Polybutadiene is a synthetic rubber that has a high resistance to wear and is used especially in the manufacture of tires. It has also been used to coat or encapsulate electronic assemblies, offering extremely high electrical resistivity. It exhibits a recovery of 80% after stress is applied, a value only exceeded by elastin and resilin. Polybutadiene is a polymer formed from the polymerization of the monomer 1,3-butadiene.
Polymerization of butadiene
1,3-butadiene is an organic compound which is a rather simple conjugated diene hydrocarbon; the chemical structure is shown as a reactant in the diagram below. A hydrocarbon diene molecule has two C=C double bonds (i. e. between two sets of carbon atoms). Polybutadiene can be formed from many 1,3-butadiene monomers undergoing free radical polymerization to make a much longer polymer chain molecule.
A chain propagating step in this chemical reaction involves a free radical near the end of a growing polymer chain forming a covalent bond with the #1 carbon in a 1,3-butadiene monomer molecule being added, resulting in a polymer chain intermediate with a substituted allyl free radical at the end of the chain. This allyl free radical, formed from the butadiene just added, can further bond to another monomer molecule at either the #2 or #4 carbons of the previous butadiene monomer. Most of the time, the new monomer bonds to the #4 or terminal carbon of the previous butadiene, resulting in a 1,4-addition of the previous butadiene unit. In a 1,4-addition, the two double bonds of the previous butadiene unit are turned into single bonds and a new double bond is formed between the #2 and #3 carbons. This new double bond may have either a cis or a trans configuration. A smaller fraction of the time (perhaps 20%), the new monomer bonds to the #2 carbon of the previous butadiene, resulting in a 1,2-addition of the previous butadiene unit. The double bond between the #1 and #2 carbons turns into a single bond in the previous butadiene unit, and the double bond between the #3 and #4 carbons remains intact in a short vinyl side group available for branching or cross-linking. Cis or trans configurations are not applicable in 1,2-additions of butadiene. See the following reaction diagram for examples of 1,2- and 1,4-addition in a polybutadiene chain.
The trans double bonds formed during polymerization allow the polymer chain to stay rather straight, allowing sections of polymer chains to line up against each other and effectively form microcrystalline regions in the material. The cis double bonds cause a bend in the polymer chain, preventing polymer chains from lining up and forming crystalline regions and resulting 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 which depend on temperature. There are different catalysts available which can result in polymerization either in the cis or the trans configurations.
1,3-butadiene can be copolymerized with other types of monomers such as styrene and acrylonitrile to form rubbers or plastics with various qualities. These copolymers are commonly graft copolymers, meaning that there are sections of polymer of one kind of monomer forming the main chains and grafts made of another type of monomer forming branches and cross-links bonded to the main chains. This way a copolymer material can be made with good stiffness, hardness, and toughness.