The glass transition temperature is a property of amorphous polymers that is not yet well understood. In this study united-atom molecular-dynamics computer simulations of linear and cyclic atactic polystyrene were performed for bulk and free-standing films. The free-standing films thickness ranged from 2 nm to 20 nm and the bulks chain length ranged from 10 monomers to 160 monomers. The glass transition temperature has a strong film thickness dependence on films less than 10 nm. To understand why this is the case we studied the distribution of chain ends, orientation of the backbones and phenyl rings, and the properties of the interfacial layer. To investigate which of these has a greater influence on the glass transition temperature the data from the linear polystyrene was compared to those of cyclic polystyrene. Using cyclic polystyrene it was also found that the glass transition temperature has a molecular weight dependence. This was accomplished by using different methods to calculate the glass transition of bulk cyclic polystyrene. The methods employed to calculate the glass transition temperature were tracking the particles diffusion and specific volume. The ideal transition temperature was also found by studying the orientational mobility of the phenyl bonds.