Synthesis of poly(2,3-dimethyl-1,3-butadiene) and its hydrogenation using a metallocene/n-butyllithium catalyst system

Poly(2,3-Dimethyl-1,3-butadiene) (PDMB) with varying contents of 1,4- and 1,2-structures has been anionically synthesized using either n-butyllithium or sec-butyllithium as an initiator. The addition of tetrahydrofuran could enhance the rate of synthesis and effect the microstructure. The T-m was higher for PDMB with a lower content of 1,2-structure, and the T-g was lower. This PDMB was then hydrogenated with a nickelocene/n-butyllithium catalyst system leading to the formation of HPDMB. The trans 1,4-structure unit was more difficult to hydrogenate due to its steric hindrance. Repetitive hydrogenation was necessary in order to achieve a high degree of hydrogenation. The hydrogenated PDMB is an amorphous elastomeric material. The To's were found to decrease with an increase in the degree of hydrogenation, concurrent with a gradual disappearance of the T-m's. Since a HPDMS with a low content of 1,2-structure resembles a head-to-head polypropylene, our data suggest that the T-g of an atactic head-to-head polypropylene lie between -30 and -35 degrees C.