadiene, the monomer for polybutadiene, has the C₄H₆ and a linear structure represented as CH₂=CH-CH=CH₂. This arrangement features two conjugated double bonds, classifying it as a conjugated diene, which facilitates 1,2-addition and 1,4-addition modes during to form the polymer chain. Industrially, 1,3-butadiene is primarily obtained as a from the thermal cracking of fractions, such as or production in steam crackers, yielding a mixed C₄ stream. The is isolated from this crude C₄ cut via using polar solvents like N-methylpyrrolidone (NMP), achieving a high purity level exceeding 99.5% to minimize impurities that could lead to unwanted branching in downstream . Physically, 1,3-butadiene is a colorless gas at with a of -4.4°C and a of 0.621 g/cm³ at 20°C. It is highly flammable, with a below -76°C, and exhibits reactivity toward oxygen in air, readily forming explosive peroxides that necessitate storage and handling under inert atmospheres to prevent auto-oxidation. In polybutadiene synthesis, the Diels-Alder reactivity of 1,3-butadiene as a enables its copolymerization with dienophiles like styrene or , but homopolymerization primarily involves the saturation of its conjugated double bonds through addition reactions to propagate the growing chain. Regarding safety, 1,3-butadiene is classified by the International Agency for Research on Cancer (IARC) as carcinogenic to humans (), based on sufficient evidence from epidemiological studies linking occupational exposure to increased risk.

Polymer Configurations and Microstructure

Polybutadiene, formed by the of 1,3-butadiene, exhibits diverse configurations depending on the mode of addition during synthesis. In 1,4-addition, the butadiene units link such that the repeating unit is −[CH2−CH=CH−CH2]−-\left[ \mathrm{CH_2-CH=CH-CH_2} \right]-, preserving a in the polymer backbone and resulting in a linear chain with potential for cis or trans around the . In contrast, 1,2-addition produces a repeating unit of −[CH2−CH(CH=CH2)]−-\left[ \mathrm{CH_2-CH(CH=CH_2)} \right]-, where the occurs at positions 1 and 2, yielding a backbone resembling with vinyl groups and unsaturation in the . These configurational differences fundamentally influence the polymer's overall architecture and physical behavior. The microstructure of polybutadiene is characterized by the relative proportions of these addition modes, specifically the percentage of 1,4-units (cis versus trans) and 1,2-units (with tacticity such as isotactic, syndiotactic, or atactic), which are dictated by the polymerization conditions including catalyst type, temperature, and solvent. For instance, coordination catalysts favor high cis-1,4 content, while certain anionic systems with additives like TMEDA promote 1,2-vinyl incorporation. This microstructural composition is typically quantified using techniques such as 13^{13}C NMR spectroscopy, where distinct chemical shifts distinguish between cis (around 27 ppm), trans (around 32 ppm), and vinyl units. The tacticity in 1,2-units further modulates chain regularity, with syndiotactic forms exhibiting higher crystallinity than atactic ones. These microstructural features profoundly affect the polymer's thermal and mechanical properties. Predominantly cis-1,4 polybutadiene forms amorphous elastomers with excellent flexibility and low temperatures, ideal for rubber applications. Trans-1,4 configurations lead to semi-crystalline structures with higher melting points and greater rigidity, enhancing strength but reducing elasticity. High 1,2-vinyl content results in glassy polymers with elevated temperatures due to the sterically hindered side chains, contributing to at low temperatures but improved compatibility in blends. Specific variants, such as high-cis forms, demonstrate superior resilience in treads compared to trans-rich ones. Molecular weight in polybutadiene is controlled primarily through the initiator concentration and reaction termination, yielding typical number-average values ranging from to 500,000 g/mol for industrial elastomers, which directly impacts and processing ease—higher weights increase melt , complicating extrusion but enhancing mechanical strength. In systems like anionic methods, the (DP) is given by DP=[Monomer][Initiator],\mathrm{DP} = \frac{[\mathrm{Monomer}]}{[\mathrm{Initiator}]}, allowing precise chain length tuning. The polydispersity index (PDI) varies from approximately 1.1 in controlled anionic polymerizations to around 2.0 in free-radical processes, reflecting narrower distributions in that improve uniformity and performance predictability.

