Anionic Polymerization of 2,3-Dimethylbutadiene

2,3-Dimethylbutadiene was polymerized in tetrahydrofuran with n-BuLi. The content of the 1,2-unit in the polymer varied with the polymerization temperature as follows: 86% at −78°C, 69% at −45°C, 54% at −30°C, and 48% at 0°C. The relationship between log (k1,2/k1,4) and 1/T showed a break at −30°C, where k1,2 and k1,4 are the propagation rate constants leading to the denoted structures, respectively, and T is the polymerization temperature (°K). It was postulated that the continuous arrangement of 1,2-units, which is preferential at low temperature, becomes impossible at higher temperatures due to steric hindrance, and the insertion of the 1,4-unit in the sequence of 1,2-units takes place more frequently, resulting in alternating arrangements of the 1,2- and 1,4-unit above −30°C. The NMR spectra of the polymers showed that the chemical shifts of each monomeric unit varied with the difference of the neighboring monomeric units in both sides. The spectrum of the polymer having a 1,2- and 1,4-unit in 1:1 suggested the 1,2-unit being held between the 1,4-units, i.e., both the units being alternate. The polymerization of 2,3-dimethylbutadiene in hydrocarbon solvent gave a polymer rich in the 1,4-unit with n-BuLi at 30°C, but formed a similar polymer with K-dispersion as that obtained with n-BuLi in tetrahydrofuran at 30°C.