Micro-kinetics of pitch polymerization with regards to molecular weight distribution

Mesophase pitch is made via thermal polymerization from raw pitch with smaller molecular size. However, the molecular weight distribution (MWD) is difficult to control in a narrow range for better spinnability. Deep understanding of the polymerization process is needed. In this study, a micro-kinetics model for pitch polymerization is proposed in terms of free radical generation, free radical combination and molecular structure transformation that occur cyclically. The reactions result in molecular size increase and distribution widening. The MWD and average molecular weights are simulated and fitted to the experimental data examined via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. The results show that the so-called autocatalysis that makes the MWD widen is acceleration of the reaction speed as molecular size increases since the activation energy of free radical generation decreases. Furthermore, the molecular size of specific solvent insolubility becomes smaller as the reaction progresses. This model can be helpful for the control of the composition of pitch by adjusting the reaction temperature, time and other processes purposefully.