Polymerization Methods

Free-Radical Polymerization

Free-radical polymerization of was historically pivotal during for producing on a large scale, particularly through the development of government rubber-styrene (GR-S) copolymers in the United States to address shortages. This method enabled rapid industrial scaling via processes, yielding high volumes of elastomer for tires and military applications. The mechanism begins with initiation, where water-soluble peroxides such as decompose thermally in aqueous to generate primary radicals that enter micelles and react with monomers. Propagation proceeds via allylic radicals formed by addition to the diene's , allowing resonance-stabilized growth through 1,2- or 1,4-addition modes; the allylic nature arises from delocalization over carbons 2 and 4 of the unit. Termination occurs primarily by radical recombination (coupling), though and can also contribute, leading to branching or low-molecular-weight species. Typical conditions involve aqueous at around 50°C, with soaps or above the to stabilize particles, a water-to-monomer ratio of 70:30 to 40:60, and initiator concentrations yielding 90% conversion. The resulting polybutadiene exhibits a random microstructure, approximately 20-30% 1,2-vinyl units and a mixed 1,4-content (around 35-45% cis-1,4 and 45-50% trans-1,4), influenced by where higher values favor more cis and vinyl units over trans. This approach offers advantages including low cost due to inexpensive initiators and water-based media, high yields approaching 90% conversion, and simultaneous achievement of high molecular weight and reaction rates in emulsion systems. However, it suffers from disadvantages such as broad polydispersity index (PDI >3) from irreversible termination and transfer reactions, and poor control over microstructure, resulting in inferior elasticity compared to stereospecific methods. The polymerization rate follows the standard form for radical chain growth, Rp=kp[M][R∙]R_p = k_p [M] [R^\bullet] where RpR_p is the rate of , kpk_p the rate constant, [M] the concentration, and [R•] the total radical concentration; at , [R•] = (R_i / 2k_t)^{1/2}, with R_i the rate. A effect (Trommsdorff-Norrish effect) accelerates the rate at high conversions (>30-50%) as increased hinders termination more than , leading to autoacceleration and potential runaway reactions. The output from free-radical polymerization is primarily incorporated into styrene-butadiene rubber (SBR) copolymers rather than pure polybutadiene, as the latter's random microstructure renders it tacky and difficult to process alone. In contrast to stereospecific methods, this process yields non-regular polymers suited for general-purpose rubbers but with limited resilience.

Coordination Polymerization

Coordination polymerization of employs catalysts, such as Ziegler-Natta systems, to produce stereoregular polybutadiene with controlled microstructures, particularly high cis-1,4 or trans-1,4 content. These methods enable precise insertion of the into the growing chain, yielding polymers suitable for applications requiring specific mechanical properties like elasticity in tires. Key catalysts include titanium-based systems like TiCl₄ combined with AlEt₃, which favor trans-1,4-polybutadiene with up to 95% trans content by promoting selective monomer coordination and insertion. For high cis-1,4 selectivity, dichloride (CoCl₂) complexes with organophosphine s, such as PPh₃ or PRPh₂ (R = alkyl), activated by alkylaluminum compounds like AlEt₂Cl or EASC, achieve cis-1,4 contents of 96% or higher, with depending on ligand sterics and hindrance. -based catalysts, notably neodymium versatate (NdV₃) with co-catalysts like DEAC and TIBA in ratios such as NdV₃:DEAC = 1:9, produce ultra-high cis-1,4 polybutadiene exceeding 98% cis content, offering superior and low formation. The choice of catalyst and ligands directly influences , as demonstrated by cis content reaching 96% in Co systems versus >98% in Nd systems. The mechanism involves coordination-insertion, where the butadiene monomer coordinates to the metal center, forming a π-allyl intermediate that inserts into the metal-carbon bond of the growing . This process enables 1,4-cis selectivity through stabilization of the syn-butenyl configuration in the π-allyl , with activation barriers around 12 kcal/mol for cis-1,4 insertion in analogs, adaptable to other metals like Nd or Co. propagation occurs via repeated binding and migratory insertion, maintaining high stereoregularity without significant branching. Polymerizations are typically conducted in solution using hydrocarbon solvents like or at 50–70°C under 1–5 , yielding high molecular weight polymers with M_w > 300,000 g/mol. These conditions, often with concentrations of 10–20 wt% and reaction times of 40–120 min, support efficient conversion while controlling . This approach provides excellent control over elasticity and microstructure for enhanced tire performance, including low and high abrasion resistance.

