Around the turn of the century, in the early days of synthetic rubber research, attempts were made to polymerize not only isoprene (methyl butadiene), the structural unit of NR, but also its simplest analogue, butadiene (C.D. Harries, F. Hofmann, 1911). Polymerization of butadiene using sodium, from which the name Buna (1926) was derived, has led to the development of numbered Buna grades. These early research efforts and the first commercial grades that were marketed in Germany and Russia, were only a temporary success, and no substantial markets developed. /1/

Only through the use of coordination catalyst of the Ziegler-Natta type, and the use of alkyl lithium catalyst was it possible to produce solution polymers, which are widely applied in tyre compounds, particularly in blends with NR or SBR. Today, the by far largest proportion of the globally produced BR is obtained by solution polymerization. The initiators used are primarily coordination catalysts, namely titanium, cobalt, nickel and neodym compounds or alkyl lithium compounds. When BR is made by the solution process the microstructure (cis, trans, vinyl) of the polymer can be varied. From the free radical polymerization of butadiene in emulsion, a less uniform BR is obtained. /1/

Structure
BR is composed of butadiene units which may have joined linearly by 1,4 (preferred in cis-1,4, but also in certain measure, trans-1,4 conformation), as well as by 1,2-addition (vinyl structure of butadiene). The structure of BR (cis/trans structure, MWD, branching) depends in great measure on the polymerization process and used catalyst. The glass transition temperature Tg is about -100^oC, among the lowest of all rubbers. Pure syndiotactic or isotactic 1,2-polybutadienes are not rubbery. They are, instead, impact resistant thermoplastics. /1,2/

Figure 1 - The structural formula of BR

The higher the cis-1,4 content of BR, the lower is its glass transition temperature Tg. Pure cis-1,4 BR grades have a Tg temperature of about -100^oC, while commercial grades with about 96% cis-1,4 content have one below -90^oC. Pure cis-1,4 polymers have a melting point of +1^oC and do not exhibit strain crystallization at room temperature. The glass transition temperature rises linearly as the concentration of 1,2 structure (vinyl content) increases. The tendency towards crystallization is determined by the 1,2 content. /1/

Pure cis-1,4 BRs have the best abrasion resistance. They have also low hysteresis and will give good snow traction in the tread area of the tire, but poor wet traction. As the 1,2 content increases, the abrasion resistance becomes poorer and the wet traction improves, so that a compromise has to be found for spesific applications. A high vinyl (70%) BR will give good traction and handling in the tread area of a tire but has poor snow traction and very poor tear resistance. With the exception of ENR (epoxidized BR), the general rule applies that abrasion resistance is gained at the expence of wet traction. /1,5/

Processability and properties
Because of its poor behaviour on mills and for vulcanizate property reasons, especially wet traction, BR is mostly used in blends with NR or SBR. BR can be easily blended with all non-polar diene rubbers. /1/

Dynamic properties, such as heat build-up and resistance to groove cracking, are improved for NR and SBR vulcanizates, when they are compounded as blends with BR. In addition, the reverse resistance on overcure and the aging resistance of NR vulcanizates are improved, when they contain BR as blend component. /1/

By increasing the BR content in a blend with NR or SBR, the rolling resistance of tyres built from the resulting vulcanizates becomes smaller. This is particularly advantageous for the fuel consumption of cars. At the same time, however, the traction, and in particular, wet traction, becomes poorer, making optimized balance of properties necessary. On the other hand, a higher BR content in these blends (about 40%), has a favourable influence on ice traction, which is important in formulating tread compounds for winter tires. /1/