The increased demand for low dielectric constant (k < 3.0) chemical vapor deposited polymer thin films to replace SiO2 (k = 3.9-4.3) to reduce RC-delay in ultra large scale integration (ULSI) devices has prompted the synthesis of many new polymers. However, the ultimate properties of the polymer thin film are determined by its molecular structure that for polymers is often anisotropic. A basic understanding of the structure-property relations and how they are influenced by the molecular architecture is imperative for the future development of polymer thin films in a large number of applications including those in the electronics industry. The study here utilizes X-ray diffraction and variable angle spectroscopic ellipsometry to characterize the optical birefringence and its change as a function of successive post-deposition anneals with poly(p-xylylene), poly(chloro-p-xylylene), and poly(tetrafluoro-p-xylylene). beta-PPXN and VT-4 are shown to have a large negative birefringence after a post-deposition anneal. However, PPXC and PPXD show the opposite behavior, exhibiting a small positive birefringence after anneal. Possible reasons for this difference are given.