Anionic Polymerization

Anionic polymerization of is a living chain-growth process that utilizes organolithium initiators to produce polybutadiene with precisely controlled molecular weight and narrow polydispersity, enabling the synthesis of well-defined block copolymers. The most common initiator is (n-BuLi), which is employed in non-polar solvents such as or to generate active chain ends that propagate by to the . This method ensures minimal termination or , maintaining the "living" character of the chains throughout the reaction, which allows for quantitative initiation and predictable polymer architecture. The mechanism involves the of n-BuLi to form a butyl anion, which adds to the conjugated double bonds of , creating an allylic that propagates via repeated 1,2- or 1,4-addition steps. In non-polar solvents, the polymerization predominantly yields 1,4-addition products (approximately 92 wt%), with 1,2-vinyl units comprising the remainder (about 8 wt%). Within the 1,4-microstructure, the cis isomer becomes dominant at lower temperatures, such as -20°C, due to favoring the more stable cis configuration, while higher temperatures increase the trans content. The reaction is conducted under strict inert atmosphere (e.g., or ) to prevent of the sensitive organolithium , typically at temperatures between 20°C and 50°C, though lower temperatures enhance cis selectivity. This temperature range balances rate with control, achieving near-quantitative conversion in 1-2 hours. Molecular weight is directly controlled by the monomer-to-initiator ratio, following the relation: Mn=[M]×MWm[I]M_n = \frac{[M] \times MW_m}{[I]} where [M][M] is the initial monomer concentration, MWmMW_m is the molecular weight of butadiene (54 g/mol), and [I][I] is the initiator concentration, yielding number-average molecular weights (MnM_n) from thousands to hundreds of thousands g/mol. The polydispersity index (PDI) is typically below 1.2, reflecting the living nature and absence of side reactions under optimized conditions. The active chain ends can be selectively functionalized post-polymerization; for example, reaction with CO2_2 introduces carboxylic acid groups, enabling further coupling or compatibilization in composites. This polymerization technique is particularly valued for producing high-purity polybutadiene precursors free of metal residues, serving as the midblock in styrene-butadiene-styrene (SBS) triblock copolymers used in adhesives, footwear, and thermoplastic elastomers. Compared to coordination polymerization, anionic methods offer superior control over chain ends and molecular weight distribution but generally lower cis-1,4 content without additives.

Microstructural Variants

Cis-1,4-Polybutadiene

Cis-1,4-polybutadiene with a high cis content exceeding 90% is predominantly produced through employing (Co) or (Nd)-based Ziegler-Natta catalysts, which enable the formation of polymers with 96-99% cis-1,4 microstructure. These catalysts facilitate stereospecific insertion of the , resulting in the desired high-cis configuration essential for elastomeric performance. Key properties of high-cis cis-1,4-polybutadiene include a temperature (Tg) of -102°C, which contributes to its flexibility at low temperatures, along with exceptional resilience characterized by values greater than 90% at 50°C and low that minimizes heat buildup under . A distinctive feature is its ability to undergo stress-induced under strain, which improves green strength and processability by providing temporary structural reinforcement in the uncured state. In copolymers incorporating cis-1,4-polybutadiene, resilience correlates with the low Tg, predictable via the Fox equation for estimating the overall temperature: 1Tg=w1Tg1+w2Tg2\frac{1}{T_g} = \frac{w_1}{T_{g1}} + \frac{w_2}{T_{g2}} where w1w_1 and w2w_2 represent the weight fractions of the components, and Tg1T_{g1} and Tg2T_{g2} are their respective temperatures in .

