A series of high molecular weight polymethylsilsesquioxanes (PMSQs) were synthesized through polymerization of an isolated hydroxyl-substituted cyclic siloxane stereoisomer, cis-trans-cis 1,3,5,7 tetramethyl 1,3,5,7 tetrahydroxyl cyclosiloxane, for interlayer dielectric (ILD) application as low dielectric materials. The molecular weights of PMSQs were controlled by varying polycondensation time. Structural analyses of the obtained PMSQs were carried out by gel chromatography (GPC), FTIR, $^{29}Si$ NMR, static light scattering (SLS), solution X-ray scattering, and X-ray diffraction. The change in molecular shape of the PMSQs went from linear to branched in structure as molecular weight increased. Furthermore, thin film properties were investigated for application as interlayer dielectric materials. As structural branching increased, its properties showed isotropic tendencies, relatively high modulus (5.4 GPa), and low dielectric constant (2.74) with increase in the inter-molecular space.