Types of Polybutadiene
High Cis Polybutadiene
The alkyllithium and transition metal catalysts make very different products. The transition metal, or so called Ziegler catalysts produce very “stereoregular” polybutadienes with one type having the main polymer chain on the same side of the carbon-carbon double bond contained in the polybutadiene backbone. This is called the cis configuration.
Figure 2. Schematic representation of a high cis polybutadiene.
High cis polybutadiene will usually have cis content >95% which gives rise to better “green strength” and increased cut growth resistance in the cured product. Green strength, which is the strength of the uncured rubber compound, is important for the tyre building process and cut growth resistance is necessary for tyre performance. Cut growth resistance is the resistance to the propagation of a tear or crack during a dynamic operation like the flexing of a tyre in use. High cis polybutadiene also shows lower Tg compared to alkyllithium-based BR because it has almost no vinyl structure. As mentioned earlier, vinyl tends to increase the Tg of the polymer. The low vinyl content and low Tg makes high cis polybutadiene ideal for golf ball cores. Golf ball cores are cured with peroxides, which tend to “over cure” the vinyl units making a very hard and slow golf ball. The neodymium catalyst system produces the highest cis content of about 99% and also makes the most linear chain structure (no branching) producing a polymer with the best tensile and hysteresis (low heat build-up) properties of all the high cis types. The cobalt system produces a highly branched BR with a low solution viscosity that makes a good polystyrene and acrylonitrile-butadiene-styrene modifier. The nickel catalyst makes polybutadiene with an intermediate level of branching.
Figure 3. Schematic representation of vinyl.
Lithium-based Polybutadiene
The alkyllithium or “anionic” catalyst system produces a polymer with about 40% cis, 50% trans and 10% vinyl when no special polar modifiers are used in the process. The alkyllithium process is probably the most versatile, because the growing chain end contains a “living” anion (negative charge) which can be further reacted with coupling agents or functional groups to make a variety of modified polybutadienes. It also produces gel-free polybutadiene making it ideal for plastics modification. Vinyl increases the Tg of the polybutadiene by creating a stiffer chain structure. Vinyl also tends to crosslink or “cure” under high heat conditions so the high vinyl polymers are less thermally stable than low vinyl. Note above, that in vinyl units the double bonds are pendent to the main chain, giving rise to the special properties of high vinyl polymers. Vinyl units can be increased in lithium-based anionic polymerisation through the use of polar modifiers, which are usually nitrogen or oxygen-containing compounds. The modifiers direct the attack of the propagating anion on the “living” chain end to give a 1,2 addition to the butadiene monomer.
Growing “living” anion (negative charge) on end of live polybutadiene chain with Lithium counterion (positive charge)
High Trans Polybutadiene
High trans polybutadiene is a crystalline plastic material similar to high trans polyisoprene or balata, which was used in golf ball covers. Note below, that in the trans configuration the main polymer chain is on opposite sides of the internal carbon-carbon double bond. Trans polybutadiene has a melting point of about 80°C. It is made with transition metal catalysts similar to the high cis process (La, Nd, and Ni). These catalysts can make polymers with >90% trans again using the solution process.
Figure 4. Schematic representation of trans 1,4 polybutadiene.
Conclusion
Polybutadiene is and will continue to be a high volume rubber for use in tyres, toughened plastics, and golf balls due to its low cost, availability and unique properties. As new markets develop, there will be a need to develop new, higher performance grades of polybutadiene using both the alkyllithium and Ziegler systems.