Trans-1,4-Polybutadiene

Trans-1,4-polybutadiene refers to the stereoregular variant of polybutadiene featuring over 80% trans-1,4 linkages in its microstructure, resulting in a predominantly linear backbone that promotes semi-crystalline behavior. This configuration imparts characteristics, with the exhibiting rigidity and processability distinct from the more elastic cis-1,4 counterpart. The high trans content enables ordered packing into lattices, influencing its mechanical and responses in applications requiring balanced strength and flexibility. Synthesis of high-trans-1,4-polybutadiene typically employs titanium-based Ziegler-Natta catalysts, such as supported on MgCl₂ combined with alkylaluminum cocatalysts like Al(i-Bu)₃, in processes. These systems achieve trans-1,4 selectivities exceeding 90%, often up to 97.7% with optimized titanium loading, under conditions including temperatures around 70°C to control reaction kinetics and microstructure. The process favors trans addition through coordinated insertion at active titanium sites, yielding polymers suitable for subsequent . Key properties of trans-1,4-polybutadiene include a glass transition temperature (T_g) of approximately -90°C, reflecting its flexible chain segments below ambient conditions, and a solid-solid phase transition temperature of approximately 70°C from the monoclinic low-temperature crystal form to the hexagonal high-temperature form, with a melting point of about 145°C for the hexagonal structure. Crystallinity levels reach up to 40%, driven by the regular trans configuration, which enhances stiffness with a Young's modulus around 10 MPa—significantly higher than the 1-2 MPa typical of cis-1,4-polybutadiene. The degree of crystallinity (χ_c) is quantified via differential scanning calorimetry using the relation: χc=ΔHmΔHm0×100\chi_c = \frac{\Delta H_m}{\Delta H_m^0} \times 100 where ΔH_m is the observed melting enthalpy and ΔH_m^0 ≈ 210 J/g represents the value for a hypothetical perfect crystal. These attributes stem from the polymer's ability to form ordered domains, providing thermal stability and mechanical reinforcement without full elastomeric recovery. In contemporary use, high-trans variants (>80%) find niche roles in adhesives and soles, where their abrasion resistance and rigidity improve wear performance under stress. Conversely, low-trans content (<20%) is blended into compounds to optimize and durability alongside predominant cis structures.

1,2-Vinyl-Polybutadiene

1,2-Vinyl-polybutadiene, also known as high-vinyl polybutadiene, features a microstructure with greater than 50% 1,2-addition units, resulting in a backbone of alternating methylene and methine groups with pendant vinyl side chains (-CH=CH₂). This configuration contrasts with predominant 1,4-addition forms by introducing branching that restricts chain mobility and promotes rigidity. The of these 1,2-units—whether syndiotactic, isotactic, or atactic—significantly influences crystallinity and mechanical behavior, making it suitable for applications rather than elastomeric ones. High-vinyl 1,2-polybutadiene is primarily synthesized via using chromium-based catalysts, such as Cr(acac)₃ combined with alkylaluminum compounds, which selectively promote syndiotactic 1,2-insertion of monomers to yield polymers with over 90% 1,2-units and vinyl groups. Free-radical methods, often in systems, can also produce high 1,2-content variants, though these tend to be atactic with less regularity in side-chain orientation. For isotactic forms, anionic with organolithium initiators enables stereospecific 1,2-addition, producing highly regular chains. The properties of 1,2-vinyl-polybutadiene are characterized by a temperature (T_g) of approximately -20°C, varying slightly with ; syndiotactic variants around -25°C to -15°C due to partial crystallinity, while atactic forms are similar, around -20°C. These materials display high , approximately Shore D 70 in their state, but remain brittle without plasticizers to mitigate chain entanglement limitations; molecular weights (M_w) generally fall between 50,000 and 200,000 for processable resins. Isotactic 1,2-vinyl-polybutadiene, achieved through anionic methods, forms crystalline thermoplastics with melting points up to 90°C, and was investigated in the by researchers like as a potential alternative to conventional plastics due to its stereoregular and mechanical potential. The vinyl content directly impacts T_g through steric hindrance from side chains, leading to a linear relationship where T_g increases with percentage of 1,2-units: approximately T_g = T_{g,1,4} + k × (% vinyl), with k ≈ 0.8–1.0 °C per % vinyl relative to cis-1,4 baselines around -100°C. In applications, 1,2-vinyl-polybutadiene serves as an impact modifier in high-impact (HIPS), where its dispersed particles enhance toughness by 2–3 times compared to unmodified through improved energy dissipation, particularly when microstructures are tuned for compatibility during .

