Due to the presence of two alternating bond angles in the backbone of the polymeric chains, polyisobutylene (PIB) possesses four different rotational isomeric states. Accordingly, conformations of PIB could be represented by a four-state rotational isomeric scheme with only one adjustable statistical weight parameter. Rotational isomeric states (RIS) were determined at +25°, −25°, +120°, −120°. Conformations were divided into a “+class” and “−class,” with bond conformations tended to be followed by those from the same class and with changes from one class to the other rare. Since the configurational and thermodynamic properties of PIB depend to a great deal on the conformational characteristics of the polymer, a modified rotational isomeric state approximation was used to generate the initial configurations of the polymeric chains without allowing for any segment-segment overlap. Attempts were made to test the configurational properties of these systems against those determined experimentally to ensure that these configurations do represent realistically the polymeric system. Furthermore, these configurations were used to perform subsequent molecular dynamics runs to elucidate the effect of the molecular weight of the polymer and the temperature on some of its important thermodynamic properties, such as self-diffusion coefficient, thermal pressure coefficient, heat capacity, and dielectric constant.