Industrial Production

Synthesis Processes

The dominant industrial synthesis route for polybutadiene is , which accounts for the majority of global production and employs continuous stirred-tank reactors with (Nd)- or (Co)-based catalysts to achieve high cis-1,4 content. In this process, 1,3-butadiene is fed at concentrations of 20-30 wt% in an organic solvent such as or , with the catalyst system typically comprising Nd versatate, alkylaluminum compounds like diisobutylaluminum hydride, and a source like ethylaluminum sesquichloride. The reaction proceeds at temperatures of 50-70°C under inert atmosphere, yielding polymers with over 96% cis-1,4 microstructure suitable for applications, and the process is optimized for continuous operation to ensure steady-state conversion rates exceeding 90%. An alternative route, , is utilized for producing low-cis polybutadiene grades, primarily in batch reactors where is emulsified in with and free-radical initiators like persulfates. Following polymerization at 5-10°C, the is coagulated using salt solutions or acids to precipitate the , and unreacted and residuals are recovered via stripping. This method, though less common for high-performance polybutadiene due to broader microstructure distribution, supports production of specialty latices for blends like ABS resins. Commercial polybutadiene plants operate at scales of 100,000 to 300,000 tons per year, with notable facilities including the PT Synthetic Rubber Indonesia plant in , a with a capacity of 120,000 tons annually, and similar installations in and by producers. Process optimizations focus on , recovering over 95% of unreacted through for reuse, alongside catalyst efficiencies where turnover numbers (TON) exceed 10,000 moles of butadiene per mole of Nd catalyst, and control of 1,2-vinyl content below 5% via precise catalyst aging and . consumption is approximately 1.05 kg per kg of produced, reflecting high conversion in modern plants. Energy requirements for polybutadiene synthesis range from 70-100 MJ per kg of , encompassing heating, recovery, and downstream separation. Laboratory-scale methods, such as small-batch anionic or coordination polymerizations, inform these industrial designs but are not directly scaled for commercial output. As of 2025, expansions such as Evonik's increase in production capacity for silane-functionalized polybutadienes underscore ongoing advancements in specialized synthesis.

Processing and Compounding

Polybutadiene rubber (BR) is typically compounded using internal mixers such as the Banbury mixer to incorporate fillers, oils, and other additives, ensuring uniform dispersion and optimal processing characteristics. In standard formulations for treads and other applications, is added at levels around 50 parts per hundred rubber (phr) to enhance reinforcement and abrasion resistance, while processing oils are incorporated at 10-20 phr to improve flow and reduce during mixing. Accelerators, such as sulfenamides or thiazoles, are included alongside to facilitate efficient , with typical accelerator loadings of 1-2 phr to promote formation without excessive scorch. Following , the rubber mixture is processed into usable forms via methods like for creating profiles, hoses, or treads, and calendering for producing sheets or plies. These techniques rely on the compound's Mooney viscosity, which is controlled in the 40-60 range (ML 1+4 at 100°C) to ensure adequate flow and prevent defects during shaping; higher viscosities may lead to poor dispersion, while lower ones can cause excessive tackiness. The mixing process typically operates at temperatures below 120°C to avoid premature crosslinking, with cycle times of 4-6 minutes for complete incorporation of additives. Vulcanization transforms the compounded BR into a durable by heating at approximately 150°C for 10-30 minutes, depending on thickness and formulation, using 1-3 phr to form polysulfidic crosslinks that boost tensile strength to around 20 MPa. This level provides a balance between elasticity and modulus, with zinc oxide (3-5 phr) and (1-2 phr) acting as activators to accelerate the reaction. Post-vulcanization, the material exhibits improved mechanical integrity suitable for high-stress applications. Quality control in BR processing emphasizes purity and stability, with ash content limited to less than 0.5% and volatiles below 0.3% to minimize impurities that could affect performance or aging. Antioxidants like N-isopropyl-N'-phenyl-p-phenylenediamine (IPPD), added at 1-2 phr during compounding, are critical for preventing oxidative degradation in polybutadiene's unsaturated structure, particularly in high-cis variants prone to thermal oxidation. Routine tests include rheometer analysis for cure characteristics and viscometry for consistency. Industrial handling generates 5-10% scrap from trimming and rejects, which is often recycled through devulcanization processes like or thermomechanical methods to break sulfur crosslinks and reclaim the for in lower-grade compounds, reducing and environmental impact. This recycling step maintains material value while adhering to goals in rubber production.

Physical and Chemical Properties

Mechanical Properties

Polybutadiene exhibits a range of mechanical properties that vary significantly with its microstructure, particularly the cis-1,4, trans-1,4, and 1,2-vinyl content, as well as and . cis-1,4-polybutadiene, the most common variant for elastomeric applications, demonstrates tensile strength in the range of 15-25 MPa, with elongation at break typically between 400% and 600%. Its is relatively low at 2-5 MPa, reflecting the material's high flexibility and rubbery nature under low strain. In contrast, trans-1,4-polybutadiene shows higher with a approaching 3 MPa but lower elongation, making it less suitable for high-deformation uses. The 1,2-vinyl variant tends to have intermediate properties, with reduced elongation compared to cis due to its more rigid chain segments. Fatigue resistance is notably high in cis-1,4-polybutadiene, capable of enduring up to 10^6 cycles at 100% in vulcanized forms, attributed to its ability to dissipate efficiently without crack propagation. This performance stems from low , where loss per cycle is less than 10%, minimizing heat buildup and structural damage during repeated deformation. Trans-1,4 variants exhibit poorer life due to higher crystallinity even at rest, leading to brittle failure under cyclic loading, while 1,2-vinyl structures show moderate resistance but increased from polar side chains. Abrasion resistance is a key strength of polybutadiene, particularly in applications, where cis-1,4 variants achieve a DIN abrasion index of 150-200, indicating low volume loss under standardized conditions. This outperforms in wet conditions, where polybutadiene maintains structural integrity better due to its lower coefficient of friction and resistance to hydrodynamic tearing. Trans-1,4-polybutadiene offers comparable dry abrasion resistance but underperforms in wet scenarios owing to its reduced resilience. A distinctive feature of cis-1,4-polybutadiene is , which occurs above 400% strain and dynamically increases the modulus, enhancing load-bearing capacity during extension. This can be modeled as σ=Eε+f(χ)\sigma = E \varepsilon + f(\chi), where σ\sigma is stress, EE is the initial modulus, ε\varepsilon is , and f(χ)f(\chi) represents the contribution from crystallinity χ\chi. Trans and vinyl variants lack this pronounced effect, resulting in more linear -strain behavior without upturn. Tensile and dynamic properties are evaluated using ASTM D412, which specifies dumbbell-shaped specimens tested at controlled rates. Performance is optimal between 0°C and 60°C, where cis-1,4-polybutadiene maintains elasticity without excessive softening or premature . Outside this range, modulus decreases at higher temperatures due to increased chain mobility, while low temperatures promote static , reducing .

Thermal and Chemical Stability

Polybutadiene exhibits varying thermal stability depending on its microstructure. The glass transition temperature (Tg) for cis-1,4-polybutadiene is approximately -100°C, enabling flexibility at low temperatures, while trans-1,4-polybutadiene has a Tg around -95°C. These low Tg values contribute to the polymer's utility in applications requiring resilience across a broad temperature range, with typical service temperatures spanning -50°C to 100°C. Thermal decomposition begins above 300°C via random chain scission, leading to fragmentation and volatile products, with significant mass loss observed around 400-500°C under inert conditions. The degradation kinetics follow the Arrhenius equation, k=Ae−Ea/RTk = A e^{-E_a / RT}, where the activation energy EaE_a for cis-polybutadiene is approximately 215 kJ/mol. Oxidative stability of polybutadiene is inherently poor due to its unsaturated backbone, which readily forms hydroperoxides upon exposure to oxygen, accelerating chain scission and embrittlement without protective additives. exposure causes surface cracking in polybutadiene elastomers, but this vulnerability can be mitigated by incorporating waxes that bloom to the surface and form a protective barrier. High-vinyl (1,2-) polybutadiene grades show enhanced UV stability compared to cis-1,4 variants, attributed to their potential for saturation or crosslinking that reduces photo-oxidative degradation sites. Chemically, polybutadiene demonstrates resistance to and dilute acids, with negligible due to its hydrophobic nature, making it suitable for non-aqueous environments. However, it swells significantly in hydrocarbons and is fully soluble in , limiting its use in solvent-exposed applications.

Applications

Polybutadiene rubber (BR) plays a pivotal role in , particularly in tread compounds where it is typically incorporated at 20-30% by weight, blended with (NR) or rubber (SBR) to enhance overall performance. In passenger car tires, common formulations use around 25% high-cis BR with 75% SBR, while truck tires often employ 20-30% high-cis BR with 70-80% NR to balance durability and efficiency. This incorporation significantly reduces , contributing to improved fuel economy by lowering energy dissipation in the tread. High-cis polybutadiene, with over 90% cis-1,4 content, is favored for its contribution to wet grip through enhanced viscoelastic properties. It also exhibits low heat buildup, with temperature rises typically under 50°C during use, which minimizes losses and extends life. Typical tread formulations include 20-70 parts per hundred rubber (phr) of BR, combined with 50 phr for reinforcement, along with curatives like and accelerators. Post-1990s advancements introduced silica-reinforced variants, where 30-50 phr silica replaces or supplements in BR/SBR blends, improving wet traction while maintaining low , as pioneered in "green tire" technologies. In truck tires, high-trans BR blends are utilized to boost longevity, enabling mileage exceeding 100,000 km through superior abrasion resistance and cut growth prevention. The global industry consumes about 75% of polybutadiene production, totaling approximately 3.3 million tons annually as of 2024, underscoring its dominance in vehicle applications.

Polymer Blends and Plastics

Polybutadiene functions as a key impact modifier in thermoplastics, particularly in high-impact polystyrene (HIPS), where it is typically incorporated at 3-20 wt% to form dispersed rubber particles that significantly enhance material toughness. These rubber domains, ranging from 1-5 μm in size, create a bimodal morphology when combined with sub-micrometer core-shell particles, promoting energy absorption through and shear banding mechanisms that prevent brittle . For instance, maintaining polybutadiene content at around 17 wt% in HIPS blends yields impact strengths of around 150-200 J/m, far surpassing unmodified . In acrylonitrile-butadiene-styrene (ABS) copolymers, polybutadiene serves a similar role, with typical contents of 10-20 wt% enabling notched impact strengths of 70-370 J/m, representing up to a 10-fold improvement over pure polystyrene's range of 19-45 J/m. This enhancement stems from the rubber phase's ability to initiate multiple deformation zones, dissipating fracture energy effectively while preserving the rigidity of the styrene-acrylonitrile matrix. Reactive is commonly employed for processing these blends, facilitating in-situ of onto polybutadiene to improve interfacial , with melt temperatures around 200°C ensuring efficient reaction kinetics. degrees in such systems typically range from 5-10% by weight on the polybutadiene backbone, stabilizing the phase morphology during . High-vinyl polybutadiene variants, featuring over 50% 1,2-vinyl content, offer superior compatibility in blends like /ABS (PC/ABS), reducing and enhancing overall impact resistance in demanding applications. Approximately 10% of global polybutadiene production is directed toward consumer goods and , including housings and , where these blends provide shatter resistance and durability.

Sports Equipment

Polybutadiene plays a critical role in sports equipment requiring high resilience and energy return, particularly in golf balls where it forms the core to optimize rebound and distance. The core is typically composed of 100% cis-1,4-polybutadiene rubber, crosslinked with zinc diacrylate at approximately 30 parts per hundred rubber (phr) to achieve a balanced hardness and elasticity. This crosslinking enhances the material's durability while maintaining flexibility, resulting in a core compression rating of 70-90, which suits a wide range of swing speeds. The properties of these polybutadiene cores include a (COR) ranging from 0.75 to 0.80, enabling efficient energy transfer upon impact and contributing to greater ball flight distance. Compared to older balata-covered balls, modern polybutadiene-core designs provide a 10-15% increase in driving distance due to improved rebound efficiency. The high-cis microstructure of the polybutadiene is essential for this resilience, as it allows for superior elastic recovery during deformation. Manufacturing involves the polybutadiene mixture at around 180°C to form the solid core, followed by injection molding an cover for added toughness and spin control. This process, pioneered in the through patents like US3313545 by inventors such as James R. Bartsch, revolutionized golf ball performance by introducing high-cis polybutadiene for enhanced energy return. Beyond golf balls, polybutadiene is blended with (NR) to form cores, where it improves bounce consistency and pressure retention within the felt-covered shell. These blends typically incorporate 5-40% 1,2-polybutadiene with high-cis NR or cis-1,4-polybutadiene to meet international standards for rebound height and durability.

Other Industrial Uses

Polybutadiene, particularly low-trans variants such as (HTPB), serves as a key component in pressure-sensitive adhesives, where it provides flexibility and compatibility with tackifiers like esters to enhance under light pressure. These formulations leverage HTPB's low temperature and viscoelastic properties, often achieving tack values in the range of 10-20 N/cm when blended with esters such as or esters, improving peel strength and shear resistance for applications in tapes and labels. In wire and cable insulation, polybutadiene is blended with (PVC) to impart enhanced flexibility while maintaining electrical performance, resulting in materials with Shore A around 60 for pliability in installation and operation. These PVC-polybutadiene composites exhibit high , typically 20 kV/mm, alongside resistance to abrasion and environmental stress, making them suitable for low-voltage wiring in industrial and automotive settings. Liquid polybutadiene, especially hydroxyl-terminated forms, is incorporated into coatings to deliver robust weather resistance and durability, with formulations maintaining integrity for over five years in outdoor exposures due to inherent hydrophobicity and UV stability. These coatings, cured via reactions, provide excellent adhesion to metal and substrates, resisting and oxidation for protective applications in and marine environments. Polybutadiene contributes to sole formulations, where it is blended for superior resilience and slip resistance with coefficients around 0.6 on wet surfaces. Its high abrasion resistance and low-temperature flexibility ensure durability in everyday , enhancing traction without compromising comfort. In the , emerging bio-based polybutadiene derived from renewable sources, produced via and catalytic processes, is gaining traction for sustainable foams in and insulation, reducing reliance on feedstocks while preserving mechanical properties like elasticity. Pilot plants, such as the one operational since 2024 by , IFPEN, and Axens for bio- production, demonstrate feasibility for large-scale production; as of 2025, bio- output has commenced, advancing toward full bio-based polybutadiene commercialization.

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